Index

Component information

    General

    Few designers spend time pondering the traits of resistors, capacitors, or other simple passive components. Determine the necessary nominal value, tolerance, and temperature coefficient for each instance in your circuit, and you're pretty much done. Consider the nonelectrical variables like package and pricing, and you're a certifiable good corporate citizen. When it comes to the active components, there are also many other things to consider.

    Resistors

    Resistors are passive electronic components used extensively on the circuit boards ofelectronic equipment. Resistors are usually used to limit current, attenuate signals, dissipate power (heating) or to terminate signal lines. Resistors are usually color coded with stripes to reveal their resistance value (in ohms) as well as their manufacturing tolerance.

    Most importans characteristics of resistor are the resistance, tolerance of resistance and the power handling capacity. Resistors are generally available from the fractions of ohms upto several megaohms (higher value special components are also available).

    Most small general purpose resistors have power handling capacity ofaround 0.25W. Most resistors used to be this type, and most electronics designs expect this kind of resistor unless the power rating is mentiones. In typical circuits you can nowadays see resistors with power handling of 0.125W up to 1W. The typical voltage rating of such "normal" resistorsis normally 125..250V. Also special power resistors are available, generallywith power rating from few watt up to 50-100W. Higherst power power resistorsare generally built to metal case which is designed to be connected to a heatsink.

    The resistors are manufactured with some tolerance. For example a typical resistor can have a 5% tolerance, which means that the resistance value can be 5% higher or 5% lower than what the color code indicated. There are special accurate resistors also available, for example resistors with 1% or better accuracy.

    There are many differetn resistor types which are characterized by the material they are made of and how they are constructed. Here are some details of different resistor types:

    • Carbon film resistor: cheap general purpose resistor, works quite well also on high frequencies, resistance is somewhat dependent on the voltage over resistor (does not generally have effect in pratice)
    • Composite resistor: Usually some medium power resistors are built in this way. Has low inductance, large capacitance, poor temperature stability, noisy and not very good long time stability. Composite resistor can handle well short overload surges.
    • Metal film resistor: good temperature stability, good long time stability, cannot handle overloads well
    • Metal oxide resistor: mostly similar features as metal film resistor but better surge handling capcity, higer temperature rating them metal film resistor, low voltage dependity, low noise, better for RF than wire wound resistor but usually worse temperature stability
    • Thick film resistor: Similar properties as metal fim resistor but can handle surges better and can withstand high temperatures
    • Thin film resistor: good long time stability, good temperature statiblity, good voltage dependity rating, low noise, not good for RF, low surge handling capacity
    • Wirewound resistors: Wire would rsistors are built just by winding a thin wire (made of material that has considerable resistance) over some insulating material (so that different wire sounds do not touch each other). Wirewound resistors are the quietest resistor type having only thermal noise. The downside of them is that because the wire in resistor makes a coil with quite many turns, they usually have considerably high inductance (not suitable for high frequency operation). Wire wound construction is mainly for high power resistors and high power potentiometers. Wire wound construction is also used to make very accurate resistors for measuring circuit.

    In some applications the temperature dependance of resistor value is important. Metal oxide thick film resistors typically have temperature coefficients of +/-200ppm/C and can be run at up to 125C, which implies a resistance change of up to 20,000ppm, or 2% on that temperature range. Regular metal film resistors, at +/-100ppm/C, are a bit better. A carbon film resistor has typically a negative temperature coefficient ofaround 500ppm/C.

    Resistors have a limited voltage handling capacity. If there is higher voltage that the rated votlage over the resistor, it can fail and arch-over. The voltage rating of typical 0.25W general purposes resistors is typically in 200-300V range. If higher voltage handling is needed, then depending on application a special high voltage resistor or several lower voltage resistors are wired in series. There are high voltage resistors that can handle high voltages up to many kilovolts.

    In some applications resistors are used like a fuse (for example in some power supplies and telecom applications). In those applications theresistor burns up when it is overloaded. In this type of application non-flammable resistor are used to avoid the flames and risk of fire. If the application calls for non-flammable resistor (usually haswhite case), do not replace it with any other type. Sometimes special resistors designed to be used as fuses are called fusible resistors.

    Sometimes you can see a zero ohm components on the circuit board. Zero "0" ohms means that there is no resistance. In theory, it means total conductance. In practice, it can be a piece of wire. Zero ohm resistors are typically used as jumpers on boards. Sometimes those zero-ohm resistors can act like fuse links or as local RF stand off chokes with a by pass cap on the other side.

    Potentiometers are adjustable resistors. The resitance value of potentiometer is adjusted by moving the potentiometer control. The potentiometer control is usually rotating control, but can be sometimes linear slider. Potentiometers are usually available with aroud 6 mm axles where user can attach knobs. In USA those potentiometer control diameter is typically 1/4 inched (6.35 mm) and in Europe it is generally 6 mm. Potentiometers are used often as variable resistor or where only a portion of an output voltage from a signal source is needed (for example audio volume controls). A potentiometer generally has 3 terminals. 2 of the terminals are connected to the opposite ends of a resistive element. The 3rd terminal (usually, is physically in-between the other 2 terminals) is called the wiper. The wiper is a contact that slides along the resistive element. Most potentiometers are constructed so that they are controlled with a rotatable shaft, but there are also potentiometer where the control movement is linear movement (used for example in audio mixer sliders). The potentiometers with rotatable shaft in them typically turn their whole range at around 270 degrees or turning, but there are also other constructions with different turns rations and even special multi-turn potentiometers (for example 10 turns range). The potentiometer response from control resistance to resistance value does not need to be linear. There are for example potentiometers with logarithmic, semilogarithmic and inverse-logaritmic response available (many of them used in audio circuits adjusting audio signal attenuation or amplification). In addition to the normal potentiometers, there are also similar components called "trimmers". Those work in the same way as potentimeters, but are generaly made adjustable with a crewdriver and not designd for continuous adjustation (limited mechanical workind life). Trimmers are generally put to circuit boards in places where there is need for factory adjustments (for example calibration) and repair adjustments. Overview of different potentiometer types:

    • Potentiometer: Designed for frequent changes, for example a volume control or other user control
    • Trimmer: Designed for less then frequent changes (occasional), for example a trim pot on a PCB to finalize a circuit resistance
    • Rheostat: A three terminal potentiometer which only uses two terminals.
    Overview of different potentiometer contructions / materials:
    • Carbon composition: This is the most commonly used material, cheap to make, usually medium power handling capacity, the secular change of the resistance as a result of exposure to high temperature or humidity is rather great (but very uniform over the entire resistor), typical available resistance 50ohm - 50Mohm. In the manufacturing a coating of carbon material is applied to a substrate and cured.
    • Resistive wire (Wirewound): Used to make medium or high power potentiometer, properties similar to wirewound resistors, available resistance range is typically quite low resistance values, typical available resistance range 10ohm - 100Kohm. The potentiometer is contructed by winding resistor wire to base material. Please note that the resistance of wirewound potentiometer is changed in discrete steps as the wiper moves from one wire to another.
    • Conductive Plastic: Humidity can have large effect on resistance, low power handling capacity, low noise, typical available resistance range 100ohm - 5Mohm. A conductive plastic material is generally molded into the desired resistive element shape.
    • Cermet (a mixture of ceramic and metal): Quite immune to humidity, works well for high frequency, low and high power constructions available, typical available resistance range 10ohm - 5Mohm.
    • Thin film: A thin coating of resistive metal is evaporated or sputtered onto a substrate. The available resistance range of the element can be limited.

    In carbon composite, conductive plastic, and cermet potentiometers the resolution is essentially infinite, however contactresistance between the wiper and the cermet element must be taken into consideration.

    The subject of resistor types comes up quite often in audio circuit design. Some people will recommend only using carbon composition resistors, others will tell you that metal film resistors are better. Who is correct? Well, the answer depends on what your design goals are. From a noise aspect, there are several things to take into consideration. Resistor noise is made up of three main types: thermal, contact, and shot noise. Thermal noise is mainly dependent on temperature, bandwidth, and resistance, while shot noise is dependent on bandwidth and average DC current, and contact noise is dependent upon average DC current, bandwidth, material geometry and type. Wirewound resistors are the quietest, having only thermal noise, followed by metal film, metal oxide, carbon film, and lastly, carbon composition.

    The thermal noise of a resistor is equal to: Vt = SQRT(4kTBR)
    where: Vt = the rms noise voltage, k = Boltzmann's constant, T = temperature(Kelvin), B = noise bandwidth, R = resistance
    Since the characteristics of thermal noise have a Gaussian probability density function, and the noise of the two separate sources is uncorrelated white noise, the total noise power is equal to the sum of the individual noise powers.

    In general, the thermal noise of any connection of passive elements is equal to the thermal noise that would result from the real part of the equivalent total impedance. If we are dealing with pure resistances, the thermal noise is equal to the thermal noise produced by an equivalent resistance. Therefore, the thermal noise of a 1K carbon resistor is the same as a 1K metal film; it is independent of material. The only way to reduce this noise is to reduce the resistance value.

    Contact noise is dependent on both average DC current and resistor material/size. The predominant noise in carbon comp, carbon film, metal oxide, and metal film is composed of contact noise, which can be very large at low frequencies because it has a 1/f frequency characteristic. Wirewound resistors do not have this noise, only resistors made of carbon particles or films. This noise is directly proportional to both the current flowing in the resistance and a constant that depends upon the material the resistor is made of. If no current (AC or DC) flows in the resistor, the noise is equal to the thermal noise. The contact noise increases as the current is increased. The material and geometry of the resistor can greatly affect the contact noise.

    Shot noise is dependent upon current, so the more average DC current through a resistor, the more noise you get. In order to reduce this type of noise, you must keep the DC current to a minimum

    In general, for low-noise design:

    • Keep resistance values low, because thermal noise is directly proportional to resistance value.
    • Wirewound resistors are the best choice for noise, followed by metal film, metal oxide, carbon film, and lastly, carbon composition.
    • Use the largest practical wattage resistors (unless you are using wirewound resistors) because contact noise is decreased in a larger geometry material.
    • Keep the DC and AC currents to a minimum because contact noise is proportional to current.
    • Don't forget that potentiometers are also resistive elements, and are almost always carbon composition, and generally are large values. Those can be a major source of noise.

    Since high-quality metal film resistors and wire wound resistors are more expensive than cheaper carbon films, there are also costs that needs to be considered in the selection of best resistor type for the application.

    Please note that resistors have a maximum voltage rating that needs to be looked at on applications where high voltages are present or can be present. For example many 0.25-0.5W resistors and some 1 watt resistors are only rated for 250-350V. Be sure to get a resistor rated for the appropriate voltage that is used in your circuit.

    Integrated circuits

    Integrated circuits are miniaturized electronic devices in which a number of active and passive circuit elements are located on or within a continuous body of material to perform the function of a complete circuit. Integrated circuits have a distinctive physical circuit layout, which is first produced in the form of a large scale drawing and later reduced and reproduced in a solid medium by high precision electro chemical processes. The term "integrated circuit" is often used interchangeably with such terms as microchip, silicon chip, semiconductor chip, and micro-electronic device.

      Power supply ICs

      Voltage regulators are very commonly used ICs in small power supplies. There are many easy to use regulators in the market, meaning tha you can usually just put one to circuit without too much design work. But when you use voltage regulator, remeber the decoupling capacitors (on both input and output), because many regulators will oscillate if they don't have enough outputcapacitance.

      Popular general purpose ICs

      Operational amplifiers

      Op-amps are generic term for a generic device that has a differentialinput of nearly infinite gain. The circuits using them rarely depend ona specific type. Use any basic circuit and any good op-amp. The operational amplifier is the work horse of the analog world. It is found in applications ranging from cellular phones to laptop computers to smoke detectors. Operational amplifiers are the child of the analogue signal processing age. Ironically, perhaps, today's emphasis on digital systems shifts such computational duties from continuous-time to clocked-circuit operation, but systems engineers require more op amps than ever before to bridge the analogue-to-digital divide.

      The classical ideal operational amplifier is characterized by infinite input impedances (zero input currents), zero output impedance (ideal voltage source output), and an infinite open-loop voltage amplification factor, A. OpAmps with very large A factors (100-120 dB being quite normal) and a surrounding negative-feedback circuit (a circuit path between the output terminal and the inverting input terminal) will lead to the simplification in circuit design known as the virtual-ground principle: The output voltage remains finite, whereas the input voltage difference is suppressed by the feedback mechanism to virtually zero! The output voltage of any real opamp will of course be restricted by the limited supply voltages of the opamp.

      Two main factors now challenge semiconductor-device and end-user equipment designers alike: the trend toward single-supply operation and the explosive growth in mobile devices. Each of these factors adds its own requirements, but both share the ever-lower power-consumption requirements that contemporary designs demand. Single-supply operation now dominates op-amp applications for several reasons. First, it's convenient; you no longer have to design and accommodate multiple power supplies. Just lowering the supply voltage from the traditional ?15V to, say, 5V helps you to conserve energy and minimise power dissipation.

      Certain applications, such as audio, demand low-noise performance. The amplifier itself generates internal, or amplifier, noise. The designer must account for the effects of amplifier noise, because the wrong instrumentation amplifier can make amplifier noise dominant. The most important parameter in low-noise design is the source impedance. Low source impedance dictates selection of a low-voltage-noise amplifier. High source impedance dictates that you select a low-current-noise amplifier. And medium source impedance means that the amplifier selection is a compromise between voltage- and current-noise performance. JFET is usually a better choice than CMOS for low-noise performance in the 20-Hz to 20-kHz frequency range.

      There are two different classes of operation amplifiers in use today: voltage-feedback amplifiers (VFA) and current-feedback amplifiers (CFA). The name, operational amplifier, was given to voltage-feedback amplifiers (VFA) when they were the only op amps in existence. These new (they were new in the late .40s) amplifiers could be programmed with external components to perform various math operations on a signal; thus, they were nicknamed opamps. Current-feedback amplifiers (CFA) have been around approximately twenty years, but their popularity has only increased in the last several years. Two factors limiting the popularity of CFAs is their application difficult and lack of precision.

      The VFA is familiar component, and there are several variations of internally compensated VFAs that can be used with little applications work. Because of its long history, the VFA comes in many varieties and packages, so there are VFAs applicable to almost any job. VFA bandwidth is limited, so it can't function as well at high signal frequencies as the CFA can. The VFA has some other redeeming virtues, such as excellent precision, that makes it the desirable amplifier in low frequency applications.

      Fortunately, precision is not required in most high frequency applications where amplification or filtering of a signal is predominant, so CFAs are suitable to high frequency applications. The lack of precision coupled with the application difficulties prevents the CFA from replacing the VFA in many traditional opamp application.The CFA circuit configuration was selected for high frequency amplification because it has current-controlled gain and a current-dominant input. Being a current device, the CFA does not have the Miller-effect (resulting from stray capacitance) problem that the VFA has. The input structure of the CFA sacrifices precision for bandwidth, but CFAs achieve usable bandwidths ten times the usable VFA bandwidth.

      The input impedance of a VFA and CFA differ dramatically because their circuit configurations are very different. The VFA input circuit is a long-tailed pair, and this configuration gives the advantages that both input impedances match. Also, the input signal to VFA looks into an emitter-follower circuit that has high input impedance. So far, the implicit assumption is that the VFA is made with a bipolar semiconductor process. Applications requiring very high input impedances often use a FET process.The CFA has a radically different input structure that causes it to have mismatched input impedances. The noninverting input lead of the CFA is the input of a buffer that has very high input impedance. The inverting input lead is the output of a buffer that has very low impedance. There is no possibility that these two input impedances can be matched. The CFA is practically limited to the bipolar process because that process offers the highest speed (FET process is not attractive today).

      Stability is important in opamp circuits. The op amp contains many poles, and if it is not internally compensated, it requires external compensation. The op amp always has at least one dominant pole, and the most phase margin that an op amp has is 45?. Phase margins beyond 60? are a waste of op amp bandwidth. Wiring the op amp to a printed circuit board always introduces components formed from stray capacitance and inductance. Stray capacitance causes stability to increase or decrease dependingon its location. Stray capacitance from the input or output lead to ground induces instability, while the same stray capacitance in parallel with the feedback resistor increases stability. Stray inductance becomes dominant at very high frequencies. The CFA stability is not constrained by the closed-loop gain, thus a stable operating point can be found for any gain, and the CFA is not limited by the gain-bandwidth constraint. If the optimum feedback resistor value is not given for a specific gain, one must test to find the optimum feedback resistor value. Stray capacitance from any node to ground adversely affects the CFA performance. Stray capacitance of just a couple of pico Farads from any node to ground causes 3 dB or more of peaking in the frequency response. Stray capacitance across the CFA feedback resistor, quite unlike that across the VFA feedback resistor, always causes some form of instability. CFAs are applied at very high frequencies, so the printed circuit board inductance associated with the trace length and pins adds another variable to the stability equation. The wiring of CFAs is critical, so stay with the layout recommended by the manufacturer whenever possible.

      A quite often mentioned opamp type in hobby circuits is 741. 741 has been around for awhile (introduced 1968), it is still made because it is such a good,reliable general purpose device. It's an ancient op-amp with quirks that canbe frustrating unless you know how its internals work (those relate to internal compensation). If you are designing anything new demanding yourself, I recommend to use something better (there is wide choise of those). The LM324 quad and the LM358 dual op amps are cheaper, more compact,just as crummy and work as single supply op amps. For not so demanding simple application 741 is still useable workhorse, and it is widely available also in well shielded cases. For military applications, and the occasional harsh industrialenvironment, but even there, using the 741 is generally a cop-out.These days there are always better amplifiers for any specific job.

      Some well known operational amplifiers:

      • ?A702 is the first solid-state monolithic op-amp introduced 1963. It was manufactured by Fairchild Semiconductors.
      • ?A709 is first widely successfull opamp (introduced 1965), +/- 15 Volt DC power supply
      • LM101 first opamp with 'short-circuit' protection, and simplified frequency compensation (external capacitor across selected connection pins), increased gain (up to 160K)
      • ?A741 was internally an internally compensated op-amp introduced 1968, short-circuit protected output, this device is still made because it is such a good, reliable general purpose device
      • RC4558 in 1974, characteristics similar to the ?A741
      • LM324 quad op amp from National Semiconductor in 1974, similar in characteristics in comparison with the ?A741 in speed and input current, low-power consumption, single-power-supply requirement, four opamps in one case, this is still made and widely used
      • CA3130 was the first FET input op amp (1974), can be supplied by a +5 to +15vdc single supply system
      • TL084 op amp in 1976, a quad JFET input op amp, low bias current and high speed
      Most of the mentioned op-amps have of course been replaced over time, keeping the same model number, with cleaner and low-noise types.We now enjoy a variety of op amps that will provide the user essentially with anything s/he needs, such as high common-mode rejection, low-input current frequency compensation, cmos, and short-circuit protection. All a designer has to do is expressing his needs and is then supplied with the correct type. Op-Amps are continually being improved, especially in the low-noise areas.

      Comparators

      Comparators compare two voltage levels and provideo digital 1/0 output depending on the input voltage levels. Comparators have an op-amp front end and a digital back end that operates like a gate. The comparator output stage may be an open collector transistor, so it often connects to the logic supply through a pullup resistor. Regardless of the input voltage, the output voltage is saturated at either power-supply rail because the analog front end amplifies input voltages with an almost infinite gain. Manufacturers employ digital semiconductor techniques to make the output circuits in a comparator; thus, the comparator comes out of saturation very quickly. The response time of a comparator is so important that it is a data-sheet specification.

      In some cases you can use an opamp as a comparator. If you leave the feedback resistor out of an op-amp circuit, it operates like a comparator, but you shouldn't use op amps to perform comparator functions except under limited conditions. Opamp manufacturers employ analog semiconductor techniques to make the output circuit of an op amp. The opamp designers assume that the output never saturates, hence, the response time of an op amp driven into saturation is uncontrolled.

      • Adding hysteresis to comparators - Comparators have very high open-loop gain, and, without some type of positive feedback, they have no noise immunity. This column adds hysteresis to comparators to eliminate multiple switching on the output.    Rate this link
      • Designing with comparators - Comparators have an op-amp front end and a digital back end that operates like a gate. The comparator output stage may be an open collector transistor, so it often connects to the logic supply through a pullup resistor. Regardless of the input voltage, the output voltage is saturated at either power-supply rail because the analog front end amplifies input voltages with an almost infinite gain. If you leave the feedback resistor out of an op-amp circuit, it operates like a comparator, but you shouldn't use op amps to perform comparator functions except under limited conditions.    Rate this link

      Other analogue ICs

      • Analog ICs for 3V Systems - Single 3V operation is available for many op amps, comparators, and microprocessor supervisors, and for some RS-232 interface ICs.    Rate this link
      • How did analog ICs get that good? - building blocks available on a typical IC fabrication process are really not very good in absolute terms because the key transistor parameters such as transconductance, input threshold voltage and output impedance vary by at least plus or minus 20% and are not as good as can be produced in discrete form but with correct desing it is possible to make very high performance analogue ICs    Rate this link
      • Reinventing The Role Of Analog/Mixed-Signal - not long ago, analog and mixed-signal functionality were treated as though they were an afterthought in the system design process but now markets move towards mixed-signal technology which combines analog and digital functionality    Rate this link
      • Selecting the Right CMOS Analog Switch - First developed about 25 years ago, integrated analog switches often form the interface between analog signals and a digital controller. This tutorial presents the theoretical basis for analog switches and describes some common applications for standard types.    Rate this link

      Digital to analogue converters

      • Take the rough edges out of video-filter design - Incorrectly processed image-frequency information can distort displays generated from digital-video sources. Oversampling and well-implemented video-DAC-output filters can save the day, but improperly designed filters can make matters worse. Before you design your next digital-video system, take some time to investigate video-reconstruction-filter design and trade-offs in oversampling.    Rate this link

      Voltage references

      • A quick guide to voltage references - A review of reference topologies and a quick look at the various ways that manufacturers specify references will help you pick the best part for your next design.    Rate this link
      • Selecting Voltage References - Voltage references are simple devices, but making the right choice for a given application can be a chore if you don't take an orderly approach. This article simplifies the task with a review of the available reference types and a discussion of the specifications manufacturers use to describe them.    Rate this link

    Capacitors

    A capacitor is simply two charged plates placed close together with a dielectric (non-conducting)material sandwiched between the plates. When a charge is applied to one plate, it repelscharges on the opposite plate, until an equilibrium is established. For direct current, the capacitorcharges up with a time constant that depends on the capacitance value and the impedance throughwhich the current flows into the capacitor. Once the capacitor is fully charged, no more current flows. This means that the capacitor is an effective block for direct current. For alternating current (like audiosignals), the response is more complicated. The charge that develops on the capacitor depends on howfast the current is changing. It takes time for the charge to build up, and that time results in a frequency dependent delay (or phase shift) in the output signal.

    Capacitor device is often used to store charge in an electrical circuit. A capacitor functions much like a battery, but charges and discharges much more efficiently. A basic requirement for all electronic circuits is the inclusion of bypass, or decoupling, capacitors. These devices reside across the positive supply to ground, as close as possible to the supply pin of the active device. You may get away with excluding these capacitors in low-frequency circuits, but many low-frequency active devices have high-frequency entities inside the active devices. Digital devices are not the only chips that require bypass capacitors. Analog circuits also benefit from including bypass capacitors but in another way. Although bypass capacitors in digital systems control fast rising- and falling-time glitches from the device, bypass capacitors in analog systems help reduce power-supply noise at the analog device. Typically, analog devices have built-in, preventive power-supply filtering or line-rejection capability. These noise-rejection mechanisms effectively reduce low-frequency power-supply noise, but this scenario is not the case at higher frequencies. Typically, manufacturers include suggested bypass-capacitor values in their data sheets, but you can also determine the proper value on your own.

    A basic capacitor is made up of two conductors separated by an insulator, or dielectric. The dielectric can be made of paper, plastic, mica, ceramic, glass, a vacuum or nearly any other nonconductive material. Capacitor electron storing ability (called capacitance) is measured in Farads. One Farad is actually a huge amount of charge (6,280,000,000,000,000,000 electrons to be exact), so we usually rate capacitors in microfarads (uF = 0.000,001F) and picofarads (pF = 0.000,000,000,001F ). Capacitors are also graded by their breakdown (i.e., smoke) voltage.

    There are very many different capacitor types. You have to realize that not all capacitors are equal. A 1uF ceramic definitely is NOT the same thing as a 1uF tantalum. You choose the device according to the application.

    Two 'parasitic' effects of capactitors are 'effective series resistance' (ESR) and series inductance. High ESR will cause power loss in higher-frequency applications (caps will get hot) especially in switching power supplies. High ESR also limits the effective filtering (your power supplies end up with more ripple). Except for very high frequency (multi-megahertz)applications a high inductance isn't quite so critical.

    The rated DC voltage is also very important. Usually it is a good idea to select capacitors rated at least 1.5 times or twice the maximum voltage you think they'll ever see. Temperature ratings also exist.

    The most common capacitor types are ones built using standard capacitor plates + insulator and then there are electrolytic capacitors. Typical capacitors consists of some form of metal plates and suitable insulation material in between those plates. This insulation can be some form of plastic, paper, mica, ceramic material, glass or air (some physical separation between layers). Those metal plates used in capacitors are usually thin metal foils. This type of capacitors have usually very good propertied otherwise, but the available capacitance is usually quite small (usually goes from pF to few microfarads). This kind of capacitors can take easily DC at both polaritied and AC without problems. This type of capacitors are available with various voltage ratings from few tens of volts up to few kilovolts as ready made components. For special application same technique can be used for very high voltage capacitors.

    Here is overview of most common capacitor types:

    • Ceramic: Fairly cheap but not available in really high capacitances - 2uF-10uF are about the max for any practical devices. Extremely low ESR. Surface mount devices have essentially no series inductance and are commonly used to bypass high-frequency noise away from digital IC's. Not polarized.
    • Electrolytic: Cheapest capactitance per dollar, but high ESR. Mostly used for 'bulk' power supply. Typical values 1uF-5000+uF. Polarized. Fairly durable, but will literally explode if reverse-biased. Tolerances of +-10% and +-20% are not uncommon. A tolerance of +80% and -20% is common for capacitors used in power supplies.
    • Tantalum: The 'cadallac' of capacitors. Very low ESR (not as low as ceramic, though), very high capacitance values available, but expensive (10x electrolytic). Usually used where one might use electrolytics. Polarized.
    • Polyester: Kinda expensive, not very high capacitance values, ESR not too bad. Polyester capacitors have very very stable temperature characteristics (capacitance change is very small as temperature changes). Used where stable capacitance is important like oscillators and timers. NOT polarized.
    There's others, of course, such as 'X' caps made to connect directly across mains AC power supplies that literally 'heal' themselves after an overvoltage. There are also so called 'Y' capacitors which are used in mains filters where they are connected between ground and live+neutral connectors. Y-capacitors have special safety regulations related to them.

    When selecting capacitor there is always a among size, dielectric, and value. Use the better grades of ceramic capacitors to obtain low impedance at high frequencies. Mica has the highest frequency response, but mica capacitors have the least volumetric efficiency. Aluminum and tantalum-electrolytic capacitors pack lots of capacitance to a small package but are useless at high frequencies (greater than 1 and 10 MHz, respectively).

    Electrolytic capacitors are constructed using a metal electrodes put into some form of electrolytic liquid. This kind of capacitorcan give high capacitances (from microfarads to tens of thousands of microfarads). Fixed Aluminum Electrolytic consists of two conducting electrodes, with the anode having an aluminum metal oxide film acting as the dielectric material or insulating medium. The typical voltage rating of electrolytic capacitor varies from few volts to few hundred volts. The biggest disadvantage if electrolytic capacitors is that they are polarity sensitive: you are only allowed to charge them only on one way. The capacitors have the positive / negative terminals marked. The capacitor must be put in the right way to the circuit (putting it wrong way will cause serious damage to the capacitor).For power supply smoothing capacitor applications, where large capacitances are needed, aluminium electrolytic capacitors arethe most common choise. There is aging thign to consider with electrolytic capacitors. Electrolytic capacitors contain a wet electrolyte that gradually dries with time and leads to an increased ESR (EquivalentSeries Resistance) and reduced capacitance, and this causes increased voltage ripple and possible timing problems in analogue circuitry. The of drying depends on the capacitor temperature (hotter they run, faster they will fail). A good example of electrolytics drying out is the appearance of a strong visual ripple or image tearing on old video monitors. The fix in this case is to change the main PSU electrolytics.

    Tantalum capacitors have a good capacitance versus size ratio but start to behave like an inductor at very low frequency due to high-parasitic inductance called equivalent series inductance (ESL). Standard tantalum capacitors have quite a high equivalent series resistor (ESR). Tantalum capacitors lose their intrinsic capacitance value when working at frequencies in excess of tens of kHz. Tantalum capacitors are polarity sensitve like all electrolytics and can be damaged easily if you apply wrong polarity voltage to the capacitor.

    Fixed Ceramic Monolithic capacitors are constructed by co-firing alternate layers of metal (electrodes) with ceramic (dielectric) materials. Used in filters, timing elements and HF coupling. Ceramic capacitors offer smaller global impedance (ratio 100:1) when compared to standard tantalum capacitors. The intrinsic capacitance value is kept more constant with variations in frequency compared to tantalum devices. In filtering applications the ripple voltage generated by ceramic capacitors is lower than those generated by tantalums. There is a de-rating specification that designers must apply to ceramic capacitors. The manufacturer measures the specified capacitor value at 0 VDC. When polarized, the capacitance value decreases according to the polarization applied to the capacitor (5 percent to 30 percent losses for X7R). It is trongly recommended if you are using using X5R/X7R to take the nominal voltage needed and multiply it by a factor of two when selecting the capacitor (tantalum 1 μF used on a 3.3-V device can be replaced by a ceramic X5R/X7R 1 μ/6.3 V). There are various ceramic family members that offer different characteristics.

    • Y5V, Y5U, Z5V and Z5U (class III grade) have very high dielectric constants and allow manufacturers to offer price-competitive, high-capacitor value in small packages (up to over 100 uF). Y5V, Y5U, X5R and X7R cannot be used in distortion sensitive applications because of their high dielectric absorption. They cannot be practically used in tuning applications because they are not practical as tight tolerance components.
    • X7R and X5R are materials suitable for more severe temperature atmospheres. They offer practically the the same dielectric constant as class III grade types. Values for X7R can be found up to 47 μF/6 V. X5R and X7R cannot be used in distortion sensitive applications because of their high dielectric absorption. They cannot be practically used in tuning applications because they are not practical as tight tolerance components.
    • NPO (strontium titanate?class I grade) ceramic capacitors are the most stable with respect to temperature, frequency, aging and tolerance. Typically, NPO ceramic components are used in tuning (filters, etc.) applications where aging, tight tolerance, stability versus temperature, and voltage are a must. However, because the constant dielectric is very low the capacitors tend to be big and there is limitation on what are the maximum values available.

    Film capacitors can be made at a very tight tolerance, while exhibiting a very stable behavior over frequency, temperature and aging, without showing any voltage de-rating. They are available in high-voltages, making them well suited for applications like filter. Film capacitors also offer the ability to sustain voltage surges well above their nominal voltage for a small amount of time, without loosing any of their properties. This makes them ideal for use with the line transformer that must deal with the high telephone line voltages (anywhere between 250 to 630 V) and in mains voltage circuits. Fixed Film Snubber capacitors are generally used a part of a suppression network to reduce spikes and EMI in power supply designs. You should keep in mind that the film capacitor family is extremely sensitive to the soldering process and that care must be taken during manufacturing, specifically for surface mount device (SMD) types of film capacitors. The manufacturers typically give strict rules concerning the temperature and time of soldering.

    High Voltage capacitors are the ones that have a working voltage exceeding 500V. They are found in high voltage power supplies, X-ray devices, pulse applications and voltage multipliers.

    Fixed AC (Incl. Motor Start) capacitors can be found in motors in conjunction with the windings. Usually, required where motor starting and/or running torques must be relatively high in relation to running torque.

    For power signal wire and power plane decoupling in digital electronics ceramic and tantalum capacitors are considered as the best solutions.

    Low Equivalent Series Resistance (ESR) refers to capacitors that have a low equivalent internal AC series resistance. Used in power supplies, high-current pulse circuitry, RF/microwave elements.

    For RF applications ceramic capacitors are common. Ceramics do not suit for all applications, because most of ceramics have strange effects, like changing capacitance with bias voltage.

    In audio applications type of insulation material does make a difference. For audio applications IIRC, ceramic, paper, mica, electrolytic and tantalum are all considered inferior by high-end hifi people. The plastic-film kind (especially polystyrene) are the preferred dielectric in very high quality audio applications.

    The newest type of capacitors are supercapacitors. A new category of capacitors called supercapacitors offer the high power delivery of capacitors and the high energy storage capacitry approaching batteries (nowadays energy densities only a small fraction of batteries). Supercapacitors are a cross between traditional capacitors and batteries, supercapacitors uniquely combine battery-like energy storage with capacitor-like power discharge in a small package. Supercapacitors hold more electricity than capacitors and transfer and recharge faster than batteries. Supercapacitors store energy as electricity like normal capacitors (not using chemistry like batteries). Supercapacitors are expected to be the next evolution of energy and power storage devices. They are replacing conventional batteries and capacitors, or being used to complement them. The main component in supercapacitors is activated carbon and organic electrolyte. Supercapacitors are manufactured using precisely controlled physical reaction that results in a fantastic carbon sponge filled with molecular-size pores, making almost every atom of carbon part of an available surface to store or release a charge. There are also very high capacitance capacitors made using gold based technology (Panasonic Gold Capacitors is a small 0.33F capacitor) and using aerogel (1 Farad capacitors). High capacity supercapacitors are generally low voltage devices (for example 2.5V or few volts voltage ratings).

    Capacitors used in bypass applications are implemented as shunt elements and serve to carry RF energy from a specific point in the circuit to ground. Proper selection of a bypass capacitor will provide a very low impedance path to ground. Satisfying capacitive bypass application requirements entails careful analysis of various frequency dependent capacitor parameters such as series resonant frequency (FSR), equivalent series resistance (ESR), and the magnitude of the impedance. The ESR and impedance should always be evaluated at the operating frequency.Nowadays a lot of talked about capacitor feature is ESR. ESR is an abbreviation for Equivalent Series Resistance, the characteristic representing the sum of resistive (ohmic) losses within a capacitor. The ESR rating of a capacitor is a rating of quality. A theoretically perfect capacitor would be loss less and have an ESR of zero (=no in-phase AC resistance). ESR is the sum of in-phase AC resistance. It includes resistance of the dielectric, plate material, electrolytic solution, and terminal leads at a particular frequency. ESR acts like a resistor in series with a capacitor (thus the name Equivalent Series Resistance). This resister can cause circuits to fail that look just fine on paper and is often the failure mode of capacitors. While ESR is undesirable, all capacitors exhibit it to some degree. Materials and construction techniques used to produce the capacitor all contribute to the component's ESR value. ESR is a frequency dependent characteristic, so comparison between component types should be referenced to same frequency. Industry standard reference for ESR is 100kHz at +25?C. Power dissipation within the capacitor, and the effectiveness of the capacitor's noise suppression characteristics will be related directly to the ESR value.

    Another important thing to keep in mind is ESL. ESL (Equivalent Series Inductance) is pretty much caused by the inductance of the electrodes and leads. The ESL of a capacitor sets the limiting factor of how well (or fast) a capacitor can de-couple noise off a power buss. The ESL of a capacitor also sets the resonate-point of a capacitor. Because the inductance appears in series with the capacitor, they form a tank circuit which is tuned to some frequency.

    When selecting a capacitor, designers should be sure to carefully consult the capacitor's data sheet before selecting the part. Failure to do so could result in a significant modification of the characteristics of the capacitor, or even in the destruction of the capacitor. Ultimately, the price and package size will be major factors in the designer's final choice.

      Electrolytic capacitors

      Name electrolytic capacitor refers to capacitors where the dielectric is formed by an electrolytic process. Wet electrolytic capacitors have an actual moist electrolyte, while dry or solid electrolytic capacitors don't. Most electrolytic capacitors have dielectric that is made up of a thin layer of oxide formed on a aluminum or tantalum foil conductor.Aluminium electrolytic is the term used by capacitor manufacturersfor electrolytic capacitors constructed with aluminium electrodes. This is the most commonly used type, and most often then peopletalke about "electrolytics" they mean aluminium electrolytic capacitors. Tantalum electrolyticis the term used by capacitor manufacturersfor electrolytic capacitors constructed withtantalum electrodes. The largest advantage of electrolytic capacitor is that they can fit large ampunts of electricity (large capacitance) to a very small size component.Electrolytic capacitors have several undesirable properties. They are inherently polar devices, meaning that the anode of the capacitor must be more positive than the cathode (There are also special true bipolar electrolytic capacitors available). Most electrolytic capacitors can withstand small and brief amounts of reverse voltages, but this is not recommended. The main concern is internal heat and gas generation. You need to pay attention to correctly hooking a polarized capacitor like electrolytics. If you "push" a polarized capacitor hard enough, it is possible to begin "electrolyzing" the moist electrolyte. Modern electrolytic capacitors usually have a pressure relief vent to prevent catastrophic failure of the aluminum can. Be warned that large value capacitors may explode if abused very badly. Leakage currents are higher ESR's are higher and operating voltages and failure rates are higher than non-electrolytic capacitors. Electrolytic capacitors have low self-resonance frequencies and are unsuitable for high frequency work. Electrolytic capacitor tolerances are normally high. The one factor that outweighs all these undesirable properties is the very high volumetric density that electrolytic capacitors exhibit. This means that you get lots of capacity in small size package. Several metals, such as tantalum, aluminum, niobium, zirconium and zinc, can be coated with an oxide film by electrochemical means. These metal oxides are remarkable dielectrics under the proper conditions. However, the metal-metal oxide interface is rectifying. That is, in one direction it is a good insulator, and in the other direction it is a conductor. This is why capacitors are polar. Non-polar electrolytic capacitors are made by using two oxidized films back-to-back. Please note that with electrolytic capacitors the operation voltage can have effect on the capacitance. Some electrolytic capacitors can show reduced capacitance values when operated very much below their designed operating DC voltage.

      • Capacitor Reforming - or : How to avoid the Big Bang!    Rate this link
      • Electrolytic Capacitors - What is an electrolytic capacitor?    Rate this link
      • Guidelines For Using Aluminum Electrolytic Capacitors - When using Aluminum Electrolytic Capacitors, please observe the following points to ensure optimum capacitor performance and long life.    Rate this link
      • Reforming Electrolytic Capacitors - Aluminum electrolytic capacitors can develop internal short-circuits over time. When an aluminum electrolytic capacitor is first manufactured, the internal materials are not ready for use. The capacitor must first go through a process called formation to activate and condition the capacitor. This process generally involves charging the capacitor at very low currents until the rated voltage is reached. The aluminum oxide layer is maintained every time the capacitor is used. The capacitor uses a tiny amount of current called leakage current to maintain its oxide layer. Over time, the aluminum oxide layer can partially or completely dissolve. If a capacitor with a partially or fully dissolved oxide layer is placed into operation, failure can occur. Reforming is a process of rebuilding the oxide insulation layer back to original specification. The easiest way to reform a capacitor is to charge the capacitor to its rated voltage over 12-24 hours. This allows the oxide layer to build up in a uniform manner. This is a very simple process with the instructions given in this page.    Rate this link
      • UltraCap technology - Basically, an UltraCap is an electrochemical double layer capacitor consisting of two electrodes, which are immersed into an electrolyte. The high energy content of UltraCaps in comparison to aluminum electrolytic capacitors originates in the activated carbon electrode material, which has an extremely high specific surface area of about 2000 m2/g and the extremely short distance between the opposite charges of the capacitors, which is of the order of a few nanometers (2 ... 5 nm). Since the dielectric is extremely thin ? it only consists of the phase boundary between electrode and electrolyte ? capacitance of a few thousand Farads can be realized in devices as small as a soda can.    Rate this link

      Capacitor markings

      There is difference how different capacitors can be marked.Large capacitor have usually the value printed plainly on them, such as 10 uF (Ten Micro Farads).Many mall disk types along with plastic film types often have just 2 or three numbers on them. First, most will have three numbers, but sometimes there are just two numbers. These are read as Pico-Farads. An example: 47 printed on a small disk can be assumed to be 47 Pico-Farads (or 47 puff as some like to say). Here is short introduction to markings you might see on circuit digrams:

      • 1 F = 1 Farad
      • 1 mF = 1 milli Farad = 1/1,000th of Farad or .001 Farads
      • 1 uF = 1 micro Farad = 1/1,000,000 of Farad or 0.000 001 Farads (10-6 )
      • 1 nF = 1 nano Farad = 1/1,000,000,000 of Farad or 0.000 000 001 Farads (10-9)
      • 1 pF = 1 pico = 1/1,000,000,000,000 of Farad or 0.000 000 000 001 Farads (10-12)
      Sometimes you might see combination markings like 1n5, where decimal dot is marked with letter. Here 1n5 means 1.5 nF. In the same way 2p2 means 2.2 pF. This is a common practice by some manufactures and the reason for thisis quite simple. By putting the "letter" in place of the "Tiny Decimal Point"it eliminates the chance of missing it on a poorly photo-copied or printedcopy of a schematic.

    Coils

    An typical inductor is simply a coil of wire, which can be wrapped around either air or metal cores. As current flows into an inductor, a magnetic field is created around the coil. When the current stops, the magnetic field collapses, generating an induced current flow in the coil. Low frequency currents flow easily into the inductor, but as the alternating current frequency increases, the impedance of the inductor increases. The inductor introduces a phase shift to AC signal going through it. Inductors allow direct current to flow, but as the frequency of oscillation increases, so does the inductor's impedance. A coil (of any sort) is an inductor

    Inductors behave to electricity as mass does to a mechanical system. Inductors resist change in current flow, just as masses resists change in physical movement. Stand in front of a moving car and try to stop it: its mass keeps it going. In the same way, if you suddenly try to stop the current flowing in an inductor - the inductor will resist the change in current. The same way the mass of the car resisted the mechanical stopping, so will the inductance of the coil resist the stopping of the electrical movement - the current flow.

    An inductor is an energy storage device. A coil/inductor can be as simple as a single loop of wire or consist of many turns of wire wound around a special core. Energy is stored in the form of a magnetic field in or around the inductor. By placing multiple turns of wire around a loop, we concentrate the magnetic field into a smaller space where it can be more useful. When you apply a voltage across an inductor, a current starts to flow. It does not instantly rise to some level, but rather increases gradually over time. The relationship of voltage to current vs. time gives rise to a property called inductance. The higher the inductance, the longer it takes for a given voltage to produce a given current.

    Whenever there is a moving or changing magnetic field in the presence of an inductor, that change attempts to generate a current in the inductor. An externally applied current produces an increasing magnetic field which in turn produces a current opposing that applied externally, hence the inability to create an instantaneous current change in an inductor. This property makes inductors useful as filters in power supplies.

    In most practical circuits inductive devices, operating in d.c. circuitry, which are switched on and off should have a diode or other suitable protection component connected across their coils to catch the inductive fly back.

    Most simple coils are air-core coils. They consists just winded copper wire. Air-core coils canproduce stable inductance over wide range of DC bias currents and work up to very high frequencies. The biggest downside od air-core coils is that very many turns are needed to produce large inductances. Other downside is that they produce somewhat large magnetic fields around them.

    Larger inductance coils can be produced by using suitable magnetic material core. With this approach large inductances are possible. Many types of cores are commonly used in inductors. magnetic material in coil core tends to concentrate the inductor's magnetic field inside the core and increases the effective inductance. While a magnetic core can providegreater inductance in a given volume, there are also drawbacks. A magnetic core can contain only a limited magnetic field. The limitations of the cored coils are the usually limited operating frequency range and possibility of core saturation because of excessive AC current or large DC current. All those characteristics depend on core material characteristics ans coil design and coil core type. Toroid inductors minimize the magnetic field around the coil.

    Inductors are often used in filter circuits and switch mode power supplies. Inductors have a bad reputation as filter components in some applications. They not only transmit EMI, but they act as antennas for receiving EMI as well (unless they are made wo a very good magnetic core).

    The markings of inductors can vary. Molded inductors usually follow the same coloring scheme as resistors except the units are usually microhenries. A brown-black-red inductor is most likely a 1000 uH. Sometimes a silver or gold band is used as a decimal point. So a red-gold-violet inductor would be a 2.7 uH. On some inductors you can see a wide silver or gold band before the first value band and a thin tolerance band at the end. Some inductors have their values marked to them and some have no markings in them. If you are unsure of some inductor value, then it is a good idea to measure it with an inductance meter (usually inductance scale on RLC meter).

    Inductors are useful components, but generally they are less common and harder to get than most other basic components (like resistors, capacitors, transistors etc.). In some applications the function of an inductor is replaced with an electronic circuit. A simulated inductor circuit is geneallhy designed as a circuit that reverses the operation of a capacitor with the aid of a transistor or an operational amplifier. The circuit acts as a synthetic inductor with one end connected to ground.

    Coils are used in the EMI filtering applications. A typical EMI filter coil consits of signal cable wrapped around toroidal ferrite core several times or it goes through ferrite core (can be considered as a coil with one turn). The aim is to generate high enough impedance on the noise frequency so that the not much of that signal gets through this coil. Ideally, impedance would be proportional to frequency and the square of the number of turns regardless of the magnitude of either. Also the properties of the core material has it's effect (usually somewhat frequency dependent). This is generally the case at very low frequencies, but becomes less valid as frequency increases. The predominant cause of such behavior is interwinding capacitance. Capacitance is directly proportional to the area of the conductor and inversely proportional to the distance between the conductors. As the number of turns increases, the area of the conductor (the length of the wire) increases and the distance between the conductors (the spacing between turns) decreases. The end result is an LC resonance above which capacitive reactance decreases impedance. The number of turns, their spacing, and the uniformity of their spacing are major factors in the frequency response of wound toroidal filters and must therefore be carefully considered in their assembly.

    Transformers

    A transformer is a static piece of apparatus with two or more windings which, by electromagnetic induction, transforms a system of alternating voltage and current into another system of voltage and current of same values or of different values, and at the same frequency for the purpose of transferring electrical power.

    Transformer is an integral component of the power supply that pulls power from the wall outlet and transforms it or makes it into power that can be used by the electronic device.The transformer outputs its power as alternating current as it receives power from the wall outlet. In power supply application this output is sent to the rectifiers in a power supply that change the alternating current to direct current.

    A transformer transfers AC signals only by means of a magnetic field at low loss. A transformer consists of two separate coils which have overlapping magnetic fields, so that current flowing in one circuit is coupled to the other. Often, transformers consist of an iron core with two or more coils, which couple magnetically.

    Transformers are used for many different applications. Most often transformers are used to transfer mains voltages tolower voltages inpower supplies and to provide galvanic isolation for signal lines. Transformers are sometimes used to get voltage gain (at the expense of current reduction) and to step down power line voltages for power supplies. Transformers are also used to match impedances between devices and to provide ground isolation.

    Transformers are also used to do unbalanced-balanced signal conversion. Sometimes in this conversion a transformer and a common-mode choke are used together. A common-mode choke can be used to reduce susceptibility to line imbalance from a poorly designed transformer. The common-mode choke is basically a pair of inductors sharing the same magnetic core, but that is in series with the line. Also, the windings are placed in a way that the two generated magnetic fields oppose each other. A common choke will efficiently attenuate common mode noise, up to 10MHz or higher.

    A typical transformer is layer wound on transformer core (usually so called "E" core). A layer-wound coil consists of single layers of wire separated by layers of insulation. Here, the insulation serves a dual purpose: it is a support platform for the wire and electrical isolation from other transformer parts made of conductive materials (i.e., core, other windings). Although transformers look simple, its design involves a few very important electromagnetic laws and formulas, which were developed by Joseph Henry and Michael Faraday. The formula below is often used to calculate the induced voltage in the secondary winding:

    E = kAcNBmf(10-4)

    Where:

    • E = induced voltage
    • k = constant (4.44 for sinewave, 4.00 for squarewave)
    • Ac = cross section area of core in cm2
    • N ? number of turn which induces the voltage
    • Bm ? maximum flux density of the core in Tasla
    • f ? frequency in Hz

    Armed with the above formula, one can increase or decrease the amount of turns of the winding to increase or decrease the induced voltage. Indeed, there are more turns on the secondary winding than in the primary winding for a step-up transformer and vise versa for a step-down transformer. If the windings are not correctly designed and manufactured, they may cause harm to the system in which the transformer is used. When a power tranformer is nor properly designed there is a risk of electrocution, burns, and fires. In the low power low voltage circuits bad transformer generally causes that the circuit does not work properly as designed.

    Nowadays also so called planar transformers have became popular in many pulse transformer and switched mode power supply applications. Those planar transformers use typically a low-profile E-core ferrite core, which mounts on the board and lets you use board tracks for windings of magnetic components, such as transformers and output chokes in power supplies and chargers. This kind of planar transformers are typically designed to operate ataround 200 kHz to 1.5 MHz frequency.

    Transformers are not ideal devices. Transformer have losses (typically 5-20% depending on design) when they operate. Those losses heat up the transformer. Let's pick up a normal mains power transformer as an example. There are two kinds of major losses: copper losses and iron losses. Copper losses are the losses which are caused by the wire resistances in the transformer primary and secondary. Copperlosses are related to wire resistance (wire thickness) and the current trough the wire. The losses increase to the square of the current travelling through transformer. Iron losses are generated in the transformer core material (iron in mains transformer) due magnetic reluctance, induced current circulating in the core and magnetic leakage. Iron losses on transformer core are proportional to the voltage fed to the transformer primary (quare to voltage). The operating frequency does not effect the copper losses, but it has effect on iron losses (higher frequency gives higher losses). Generally iron losses dominate the losses when transformer is not loaded and copper losses dominate the transformer losses when the transformer is heavily loaded. When talking about signal transformers you can sometimes see term insertion loss. The insertion loss of a transformer is simply a measure of the efficiency. It shows how power is consumed by the transformer. The result is the temperature rise, or how hot the transformer gets. The majority of the losses are the DC resistance in the windings. However, the core loss can be quite high if the flux density is great.

    In high frequency transformers the effects described above are the same. In addition to effects above you need to take into account the "skin effect" in the wires and the capacitive losses in the winding. Genrally in transformer design more iron and the finer the grade of iron(smaller particles = less eddie currents) the more efficient the transformer. Higher frequencies require finer laminations, then particlecore then air core to keep heating caused by losses down.

      Audio transformers

      A transformer is an electrical device that allows an AC input signal (like audio) to produce a related AC output signal without the input and output being physically connected together. This is accomplished by having two (or more) coils of insulated wire wound around a magnetic metal core.Audio transformers are used in many audio applications wheresignal needs to be converted (balanced-unbalanced converting), isolated (audio isolation transformers) or impedance needsto be converted (impedance conversion transformers, tubeamplifier output transformers).Audio transformers can:

      • Step up (increase) or step down (decrease) a signal voltage.
      • Increase or decrease the impedance of a circuit.
      • Convert a circuit from unbalanced to balanced and vice versa.
      • Block DC current in a circuit while allowing AC current to flow.
      • Electrically isolate one audio device from another
      • Convert an unbalanced signal to balanced signal and vice versa
      • Block Radio Frequency Interference (RFI) in some applications

      Unity 1:1 transformer, often called an isolation transformer, hasthe same number of windings on each coil. As the impedance is identical for the primary and secondary, the signal level does not change. A unity transformer allows an audio signal to pass unmodified from the primary to the secondary while blocking DC voltage and radio frequency interference (RFI). Also, since the primary and secondary are insulated from each other, a unity transformer will electrically isolate different pieces of equipment. This can solve hum problems by isolating ("lifting") the grounds of different devices. Other unity transformer applications include providing multiple outputs from a single mic input by using multiple secondary windings, and changing balanced signals to unbalanced signals or vice-versa.

      In a step-up / step-down transformer, the primary and secondary have a different number of windings, thus they have different impedances. Different impedances cause the signal level to change as it goes through the transformer. If the secondary has a higher impedance (more windings) than the primary, the signal level at the secondary will be a higher voltage than at the primary. Many microphones have step up or impedance matching transformers at their output.

      In audio application the transformers are generally divided to two different groups: output transformers and input transformers. Most simply stated, output transformers are used at the lowimpedance or driven end of a balanced line and input transformersare used at the high impedance or receiving end. The technical requirements, and as a result, the designs and physicalconstructions, of the two transformer types are very different.

      An OUTPUT transformer is driven by an amplifier and typically loaded by several thousand pF of cable capacitance plus the 20 koh,of a "bridging" line receiver. An outputtransformer must have a low output impedance, especially at highfrequencies. This requires low DC resistance windings and verytight magnetic coupling, since the sum of the windingresistances and the "leakage inductance" resulting from imperfect coupling are effectively placed in series between amplifier and load.To maintain the impedance balance of the output line, the transformer must also have balanced output capacitances. In audio applications there are usually two kinds of different output transformer used: line output transformers (used in mixer and other equipment line outputs) and power amplifier output transformers (used on tube amplifiers to drive speakers).

      An INPUT transformer is driven by the balanced line and is typically loaded by the input of an amplifier stage. Its primary must have ahigh impedance to the differential voltage between the lines and thisrequires more turns of smaller wire producing relatively higher resistance windings. The transformer must also suppress anyresponse to the common-mode voltage. A Faraday shield,connected to ground, is used to prevent capacitive coupling of the common-mode voltage from primary to secondary. Sometimes also a thin copper foil between windings is also used to reduce magnetic coupling. There are usually two groups of input transformers: microphone input transformers and general purpose input transformers. The difference in those is the maximum signal level those can handle (microphone transformers handle only millivolts signals well, while larger general purpose transformers cna handle up to few volts).

      Audio transformer have their limitartions.

      • The first limitation is frequency response. By design, audio transformers only pass audio signals. Therefore, an audio transformer will block signals that are below or above the audio range of 20 - 20,000 Hz. This can be a limitation or a benefit depending on the situation.
      • A second limitation is that audio transformers have a maximum input level that cannot be exceeded without causing a distorted signal. When the maximum level is exceeded, the transformer is said to be "saturated", i.e. it cannot hold any more signal. Especially miniature transformers have very strict limits as to how strong (voltage-wise) a signal they can pass, particularly at the lowest audio frequencies, and particularly if the transformers include a voltage step-up (sometimes sold as "Low impedance to High impedance" transformers). If transformer is overloaded, any such overload should be audibly apparent when the kick drum gets loud or with any very strong signal. There would be a muffling effect at the moment of impact, and possibly other forms of temporary distortion.
      • A third limitation is that audio transformers cannot step up a signal by more than about 25 dB when used in typical audio circuits.
      Audio transformers have always some losses in them. The insertion loss of a transformer is simply a measure of the efficiency. It shows how power is consumed by the transformer. The result is the temperature rise, or how hot the transformer gets. The majority of the losses are the DC resistance in the windings. However, the core loss can be quite high if the flux density is great.

      The "impedance" specification of audio transformers seems to confuse many engineers. Although they tend to produce optimumresults when used with specified external impedances, thetransformer itself has no intrinsic impedance. Audio transformer impedancs is really no more than a label which can be attached to a transformer or a winding. A transformer simply reflects impedances, modified by the square of the turns ratio, from one winding to another. Keeping in mind that input and output power are equal (minus the losses in transformer). If you measure the impedance of the primary winding you will see the "reflected" impedance of the load you connect to the secondary winding. "Reflected" means multiplied by the turns ratio squared. Transformer simultaneously reflects two different impedances. One is the impedance of the driving source, as seen from the secondary, and the other is the impedance of the load, as seen from the primary. The impedance rating of transformer can usually read that in the normal operating range the impedance cause by transformer shelf-inductance (uloaded) does not fall below the rated impedance in the specified operating range. Usually the audio transformers work best when used i circuit with impedances that are near to transformer specifications, but usually some change from specifications (few times higher or lower) does not hurt performance very much,

      Sometimes you have a situation where you need to replace an existing audio tranformer on some application with an new one. If you can't fid the data on the original transformer, the following measurements could be helpful in determining what kind of transformer the original one was and what could be a suitable replacement:

      • Measure DC resistance of the windings with an ohm meter.
      • Measure turns ratio by applying a small audio voltage to one winding and measuring the voltage across the other winding(s).
      • You can measure transformer inductances with RLC meter. Measure the inductances from all coils when all other coils are open, this can be used to get some idea of impedance range when you know the frequency on application. And then measure also the inductances other cols shorted to get idea of leakage inductances.
      • You can use a scope and an amplified signal source to find out what levels it takes to saturate the cores.

      Power transformer design is a pure math science, audio transformer design is a creative art.The physical size of both audio transformer designs are dependent upon the lowest frequency and the power available at that frequency. If you choose a low end frequency of 50Hz and then pump heavy 30 Hz signal into the transformer you may develop a transformer saturation condition and the amplifiers will see a shorted output.

      RF transformers

      RF transformers are widely used in electronic circuits for maximum power transfer, impedance matching, signal voltage level matching, DC isolation and balanced/unbalanced interfacing.RF transformers are generally used for signal isolation, for balanced-unbalanced conversion, for signal level conversion and for impedance conversion in RF applications. Essentially, an RF transformer consists of two windings linked by a mutual magnetic field. By designing the number of turns in the primary and secondary windings, anydesired step-up or step-down voltage ratio can be realized. Mutual coupling is accomplished simplywith an air core but considerably more effective flux linkage is obtained with the use of a core of iron or ferromagnetic material with higher permeability than air.The basic phase relationship between the RF signals at the transformer input and output ports may be in-phase, 0 degrees, or out-of-phase, 180 degrees. In some applications there is a need to pass a relatively high DC current (or low frequency AC) thrugh primary winding.In this case, the transformer core may saturate resulting in reduced transformer bandwidth and power handling capability. For this type of applications special transformers that can handle the needed current must be used.

      Telecom transformers

      Transformers are very much used in telecommunication devices. The most common use for a transformer is to form the galvanicisolation between the terminal equipment and the telephone line. Transformers are used in this applications in almost any equipmentwhich connects to a telephone line and to mains power (for examplein modems, ISDN cards, ADSL cards etc.). Most often used signalisolation transformer in telephone line application is 600:600 ohmtelecom isolation transformer.In addition to signal isolation transformers are alsoused for signal balancing (balun), impedance conversion (matching different impedance signal lines) and they werecommonly used to build telephone hybrid circuits in older telephones. Normal telephone line interfacing applications rely on 600:600 ohm telephone transformer that is designed to pass the telephone frequency range (300Hz to 3400 Hz). Telephone line transformers are generally available on two versions: "wet" and "dry". The "wet" type is designed to be able to handle the normal telephone line current (typically around 20 mA DC) going through the transformer primary without problems (the current does not saturate the transformer core). A "dry" transformer is designed to operate in such way that there is no DC going through it. The "dry" transfermers are generally smaller, less expensive and have other ways better performance than "wet" types. "Wet" transformer used to be traditionally used in old telephones and modems, while many modern designs (for example modem modems) are generally built using "dry" transformers. The telephone transformer are generally used for the telephone line isolation from the other parts of the electronics. The typically neededisolation level needs to be from around 1.5 kV (this is used in many normal modems) up to around 4 kV. Typical characteristics of 600:600 ohms telephone transformer are the following: primary resistance 60-120 ohms, secondary resistance 60-120 ohms, primary inductance when secondary is not loaded 300 millihenries or more (up to severam henries on some tranformer) and insulation withstand voltage 1.5 kV or more. Isolation transformers with different specifications are used in the other telecom applications. Application liks ISDN, ADSL and other high speed data applications need other types of transformers than normal telephone line. Generally those datacom transformers are designed to operate at higher frequencies (from tens of kHz up to several MHz range) and at wider bandwidth. Generally the needed signal isolation level is the same as in normal telephone applications (at least 1.5 kV) because they are connected to the same telephone cables as normal telephones, so are subjected to same conditions. The transformerd used in datacom applications are generally matched to impedances of 120 or 100 ohms, because this is the impedance level of telephone wire at those higher frequencies. In telecom applications the main application of transformer is signal balancing and isolation. An ideal transformer is a notional perfect circuitelement that transfers electrical energy betweenprimary and secondary windings by the action ofperfect magnetic coupling. The ideal transformer willonly transfer alternating, differential mode current.Common mode current will not be transferredbecause it results in a zero potential difference acrossthe transformer windings and therefore does notgenerate any magnetic field in the transformerwindings. Any real transformer will have a small, but non-zerocapacitance linking primary to secondary windings.The capacitance is a result of the physical spacingand the presence of a dielectric between the windings. For common-mode current, capacitance between winding offers a path across the transformer, the impedance of which isdependent on the magnitude of the capacitance and the signal frequency.

      Current transformers

      When measuring high currents on mains cables devices called"current transformers" are used. Their main purpose is to produce, from the primary current, a proportional secondary current that can easily be measured or used to control various circuits. The primary winding is connected in series with the source current to be measured, while the secondary winding is normally connected to a meter, relay, or a burden resistor to develop a low level voltage that is amplified for control purposes. In many high current applications the primary coil is just wire going through the toroidal core of the current transformer (=equivalent to one turn primary coil). When using just one wire going through the core, that wire can easily made thick enough to be able to handle large currents. Current transformers are relatively simple to implement and are passive devicesthat do not require driving circuitry to operate. The primary current (AC) will generate a magnetic field that is coupled into a secondary coil by Faraday?s Law. The magnitude of the secondary current is proportional to the number of turns in the coil, which is typically as high as 1000 turns or even more. The secondary current is then sensed through a sense resistor toconvert the output into a voltage. The voltage measured over selected burden resistor resistor connected between the current transformer output coil outputs gives the indicationof the current (voltage directly proportional to the current). The selected burden resistor value is usually defined with help of transformer data and experimenting. When a suitable burden resistor value is selected, a general (experimental) transformation ratio is calculated for thisapplication (ratio from input current to output voltage with given current transformer and burden resistor). In some SMPS designs current transformer (usually made using a ferrite toroid) helps to track the current in the control circuit's feedback loop. This current is then used to determine how the future behavior of the SMPS will be modified.Many clamp-on multimeters and clamp-on current measuring adapters that can measure AC current are built as current transformers. A simple current adaptor can only consist of the transformer core (which can be opened), the transformer secondary coil and suitable burden resistor.

      Transformer applications and circuits

      Mains power transformers

      Transformers convert AC electricity from one voltage to another with little loss of power. Transformers work only with AC and this is one of the reasons why mains electricity is AC. Step-up transformers increase voltage, step-down transformers reduce voltage. Most power supplies use a step-down transformer to reduce the dangerously high mains voltage (230V in UK and 110V AC in USA) to a safer low voltage. The input coil is called the primary and the output coil is called the secondary. There is no electrical connection between the two coils, instead they are linked by an alternating magnetic field created in the soft-iron core of the transformer. The ratio of the number of turns on each coil, called the turns ratio, determines the ratio of the voltages.

      Power transformers are available in a variety of configurations, primarily determined by the type of core selected. For the most part, they boil down to one of two types: EI laminations and tape- wound toroidal cores. The tradeoffs involved in selecting one over the other usually include cost, circuit application, weight efficiency, shape and volume. Regardless of which type is chosen, the electrical function is the same: one or more electrically conducting coils coupled together through magnetic induction. All power transformers, should have approved insulation systems suitable for the user's application. A transformer with an inadequate insulation system can be a potential fire hazard.

      National and regional transformer requirements and specific applications require the system manufacturer to be aware of the appropriate standards. One important IEC document is IEC 950, which consolidates the requirements in the former IEC 380 (Safety of Electrically Energized Office Machines) and the former IEC 435 (Safety Data Processing Equipment). IEC 950 is embodied in several other national and regional standards, including UL 1950 (U.S.), EN 60950 (European Community), VDE 0805, Part 100 (Germany), BS 16204 (U.K.) and CSA C22.2950 (Canada). In general, the major portions of these individual standards are the same as IEC 950.

      Many modern transformers nowadays in use in Europe are designed according standard EN 60742 (similar to IEC 742). EN60742 is based on the International standard IEC 742, which is also known as BS3535 in the UK and VDE 0551 in Germany. It is the CENELEC standard for Isolating Transformers & Safety Isolating Transformers. Other inportant newer standard is IEC/EN 61558 - 1: Safety of power transformers, power supply units and similar.This standard has the following subparts:

      • IEC 61558-2-1: separating transformers for general use.
      • IEC 61558-2-2: control transformers for general use.
      • IEC 61558-2-3: ignition transformers for oil burners.
      • IEC 61558-2-4: isolating transformers for general use.
      • IEC 61558-2-5: shaver transformers and shaver supply units.
      • IEC 61558-2-6: safety isolating transformers for general use.
      • IEC 61558-2-7: transformers for toys.
      • IEC 61558-2-8: bells and chimes transformers.
      • IEC 61558-2-9: transformers for Class lll handlamps incorporating tungsten filament lamps.
      • IEC 61558-2-10: high insulation level transformers with working voltage above 1000 volts

      A tranformer needs to be protected agains overloads, because loading atransformer for long time above it's rated power will cause it to overheat, that will lead to destruction of transformer and even sart an electrical fire. There are different ways to protect transformers agains overload. The most common methods for protection are overcurrent protection devices (fuses, circuit breakers, current limiters etc.) wired in series with the transformer primary and/or secodnary. There are also transformers with built-in overheating protection fuses. Here is a quick overview of different transformer types and their protection:

      • A transformer which has to be inherently short-circuit-proof as per IEC 61558 is constructed without protection. This kind of transformer can withstand short circuits without damage. Usually only some very low power transformers are designed to be this type.
      • A non-inherently short-circuit proof transformer as per IEC 61558 is equipped with a cutout to protect against short-circuit and overload. In this case, the transformer should be equipped with a thermal cutout. This is propably the most often used transformer type on low power and average power applications (normal appliances).
      • There are also transformers which are not short-circuit-proof as per IEC 61558 and not equipped with a cutout. When slling this kind of transformer, the manufacturer is obliged to inform the user of the required safety measures by means of which the transformer must be protected in operation. In this case, the transformer should be protected by means of a miniature fuse as per IEC 127: the type and current rating of the fuse must be stated on the transformer label.
      Transformers are designed for different operating temperatures in mind. Usually the rating of temperature is based on the IEC 85 norm which defines the temperature ratings of insulation materials:
      • Y = 90 ?C
      • A = 105 ?C
      • E = 120 ?C
      • B = 130 ?C
      • F = 155 ?C
      • H = 180 ?C
      • 200 = 200 ?C
      • 220 = 220 ?C
      • 250 = 250 ?C

      Doughnut shaped transformer commonly used in high quality electronics and amplifiers in particular for its low noise, low resistance to current flow, and power output for its size. Typical mains power transformers have around 90% effiency (some small ones have worse and some very large ones have usually better effiency).

      Toroidal mains power transformers are generally made with tape wound cores and high frequency toroidal transformers use generally ferrite core. The tape wound toroid cores provide an almost perfect magnetic circuits to minimize losses, fringing, leakage, distortion, and provide good magnetic shielding. It also decreases the magnetization force required to produce a given flux density. It is much more efficient than E-type lamination cores, but will have somewhat higher cost as the windings need to be done on the core itself. Toroidal transformers generally weigh around a pound for every 30 watts of output they can produce. Thus a toroidal transformer capable of outputting 600 watts would weigh around 20 pounds.

      For transformers with power ratings less than 1 kVA, the trend has been away from layer-wound to bobbin-wound coils. A bobbin-wound coil has layers of wire precision-wound on a rigid form. Most typical power transformers are constructed either as traditional E-core transformers and toroidal transformers.

      The main problem in equipment powered by a transformer is overheating due to excess current. Typical causes of excess current are a short-circuit in the load connected to transformer ortoo much load connected to the transformer. The result can lead to smoke, fire, burned wiring, and connectors unless the transformer is protected agains this kind of occurence. Typical protection methods are use of fuse (primary side and possibly on secondary side), over tempraturefuse inside tranformer or other similar overvoltage protection methos. Typically the transformer primary fuse is used as the protection against short circuits in transformer (the fuse must generally be rated tohave few times higher amperage than the transformer power would indicate to be able to handle the transformer start-up surges that can be quite hige especially with toroidal transformers). If transformer needs to be accurately protected against overload with fuses, fuses rated per transformer power are usually needed on transformer secondary size. Nowadays many modern transformers have internal overheating protection fuse to protect the transformer agains dangerous heating (caused by poor ventilation or overload).

      If the output of a mains transformer is short circuited, then quite high currents can be seen on secondary of the transformer(up to many times the transformer power rating). In short circuit situation the secondary current is limited by the impedance of the transformer and the impedance of the electrical network powering the transformer. In case of small mains transformer foind on equipment (less than 1 kW) the impedance of the transformer is the one that counts and the details of electrical network can be ignored (because the impedances there are much smaller than the transformer impedances). In most practical cases the maximum secondary current is limited almost entirely by the primary and the secondary coils resistances. The saturation of the core will not occur under short circuit conditions (the core flux will be roughly half normal or lower).

      Applying too high input voltage to a mains transformer will cause more than normal magnetig flux on the transformer core. If there is enough material in the core to keep it from saturating, it will. Once the core saturates the impedance of the primary will drop to a very low value. Core saturation can cause very high transformer primary currents, where the the current through the primary will only be limited by the resistance of the primary coil. This will cause that sooner or later either the primary coil or the protecting circuit breaker will open. This same thing can happen with the rated voltage of the mains frequnecy drops very much below rated frequency.

      The ratio of the number of turns on each coil, called the turns ratio, determines the ratio of the voltages. In ideal theoretical transformer this will hold true exactly, but in real-life transformers there are losses that cause that the secondary voltage is somewhat lower than calculated from primary and secondary turns ratio. The voltage drop is causes by resistive losses on the coils (both primary and secondary) and by the losses caused by leakage inductance. Those losses with get higher when the current taken from transformer gets higher.

      The mains power transformer will give it's rated output voltage when it is powered with the designated input voltage and loaded with it's rated power. If the load is lower than the rated maximum power, the output voltage is somewhat higher (usually 10-20% on small transformers, but can be even higher on some special transformers) than the rated voltage. If the output is loaded more than at rated power (transformer is overloaded) the output voltage will drop below the rated voltage. The mains transformer output is generally rated at continuous full load. This means that at the rated load it gives the rated output voltage. Lighter loads will result in higher output voltages. Heavier or intermittent loads will result in proportionally lower voltage dips. Generally Larger transformers have better load regulation than smaller transformers. Typically the regulation accuracies are 500VA = 5%; 100VA = 10%; 50VA = 15%; 10VA = 20%; 5VA = 30%. If better regulation is desired on the lower VA ratings, a special larger design can be specified to accomplish this.

      If the input voltage is something else than the rated voltage, the output voltage will chance in approximately the proportion as the input voltage changes. If input voltage get higher than rated, the output voltage of the transformer will rise until the transformer core saturates or transformer insulation cannot handle the high voltage.

      In mains transformers there is always some capacitive coupling from the primary to secondary of the transformer. A typical capacitance here is in range 10-100 pF. This capacitance causes that some input signal leaks to the output coil (mostly as common mode noise). This capacitance cause some small leakage current at mains frequencies to transformer secondary and to transformer core. AC power line, high frequency, noice and transients can appear on the secondary of the transformer via the capacitive coupling between the primary and secondary windings. Depending on the application the leakage to the secondary or to the transformer core (usually connected to equipment case) can cause noise to the system. The actual leakage current is determined by voltage, frequency and capacitance. The capacitance from the differnet parts of transformer to other parts can vary. There is usually some difference in the capacitance from the different ends of promary coil to different transformer parts (so sometimes the way the transformer is connected to live and neutral can affect somewhat to the amount leakage current from mains supply to device). In some applications where even a small leakeage is undesired, special transformer constructions are use. Typical solutions to redice the leakege current are completely separate primary and secondary coil connected to each other only through a grounded transformer core or using an electrostatic shield between primaty and secondary coil (typically copper or aluminium foil). Specifying an electro-static shield with a ground lead, on transformers with concentric windings can minimize this effecct. Some approval standards require a ground screen for safety between primary and secondary. Non concentric windings (side by side) generally do not require an electro-static shield, provide ideal isolation, and extra insulation of primary to secondary.

      In some applications it is desirable to reduce the stray magnetic field escaping from the transformer. A copper flux band in the plan of the coil, and/or a grain oriented steel band around the core can be specified to acoomplish this. Those bands act as an electro-magnetic shield around the transformer. Typical cases where this kind of extra shield are needed are power transformers operating close to ray tubes causing a distorted display; or high frequency transformers which are emitting undesirable RFI (radio frequency interference).

      When obtaining a CSA or UL certiification on equipment which contains a power transformer, a specific CSA or UL standard will apply to that type of equipment. The stadnards generally specify the maximum temperature rise at different parts of the transformer. For a proper transformer design it is very important for the transformer manufacturer to know the temperature of the air inside the equipment in the vicinity of the transformer as well as the maximum allowed temperatures. When ordering a transformer be sure to specify your needs clearly and comletely to save a second sample submission to the approval agency.

      There are also some special applications where the transformers are specially optimized. Here are few examples of them:

      • Energy limited transformers are used in chimes, door bells, security systems, decorative lighting, furnaces, humidifiers, appliances, vending machines, intercom system type applications. Those transformers are designed so that the output current from them is limited to some safe value (somewhat higher than normal maximum operating votlage) even if the transformer output is shorted. This built-in limitation in the transformer helps in circuit protection design (on many cases this will avoid need of some fuses to protect the wiring connected to tranformer secondary). There are many low power transformers (power 20VA or less) that are are inherently limited - no fuses or circuit breakers needed. Many transformers in this application field have some current limiting and then built-in thermal fuse (or slow-blow normal fuse).
      • Microwave transformers only operate at full load for short periods. Under full load the core losses are negligible. Nearly all real power is coupled through the windings' magnetic fields. And the heating, primarily due to resistive loss, is only shortterm. So, to save money, the transformer primary can be of low inductance. This also keeps loaded resistive losses low. But,that causes high unloaded primary current and thus high core loss along with the unavoidable primary resistive loss.
      • Neon and boiler ignition transformers are specal tranformers designed to generate a current limited high voltage (few kV) output that is needed to drive nenon tubes or spark gap. Current limiting in Neon Sign and Boiler Ignition Transformers is due to the relatively high flux leakages deliberately built into them. This is usually achieved with magnetic shunts. These shunts bypass some of the primary coil generated magnetic flux around the secondary coil making the magnetic flux linkage between the primary and secondary coils less than unity. It is specified as the coefficient of coupling (k).

      Transformers operating continuously or under varying loads must have enough primary inductance to keep unloaded current low enough to keep core losses acceptable for all load conditions.

      There are some applications where autotransformers are used to change the mains voltage. Using an auto-transformer is always an economical solution when you need to step down the voltage from 230V to 120 V or vice versa, and the equipment already has a fully isolated transformer or when the components do not require an isolated transformer. Autotransformers are generally smaller than normal transformer. They can change the mains voltage as well as any normal transformer, but they don't provide the isolation between input and output (one pin on input and output is common in autotransformers). With its small size and weight, the auto-transformer can be usually installed easily in existing equipment or supplied in an optional enclosure with or without cord, plug and receptacle.

      The Balanced Power Transformers are designed with a center-tapped secondary winding which consists of two identical, mirrored secondary windings in series with an electrostatic shield. When the center tap is grounded, these two identical windings short circuit the equal amplitude but opposite phased noise current and greatly attenuate (low to medium frequency) common mode noise. The ground noise amplitude goes to zero, which eliminates audio to be free of noise interference. balanced trnsformers are used to generate balanced mains for audio recording and production studios.

      The primary winding of a transformer must be designed to match the voltage and the frequency from the power source to the transformer, and must be able to transfer the necessary VA to the secondaries without overheating. The current carried by the primary winding thus depends on the secondary VA, but the voltage depends only on the power source. Ideally, the nominal primary voltage for a transformer should be equal to the nominal supply voltage, and the rated frequency should be equal to the supply frequency. The transformer design does not impose any lower limit on the supply voltage. The output voltage from the transformer does, however, decrease with decreasing supply voltage, so a lower limit is set by the tolerance required for the secondary voltages. This tolerance is usually in the order of -10% At the rated frequency, supply voltages higher than the nominal primary voltage will eventually cause noticeable mechanical hum or saturation effects, but a properly designed transformer should allow for more than + 10% headroom before this happens. As a general rule it can be assumed that it is safe to use a transformer for nominal supply voltages between about 95% and 105% of the transformer's nominal primary voltage.

      The transformer design does not impose any practical upper limit on how much the supply frequency can exceed the rated frequency, but a supply frequency lower than the rated frequency has the same effect as an overvoltage. The voltage headroom will be decreased by the percentage reduction in supply frequency below rated frequency, and a frequency larger than the rated frequency will increase the headroom. (A 50Hz transformer will have 6% headroom when run at 48Hz, but more than 32% headroom at 60Hz.)

      A transformer must be provided with more than one primary winding if it is to be used for several nominal voltages. One way to do this is to make a winding with several taps, but other methods are often more economical. Usually the optimum tranformer for 230V/115V is to make the transformer with two primary windings, each for 115V, we can connect the primaries in parallel for 115V supply, and in series for 230V supply. In both cases all the wire in the primary windings contributes to the output, so winding space is not wasted. As a result a smaller size transformer can be made than with coil taps (where the whole primary could would have been rated for that higher current). A parallel/series connection works only when the high voltage is a whole multiple of the low voltage. In case of other voltage like 100/120V or 220/240V are needed, those can be made with extra taps (the wasted winding space is quite small in these cases). If the primary must cover a wide range of voltages, it is always best to combine small taps with the series/parallel connection of the tapped windings.

      In many items that are made in small volumes, the mains transformer is selected from the available trandard transformer models (with right output voltage and high enough power rating). When making very many devices or needs are special, designing an application specific transformer makes sense. Correct specification of the secondary data is the most important rule for cost and size reduction of a transformer. The cost of a transformer increases with the size of the transformer and with the number of primary windings, so savings can also be made by following these rules for specifying primary data:

      • Do not specify a lower rated frequency than needed. (A 60Hz transformer has 20% higher rated power than a 50Hz transformer of the same size and weight.)
      • Do not specify more primary windings than needed. (Taps in the primary windings waste winding space, and thus make it necessary to make the transformer bigger.)
      • Do not specify more voltage headroom than needed. (The size of the transformer increases if the maximum supply voltage specified is larger than 110% of the nominal primary voltage.)
      Do not include too much extra safety margins. Tell your transformer manufacturer/designer what you need, and let the transformer designer to specify the needed margins. You can save money on mass produced by using a single-primary 60Hz transformer in USA and Canada, and using a larger 50Hz Dual primary or Quad primary only in other markets.

        Isolation transformers

        Isolation transformers are often installed to isolate and protect sensitive, expensive equipment from noisy electrical system grounds, ground loops, power line spikes and other power line disruptions. Many instances arise when it is desirable to incorporate an isolation transformer within an electronic product. Usually the reason for this is increased safety or noise isolation. This may be desirable for special applications or designs, such as a demonstration, display or design prototype.

        Isolation transformers are also available as separate units. Those are generally used in laboratory environment and dangerous environments to increase the electrical safety. Sometimes isolation transformers are needed to fight against power line noise or ground induced noise in sensitive electronics systems.

        Medical Grade Transformers generally refer to the transformers used in medical devices as well as hospital, biomedical and patient care equipment. There are a number of strict safety rules, guidelines and laws governing the design, construction and the test of these transformers. One can find these rules and regulations in UL 2601-1, IEC 601-1, CSA C22.2 No. 601 and EN 60601-1 standards. The majority of medical grade transformers are isolation transformers.

        There are generally two types of isolation, one that relies on Safety Ground (Protective Earth), and one that relies on Double or Reinforced Insulation. A transformer that relies on safety ground uses a basic isolation between the primary and the safety shield and shield to secondary. This shield has to be thick to be able to meet required tests in the safety standard. If the isolation breaks, the electrical path goes directly to ground, providing safety. For a transformer that relies on double or reinforced Insulation there is no "safety" shield, but the insulation as indicated by the name, is much thicker. It is designed so that all layers of the insulation can pass the thickness and high potential voltage tests required by the standard. If one insulation layer breaks, the next layer will be able to provide the required safety. In both the design options, the construction design must still meet the required creepage and clearance requirements.

        Some isolation transformers include a static shield for noise reduction. When working with those transformers it is important to note that this shield does not provide the transformer with Safety Ground but it working as a functional earth. When the shield is grounded, it attenuates the common mode noise and can reduce the leakage current from the primary to the secondary winding (please note that this leakage reduction is not the same as when measured Primary to Ground). A Safety Shield wire should be connected to the Protective Earth terminal in the equipment while the Static Shield, if included, should be connected to the Functional Earth terminal.

        In Europe, the IEC either directly or indirectly sets the electrical safety standards for a great many individual nations. The IEC?s chief standard for Safety Isolation and Safety Isolating Transformers is the IEC 1558 (recently replacing the IEC 742). In contrast, the European Community (EC) version of the IEC 1558 is EN61558. An additional IEC standard, IEC-601-1, is generally accepted throughout Europe as the standard by which medical electronic equipment must comply (such as UL 544 in the United States and C22.2 No. 125 in Canada).

        Worst-case conditions should always be considered when trying to comply with one or more international standards. The following recommendations can be applied to the selection of a transformer for working voltages of 250 V or less. For example, a transformer should be specified with high dielectric strength of 4 kV or more. This ensures that the level of isolation will meet general as well as specific medical standards. Transformers should also meet minimum requirements for creepage and clearance, say 10 mm. The transformer?s minimum insulation temperature should be at least +130 ?C, and the primary-to-secondary current leakage should be no more than 30 ?A. By meeting these minimum provisions, and evaluating related requirements, such as the type of operating environment (indoors, outdoors, surrounded by hazardous materials, etc.) and the number of fuses and circuit breakers in the remaining circuitry, product developers can ensure compliance with a large number of international standards for operating voltages of 250 V or less.

        Making own power transformers

        • Using Old Transformers - Transformers can be a tedious thing to tackle and in many cases it's not worth the trouble to make them when you can just go out and buy them. But in some cases it can be practical to rewind the secondary coils of already existing transformers in order to get the exact voltage you want. In most cases it is only the secondary windings that need to be changed and doing this requires very little mathematical skill. Fortunately, the secondary windings are usually on the outside of the primary windings.    Rate this link
        • European Style Transformer Design Equations - a few equations which are useful in transformer design    Rate this link

      Toroidal transformers

      There is no dramatic technical difference between a toroidal transformer and a conventional transformer. The only main difference is the formof transformer. In principle a perfect toroidal winding has no external magnetic field,and in practice toroidal transformers do have lower external fields,but transformer designers tend to design toroids to run closer tosaturation, which increases the external field, largely eliminatingthe advantage. If designed to do so, a toroidal transformer canprovide higher inductance, tighter coupling , higher efficiency, and higher Q, and on and on comapred to traditional transformer. Toroids are popular in hi-fi amplifiers because they allows claimsabout low external field, and because thesize of wound toroidal transformer is lower than than equivalentconventional transformer.The "squashed" profile of the toroidal transformer also gives itmore surface area per unit VA than a conventional transformer, soit dissipate more heat per unit temperature rise, which thedesigners exploit by running them at higher current density. There are two disadvantages associated with toroidal cores. The first is price. The nature of a toroidal core necessitates slower, more complex winding techniques, particularly for high-voltage or multi-output transformers. The price differential is most significant for sizes up to 300 VA.High power (1500W and up) toroidal transformer can have a very high inrush current because of low air gap in transformer. EI laminations offer inherently lower inrush current, and the problem can be further reduced by introduction of an air gap into the construction. This is far more difficult and expensive to do with a toroid. It sometimes becomes necessary to add a resistor in series with a primary of a toroidal transformer to prevent destruction of overload protectors on turn-on.

    Magnetic materials

    Magnetic materials are used in applications such as power supply transformers, audio transformers, AC and RF Filter inductors, broadband and narrow band transformers, damping network, EMI/RFI suppressors, etc. The basic characteristic of magnetic materials is the permeability (?). It is a measure of how superior a specific material is than air as a path for magnetic lines of force(Air has a ? of 1). Another characteristic of magnetic material is saturation. It is the maximum value of magnetic induction at a specified field strength. When a material saturates, it losses its linearity. Magnetic materials are available in many different types and sizes.There are many different magnetic materials with different characteristics.Laminated or tape wound cores are manufactured by using different steel grades with different widths and thickness, wound in circular manner. Tape wound cores have very high permeability and are used primarily in power transformers, reactors in 60 Hz to 400 Hz, DC to DC converters, and current transformers.Iron powder cores are composed of finely defined particles of iron which are insulated from each other but bound together with a binding compound.Iron powder cores are suitable for applications such as narrow band filter inductors, tuned transformers, oscillators and tank circuits.Ferrites are ceramics materials that can be magnetized to a high degree. The basic component is iron oxide combined with binder compounds such as nickel, manganese, zinc or magnesium. Two major categories of ferrites are manganese zinc (MnZn), and nickel zinc (NiZn). Ferrites can be manufactured to very high permeability (over 15000) with little eddy current losses. However, the high permeability of the ferrite makes it unstable at high temperatures, and saturates easily (even could be damaged by high saturation). Ferrites are suitable for applications such as DC to DC converters, magnetics amplifiers, EMI/RFI suppressors, transformers and inductors.Ferrite cores can be gapped to avoid saturation under DC bias conditions.

    Relays

    The relay is an automatic control element whose output variable undergoes a change by leaps and bounds when its input variable (electric, magnetic, sound, light, heat) reaches a set point. A typical relay is a remotely controlled operated switch; it consists of one or more contact pairs that serve to open, close or transfer external circuits. The relay is just a switch, activated by electricity.A relay is a simple electromechanical switch made up of an electromagnet and a set of contacts. Relays can be classified into many different categories according to their working principle, physical dimensions, protective features, contact loads and product applications.

    Here is one classification of relays according to working principle:

    • Electromagnetic Relays: Relays in which the relative movements of their mechanical components produce preset responses under the effect of the current in the input circuit are called electromagnetic relays. Relays in this category include DC electromagnetic relays, AC electromagnetic relays, magnetic-latching relays, polarized relays, and reed relays.
      • DC electromagnetic relays: relays whose control current in the input circuit is DC.
      • AC electromagnetic relays: relays whose control current in the input circuit is AC.
      • Magnetic-latching relays: relays, after the magnetic steel is introduced into the magnetic loop, even the relay coil is de-energized the armature iron steel still maintains its state as that when the coil is energized, with two steady states.
      • Polarized relays: DC relays whose change of state depends on the polarity of the input exciting variable.
      • Reed relays: Relays that rely on the movements of the reed which is built in the tube and has dual functions as contact reed and armature iron magnetic circuit for connecting, breaking or switching circuits. A reed relay has a set of, usually normally open, contacts inside a vacuum or inert gas filled glass tube. This protects the contacts against atmospheric corrosion. The two contacts are closed by magnetism from a coil around the glass tube.
      • Mercury wetted relays: A mercury wetted relay is a form of reed relay in which the contacts are wetted with mercury. Such relays are used to switch low-voltage signals (one volt or less) because of its low contact resistance, or for high-speed counting and timing applications where the mercury eliminated contact bounce. Mercury wetted relays are position-sensitive and must be mounted vertically to work properly. Because of the toxicity and expense of liquid mercury, these relays are rarely specified for new equipment.
      • Latching relays: Latching relays are available that have two relaxed states (bistable). These are also called 'keep' relays. When the current is switched off, the relay remains in its last state. This is achieved either with a solenoid operating a ratchet and cam mechanism or by having two opposing coils with an over-center spring or permanent magnet to hold the armature and contacts in position while the coil is relaxed. In the ratchet and cam example, the first pulse to the coil turns the relay on and the second pulse turns it off. In two coil model a pulse to one coil turns the relay on and a pulse to the opposite coil turns the relay off. This type of relay has the advantage that it consumes power only for an instant, while it is being switched, and it retains its last setting across a power outage.
    • Solid-state Relays Relays whose input and output functions are performed by electronic elements without mechanical movement components.
    • Time Relays Relays whose controlled circuit connects or breaks when the output part is delayed or timed to a preset time after the input signal is added or erased.
    • Temperature Relays Relays that get into motion when the outside temperature reaches a present point.
    • Wind-velocity Relays When the wind velocity reaches a certain point, the controlled circuit will connect or break off.
    • Acceleration Relays When the acceleration of the moving object reaches a preset point, the controlled circuit will connect or break off.
    • Relays in Other Categories Including photo relays, sound relays, and heat relays.
    This relay description mainly concentrates on electromagnetic relays and solid-state relays. Classification of the Relays according to Physical Dimensions:
    • Mini Relays: Relays whose maximum edge is no larger than 10mm
    • Super Mini Relays: Relays whose maximum edge is larger than 10mm but no larger than 25mm
    • Compact Relays: Relays whose maximum edge is larger than 25mm but no larger than 50mm
    Classification of the Relays according to Protective Features:
    • Sealed Relays: Relays whose contact and coil are sealed in a metal case by welding or other methods and therefore enjoy low leakage rates
    • Plastic-cased Relays: Relays whose contact and coil are sealed in a plastic case by gluing and have somewhat higher leakage rates than sealed relays
    • Dust-proof Relays Relays whose contact and coil are sealed in a case for protection purposes
    • Open Relays: Relays whose contact and coil are not protected with a case
    Classification of the Relays according to Contact Load:
    • Micro-power Relay: Relays whose current is smaller than 0.2A
    • Low-power Relays: Relays whose current is in the range of 0.2-2A
    • Medium-power Relays: Relays whose current is in the range of 2-10A
    • High-power Relays: Relays whose current is larger than 10A
    Classification of the Relays according to Applications:
    • Communication Relays: Relays with contact load ranging from low level to medium current
    • Machine Tool Relays: Relays used on machine tools with high contact load power and long service life. A machine tool relay is a type standardized for industrial control of machine tools, transfer machines, and other seqential control. They are characterized by a large number of contacts (sometimes extendable in the field) which are easily converted from normally-open to normally-closed status, easily replaceable coils, and a form factor that allows compactly installing many relays in a control panel.
    • Household Appliance Relays: Relays used in household appliances must meet a high safety standard
    • Automobile Relays: Relays used in automobiles with high switching load power and high impact and vibration resistance, automotive relays are often designed to operate at 12V or 24V DC
    • High frequency relays: Special relays used to switch high frequency signals (for example coaxial relays)
    • Contactor: A contactor is a very heavy-duty relay used for switching electric motors and lighting loads. Contactors are often used for motor starters.
    • Safety relays: A forced-guided contacts relay has relay contacts that are mechanically linked together, so that when the relay coil is energized or de-energized, all of the linked contacts move together. If one set of contacts in the relay becomes immobilized, no other contact of the same relay will be able to move. The function of forced-guided contacts is to enable the safety circuit to check the status of the relay. Forced-guided contacts are also known as "positive-guided contacts", "captive contacts", "locked contacts", or "safety relays".

    Idea in the simple electromechanical relay operation is that the relay output contacts are normally open, typically a spring keeps them in this position. When you wnat to close the relay, you feed current to the control coil. The current at that coil creates a magnetic field that moves the output switch mechanical contacts to physically different position, to such position that the output contacts close the output circuit.

    Relays usually have several contacts. A common type is Dual-Pole Dual-Throw, which means that it has two sets of contacts, and that both sets have two positions. For each set, there will be a 'common' line, and one which is normally connected to the common line (when power is off), and one which is normally open.If you supply power to the coil (at rated coil voltage), the relay will engage, and the normally open contact will be connected to common. If you connected the live wire to the common pin, and the load (VCR, TV)to the normally open pin, then it would go on when you supplied power to the coil. The industries using relays are many and varied.

    Since relays are switches, the terminology applied to switches is also applied to relays. According to this classification, relays can be of the following types:

    • SPST - Single Pole Single Throw. These have two terminals which can be switched on/off. In total, four terminals when the coil is also included.
    • SPDT - Single Pole Double Throw. These have one row of three terminals. One terminal (common) switches between the other two poles. It is the same as a single change-over switch. In total, five terminals when the coil is also included.
    • DPST - Double Pole Single Throw. These have two pairs of terminals. Equivalent to two SPST switches or relays actuated by a single coil. In total, six terminals when the coil is also included. This configuration may also be referred to as DPNO.
    • DPDT - Double Pole Double Throw. These have two rows of change-over terminals. Equivalent to two SPDT switches or relays actuated by a single coil. In total, eight terminals when the coil is also included.

    NEMA Definitions for Contact Arrangements:

    • Form A Contacts: A Form A contact arrangement is one that has single-pole, single-throw, normally open contacts. The function of this arrangement is to close a circuit when actuated.
    • Form B Contacts: A Form B contact arrangement is one that has single-pole, single-throw, normally closed contacts. The function of this arrangement is to open a circuit when actuated.
    • Form C Contacts: A Form C contact arrangement is one that has single-pole, double-throw contacts with three terminals - one for normally open, one for normally closed, and one common. The function of this arrangement is to transfer a circuit when actuated.
    • Form X Contacts: A Form X contact arrangement is one which has single-pole, single-throw, normally open double-make contacts. The function of this arrangement is to close a circuit when actuated.
    • Form Y Contacts: A Form Y contact arrangement is one that has single-pole single-throw normally closed double-break contacts. The function of this arrangement is to open a circuit when actuated.
    • Form Z Contacts: A Form Z contact arrangement is one that has single-pole, double-throw, contacts with four terminals ? two for normally open and two for normally closed. The function of this arrangement is to open one circuit and close the other.
    • Repeat Cycle or (Flicker): Power is applied continuously. When a start signal is applied, the timing cycle begins. When the time delay is completed, the output contacts change state and the next timing cycle begins. This cycle will repeat until a reset signal is applied or power is remove.

    Designers often use relays as electrically controlled switches. In a relay the switch contacts are electrically isolated from the control input which is a very useful feature on many applications. So called "light duty" electromagnetic relays are used in applications like communication, control, monitoring, or alarm switching circuits in which load currents are normally fractions of an ampere to 25 amperes. Relays are very much used in automotive applications and mains switching applications where considerable currents needs to be switched. Relays are also used for analogue signal switching (hifi equipment, measurement devices), telecommunications application (telephone line on/off hook relay) and for RF signal switching (special coaxial cable relays). Relays are available with AC and DC coils for various voltages (usually anythign from few volts of DC up to 230V AC).

    The most common form of actuator or motor system for electromagnetic relays consist of an energizing coil and a permeable iron circuit. It has both a fixed portion (open loop) and a movable member, called the armature, that completes the magnetic circuit by closing the air gap. The movement of this armature causes the contacts of the controlled circuit to perform a switching function. A typical relay has a spring for the return stroke and for holding selected contacts closed when the relay coil is in the de-energized. The operation speeds of relays vary dependign on the relay type. The relay speed is listed in the relay technical specifications somewhere. Usually the time for the relay to close and open is different. And how the relay is wired to circuit can have effect on relay opening speed (a protection diode slows the relay coil power turning off somewhat). For reference around 15mS is quite typical speed opening time for the type known as 'Continental Cradle' relays, with a diode directly across the coil. You can measure relay speed very roughly, by wiring up the relay as a buzzer and measuring the frequency (usually 200-800 Hz range for small relays).

    Typical specifications you get from a DC relay coilis the coil resistance and intended operation voltage (typically voltage range). Coil resistance specifications are typically given for an ambient temperature of 25 degrees C. The coil operation voltage should be checked, because lowerthan minimum operating voltage will not reliably operatethe relay and higher then rated voltage can damage the relay (typically heats the coil too much). When selecting the relay, keep in mind that you need to determine if you power it with AC or DC, because they need different relay constructions. You can't use a DC coil relay on AC as it may chatter since itmissing the "shading" ring that would be present on an AC relay. And,of couser, the DC resistance of the coil doesn't directly apply to AC. For AC applications you need a relay with could designed for AC and the voltage / frequency you use. For DC applications select a DC coil relay with suitable coil voltage rating. In some cases you may want to use AC relay with DC also. Driving AC relay with DC generally activates the relay nicely, but you should be careful in the driving voltage because the maximum coil you can apply safely to coil with DC can be considerably lower than the nominal AC voltage (at AC the coil inductance limtis the current, at DC there is only coil resistance to limit the current). AC relays need somewhat specific constructions. Shaded pole AC relays are generally constructed like simple DC electromagnetic relays with a portion of the core pole face separated from the rest of the pole face and enclosed in a loop of copper. This loop produces a lag in the timing of the ac magnetic flux in one portion of the pole face with respect to that in the unshaded portion. While the current in the coil passes through zero twice each cycle, the flux in the armature gap remains at a high enough level to hold the armature operated.

    When using relays with electronics control circuitry please note that relay coils can generatequite high self-induced voltage when the relay is switched off. Because this voltage can damage electronic componentslike switching transistors, typically protective componentsare used to avoid it (most typically used component is a reverse-polarized diode in parallel with the relay coil).

    Relay contacts in the industrial world are normally labeled with NO (Normally Open), NC (Normally Closed), and C (Common). The first thing you need to know is that a relay contact is a switch, nothing more, nothing less. It does not provide power; it simply opens and closes an electrical circuit, just like the light switch on a wall. When the relay is de-energized or turned off there is an electrical connection between NC and Common, hence normally closed. In the off state there is not a connection between NO and common, hence normally open. When the relay is energized or turned on the NO and C make an electrical connection, and the electrical connection between NC and C is removed.

    Here is a sample of connecting a simple 110 VAC light to a relay so that it will turn on when the relay is energized. Wire the hot 110VAC to the Common (C), Normally Open (NO) to hot side of the light bulb, Neutral from the light bulb to the neutral of the 110VAC wiring. Now when the relay is energized the NO will close connecting to the C and allowing power to flow through the bulb.

    Dielectric ratings for relays are a function of size, the separation between contacts, and the separation between various parts of the structure. The ability of a relay to withstand impressed voltage depends on the type of insulation employed and the severity of the in-service environment. The periodic polarity reversal that is characteristic of AC voltages applies greater stress to most insulating material than does an equivalent DC voltage. The result is that a given dielectric material will likely breakdown at a lower peak ac voltage than DC voltage. Please note the relay voltage ratings when specifying the relay for a specific use.

    When switching electrical loads on an off using relays, you must take into account the relay ratings. The relay contacts need to withstand the current to the load (including potential high inrush current) and theswitched voltage. When selecting relay rating, please note that the current and/or voltage ratings for relay contacts can be different for AC and DC switching applications. When switching mains loads like electronic devices and lamps, usually a large inrush current can go through relay contacs for brief time (can be easily up to 80A). If the relay contacts are not rated to handle the inrush current, the relay contacts can be weld shut, which means that the relay cannot switch off, and is rendered useless.

    Relays have many good features, but relays have also some downsides .First thing is that many relays are mechanically quite largecompared to very many other electronic component.The relays have the power dissipation in a relay coil may render the device unattractive in battery-powered applications. A relay coil is a highly inductive load, which means than when driving a raly from electronics circuit you need todesign the driver circuit such that it is protected against"inductive kick-back" when current to relay coil is stopped or you need to add extra protection diode in parallel with relay coil. Because a relay is an electromechanical device, it has limited life both in mechanical and electrical contacts. The bouncing relay contacts can produce arcs that threaten system reliability, can cause RFI problems and can be dangerous in some application.

    Power Relays or Contactors are used in industrial and military applications used for switching heavy contact loads that may be highly inductive, such as motor, generator, and transformer loads. These devices are also used to switch the heavy resistive and lighting loads. Most typical use for contactors are motor starters. Across-the-line industrial motor starters are made in sizes up to those capable of carrying 600 amperes. Contacts of power relays used for motor control must be capable of opening at six to eight times the rated steady current in case a motor should stall. Wattage dissipation is greater in these relatively large units than in the general purpose relay.

    Solid state relays (SSR) are the electronic equivalents of a mechanical relay with some notable advantages. Solid State Relays (SSR) is a relay w put and output whose functions are achieved by means of electronic components without the use of moving parts as found in Electromechanical relays. Solid state relay (SSR) and semiconductor relay are both names of relay like device which works like a normal relay. A basic definition of a totally solid state relay is a device that operates a load circuit without the use of physical contacts. This relay contains a transistor or triac which turns on a load circuit. An SSR is a semiconductor device that can be used in place of a mechanical relay to switch electricity to a load in many applications. Solid-state relays are purely electronic, normally composed of a low current control side (equivalent to the coil on an electromechanical relay) and a high-current load side (equivalent to the contact on a conventional relay). Advantages of SSRs are quieter operation, longer life and faster repetitive operations, especially where counting or numerical operations are concerned. SSRs are also more immunite to physical shock than electro-mecahnical relays (EMRs). Disadvantages are cost and higher currents may require external heat sink components. A typical SSR consists of an LED input, which is galvanically isolatedfrom an output switch circuit. The output switch uses aphoto diode stack to detect the LED optical signal and thendrives a pair of common source power MOSFET's or one TRIAC which short or open the output depending on the state of the input. This arrangement offers a number of importantadvantages over mechanical relays. These include high input-output isolation as a result of the optical coupling,high reliability because of the elimination of contacts, immunity to magnetic field coupling and very smallpackaging. SSR's are widely used in a number of applications ranging from modems to candy machines. Triacs are used in relays ment for only AC operation. FETs are used in relays which must be capable to switch AC and DC.

    Optoisolator Relay is a name for an electronics component most often just called optoisolator or optocoupler. The optoisolator, sometimes called an optocoupler, is an assembly that contains a light emitting diode and a solid state photosensitive device. These are placed in close proximity to each other so that light generated by the LED will be impressed upon the photosensitive device, which may be a transistor, SCR or triac that is normally non-conducting. An input signal fed to the LED causes it to glow, emitting light. When the light energy is impressed upon the solid state device it becomes conductive, allowing the output circuit to be energized. Since the coupling medium is light, the optoisolator can be designed to attain an isolation voltage rating of several thousands of volts.

    Safety relays are relays that are used for safety engineering applications in machines and systems. The safety relays are typically used to monitor safety-related control systems such as emergency-stop circuits, safety mats and bumpers, safety guards and two-hand controls. There are both electromechanical and electronic safety relays. Electromechanical realys are just pretty normal electromagnetic relays that are deisgned to work very reliably and fail at predictable way (fails to "safe" state). The current through the safety sensors (switches, etc..) activates the relay when it is considered safe to operate the device, and relay contacts then allow powwrr going to that device. There are various international standards and regulations on safety related relays (EN 954-1 as well as IEC/EN 60204-1). The most common safety products are traditional, hard-wired devices, such as emergency-stop push-buttons. The electronic safety relay consists of a power section, electronic section and two redundant relays with positive-guided contacts for enabling and signalling paths. In fault-free operation the safety circuits are controlled by the electronic unit and the enabling paths are activated via the relay. In the event of a fault, or at switch-off, the enabling current paths are interrupted immediately (Stop category 0) or with delay (Stop category 1) and the motor is disconnected from the power supply.

    The types of contact loads to be considered in relay design may be divided into four broad categories (each category has different need for relay contacts):

    • 1. Dry circuits: By definition, a contact is considered to be dry if it does not make or break current. There are, however many applications falling within this category in which contact may be required to carry appreciable current. Dry circuits are usually considered to be loads that are not opened or closed by the contacts, that is, currents may flow through the contacts after closure and before opening, but the contact does not directly control the load.
    • 2. Low level loads: Low level switching ordinarily is considered to be in range of microamperes or a few milliamperes, with the open-circuit voltage below the melting voltage of the contact material.
    • 3. Intermediate loads: Intermediate contact loads are those for which the current is below the minimum necessary for a momentary arcing condition. Fifty to 400 milliamperes at 26 Vdc is representative for this range. In the intermediate load range, slight arcing may occur on closure or opening of contact.
    • 4. Heavy loads in the so-called rated-load range: Heavy contact loads are those that cause some degree of contact arcing under normal operation. Ordinarily, contact must operate at or close to, the rated load function satisfactorily for their required life.
    There are several classifications of relays.There are basically three types of relays, a Form A which isnormally closed, Form B which is normally open and form C which is a Form A and Form B both triggered by a common input. The Form C is widely used intelecommunications circuitry.

    There are also specific devices called relays which contain relay and othe relectronics. The name relay come to those device from the fact that those used to be once electro-mechanical devices. Nowadays the same or more advanced functionality is usually implemented with electronics, and the devices can look anythign but traditional relays, but the name relay is still used because of historical reasons.

    • Analog/Digital Electromechanical Time Delay Relay is a device that provides a predetermined delay after power is applied before the contacts of an electromagnetic relay transfer. This kind of device is typically constructed so that the electromagnetic relay is operated by a signal given by analog discrete components or digital-operated integrated circuits.
    • A polarized relay is one that responds to the polarity as well as the magnitude of the energizing current. One way of accomplishing this type of operation is by connecting a blocking diode, either in series or in shunt, with the coil of a conventional dc relay. When the energizing voltage is of the correct polarity, operation takes place as in a conventional relay; with opposite polarity applied voltage there is no response.
    • Thermal relay consists of a heater element, a moving bi-metallic heated member, and an actuating linkage that operates normally open or normally closed contacts. Thermal relays are typically used for overcurrent protection (high current heats bi-metallic heated member and thus operates relay). Thermal relays typiclly provide operate time delays of 0.1 second to 5 minutes, the operate time for a particular design being a function of adjustment and power dissipation or applied voltage.
    • Overcurrent and earth fault relays are electrical network protective components which typically consists of switching, current measuring and electronic controlling parts.

    Some precautions that must be observed when using relays:

    • The condition of the relay tends to be unstable with the slow rise or decline of the coil voltage; therefore, it is necessary to test the relay in actual application.
    • Make sure the switch frequency of the contacts is lower than that in the specification sheets.
    • Make sure the ambient temperature is within the permissible temperature range.
    • Ensure the voltage applied on both ends of the relay conform to the rated voltage of the relay employed.
    • Should there be any vibration source in the appliance itself or in the vicinity of the installation, make sure the installation direction is correct.
    • Relays intended for application in heavy-current control appliances must meet voltage withstanding and insulation requirements set by safety regulations.
    • When the load is motors, lamps, solenoids or contactors, the load value indicated on the relay must correspond to the specific type of load
    • The working voltage of the relay must be within the user's voltage fluctuation range.
    • The relay must not be used or stored in a location with organic gas or sulphide gas in order to avoid the sulphurization or contamination of the relay contacts
    • If the relay is to be applied in an environment of high temperatures, choose a plastic-sealed relay. Avoid an open-type relay, which is prone to a burnout.
    • When an AC relay is used, the coil voltage must be higher than the action voltage to prevent adhesion of the relay contacts, which may lead to a burnout.
    • Make sure there is no interference from an intense magnetic field existing in the vicinity of the relay, such as a transformer or speaker.
    In some cases when switching the loads the relay can fail unless protected in some way.If the contacts are welding shut, then there's usually some inductiveeffects included in the load or power source. You might look into putting a snubber circuit across the contacts. A snubber is a series resistor-capacitor combination that absorbs a lot of theenergy when a contact opens. Search Google for snubbers and see what they do. Typical values for snubber components in motor circuits are 100 ohm inseries with 0.05uF and 100 ohm in series with 0.1uF. Experiment here. Another alternative is to use a suitably rated varistor across the contacts.Google again for varistors and relay contact protection. Please note that when you put the subber or VDR across the relay contacts, then the AC current through the relay is not completely cut off when relay contacts are open because there will be some leakage current through the snubber circuitry. You need to take this into account on your application.

    When checking the data for relay contact voltage and current ratings, be careful when reading them. The same relay can have different ratings on different applications. Usually the maximum current allowed depends on the operating voltage, because the voltage has effect on the size of spark made when the relay contacts make and break the contact. On some relays there can be different rating for switching resistive and inductive loads. Please also note that it is very typical that the relay contact voltage ratings on AC and DC are very different. The reason for this is that the spark between relay contacts acts differently on AC and DC. Typically the DC voltage ratings of a typical relay is lower. So whenever selecting a relay, be sure to check what you are switching and that you read the right (AC or DC) specifications. If you do not know if you will be switching AC or DC, then select a relay that has both AC and DC specifications good enough.

    Very many SSRs use Zero-Crossover Switching to reduce the RFI that could be caused by the relay switching (the relay turns on and off only at mains voltage/current zero crossing). For most applications Zero-Crossover Switching is well suitable, but not for all. A zero-crossover solid-state relay may be the worst possible method of switching on a transformer or a highly inductive load. A zero-crossover turn-on of highly inductive can cause a surge current of perhaps 10 to 40 times the steady state current, whereas turn-on at peak voltage results in little or no surge. The cause of inrush currents of such magnitude is core saturation. Surge currents of such magnitude can seriously shorten the life of the zero-crossover SSR, unless the SSR has a current rating well in excess the load. Surge currents create thermal and mechanical stress on the windings of the inductance and on the transformer core laminations. Surge currents also create EMI and RFI. The best method of turning on transformers and other saturable, highly-inductive loads is by use of a peak voltage turn-on device. Turn-on at peak voltage results in minimal surge, if indeed any surge is present at all.

    Solenoids

    A solenoid is simply a specially designed electromagnet. When current flows through a wire, a magnetic field is setup around the wire. In solenoid this there is a moving iron part (usually called "T" or plunger) that is moved by the magnetif field caused by the coil. So an solenoid simply translates electrical power to mechanical movement. You can cause mechanical movement by putting power to the solenoid. Solenoids are available in models that are designe for AC or DC use. When you slect a solenoid for an application, select a solenoid rated to the power you use (voltage, AC or DC etc) and the force needs you have. The mains voltage frequency has effects on solenoid operation. So some solenoids are designed to work best on 50 Hz or 60 Hz mains voltage.So-called "dual frequency" (50-60 cycle) coilsare actually wound for 50 cycles and their useon 60 cycles is limited to applications wheretheir reduced force is adequate to operate themechanism.Sometimes one coil can serve as a .dual frequency.coil if the 50 cycle operating voltageis lower than the 60 cycle nominal voltage.Solenoids are available in two grades: industrial grade and appliance grade. Today's APPLIANCE GRADE solenoids willperform adequately in applications for whichthey are intended.They can be used where anticipated servicelife does not exceed approximately 100,000cycles, and where high operating temperatureor a definite holding buzz are not objectionable.INDUSTRIAL GRADEsolenoids, though higher in price (roughlydouble for equal ratings), can be used wheremuch longer service life, cooler operating temperatureand quiet operation are desired.Due to the lower cost lamination materialused in APPLIANCE GRADE solenoids, socalled "break away springs" must be used toovercome the residual magnetism built up inthe metal parts, and to break the plunger awayfrom the field assembly when the solenoid is de-energized. INDUSTRIAL GRADE solenoids are typically made with more expensive silicon steel laminationswhich will not develop residual magnetismand require no "break away springs" tofree the plunger.Typically appliance grade solenoids require around twice the wattage to produce thesame force. The extra wattage was required bythe cheaper steel used in the laminations, to overcome "break away springs".The pull-in force of a solenoid decreasesrapidly as the voltage decreasesbelow the coil nominal rating. On theother hand, as the voltage increases overthe nominal value, the pull-in force increases,but the solenoid temperaturemay also rapidly increase. A solenoid operating on AC draws a high.inrush current. when the solenoid is open.As the solenoid closes, this current decreasesto a low .holding current. when the solenoidis fully closed.This current characteristic of an AC solenoidis extremely important. The high inrush currentprovides a high initial force which is usuallydesirable to overcome the load on the solenoid.A solenoid designed to operate on alternatingcurrent can also be operated on directcurrent. There are, however, some limitations.When solenoid is powered form DC the current flow isconstant regardless of whether the solenoid isopen or closed. The inrush and holding currentare the same.Because of this constant current feature ofDC, a compromise between pull-in force andholding temperature must be made.The question of whether or not agiven AC unit performs satisfactorily on DCpower depends upon how pull-in force andoverheating can be balanced.In applications where the stroke is extremelyshort, an AC unit can usually be operated successfullyon DC directly. In many cases, AC solenoids can be operatedon DC power with the addition of a switch andresistor. The switch is arranged to be openedwhen the solenoid closes. When the switchopens the resistor is in series with the solenoidcoil. The addition of this resistance reducesthe coil current so the solenoid can be heldenergized without burning out. A high currentwhich will produce a high pull-in force is thenpossible. The same idea can also be realised with electronics driving circuitry that gives first higher voltage/current and then reduces it to the holding level after some time. Typically if an AC solenoidis not permitted to close in approximately tenmilliseconds, the coil will overheat and forcewill decrease substantially.Within limits, DC solenoids can usually be operated on AC. For reasons of economy and flexibility, DCsolenoids are usually made with solid ironparts. When operated on AC, eddy current andeddy current losses are Introduced. Theselosses in solid iron parts are high, and hightemperatures can be developed.Therefore, the use of AC power on DC solenoidsshould be limited to applications where alow current is adequate, to prevent overheating.If a DC unit is to be used on both ACand DC power, it should be equipped withshading coils. These coils, common on AC designs,keep the solenoid from buzzing when theAC sine wave goes through zero.When using solenoids, remeber that solenoid force solenoid force is simply the amount ofload (or weight) the solenoid plunger can pullin when energized. Each stroke length has adifferent force rating, and force increases asstroke length decreases, so pay close attentionto stroke length during testing.

    Switches

    A switch is a device for changing the course (or flow) of a circuit. The term "switch" typically refers to electrical power or electronic telecommunication circuits. In the simplest case, a switch has two pieces of metal called contacts that touch to make a circuit, and separate to break the circuit. The contact material is chosen for its resistance to corrosion, because most metals form insulating oxides that would prevent the switch from working. The moving part that applies the operating force to the contacts is called the actuator, and may be a toggle or dolly, a rocker, a push-button or any type of mechanical linkage.

    A pair of contacts is said to be 'closed' when there is no space between them, allowing electricity to flow from one to the other. When the contacts are separated by a space, they are said to be 'open', and no electricity can flow.

    Switches can be classified according to the arrangement of their contacts. Some contacts are normally open until closed by operation of the switch, while others are normally closed and opened by the switch action. Typically switch contacts can be either Normally Open (NO), Normally Closed (NC), or change-over contacts.

    In a multi-throw switch, there are two possible transient behaviors as you move from one position to another. In some switch designs, the new contact is made before the old contact is broken. This is known as make-before-break, and ensures that the moving contact never sees an open circuit. The alternative is break-before-make, where the old contact is broken before the new one is made. This ensures that the two fixed contacts are never shorted to each other. Both types of design are in common use, for different applications.

    The terms pole and throw are used to describe switch contacts. A pole is a set of contacts that belong to a single circuit. A throw is one of two or more positions that the switch can adopt. These terms give rise to abbreviations for the types of switch which are used in the electronics industry. Some examples of switch classification:

    • SPST - Single Pole Single Throw. These have two terminals which can be switched on/off.
    • SPDT - Single Pole Double Throw. These have one row of three terminals. One terminal (common) switches between the other two poles. It is the same as a single change-over switch.
    • DPST - Double Pole Single Throw. These have two pairs of terminals. Equivalent to two SPST switches operated together. This configuration may also be referred to as DPNO.
    • DPDT - Double Pole Double Throw. These have two rows of change-over terminals. Equivalent to two SPDT switches actuated together. In total, six terminals.

    Switches with larger numbers of poles or throws can be described by replacing the "S" or "D" with a number or in some cases the letter T (for triple).

    Switches are also classified by their mechanical construction or other operation details:

    • •DIP Switches are switch assemblies that use tiny actuators to configure binary combinations of closures. DIP switches are often used in computers and peripheral devices for configuration settings.
    • Hall Effect Switches use the phenomena of the Hall effect to complete the switching action (they do not involve any traditional electromechanical movement). These switches find often applications in motion sensing and motion limit.
    • •Pressure Switches are generally electromechanical on/off switches that activated by pressure changes in liquids or mass flow movement.
    • Pushbutton Switches are mechanically activated by a plunger that is pushed down to open or close the connection.
    • Tact Switches are switched designed to give the user sensory or tactile feedback when button or switch is depressed. Tact Switches typically makes contact with the PC board beneath.
    • Sealed Switches are special switches that are used in harsh environments which require sealing or resistance to vibration. This type of switches are often found in military applications, vehicles, aviation, and robust industrial equipment.

    Sometimes people ask if it is OK to use AC voltage rated switch on DC circuit. The answer is usually no, even though the DC voltage used would be less than AC rating. When switching DC, you should look a switch with DC ratings you need. Some switches have both AC and DC ratings, some are only designed AC use. The amount of current that a device can handle is dependent on the size of the contacts. But it's the spacing of the contacts which determine if alternating and direct current can be used on the device. An AC switch is designed to have the contacts closer together. This allows for a quieter device than a DC switch. A switch designed to be used with DC has the contacts gapped farther apart. This is due to the fact that the current does not alternate.

    Special switches can be designed to respond to any type of mechanical stimulus: for example, vibration (the trembler switch), tilt, air pressure, fluid level (the float switch), the turning of a key (key switch), linear or rotary movement (the limit switch or microswitch), or presence of a magnetic field (the reed switch). The mercury switch consists of a blob of mercury inside a glass bulb. The two contacts pass through the glass, and are shorted together when the bulb is tilted to make the mercury roll on to them.

    Contact bounce (also called chatter) is a common problem with mechanical switches and relays. Switch and relay contacts are usually made of springy metals that are forced into contact by an actuator. When the contacts strike together, their momentum and elasticity act together to cause bounce. The result is a rapidly pulsed electrical current instead of a clean transition from zero to full current. The waveform is then further modified by the parasitic inductances and capacitances in the switch and wiring, resulting in a series of damped sinusoidal oscillations. Sequential digital logic circuits are particularly vulnerable to contact bounce. There are a number of techniques for debouncing (dealing with switch bounce). Simple timing based techniques rely on adding sufficient delays to prevent bounce being detected. Other common technique is to use resistor/capacitor network.

    Other electromechanic controls

    Diodes

    Diodes are non-linear circuit elements.Qualitatively we can just think of an ideal diode has having two regions: a conduction region of zero resistance and an infinite resistance non-conduction region. For many circuit applications, this ideal diode model is an adequate representation of an actual diode.The behaviour of a (junction) diode depends on its polarity in the circuit.If the diode is reverse biased (positive potential on N-type material) the current through the diode is very small. A forward-biased diode (positive potential on P-type material) can pass lots of current through it would much resistance (only a small voltage drop).Diodes are very often used in power supplies for rectifying applications.A typical method of obtaining DC power is to transform, rectify, filter and regulate an AC line voltage.In power supply applications it is common to use a transformer to isolate the power supply from the 110 V AC or 230V AC line. A rectifier can be connected to the transformer secondary to generate a DC voltage with little AC ripple.Diodes have alsways limits how much reverse voltage they can take. As the reverse voltage which is applied across the diode is increased, the stage will be reached where the diode breaksdown. Diodes which are used for rectifying purposes should not breakdown in their reverse direction. Thus the peak inverse voltage (p.i.v.) is quoted in their specification as the voltage below which they will not breakdown. It is possible to recover from the breakdown situation as long as the diode has not been overheated, i.e. burnt out, by excessive power dissipation. The reverse breakdown of diodes has its uses as a voltage reference or for protection purposes. There are several other types of diodes beside the typical junction diode. The Zener Diode is a special diode, where Zener breakdown occurs when the electric field near the junction becomes large enough to excite valence electrons directly into the conduction band. This means that a zener diode passes current through it in reverse direction when voltage is high enough (the zener voltage). Zener diodes are typically used as voltage reference components in measuring circuits, as voltage regulators in some low power power supplies and as over-voltage protection devices. Generally in zener circuits a series resistor must be used to limit the current flow through the diode. Light-emitting diodes (LED) emit light in proportion to the forward current through the diode. LEDs are low voltage devices that have a longer life than incandescent lamps. They respond quickly to changes in current (many can easily go up to 10 MHz). LEDs have applications as visible indicators in devices and in optical-fiber communication. LEDs produce a narrow spectrum of visible )many colors available) or infrared light that can be well collimated. LEDs have generally very low reverse break-down voltage (typically from 3-10V depending on LED) and can be damaged by the break-down. Most GaN and InGaN LEDs (UV, violet, blue, non-yellowishgreen, white, pink, and anything non-red by Nichia) suffer damage in theform of partial shorts (resistance often around a hundred to severalhundred ohms) when they break down. I would add a parallel diode to keep the reverse voltage out of LED in applications where LED can get considerable reverse voltage (for example when powered with AC power). I would not use just a series diode since it may leak enough current to cause breakdown or result in excessive reverse bias that some types may be aged excessively. Some LEDs have survived a veryharsh environment that had high voltage spikes in the reversedirection. But that doesn't mean that they should be exposed to thosehigh reverse voltages.Light-Sensitive Diodes indicate light of a proper wavelength.Photo-diodes or photocells can receive light signals. LEDs and photodiodes are often used in optical communication as receiver and transmitter respectively.

    Thyristor and TRIAC

    Thyristor is a component that acts like a very special diode. At the thyristor, the timing which the forward direction electric current flows through can be controlled. The thyristor has the three terminals. They are the T2(The terminal which is equivalent to the anode), the T1(The terminal which is equivalent to the cathode) and the gate. The electric current in the forward direction doesn't flow when the electric current doesn't flow through the gate. When the trigger electric current(Pulse) flows through the gate, the forward direction electric current of the thyristor begins to flow. The forward direction electric current of the thyristor to have begun to flow through once alwayses fall through even if the electric current at the gate passes away until the forward voltage passes away. The gate function is again restored when the forward voltage passes away. So, the forward direction electric current of the thyristor doesn't flow until the trigger electric current flows through the gate even if the forward voltage is applied once again.The basic principle of using a PNPN structure to producea thyristor, and a NPNPN structure (with two PNPN.s inantiparallel) to produce a triac has been known fordecades.Some thyristor types:

    • SCR: SCR (Semiconductor Controlled Rectifier) is name for "a normal thyristor" used in many different applications
    • GTO: GTO (Gate Turn Off) is a special type of thryristor that can be turned on and off with control current on the gate. A positive current pulse turns GTO on and negative current pulse turns it off.
    • RCT: This is a combination of SCR and reverse connected diodes. This will conduct current to both directions and current flow to one direction is controllable.
    • LASCR: LASCR (Light Activated SCR) is a light controller tyristor that is used in applications where very high voltages are controlled.
    • SITh: SITh (Static Induction Thyristor) is a thyristor that work usign JFET principle
    • IGCT: IGTC (Integrated Gate Controlled Thyristor)
    • GATT, GATO: Gate-Assisted Turn-Off Thyristor is a special thyristor used in some TV deflection circuits and in radar technology
    • FCT: FCT (Field Control Thyristor) is a thyristor that work usign JFET principle
    • IGCT: IGCT (Integrated Gate Controlled Thyristor) is a thyristor type that is used in high power inverter circuits.
    • TRIAC: TRIAC is a trade name of bidirectional triode thyristor. TRIAC is a component that is like two thyristors in one case on different directions. It can control the current flow in both directions with the control pulse turing it on until the next current zero crossing.
    The bidirectional triode thyristor (TRIAC) is the thyristor which can be used in the alternating current. It is possible to work the gate function in each of the alternating current sides of the positive voltage, the sides of the negative voltage. Because it is, the electric power of the alternating current can be controlled.DIAC is a trigger diode that has the special characteristic. At the trigger diode, the electric current flows through bidirectionaly. When the voltage across the DIAC is less that the DIAC norminal voltage, no current flows. When the voltage which is applied to the DIAC crosses the DIAC nominal voltage, the diode becomes the ON condition and the voltage of the both edges of the diode falls rapidly. This characteristic matches well to control the gate of the bidirectional triode thyristor, and this is the main application for DIACs.

    Transistors

    At their most basic level, transistors may seem simple. The bipolar junction transistor was the first solid-state amplifier element and started the solid-state electronics revolution. Bardeen, Brattain and Shockley at the Bell Laboratories invented it in 1948 as part of a post-war effort to replace vacuum tubes with solid-state devices. Their work led them first to the point-contact transistor and then to the bipolar junction transistor. The first transistors used germanium as the semiconductor of choice because it was possible to obtain high purity material. Since then, the technology has progressed rapidly.

    A bipolar junction transistor consists of two back-to-back p-n junctions, who share a thin common region. Contacts are made to all three regions, the two outer regions called the emitter and collector and the middle region called the base. Transistors are available in a large variety of shapes and sizes, each with its own unique characteristics. The characteristics for each of these transistors are usually presented on specifications sheets. Although many properties of a transistor could be specified on these sheets, manufacturers list only some of them. The specifications listed vary with different manufacturers, the type of transistor, and the application of the transistor. The "Absolute Maximum Ratings" of the transistor are the direct voltage and current values that if exceeded in operation may result in transistor failure. Maximum ratings usually include collector-to-base voltage, emitter-to-base voltage, collector current, emitter current, and collector power dissipation. The typical operating values of the transistor are presented only as a guide for designer using the transistor. The values vary widely, are dependent upon operating voltages, and also upon which element is common in the circuit. The values listed may include collector-emitter voltage, collector current, input resistance, load resistance, current-transfer ratio(another name for alpha or beta), and collector cutoff current (leakage current from collector to base when no emitter current is applied). Transistor characteristic curves may also be included.

    Transistors can be identified by type code designation printed directly on the case of the transistor. There are different marking schemes used sround the world. Joint Army-Navy (JAN) designation is one that is used in USA. The codes are in form of 2Nxxx. The first number indicates the number of junctions. The letter "N" following the first number tells us that the component is a semiconductor. And, the 2- or 3-digit number following the N is the manufacturer's identification number. If the last number is followed by a letter, it indicates a later, improved version of the device. For example, a semiconductor designated as type 2N130A signifies a three-element transistor of semiconductor material that is an improved version of type 130. European transistors are often indicated with semiconductor code that has wto letters in the beginning and three numbers after it. The typical letters you see on transistors are AC, AD, BC and BD. Japanise transistor markings have typically for of 2SCxxx or similar.

    Transistors are very rugged and are expected to be relatively trouble free when used properly. Encapsulation and conformal coating techniques now in use promise extremely long life expectancies. In theory, a transistor should last indefinitely. However, if transistors are subjected to current overloads, the junctions will be damaged or even destroyed. In addition, the application of excessively high operating voltages can damage or destroy the junctions through arc-over or excessive reverse currents. One of the greatest dangers to the transistor is heat, which will cause excessive current flow and eventual destruction of the transistor.

    To determine if a transistor is good or bad, you can check it with an ohmmeter or a transistor tester. In many cases, you can substitute a transistor known to be good for one that is questionable and thus determine the condition of a suspected transistor. This method of testing is highly accurate and sometimes the quickest, but it should be used only after you make certain that there are no circuit defects that might damage the replacement transistor.

    There are three basic transistor circuits. They are called according to that electrode(emitter, base, col-lector) which is common to both input and output circuit. When analyzing transistor in circuit simulation in mind, a transistor can be considered as an active four-pole network. When driven with small low-frequency signals its properties can be described by the four characteristic values of the h-matrix (hybrid), which are assumed to be real. In the transistor data sheets the h-parameters are usually quoted for the common emitter configuration and for a given operating point (bias). Whereas the network behaviour of low frequency transistors could be described by using the h-matrix (hybrid), the y-matrix (admittance) is usually employed for high frequency transistors.While bipolar transistors tend to be replaced with MOSFETs in many applications, bipolar transistors remain still important devices for ultra-high-speed discrete logic circuits such as emitter coupled logic (ECL), power-switching applications and in microwave power amplifiers.

    You can sometimes encounter a term digital transistor (dtr). Those components are are transistors with built-in resistors, so that the transistor base can bie directly connected to digital logic IC output. Some digital transistor have one resistor between base and emitter, others in series with the base. Many others have both. The values of resistors built into device depend on the transistor specifications and the logic family it is designed to be interfaced to.

    FETs, IGBTs

    FET stans for Field Effect Transistor.A regular FET pinches off (depletion mode) has input impedance around 1 megohm or more.MOSFET stands for Metal Oxide Semiconductor Field Effect Transistor. It is one type of SET.MOSFET (metal oxide semiconductor), also known as IGFET (insulated gate), has a layer of insulation above a transistor junction. A MOSFET can have very high input impedance, up to around 1E12 ohm. Most mosfets are enhancement mode (naturally off). MOSFET can give a true ohmic source-drain connection controlled by gate voltage.While the MOSFET is the most common type of Field Effect Transistor, there are several other types of FETs. One crucial difference, which in part also explains the fact that these FETs are less common, is that the isolation between the gate and the channel is obtained by a reversed biased diode. The operation of the transistor is therefore limited to the voltage range for which that diode is indeed reversed biased. Especially enhancement-mode transistors are affected by this restriction, which limits the useful voltage range. Here is overview of some common FET types:

    • J-FET: The Junction Field Effect Transistor (J-FET) uses a p-n diode to isolate the channel from the gate. No inversion layer exists in the device, instead the channel is depleted by reverse biasing the p-n diode between the channel and the gate. The advantage of a j-FET is that the carriers flow within the semiconductor rather than at the surface as in a MOSFET, yielding a higher carrier mobility and better noise performance. j-FETs can therefore be found in low noise amplifiers. One disadvantage of the j-FET is that the high-frequency behavior is adversely affected by the diffusion capacitance, due to the minority carriers which accumulate under forward bias, and the parasitic capacitance of the p-n diode.
    • MESFET: The Metal-Semiconductor-Field-Effect-Transistor (MESFET) consists of a conducting channel positioned between a source and drain contact region. The key advantage of the MESFET is the higher mobility of the carriers in the channel as compared to the MOSFET. The higher mobility leads to a higher current, transconductance and transit frequency of the device. The disadvantage of the MESFET structure is the presence of the Schottky metal gate. It limits the forward bias voltage on the gate to the turn-on voltage of the Schottky diode. This turn-on voltage is typically 0.7 V for GaAs Schottky diodes. The threshold voltage therefore must be lower than this turn-on voltage. The higher transit frequency of the MESFET makes it particularly of interest for microwave circuits. A classical example is the gallium arsenide (GaAs) MESFET, a frequently used microwave transistor.
    • MODFET: The Modulation-Doped Field Effect Transistor (MODFET) also uses a Shottky barrier diode to isolate the channel from the gate and has similar advantages as the MESFET. The primary difference is that the channel is not a simply doped region, but instead consists of carriers which come from a doped region with a higher bandgap. This device is also refered to as the High Electron Mobility Transistor (HEMT) which better highlights the fact that the electrons are physically separated from their donor atoms.
    The spectacular rise of the MOSFET market share during the last decade has completely removed the bipolar transistor from center stage. Almost all logic circuits, microprocessor and memory chips contain exclusively MOSFETs.

    The control voltage range that drives most power MOSFETs fully open (lowest resistance) is typically in 6-10V range. There are also special MOSFETs (usually more expensive than basic types) that can be turned fully on with lower voltage. MOSFETs that can be turned fully on with lower voltages, usually in 3-5V voltage range are often called digital FETs, because they can be connected ditrectly by the output of a normal 5V digital logic IC.

    Unijunction transistor

    Unijunction transistor (UJT) is a special transistor like componentwhich is used to build oscillators. It was quite commonly usedcomponent in 1970's, but nowadays quite rarely used.

    Crystals and crystal oscillators

    Typical crystal oscillates at the fundamental resonance frequency determined by the cystal mechanical characteristics (crystal materialand crystal cut).Many high frequency crystals (mostly those above 20 MHz) areovertone crystals, which need special attention in the use tomake them oscillate the nominal frequency and not the fundamentalfrequency.Fundamental frequencies are approximately one-third, one-fifth or one-seventh the overtone frequency, depending on the cut of the crystal.

    Fuses and similar protection devices

    Fuses are possibly the most misunderstood components in electronics, and often regarded as a "nuisance". In fact they playa very important role as a "safety valve" in many circuits, protecting both the equipment from costly damage and theuser from serious injury or possible loss of life. Fuses are always marked with the current level they?re designedto carry on a continuous basis before fusing will occur, at astandard temperature. The main job of the fuse in typical electrical distirbution wiring is to protect the wiring.

    Fuses should be sized and located to protect the wire they are connected to. Circuit breaker or fuse is a protective device located on an electrical circuit to interrupt the flow of abnormally large currents. A fuse is a circuit element designed to melt when the current exceeds some limit, thereby opening the circuit. A fuse is the most common means of providing overload and fault protection for customers and utilities. Fuse is a safety device, so when operating with them be sure what you do.

    When safety is involved, you shouldn't be guessing. It is very important to replace a blown fuse with one having thesame voltage rating, as well as the same current carrying capacity. Fuses are the commonly used protection devices to protect componentslike wires, transformers electronics circuit modules against overload.The general idea of the fuse is that it "burns fuse link" when current gets higher than it's rating and thus stops the current flowing.

    The most important ratings of fuses used in electronics circuits are current rating, voltage rating and interrupt capacity. The current ratingtells the current the fuse allows to pass through. If the current gets higher, then after some time the fuse link melts (the timethis takes depend on the fuse design and the amount of current flowing). For safety reasons, put only the fuse with the same current rating as the equipment indicates. Putting a fuse with higher current rating than indicated will cause a serious safety hazard (fire hazard when more current gets through than the equipment is designed for). The voltage rating of a fuse is to ensure that as the fuse link meltsand then arcs, that the arc extinguishes, does not re-strike, and that the fuse stays open. The fuse voltage rating tells how much voltage the fuse can safely interrupt.

    When replacing a fuse with a new one, select a fuse type with at least the same voltage rating as the original one. You can safely replace a fuse with a new one which has higher voltage rating than the old fuse.

    Issues related to the "blast" (fuse burning) are covered by the interrupting rating of the fuse, which is the maximum current the fusecan interrupt at rated voltage without exploding or rupturing. This interrupt rating is highly dependent on the voltage over the fuse and is the fuse operated at AC or DC. So when checking the interrupt rating, check under what conditions tose are defined for the fuse you use. The interrupting rating is not the same as the fuse current rating. Interrupt rating needs to be considered in the applications where high short circuit currents are possible (the fuse should reliably cut the current in those cases also).

    In addidion to those you might sometimes see additional fuse parameters, like fuse speed or arching time. Those define how quickly the fuse cuts the current in the case of overload.

    When purchasing the fuse you need generally the part number type that is on the fuse (spefifies size and other general details), the rating, and if it is a fast or slow-blow type.

    Some overview of different fuses:

    • Diazed Fuses: Diazed Fuses, commonly called "Bottle" Fuses, are available in five sizes, ND to 25 Amps, DII to 25 Amps, DIII to 63 Amps, DIV* to 100 Amps and DV * to 200 Amps. Fuse accessories are sized to match these five sizes. Each size fuse body has a different diameter to fit only into the appropriate Screw Cap and Fuse Base. Also, the fuse tips have different diameters, depending on their current rating. The diameter of the tip matches the diameter of the hole in the Adapter Screw to insure that no fuse with a higher rating than intended for the circuit can be installed. Diazed Fuses are commonly seen on European electrical distribution panels. The Fuse is held in place by the Screw Cap, which is screwed into the Fuse Base. When a Diazed fuse has blown, the color coded indicator on the head of the fuse will pop out, giving visible indication through a glass window in the Screw Cap.
    • Neozed Fuses: Neozed Fuses are more compact than the Diazed Fuses. Three sizes are available, D01 to 16 Amps, D02 to 63 Amps and D03 to 100 Amps. Fuse accessories are sized to match these three sizes. Each size fuse body has a different diameter to fit only into the appropriate Screw Cap and Fuse Base. Also, the fuse tips have different diameters, depending on their current rating. The diameter of the tip matches the diameter of the hole in the Adapter Ring to insure that no fuse with a higher rating than intended for the circuit can be installed. This prevents damage to the circuit or equipment the fuse protects. The Fuse is held in place by the Screw Cap, which is screwed into the Fuse Base. When a Neozed fuse has blown, the color coded indicator on the head of the fuse will pop out, giving visible indication through a glass window in the Screw Cap.
    • Cylinder Fuses: Compact Cylinder Fuses are typically used in industrial applications to protect electrical devices such as motors, drives, etc. They are available in four sizes with a current range from 1 to 100 Amps. Cylinder Fuses have metal caps at both ends.
    • NH Fuses: NH fuses are typically used for distribution applications and to large electrical devices such as motors, drives, etc. They are available in seven sizes with a current range of 3 to 1600 Amps. NH fuses have knife blades at both ends , which mount into one or three pole Fuse Bases. Fuse Bases can be panel or DIN rail mounted.
    • 5x20 mm cylindicar fuse: Small cylinder fuses used in many electronics devices. Typical ratins from tens of mA to 16A. Voltage rating typically 250V AC. The fuse specifications are normallu stamped on the metal ring of the fuse.
    • 6.3 x 32 mm cylindicar fuse: Small cylinder fuses used in some electronics devices, some car/boat electrical installations etc. Rating available typically up to 16A/250V.
    • Car fuse: Car fuse is a flat plastic unit that has two metal blades that connect to fuse holder. It is commonly used for automotive and marine applications, especially in European and Japanese cars. Maximum voltage of 32 Volts, typically used on 12V DC circuits. Fuse ratings from few ampreres to 30A commonly available. Typical values 5, 10, 15, 20, 25, 30amp.

    Fuse Operating Classes:

    • Slow Blow fuses are normally used to protect cable and conductors from damage due to overloads and short circuits. Typical Markings: 'T', Trage, gl/gG, snail symbol
    • Fast Blow Fast Blow fuses are typically used to protect equipment. Typical Markings.- "F, Flink, (the absence of the snail symbol)
    • Super Fast Blow Typically used for protecting semiconductors like diodes, SCRS, etc. Typical Markings: UtraRapid, Silzed , Recticur, gR
    • gl aM - Motor Protection Fast acting short circuit protection, but slow acting overload protection
    • gR - Semiconductor Protection Typically used for protecting semiconducors like diodes, SCR's etc. Current limiting, super fast blow.

    There are also some other type of protection devices than fuses. Some examples oof such thigns are resettable "fuses" built using bi-metallic technology and PTC-polymer devices are used in some electronics devices to guard against over current damage. Those devices act like automatically resetting fuses.

    A device called Miniature Circuit Breaker is is used to replace fuses in electrical distribution systems (mains electrical panel). Miniature circuit Breaker can be is used in lighting distribution system or motor distribution system for protecting overload and short-circuit in the system. Miniature circuit breaker has a "switch" on it, so it can be used for overload and short-circuit protection as well as for unfrequent on-and-off switching electric equipment and lighting circuit in normal case.

    The Residual Current Device(RCD) is a special electronic/electromechanicalprotection device that cuts off the fault circuit immediately on theoccasion of shock hazard or earth leakage of trunk line.Earth Leakage Circuit Breaker (ELCB) is mainly prevent eletric fire and personal casualty accident caused by personal electric shock or leakage of electrified wire netting.Residual Current Circuit Breakers (RCCB) is similar to Earth Leakage Circuit Breaker (ELCB).In mains wiring earth leakage circuit breaker is used for the protection,against electrical leakage in the circuit of 50Hz or 60Hz. When somebody gets an electric shock or the residual current of the circuit exceeds the fixed value, the ELCB/RCCB can cut off the power.

    Most electrical protection devices mentioned above are mostly designed for AC currents in mind. The needs in DC circuits can be different. Any device that is meant to interrupt high DC current must be specifically designed for that purpose. Batteries in all forms can produce an astounding amount of power when short-circuited. Because the electric current from a battery is Direct Current, special precautions should be taken when installing proper fuses, circuit breakers and disconnects. Household fuses and circuit breakers are designed to protect alternating current voltages and currents. Their specifications are generally designs for AC in mind. There is no guarantee that they will properly protect a battery powered circuit. These devices must be capable of not only protecting the circuit, but must also be able to do so without becoming a fire hazard themselves. When electing fuse of overcurrent protector for DC application, check the DC characteristics gives by the manufacturers. While the normal breaking current in DC is generally the same as in AC use, the current interrupt and voltage ratings can be considerably different with DC.

    When a high-current DC fault is interrupted, a sustained, very hot arc can be formed inside the fuse or circuit breaker. DC rated fuses usually have plastic beads inside the body of the fuse. When the fuse 'blows', the heat of the arc that is generated melts the beads, filling the void inside the fuse and preventing a sustained arc from forming. Similarly, DC rated circuit breakers have 'arc snubbers' or magnetic 'arc blow out' devices internally that protect the breaker from sustained arcs.

    While automotive-type fuses can and do protect low current circuits from short circuit overloads, they are only effective up to a point. Usually, the smaller gauge wiring in low current circuits works to limit the maximum amount of fault current available from the battery. Where heavy gauge wiring is used to carry large amounts of current, the fault current available from the battery during a short circuit event can easily exceed the safe limits of automotive-type protection devices, turning them into incendiary fire bombs. Every battery-powered system should have a master fuse or circuit breaker, one that is adequately sized to protect in case of a total short circuit of the main battery cables. Even a small engine starting battery can produce currents in the 10,000 amperes range for short periods when short circuited. The higher the total battery voltage, the more the risk of short circuit fault current problems.

    Batteries

    Batteries are all over the place: in our cars, our PCs, laptops, portable MP3 players and cell phones. A battery is essentially a can full of chemicals that produce electrons. Chemical reactions that produce electrons are called electrochemical reactions. If you look at any battery, you'll notice that it has two terminals. Electrons collect on the negative terminal of the battery. If you connect a wire between the negative and positive terminals, the electrons will flow from the negative to the positive terminal as fast as they can. Normally, you connect some type of load to the battery using the wire. Inside the battery itself, a chemical reaction produces the electrons. The speed of electron production by this chemical reaction (the battery's internal resistance) controls how many electrons can flow between the terminals. Normal batteries have generally 1.5V per cell voltage (except some Lithium cells which have 3V voltage). The batteries which have higher voltage output are built genrally from many 1.5V cells in series all put inside same "case".Although the terms battery and cell are often used interchangeably cells are the building blocks of which batteries are constructed. Batteries consist of one or more cells that are electrically connected.The world of batteries divides into two major classes: primary and secondary batteries. Primary batteries such as the common flashlight battery are used once and replaced. The chemical reactions that supply current in them are irreversible. Secondary batteries (for example, car batteries) can be recharged and reused. They use reversible chemical reactions. By reversing the flow of electricity i.e. putting current in rather than taking it out, the chemical reactions are reversed to restore active material that had been depleted. Secondary batteries are also known as rechargeable batteries, storage batteries or accumulators.

    Lamps

    Lamps come in many forms. In an incandescent bulb, current heats the tungsten filament, which glows white hot. To prevent the filament from rapidly oxidizing, the bulb is filled with an inert gas, mainly argon at low pressure. Much of the energy dissipated by the filament is heat; only a little is light. The filament of an incandescent lamp is simply a resistor. If electrical power is applied, it is converted to heat in the filament. The filament's temperature rises until it gets rid of heat at the same rate that heat is being generated in the filament. Ideally, the filament gets rid of heat only by radiating it away, although a small amount of heat energy is also removed from the filament by thermal conduction. The filament's temperature is very high, generally over 2000 degrees Celsius, or generally over 3600 degrees Fahrenheit. In a "standard" 75 or 100 watt 120 volt bulb, the filament temperature is roughly 2550 degrees Celsius, or roughly 4600 degrees Fahrenheit. At high temperatures like this, the thermal radiation from the filament includes a significant amount of visible light. The fact that a bulb uses 100 watts of energy doesn't mean it gives 100 watts of light (typically only 10% or less of the energy consumed by incandescent lamps is actually used to produce light, the rest ends up as heat). Incandescent lamps are generally available in wattage ranging from 2 to 1500 watts and above. In many cases, the light level generated by a particular luminaire can be increased or decreased simple by switching to a different lamp wattage (do not exceed the maximum lamp power allowed in the luminaire).Incandescent light bulbs are voltage-driven devices. The lamp's filament is designed and sized to offer a preset amount of resistance to current flow. This controls the amount of current passing through the lamp. As long as the bulb voltage is right for applied mains voltage, it will work well. The resistance of the light bulb filament changes on the temperature. The lower the temperature is, the lower is the resistance. This means that when the bulb is cold, it will take much more current from the voltage source than when it is operating at normal temperature. The resistance of a tungsten filament is lower when it is cold than when it is hot so if you measure its resistance with a multimeter the result obtained is a little misleading. Once you apply voltage across it so that current flows it heats up and its resistance increases. This means that the initial current is typically few times greater than the normal running current. The high current surge and the effects of very fast filament heating are reason why light bulbs usually fail at switch-on. In addition to incandescent lamps there are many other lamp typies including neon, fluorescent, LED and many other types.

    Piezo-electric components

    Piezoelectricity is a name for transformation of mechanical strain to internal electric field shifts and vice versa in a material. Piezo-electric materials are commonly used in different kinds of transducers. Transducers convert one form of energy to another. Piezo motors (actuators) convert electrical energy to mechanical energy, and piezo generators (sensors) convert mechanical energy into electrical energy. In most cases, the same element can be used to perform either task. Single sheets can be energized to produce motion in the thickness, length, and width directions. They may be stretched or compressed to generate electrical output. Thin 2-layer elements are the most versatile configuration of all. They may be used like single sheets (made up of 2 layers), they can be used to bend, or they can be used to extend. "Benders" achieve large deflections relative to other piezo transducers. Multilayered piezo stacks can deliver and support high force loads with minimal compliance, but they deliver small motions.A typical piezo-electric transducer used to generate audio or ultrasound has a mechanically resonant structure, because of the mass and stiffness of the piezo-electric material. Due to the piezo-electric effect these mechanical properties manifest themselves as electrical equivalent properties, so for example the electrical resonant frequency seen at the electrical terminals is equal to the mechanical resonant frequency.

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