Index

Measuring and testing

    Measuring instrument interfaces

    IEEE 488 is propably the mostly adopted communication bus and protocol used in electronic measusing equioment. In 1977, the IEEE adopted the bus structure and communication protocol that it named IEEE 488. Some others call it GPIB (general-purpose instrumentation bus). The bus's original name was HPIB (Hewlett-Packard instrumentation bus). Until the advent of the HPIB, no standardized methods existed for interfacing instruments with computers. IEEE 488 remained for more than two decades the industry's primary standard for enabling instruments and computers to talk with one another.IEEE 488 standard did a good job of defining the communications hardware, it initially gave short shrift to interfacing's software aspects. More than a decade elapsed before the evolution of the necessary software standards, particularly SCPI (standard commands for programmable instruments).IEEE 488 was not the only interface used. RS-232 ports have became popular on slower instruments. The two top contenders for the instrument-interfacing standard of the future are Ethernet and USB. You can find one or both in many instruments. Scopes that offer communication ports other than IEEE 488 are becoming increasingly common. The current and most likely future leader in replacing IEEE 488 is Ethernet. USB will also play a major role. The most obvious reasons for turning to computer-standard interfaces in place of IEEE 488 for instruments are cost, size, cable length of instrument networks, and increasing difficulty of installing specialized peripheral controllers in newer PCs. For test instruments, an advantage of an Ethernet connection over a USB or IEEE 488 connection is Ethernet's much greater allowable cable length. Ethernet LANs.even using gigabit-per-second Ethernet technology.can span thousands of feet. USB and IEEE 488 are limited to tens of feet. Don't be fooled by the new protocols' high nominal bit rates; instrument interfacing usually involves short messages. In such service, IEEE 488 can be significantly faster than protocols that at first appear to be much faster than IEEE 488.Using an instrument as a Web server is a new aspect in interfacing. Web-server technology is particularly well-suited to instruments that connect to Ethernet networks and that use TCP/IP (Transfer Control Protocol/Internet Protocol).

    Oscilloscopes

    Every scientist, engineer, and technician involved in any form of electronics has used an oscilloscope. Scope displays of amplitude as a function of time provide intuitive and easily interpreted pictures of signals. Oscilloscope is one of the most important test instruments foravailable engineers. It is useful for very many electronics measurement. The main purpose of an oscilloscope is to display the level of a signal relative to changes in time. You can use an oscilloscope to analyze signal waveform, get some idea of signal frequency and many other details.Scopes are ment for looking at the qualitative aspects of the signal (like signal waveform, esitence of signal, etc.).

    For making quantitative measurements, a scope is "usually" a bad choice (for example multimeter is more accurate tool to measure DC voltage levels than a scope). It is quite typical for the scope to be out by a percent or two or three but if you're counting on that kind of accuracy, you're using the wrong tool. Deviations as high as ~3% or more are considered "in-cal", and in uncalibrated scopes this can be much worse.

    Traditional oscilloscopes used a CRT screen and were completely analogue devices. Those analogue oscilloscopes are still very usable devicesnowadays. Analogue oscilloscopes work very well as general testing instrumentfor viewing repetitive signals. Many simple and cheap analogue oscilloscopes have typical bandwidth of 20 MHz. Some better ones go to 100 Mhz or higher in bandwidth. Even a 20MHz analogue scope will produce some response at a higher frequency but of course it will be at a lower level because it is outside of the calibrated specified bandwidth.

    Digital oscilloscopes are digital versions of that analogue instruments. Digital oscilloscopes sample signals using a fast analog-to-digital converter (ADC). The digitized signals aresotred to the scope memory and shown on the scope screen or at computer screen. The benefit of the digital technology is thatthe waveforms can be captured to memory and then analyzed, immediatlyor later, in many ways. Digital oscilloscopes can be used to capturerepetitive signals as well as transient signals.

    Oscilloscope bandwidth is generally listed as the -3-dB point in oscilloscope frequency response. Traditionally, oscilloscopes have exhibited a Gaussian frequency response. A Gaussian response results from the scope design's combining many circuit elements that have similar frequency responses. Analog oscilloscopes achieve their frequency response in this manner, thanks to chains of amplifiers from the input BNCs to the CRT display. (Analog oscilloscopes used the input signal to directly deflect the electron beam in a CRT. This architecture required amplifying the input signal by three orders of magnitude and driving the large capacitive load that the CRT deflection plates presented.) The properties of Gaussian-response oscilloscopes are fairly well-taught and well-understood throughout the industry. In a Gaussian-response oscilloscope, the oscilloscope's rise time is related to the oscilloscope's bandwidth by the familiar and commonly used formula, rise time=0.35/bandwidth. (Rise time is measured from the pulse's 10 to 90% amplitude points. Bandwidth is defined as the frequency at which the response is down 3 dB relative to dc. The theoretical relationship for a Gaussian system is rise time=0.339/bandwidth, but the industry has settled on 0.35/bandwidth as a practical formula.) Another commonly used property of Gaussian systems is the overall system bandwidth, which is the rms value of the individual bandwidths. You can calculate it using the familiar relationship, system bandwidth=1/(1/BWPROBE2+1/BWOSCILLOSCOPE2)0.5. "System bandwidth" refers to the bandwidth you achieve with a combination of an oscilloscope probe and oscilloscope. Oscilloscope probes are often designed to have sufficiently higher bandwidth than the oscilloscope bandwidth, so that the above formula is usually not necessary.

    Most oscilloscopes are built so that the signal input connector is BNC connector. The input impedance in the connetion is typically around 1 megaohm in typical normal oscilloscopes and 50 ohms in many high speed oscilloscopes (check what you have from scope manual). The connector ground side (outer shield) is normally connected to the equipment case ground which is generally wired to mains ground through mains connector. This means that the grounds of all channels are genrally connected together and then wired to mains ground (unless you power your scope through safety isolation transformer which isolated your scope from ground). Oscilloscopes are intended to be operated with their chassis at ground potntial. There are good technical and safety resons for this. If you are measuring some mains powered device, it is a very good idea to power the device through an isolation transformer. When working with mains powered equipment, the equipment you measure should be isolated from mains voltage for safety reasons.

    When doing the meausrement the right grounding is important for meaningful results. A good oscilloscope probe has a removeable ground lead, that allows the user to ground it to circuit board or not depending on what is needed in that specific meaurement. In general case the measurements are made better and more accurate with the ground lead connected. If you do not connect the ground lead then the display will show allthe noise the probe cable picks up (cable acts like antenna that picks up noise nearby). If you want rid of this you connect the ground lead to the low of the circuit you are trying to monitor. The oscilloscope ground lead will eventually find its way back to the mains earth of the oscilloscope.If you are trying to make measurements, you must have a reference against which to measure. Without that, "Pissing against the wind" comes to mind, as acomparison. There are some potential dangers when the circuit ground is at a potential with respect to oscilloscope ground then current will flow in the oscilloscope through the measuring cable shield. If the potential on the circuit is direction connection to mains then there will be a bang and possibly some damaged measuring hardware / circuit. Remedies are:

    • a) Double insulated oscilloscopes with no ground connection
    • b) Battery powered osciloscopes
    • c) Differential input oscilloscopes
    • d) Differential input adapter for your oscilloscope
    • e) Isolating transformers
    Using the option e is by far the cheapest and most commonly used, although not always the best. There are also some special oscilloscopes (expensive ones) with inputs that are not connected to ground (usually referred as differential inputs). This kind of scope can be safely connected to almost any electronics circuit. You can get the same performance with a normal scope also if you use a differential proble connected to a normal oscilloscope. In some cases the battery powered small oscilloscopes are very handly because those devices are completely floating.

    If you want to make accurate measurements, you need to have your oscilloscope calibrated. A calibrated scope will allow you to make considerably more accuratetime/voltage measurements, will show square waves as true step-functions(even at the highest sweep rates) and not some sort of distortedrepresentation, and most importantly it will trigger reliably on signals.There's a whole lot of difference between a calibrated and un-calibratedscope, but you wouldn't usually know it unless you have a source of precision calibration signals to compare against. Once calibrated, an instrument should be re-calibrated within 2-3 years since the adjustments can in fact vary a surprising amount over time (the time interval could vary somewhat depending on scope type and needed calibration accuracy). A scope requires significantly more maintenance than simpler measurement instruments like a multi-meter or signal generator. CRT based oscilloscopes are complex instruments. Much more complex than almost any other piece of test instrumentation and the circuitry is not selfadjusting (for the most part). Most common analog oscilloscopes require a fair amount of specialty calibration equipment and a thorough calibrationtakes at least 1/2 day and often longer (there can be up to 50 separate adjustments tha can be made on older scope, this is labor intensive process to get them right). Most scope problems are revealed in the calibrationprocedure in which the tech can choose to either ignore or repair. Sometimes the repairs are trivial, sometimes not. Becauses the cost of maintaining older oscilloscopes accurately many so-called "working" units find themselves on the surplus market.

    The oscilloscope probe used to establish a connection between the circuit under test and the measuring instrument. A probe can be any conductor used to establish a connection between the circuit under test and the measuring instrument. This conductor could be a piece of bare wire, a multimeter lead or a piece of unterminated coaxial cable. These "simple probes," however, do not fulfill the essential purpose of a probe; that is, "to extract minimal energy from the circuit under test and transfer it to a measuring instrument with maximum fidelity." There are many different kinds of probes that suit to different applications:

    • The bare wire can load the input amplifier with its high capacitance and inductance or even cause a short circuit; multimeter leads are unshielded and are often susceptible to stray pickup
    • The unterminated coax will severely capacitively load the circuit under test (100 pF per meter typically). Also, the unterminated coax is usually resonant at certain frequencies and does not allow faithful transfer of the signal to the test instrument due to reflections.
    • A simplest probe type is is "x1" probe that just consists of probe tip, grounding conductor and low capacitance coaxial cable to the oscilloscope. Typically the oscilloscope at probe setting "x1" it loads the circuit being measured with the full capacitance of probe + probe cable + oscilloscope input. The unterminated coax will severely capacitively load the circuit under test. Typical capacitance of "x1" probe is tens of picofarads. For DC measurements the input resistance is the same the resistance of the oscilloscope input (typically 1 Mohm on traditional CRO-type oscilloscopes, 50 ohm on some high frequency models).
    • Attenuating Passive Voltage Probes are the most commonly used probes today. The "x10" setting gives you reduced sensitivity and reduced capacitace (the load capacitance is around one tenth of "x1" setting). This means a typical input capacitance of around 15-20 pF. The 10X passive voltage probe presents a high impedance to the circuit under test at low frequencies (approximately 5 MHz and lower). Their main disadvantage is a decreasing impedance level with increasing frequency (i.e., high input capacitance).
    • FET probes include active components (field effect transistors or other active devices) rather than passive components. The FET input results in a higher input impedance without loss of signal, i.e., low input capacitance (typically less than 1 pF) and high input resistance values (typically higher than 20 kohms). Since FET probes have a 50 ohm output impedance, they can drive a 50 ? cable so they can be long cables between the probe and oscilloscope. Downside of FET probes are that they are typically expensive and need operating power to work (either supplied by oscilloscope using properietary methods or powered with batteries).
    • Several high voltage probes are available, and they typically provide 100X or 1000X compensated dividers. Because of the larger attenuation factors required for high voltage applications, the input capacitance is typically reduced to approximately 3 pF.
    • 50 ohm Divider Probes provide the lowest input capacitance (typically less than 1 pF for high frequency signals) and are used with high frequency, 50 ohm input scopes. The simplest 50 ohm divider probe consists of just one 1 kohm or 2.2 kohm resistor that is placed between the signal connection on the circuit and the 50 ohm ciaxial cable going to the oscilloscope.
    • Current probes provide a method to measure the current flowing in a circuit. Two types of current probes are available, the traditional AC only probes and the "Hall Effect" semiconductor type. AC only current probes use a transformer to convert current flux into AC signals. Combining a "Hall Effect" device with an AC transformer provides a frequency response from DC up to many MHz range. Because of its "non-invasive" nature, a current probe typically imposes less loading than other probe types. The AC current probes can be just passive devices, while the models with "Hall Effect" device need some operating power (typically provided by local battery on the probe).

    Proper probe selection will extend and enhance an instrument's performance, while imprudent probe selection often reduces your system's performance. When making measurements make sure not to exceed the maximum allowable input ratings of the oscilloscope input ports. This will prevent costly damage and provide reliable measurements. Rememeber also not to exceed the input voltage ratigns of oscilloscope probes as well, because this can damage the probes and cause severe safety risk to the person using those probes.

    A proper oscilloscope probe grounding is essential requirement to get meaningful measuring results with normal oscilloscope probes. The measured the current must always form a loop. The signal beign measured cannot exit the measured circuit and go to the oscilloscope input without having a path through which it may return. If you are measuring a "floating" circuit, then the return would go through a parasitic capacitance directly between the oscillator and the scope. This capacitance varies depending how the devices are positiones, which means that the position of the probe cable will have an effect on the shape of the signals you see on the scope! Another nasty artifact of a no-ground probe arrangement is the resonance associated with the combination of the rather large inductance of cable, and the input capacitance of the probe. This resonance is called a probe resonance and can cause considerable measurement errors. A short, explicit ground connection made between the scope ground and the equipment under test shunts those capacitances and inductances, eliminating their influence on the measured result and pushing the probe resonance up and out of the band of interest. All good probes come with short, tiny ground attachments to prevent such problems. For single-ended measurements, don't depend on mysterious ground connections. Always use a good, short ground connection.

    Oscilloscopes are used for very many different kind of measurements. In telecommunication and data communications applications you can often see results of eye diagram and eye pattern measurement. An eye pattern is an oscilloscope display in which a pseudorandom digital data signal from a receiver is repetitively sampled and applied to the vertical input, while the data rate is used to trigger the horizontal sweep. System performance information can be derived by analyzing the display. An open eye pattern corresponds to minimal signal distortion. Distortion of the signal waveform due to intersymbol interference and noise appears as closure of the eye pattern.

    Many modern digital oscilloscopes allow you to show you signal waveforms and even store the recorded signal for later inspection. Old analogue oscilloscopes lacked the ability to store the picture on the screen, unless you took a picture of the screen with a normal film camera (not very convient, camera settings needs to be right). If you happen to have an old analogue oscilloscope and need to store the waveform on the screen, then you might be able to use modern inexpensive digital camera connected to computer instead of old traditional film camera. You can for example have an usb pc camera mounted on a tripod at the?oscilloscope screen, focus close for a sharp picture,?camera output cable into the USB port. With the bundled software installed on your computer (Windows 98se, 2000, or never), you can view the image on your computer screen and save the image on the oscilloscope screen to you hard disk (for example to be included to your laboratory documents later). You see it all in real time (well almost...) and if you are recording it all as well, then you have the option of playback, editing and splicing the info/displays later for whatever purpose?or archiving etc.?It work, usually well. This could be an useful trick for those technicians out there with limited funds and equipment. Digital cameras and webcams are nowadays quite cheap compared to a modern digital oscilloscope.

      Differential measurements

      Most oscilloscopes can perform only single-ended voltage measurements; that is, measurements of signals referenced to earth ground. Wiring within the probe connects the probe's reference lead to the shell of the BNC. When you plug the probe into the scope, the reference lead becomes electrically common with the scope's chassis. The power cord's ground conductor connects the chassis to earth ground. In most oscilloscope applications the inability to make anything except single-ended measurements poses no problems. But oscilloscopes' single-ended inputs present challenges when you try to view signals that are not referenced to ground. A common example is the voltage across the switching device in an off-line switching power supply. Another type of signal that you must measure differentially is a balanced signal.

      Simplest way of doing differential measurements is to use two normal 10X probes conencted to two oscilloscope inputs and the "minus" operation to show the difference of signals between them. The normal 10X probe has a typical accuracy of ?1% and gives a differential measurement accuracy (when using two probes) of two parts per 100. Using this 10X probe, the common mode rejection ratio of a scope and probe combination would be no better than 50:1.

      True differential measurements are safe and accurate way to measure signals that are not ground referenced. To make those measurements you need a differential probe. Unlike a conventional scope probe, a differential amplifier ijn differential probe has an input that is only implicitly referenced to ground. As the name implies, a differential measurement produces a waveform that represents the difference in voltage between the two inputs. Ground does not enter into the measurement.Differential amplifiers ignore potentials that are equal in amplitude and phase and appear on both inputs. This characteristic is known as "common-mode rejection" (CMR). An ideal differential amplifier totally rejects the common-mode component.The other key feature of a differential amplifier is balanced input impedance (both inputs have identical impedance to ground, typically high impedance). A true differential probe has typically adjustments and electronics to provide common mode rejection ratios of 10,000:1 and higher.

      Building oscilloscope probes

      A probe can be any conductor used to establish a connection between the circuit under test and the measuring instrument. This conductor could be a piece of bare wire, a multimeter lead or a piece of unterminated coaxial cable. These "simple probes," however, do not fulfill the essential purpose of a probe; that is, "to extract minimal energy from the circuit under test and transfer it to a measuring instrument with maximum fidelity." Attenuating Passive Voltage Probes are the most commonly used probes today. They provide a convenient and extremely rugged, yet inexpensive, way to acquire signals from your device under test. FET probes include active components (field effect transistors or other active devices) rather than passive components. The FET input results in a higher input impedance without loss of signal.

      RF and EMI probes

      RF probes allow you to examine high frequency RF signals (much higher that your scope frequency response) on your oscilloscope screen. The RF probes generally form a some kind of rectifier / peak sampler, which allows you to see the signal strenght as the signal which connects to scope input. This allows you to quite easily measure signal amplitude and look at the moduleation (AM modulation). Rapidly changing voltages and currents in electrical and electronic equipment can easily result in radiated and conducted noise. Electromagnetic interference (EMI) can be difficult to locate and correct in electronic equipment. A miniature EMI "sniffer probe" and an oscilloscope can help to locate and identify magnetic-field sources of EMI. Typical EMI probles consist of some form of elecrical field sensing circuit (voltage proble) and some form of small coil (H-field probe).

      Video signal measurement accessories

      Typical oscilloscope does not usually sync well enough to video signal to be as such a convient instrument (when compared to special videomeasurement tools). With suitable accessories (usully special sync circuits), a normal oscilloscope can be used as a very nice video signal analyzing instrument.

      Building measuring accessories

      • Calibrate scope jitter using a transmission-line loop - Digital-clock-period jitter is the variation in the period of a clock cycle compared with a nominal (average of many cycles) clock period. To accurately measure period jitter using an oscilloscope, you must subtract the oscilloscope jitter from the measured jitter. However, oscilloscopes rarely have a jitter specification, so you must determine the oscilloscope jitter. One method of measuring oscilloscope jitter is to use the oscilloscope to measure the jitter of a pulse generator with known jitter. The ideal generator for measuring oscilloscope jitter would have zero jitter. This article shows a circuit for generating a calibration signal with near-zero timing jitter.    Rate this link
      • Coax connectors make low-cost test pieces - you can construct low-cost small test pices like filters, attenuators and terminators using coaxial panel jacks without pc boards or enclosures, design idea from    Rate this link
      • Counter Circuit Improves Oscilloscope Triggering - this prescaler circuit, when plugged into the scope's external trigger input, can provide reliable, low-jitter triggering for both older and modern oscilloscopes    Rate this link
      • Delay line upgrades vintage scope - Vintage triggered-sweep oscilloscopes find use in many applications. However, they have no internal delay line, so they can't display the pulse that triggers the sweep. Moreover, early laboratory scopes contain delay lines having insufficient delay to display such pulses during a uniform portion of the sweep. With such oscilloscopes, the true pulse shape remains a mystery. You can circumvent these limitations if you add an external delay line and equalizer. The scope can then display the exact trigger-point trace. The instrument then becomes easier to use, and the measurements become more trustworthy.    Rate this link
      • Matching pads - This article describes some impedance matching circuit for measurements.    Rate this link
      • Multiplexer creates mixed-signal scope input - using two multiplexer ICs and some TTL logic), you can view eight analog or digital (or some of both) signals on the oscilloscope    Rate this link
      • Simple circuit provides timebase calibration - inexpensive and quick way to check the timebase speeds and linearity in vintage oscilloscopes    Rate this link

    Using PC as a measurement instrument

    In those early years of computer-based measurement and automation, the desktop computer, linked by the General Purpose Interface Bus (GPIB), played an auxiliary role; however, the increasingly powerful PC has changed all of that. Today, the PC can acquire, analyze, and present data at increasing frequencies, resolutions, and sampling rates.In the dim and distant past, engineers recorded measurements with pencil and paper - a slow and error-prone method. Today, 20 years after the introduction of the IBM PC, two types of instruments - inboard and outboard - take measurements and move data into a host computer. PC technology has become the backbone of automated test and measurement systems.Today virtual instruments are superseding the traditional kind by revolutionizing how measurements are made and the data shared. History of virtual instrumentation began over 15 years ago as PCs started coming into use in test and measurement as instrument controllers. The PC is now the most powerful and cost-effective approach to building instruments. Virtual instrumentation leverages the power, flexibility, and programmability of the computer and thus brings a wide variety of benefits. Laptop computers have further encouraged this trend with a form factor ideal for many portable applications. Even a basic normal modern PC can be used to do many different kinds of measurements with no extra hardware. The soundcard found in most PCs can be used for various applications, althrough those applications are limited to audio frequencies and have usually quite limited absolute accuracy (PC soundcards are not designedprecise calibrated measureemnt instruments). With suitable software and soundcard you can use your PC as a signal generator that gan generate different waveform signals. You can generate practically any waveform (within audio frequnecy band limits) if you use some suitable sample editor software or mathematics software to generate the signal waveform and then play it out through soundcard.With suitable software a PC with a soundcard can be used as a multi-purpose audio frequency signal analyser. You can for example use PC as audio signal oscilloscope, VU meter, spectrum analyzer, frequency response analyzer. PC can also used as a very convient recording device that can record and play back any audio signal.There are also special measuring instruments that can be connected to PC to expand it's capabilities. There are varieties that connect to PC bus or some PC interfacing port (like parallel or serial port). The oscilloscope products that connect to PC through a slow port(serial, parallel etc.) and can sample at high rates are generallyimplemeted in the following way:The device has a buffer memory in it. When the device starts sampling(manual start or automatic trigger), it then samples it's memory fullat the given sample rare. After the data is sampled to memory itis stransferred to the PC. And the process can start all over.What comes to the software that controls commercial PC based measurign instruments there is one software that is more popular than anything else in the field: LabView from National Instruments. Agilent has it's own VEE software competing on the same field. There are also measuring instrument manufacturer specific control software that is supplied with the instruments.

    Transmission line measurements

    There are applications where you need to measure long cable lines that are used as transmission lines for various signals. There are many techniques related to transmission line measurements, because there are various factors that needs to be measured. Most commonly measured transmission line characteristics are the following:

    • Conductor and shield resistance
    • Insulation resistance
    • Capacitance between wire pairs and/or between conductor and shield
    • Characteristic impedance
    • System impedance mismatch (return loss)
    • Line attenuation
    • Amount of noise coupled to line

    Let's say you have a long cable with a problem. Part of the cable is buried under ground, some of it runs through walls and floors. You measure one end of the cable with an ohmmeter, and it reads about an ohm. So the cable is shorted. Hoping for the best, you cut off the connector and measure just the cable. Still reads about an ohm, so the short is somewhere else along the cable. But where? If you could locate the short, you could save a lot of time and money by repairing just that one spot, rather than pulling in a whole new cable. TDR to the rescue! You can use Time Domain Reflectometry to look at the characteristic impedance along the entire length of the cable.

    Cables used to carry high frequency electrical signals are generally analysed as a form of Transmission Line. The amount of capacitance/metre and inductance/metre depends mainly upon the size and shape of the conductors. The Characteristic Impedance depends upon the ratio of the values of the capacitance per metre and inductance per metre. To understand its meaning, consider a very long run of cable that stretches away towards infinity from a signal source. The result, when the signal power vanishes, never to be seen again, is that the cable behaves like a resistive load of an effective resistance set by the cable itself. This value is called the Characteristic Impedance, of the cable.

    Return loss (RL) is a measure of the reflected energy caused by impedance mismatches in the cabling system. Reflections create an unwanted disturbance signal or "noise" on the cabling link that potentially interferes with the reliable transmission over the link. As a noise source, return loss is measured and evaluated to assure that the reflected signal energy is sufficiently small in reference to the transmitted signal such that the reliability of the transmission is not negatively impacted. Return loss is an important characteristic for any transmission line because it may be responsible for a significant noise component that hinders the ability of the receiver when the data is extracted from the signal. It directly affects "jitter." Return loss is one number which shows cable performance meaning how well it matches the nominal impedance. Poor cable return loss can show cable manufacturing defects and installation defects (cable damaged on installation). With a good quality coaxial cable in good condition you generally get better than -30 dB return loss, and you should generally not got much worse than -20 dB.

    Return loss is especially important for applications that use simultaneous bidirectional transmission. Opens, shorts or less-severe impedance discontinuities have a way of showing up on cables in strange places - places you might never suspect. These can occur on coaxial transmission lines or twisted-pair lines. Such opens, shorts or other impedance discontinuities are called faults. The location of faults cannot be determined with simple ohmmeters. Even the existence of certain faults cannot be determined with an ohmmeter. Time domain reflectomer is an instrument often used ot locate such faults.

    Time Domain Reflectometry measurements (sometimes called Time Domain Spectroscopy techniques) work by injecting a short duration fast rise time pulse into the cable under test. The effect on the cable is measured with an oscilloscope. The injected pulse radiates down the cable and at the point where the cable ends some portion of the signal pulse is reflected back to the injection point. The amount of the reflected energy is a function of the condition at the end of the cable. If the cable is in an open condition the energy pulse reflected back is a significant portion of the injected signal in the same polarity as the injected pulse. If the end of the cable is shorted to ground or to the return cable, the energy reflected is in the opposite polarity to the injected signal. If the end of the cable is terminated into a resistor with a value matching the characteristic impedance of the cable, all of the injected energy will be absorbed by the terminating resistor and no reflection will be generated. Should the cable be terminated by some value different from the characteristic impedance of the cable the amount of energy reflected back to the cable start point would be the portion of the pulse not absorbed by the termination. Also any change in the cable impedance due to a connection, major kink or other problem will generate a reflection in addition to the reflection from the end of the cable. By timing the delay between the original pulse and the reflection it is possible to discern the point on the cable length where an anomaly exists. The cable type governs this signal propagation speed. For example normal Category 5 cable propagation speed is 66% the speed of light, and for most coaxial cables this value is between 66% and 86%.

    Other cable characteristics are usually easier to measure and can be done with more conventional instruments.

    Cable conductor resistance can be measured in installed cable by shorting the cable on one end (short center wire to shield on coax, short two wires in wire pair on twisted pair cable etc.), and then using a multimeter on the other end to read the resistance value.

    Cable capacitance can be measured with a capacitance meter by leaving one end of the cable not connected anywhere (all wired free) and connecting the meter to the other end of the cable.

    Cable insulation is typically measured with an insulation resistance meter. The cable is typically not connected anywhere (or connected to equipment that do not cause error in measurement and do not get damaged by measuring). Insulation resistance meter typically applies some quite high voltage DC (125V, 250V, 500V, 1000V) to the line between two wires and measure if there is any leakeage. The leakage current is measured and the result is converted to resistance (usually in megaohms to gigaohms range). The measuring voltage needs to be selected based on the ratings of the wiring (and equipment if such are connected). Low voltage telecom wiring and similar is typically tested with 125V or 250V voltage. Higher voltages are usually used when testing the insulation on the mains power carrying cables and some radio transmitter coaxial cable systems. The measurin voltage needs to be right for the intended application. Too low voltage might not reveal insulation problems, but too high voltage can damage wiring and equipment connected to it.

    Line attenuation can be measured by connecting the signal source used in the application (or test instrument generating suitable signal) and signal receiver on other end (receiving equipment or terminating resistor). Then you just mesure the signal level on the transmitting and receiving ends (using a suitable multimeter or oscilloscope or similar instrument). The difference on those tells how much the cable attenuates the signal. In some applications you need to do measurement with different frequencies, recording how cable attenuates on different freuqncies. Some cable TV system measurements use a wideband noise source as the transmitter and a spectrum analyzer as the receiver (difference on the signal spectrum on the transmitting and receiving ends tells the attenuation on different frequencies).

    Amount of noise coupled to the line is measured with the indended equipment or suitable line terminators connected to the ends of the cable. If you use equipment they need to be turned off so that they do niot send anything to the line. Any signal that is now measured on the line is the amount of coupled noise.

    Cable wiring testers

    Proper testing of wiring system after installation is essentialto guarantee good operation later. The cabling system needs to bemeasured after installation and the results of those measurementsshould be documented for later use. Measurement is also usefulduring use when cabling problems are suspected.The most common cable fault is an open circuit, usually due toproblems close to or at the ends of the cables. A simple ohm metertest generally suffices. For multiplair cables where cable ends are many wires inside, a simplemultimeter is bothersome. For those applications multi-pair cabletestes which find showrt circuits and broken wires are a good choise.In some application you need to measure the cable length. Dependingon the cable characteristics you know and the measuremenet instrumentsyou have, you can use a multimeter (resistance measurement), RLC meter(capacitance measurement). time domain reflectometer (pulse tesing)or signal ateenuation testing (signal source and level meter)to measure the lenght of the cable you have installes somewhere.

      Multi-wire cable testers

      Engineers have long known how to test a cable for continuity by simply connecting all conductors in series and checking with an ohmmeter. This method is sometimes impractical, however, because it cannot check for short circuits (or you need to make very many test to measureresistance between very many wire combinations). To solvel thos problem on multi-conductor cables, there are specialcable testing instuments designed for this.

      • Cable tester is fast and cheap - This simple microcontroller based cable tester verifies the correct wiring of the cable, up to 8 conductor cables.    Rate this link
      • Simple method tests cables - Engineers have long known how to test a cable for continuity by simply connecting all conductors in series and checking with an ohmmeter. This method is sometimes impractical, however, because it cannot check for short circuits. This simple method solves the short-circuit detection problem. Connecting LED indicators at each shorting loop provides a visual indication.    Rate this link

      Cable test tone senders

      • How to Build a Signal Tracer and Injector - This audio signal tracer/injector will undoubtedly prove to be very useful for many routine servicing operations. The unit consists of an audible signal monitor for "listening" to the signals present in an electronic device (such as an audio system, receiver, amplifier, or tape deck) at circuit points inside these devices. It also includes an RF detector probe and signal generator.    Rate this link
      • Microphone Circuit Test Oscillator - 440 Hz tone generator for testing XLR microphone lines    Rate this link

    High voltage measurements

    DMMs may not be particularly forgiving of voltages on their inputsexceeding their specifications. You need special tools and proceduresto successfuly and safely measure high voltages.A simple high voltage probe for a DMM or VOM may be constructed from a pair ofresistors. This kind of devices are sold as ready made devices(for example Tektronix, Agilent and Fluke sell those).Follow safety precautions when working around high voltages.Usually some form of equipment protection should be considered whenworking with high voltages.

    Frequency measurements

    Frequency counter is a necessary instrument to check that certain circuit operated at thr right frequency. Frequency counter is an useful tool when you need to tune oscillators, measure some input signal frequency and when youplay with radio devices.Inexpensive frequency counters that will measure frequency well into the microwave range are available to the hobbyist today. A frequency counter is an excellent means of accurately determining the frequency of unknown signals, or to see if an oscillator or a multiplier stage in a receiver or transmitter is working. However, one must watch out as what is really being measured and exactly what the counter is "seeing".

      General information

      • Frequency Counter Measurement Techniques - Inexpensive frequency counters that will measure frequency well into the microwave range are available to the hobbyist today. A frequency counter is an excellent means of accurately determining the frequency of unknown signals, or to see if an oscillator or a multiplier stage in a receiver or transmitter is working. However, one must watch out as what is really being measured and exactly what the counter is "seeing".    Rate this link

      Prescaler circuits

      Prescalers are circuit which are used to extend the meausrement range of other frequency measuring circuits. If you have for example a frequency coutner which can count up 10 Mhz, then with suitable prescaler circuit you can extend the measurement range to higher frequencies. For example suitable 1:10 frequency prescale woudl extend the measurement range to 100 Hz. And prescaler with higher division factor will enable you to measure even higher frequencies.

      • 3 GHz Prescaler - will take a 0.1 - 3 GHz signal and divide it by 1000 so you can measure frequencies outside the normal range of your frequency counter    Rate this link
      • VHF/UHF Prescaler - This prescaler is ridiculously simple. It consists of just one IC, a TV tuner prescaler, the Philips SAB6456A, which can divide by 64 or by 256. The device sensitivity is about 10mV RMS over the range 70 - 1000 Mhz, and the output is typically 1V p-p. The input resistance varies from 560 down to 30 Ohms, and the input capacitance, excluding the PCB, no more than 5pF.    Rate this link

      Frequency to voltage conversion

      Frequency to voltage converson allows you to convert input signal frequency to a voltag signal which can be fed to a normal digital multimeter imput, moving coil meter or A/D converter. Frequency to voltage converters are not usually as accurate as real frequency counter circuits, but they are still useful in many applications.

      • Frequency to voltage adapter - in pdf format, text in Finnish    Rate this link
      • F/V converter has high accuracy - This high-accuracy frequency-to-voltage converter (FVC) demonstrates how a synchronous, charge-balance, voltage-to-frequency converter (VFC) can function as a single-supply FVC given proper biasing and level shifting.    Rate this link
      • Idea for a car tachometer - A tachometer is simply a means of counting the engine revolutions of an automobile engine. In this suggested idea a NE555 timer is configured as a monostable or one shot. The 555 timer receives trigger pulses from the distributor points. Integration of the variable duty cycle by the meter movement produces a visible indication of the automobiles engine speed.    Rate this link
      • Pulse period to voltage converter - This circuit converts a square wave input signal into a voltage proportional to the time between edges (period) of the signal, not the frequency, the range is from 100uS to to 10mS, which produces a voltage from 100mV to 10 volts.    Rate this link

    Audio measurements

    Audio volume is the most commonly measured audio signal property. VU and dB meters both measure the audio power involved in recording and they both use logarithmic scales to report that power. When measuring electrical signals the following is true:

    • VU is short for "volume units" and it is a measure of average audio power. A VU meter responds relatively slowly and considers the sound volume over a period of time. Its zero is set to the level at which there is 1% total harmonic distortion in the recorded signal.
    • dB is short for "decibels" and it is a measure of instantaneous audio power. A dB meter responds very rapidly and considers the audio power at each instant. Its zero is set to the level at which there is 3% total harmonic distortion.
    Because of these differences in zero definitions, the dB meter's zero is roughly at the VU meter's +8.

    When measuring electrical signals decibel is the difference (or ratio) between two signal levels; used to describe the effect of system devices on signal strength. A signal strength or power level; 0 dBm is defined as 1 mW (milliWatt) of power into a terminating load.

    When measuring audio signal power (vibrations in air) the following measurements are made:The decibel (abbreviated dB) is the unit used to measure the intensity of a sound. On the decibel scale, the smallest audible sound (near total silence) is 0 dB. A sound 10 times more powerful is 10 dB. A sound 100 times more powerful than near total silence is 20 dB.What does 0 dB mean? This level occurs when the measured intensity is equal to the reference level. i.e., it is the sound level corresponding to 0.02 mPa. In this case we have equation: sound level = 20 log (Pmeasured/Preference) = 20 log 1 = 0 dB

    Sometimes the amount if noise needs to be measured.Most typically harmonic distortion needs to be measured.Harmonic distortion describes a nonlinear property of systemswhere the output of the system has added energy at frequenciesthat are at integer multiples of the frequencies input to thesystem. The traditional technique is to input a single frequency F into the system under test, then take the output, apply a filter thateliminates F, and measure everything that's left over. This is usually done with a twin-T, high-G notch filter centered on F. The problem with such a technique is that it measures EVERYTHING that's left over: not only the harmonic products of F at 2*F,3*F, 4*F and so forth, but all noise, uncorrelated components( line frequency noise, RF interference) and so forth. Nowadays computer techniques can be applied where a more detailed analysis can be made (usually based on FFT methods) where harmonic and non-harmonic componentscan be identified.

    "Standard multimeters" are not usually good instruments for audio measurements. Measuring audio (music) voltages on an AC voltmeter will give meaninglessresults as the voltmeter measures the average, over a fairly longintegrating time. This means that the level indicated will depend totally onthe programme content of the CD being played."Standard multimeters" (digital or analog) also often have a poor frequencyresponse and are not very useful for audio work for this reason. Most multimeters are designed for AC power line work and DC measurements, so perfomance up to 50-60 Hz or little bit over it is enough. To make any meaningful measurement, you need to us a CD with single frequency tones, and, unless you know that the meter measures well at higher frequencies, keep to a low frequency, ideally 50Hz, but generally up to acouple of hundred Hz will be OK. For reference: Most CD players give out 2 V AC from a fully modulated CD.Some of the "RMS" digital units might be useful ifthe frequency response is extended and flat enough. (please note that "extended" on some units means only respose up to 1000 Hz). The multimeters vary in performance, so it is worth to check their performance on this (even some cheap ones can perform accpetably on audio frequencies if a very good absolute accuracy is not needed).

    Radio measurements

      Power and field strength meters

      A field strength meter is perhaps the simplest piece of RF test equipment that can be built. Used for checking transmitters, antenna experimentation, and testing RF oscillators, field strength meters provide an indication of the presence of RF energy. They are generally not frequency sensitive and are useful where indication of a change in level is more important than the actual strength of the signal indicated. The meter works by converting any RF signal present at the antenna to a DC voltage. This voltage drives a meter movement to give an indication of relative RF. Usually the meter includes a control to reduce its sensitivity where required.

      • An RF field monitor - This is the oddest application of a neon glow lamp, that is used as a electromagnetic field detector. In fact the trigger voltage of these lamps is a little bit lower in presence of a strong field. A UJT transistor and a transformer (a common low power AC transformer) are used to produce a high voltage of about 200 Vac. This voltage is reduced by the trimmer to a value just below the trigger voltage of the lamp. In presence of a strong field the trigger voltage drops and the lamp lights.    Rate this link
      • 2.4 GHz RF Power Meter / SWR Meter    Rate this link
      • A simple 50 MHz microwattmeter    Rate this link
      • Build a RF Sniffer Probe! - This sniffer probe is miniature, only about 2 inches long and very usable up to 1 GHz or higher if linearity isn't a problem above 1 GHz. Beloq 1 GHz this proe has very flat response.    Rate this link
      • Clip-on RF Current Meter - Circuit description is in Japanese, but pictures and circuit diagram usable. This is an useful tool for RF interference troubleshooting! For EMC investigations, you can also clip this meter on to coaxial cables, rotator cables and other wiring in your shack, to find out where the RF currents are flowing, and how big they are.    Rate this link
      • Clip-on RF Current Meter - Circuit description is in Japanese, but pictures and circuit diagram usable.    Rate this link
      • Designing RF Probes - An RF probe is used to directly measure the level of RF voltage present at a particular point and is one of the most useful test instrument in the hands of the home brewer. It is normally used with a digital multi meter to indicate the voltage level as dc voltage which is equivalent to the RMS value of the RF voltage being measured. However, the level of RF voltage being measured provides useful information only when the probe has been designed for use with a specific multi meter. The design of the RF probe is a function of the DC input resistance of the meter we intend to use with it. If a new meter with a different input resistance is used with the probe the reading will be inaccurate.    Rate this link
      • Field-Strength Meter - Simple circuit based on old issue of "73 Radio Electronics", changed a few components to get better sensitivity.    Rate this link
      • Field Strength Meter - This RF field strength meter use only few parts, a printed circuit board is not necessary; components can simply be soldered to one another.    Rate this link
      • Field-Strength Meter I - This circuit is a electromagnetic field meter which can be used for meaturing of transmitters output power.    Rate this link
      • Field-Strength Meter II - This circuit is an electromagnetic field meter which can be used for meaturing of transmitters output power.    Rate this link
      • Field Strength Meter, VHF Band - This circuit measures radio field strength by converting the signal to DC and amplifying it. This field strength meter was designed for VHF frequencies in the range 80 - 110 MHz.    Rate this link
      • Funky Fresh? LED RF Signal Meter - a high quality RF signal meter based around the Analog Devices AD8313 0.1 GHz - 2.5 GHz logarithmic detector IC, capable of detecting signals as low as -80 dBm    Rate this link
      • KA8MAV RF Probe - This is a very simple RF probe circuit    Rate this link
      • Make a truly linear RF-power detector - This document shows a waveform-independent circuit that provides a linear measurement of RF power. This circuit uses the AD8361, a high-frequency true-power detector.    Rate this link
      • N5FC's Ballpoint RF Probe - This is a small RF probe that connects to a multimeter. This one is used in conjunction with a high-impedance-input Voltmeter or Digital Voltmeter (DVM).    Rate this link
      • N5FC's Classic RF Probe - The RF Probe is one of the handiest accessories you can have around the shack. Using only 3 electronic components, it may rank as one of the simplest and cheapest homebrew projects. When used with a high-impedance DC Voltmeter, it can be used to measure RF voltage (and power), trace RF signals in a new design, and troubleshoot malfunctioning RF circuits.    Rate this link
      • RF Probe Up to UHF band - This circuit will read pretty close to the RMS value of the voltage. The RF detector circuit has built-in scaling to give approximate RMS readings for sine wave signals.    Rate this link
      • Simple Analog Field Strength Meter - can be used from 30 MHz to over 2 GHz    Rate this link
      • Simple RF Measurement Probe - This probe is useful for any low level RF work, and simply connects to your multimeter. The voltage shown will not be accurate, since this is a rectifier probe, but the measurements are good enough for you to be able to determine where the RF stops, or if a stage is not giving the gain you think it should.    Rate this link

      RF signal generators

      • RF Signal Generator - This signal generator is intended for realignment of radio receivers. The unit is cheap and fairly basic, but perfectly adequate for its intended purpose. However, the output is not a pure sine wave. The unit covers a frequency range of 150KHz to 12MHz over five ranges (shown below). It is therefore suited to the alignment of RF and IF sections of AM (MW and LW) sets, as well as the IF sections of FM (VHF) circuits. It may also be used for RF alignment of SW circuits from 25 to 49 metres. The output may be amplitude modulated by an internal 800Hz audio tone (approx. 30% modulation) or by an external signal. The output level is adjustable in two ranges up to a maximum of about 4V pk-pk. The unit is mains powered (220V AC).    Rate this link

      Impedance measurements

      Couplers

      Couplers are passive devices used in cable systems to divide and combine radio frequency signals.Many RF systems use directional test points and non directional test points. What's the difference? Directional coupler separate inbound and outbound signals separately. A non-directional coupler allows the measuring technician to see both forward and reverse signals at the same time (sum of them). This situation allows only one test point to be used for forward and reverse, but there is potential possibility for measuring errors due to reflections from the bad cable or passive. Reflections can add or subtract to the actual levels. The non directional coupler is a device presenting a fairly high impedance to the circuit being measured, minimizing the loading effects. Non-directional couplers are generally implemented as high impedance (towards the line) resistive attenuation taps wired to the line. They typically have quite high attenuation (20-40 dB typical). Directional coupler is a transmission coupling device for separately sampling (through a known coupling loss) either the forward (incident) or the backward (reflected) wave in a transmission line. A directional coupler may be used to sample either a forward or backward wave in a transmission line. A unidirectional coupler has available terminals or connections for sampling only one direction of transmission; a bidirectional coupler has available terminals for sampling both directions. Directional couplers are used in a wide variety of applications and can satisfy almost any requirement for sampling incident and reflected RF or microwave power conveniently and accurately with minimal disturbance to the transmission line.Some general applications for directional couplers include line monitoring, power measurements and load source isolators. A directional coupler has at least three ports: line in, line out, and the tap. The signal passes between line in and line out ports with loss referred to as the insertion loss. A small portion of the signal power applied to the line in port passes to the tap port. A signal applied to the tap port is passed to the line in port less the tap attenuation value. The tap signals are isolated from the line out port to prevent reflections. A signal applied to the line out port passes to the line in port and is isolated from the tap port.

      • Building a Non Directional (bi directional) Coupler - A non directional coupler sees forward and reflected power at the same time, from either direction. A non directional coupler can be built from a common drop splitter or directional coupler. The non directional coupler is a device presenting a fairly high impedance to the circuit being measured, minimizing the loading effects. The tap output is attenuated by 30 dB.    Rate this link
      • Directional Coupler... - This page gives theoretical information and a on-line design/analysis tool.    Rate this link
      • Directional Coupler Terminology    Rate this link
      • Directinal Coupler Theoretical Information - When two transmission lines are close together, because of the interaction of the electromagnetic fields of each line, power can be coupled between the lines. Those coupled lines are used to construct directional couplers. Generally, in design of directional couplers microstrip and stripline forms are used. There are many kinds of directional couplers in different forms.    Rate this link
      • Directional & Non (Bi) Directional Test Points - This document describes the difference between them.    Rate this link
      • Dual core RF directional coupler Paten 6,114,924 - check also    Rate this link
      • RF Directional Couplers - The equations that describe the performance of transformer based directional couplers are derived. The best theoretical performance available from a directional coupler, using ideal transformers, is a function of the turns ratio, and the terminating impedances. At VHF and UHF frequencies, wire gauge and core material can be chosen to closely approximate the response based on the solution of these equations.    Rate this link
      • Stripline Directional Coupler Software - This is a stripline design software written in Ansi-C. The software comes in ascii-text and an X Windows interface using Motif.    Rate this link
      • RF Isolator Uses Differential Amplifiers - An RF isolator is a seemingly magic device that allows signals to pass in only one direction. Signals applied to the input port are sent to the test port and signals coming into the test port can only go to the output port. This one-way property is usually accomplished with special non-linear ferrite/magnet structures operating at very high frequencies. This is an active RF isolator capable of handling signals approaching 16 dBm and frequencies from well below 1 MHz to above 200 MHz. The circuit really emulates an isolator in that the actual signal energy is not passed from port to port and the signal levels must be fairly low. The circuit is well suited for testing the SWR of a variety of devices connected to the test port.    Rate this link

      RF spectrum analyzers

      A mong the many measurement tools sought by the amateur radio experimenter, the most desired - but generally considered the least accessible - is the radio-frequency spectrum analyzer. Spectrum Analyzer is intended for visual inspection of the spectrum of an investigated signal on the oscilloscope screen. The signal can be continuous or pulsed.

      Signal attenuators

      • Attenuator Pads - homebrew attenuation pads, descripes Pi style attenuator pads for 1 dB, 2 dB, 4 dB and 8 dB attenuation, also step attenuator circuit, also includes Pi and T Network Resistive Attenuation Calculator    Rate this link
      • Homebrew Attenuators - Contained herein are attenuator values for both PI and TEE types. The need for standard values can be met by using one or the other.    Rate this link
      • Fixed Attenuators - Fixed attenuators can be designed to have either equal or unequal impedances and to provide any amount of attenuation (theoretically) equal to or greater than the configuration's minimum attenuation - depending on the ratio of Z1/Z2. Attenuators with equal terminations have a minimum attenuation of 0 dB. Unequal terminations place a lower limit on the attenuation.    Rate this link
      • Pi and T Network Resistive Attenuation Calculator    Rate this link
      • Step Attenuator - This attenuator is designed for 50 ohms impedance and provides switches for 20, 16, 8, 4, 2 and 1 dB attenuation.    Rate this link

      RF signal generators

      Other RF measuring tools

      • A Simple UHF Dummy Load - A very simple and effective dummy load can be made from an old length of coaxial cable that has an impedance of the same value as the desired dummy load.    Rate this link
      • Bias Tee - Bias tees allow you to insert DC voltages into your signal path (coax) without disrupting the existing signal in that path, for example for feeding active antennas    Rate this link
      • Broaband Return Loss Bridge    Rate this link
      • Signal Tracer and Injector - This audio signal tracer/injector will undoubtedly prove to be very useful for many routine servicing operations. The unit consists of an audible signal monitor for "listening" to the signals present in an electronic device (such as an audio system, receiver, amplifier, or tape deck) at circuit points inside these devices. It also includes an RF detector probe for use with HF modulated signals, such as those found on an antenna, RF amplifier, or IF section of a receiver.    Rate this link

    Radioacivity

    There are many defices to detect radio active radiation. Geiger counters are devices to detect and measure ionizing radiation, as emitted by radioactive sources. The heart of a geiger counter is the Geiger-Mueller-Tube. This is a gas filled tube, to which a voltage of several 100V is applied. Normally, the gas insulates and no current is drawn. When a radiation particle or quantum passes the tube, it triggers a gas discharge, i.e. gas becomes conducting. The resulting current impulse can be amplified and made visible or hearable ("clicking"). Glas mantle tubes are only suitable for beta and gamma rays, as any alphas are absorbed in the glass. Window tubes have a window (usually at one end), which is sealed with a very thin foil or mica. Alphas can penetrate this window, and thus be detected, as well as betas and gammas. There are also diode based radioactive dosage meters in use.

    Temperature

    Temperature (sometimes called thermodynamic temperature) is a measure of the average kinetic energy of the particles in a system. Adding heat to a system causes its temperature to rise. While there is no maximum theoretically reachable temperature, there is a minimum temperature, known as absolute zero, at which all molecular motion stops. Temperatures are commonly measured in the Kelvin or Celsius scales, with Fahrenheit still in common use in the Unites States. There are many ways to measure temperature elecronically.

    A thermocouple is a very commonly used sensor for measuring temperature. It consists of two dissimilar metals, joined together at one end, which produce a small unique voltage at a given temperature. This voltage is measured and interpreted by a thermocouple thermometer. In practical applications the so called cold side of the junction is kept close to ambient temperature by bonding it to a temperature stable mass. The hot side of the junction is exposed to the temperature to be measured. Because thermocouples measure in wide temperature ranges and can be relatively rugged, they are very often used in industry. Thermocouple is a A temperature sensing device made by joining two dissimilar metals. This junction produces an electrical voltage in proportion to the difference in temperature between the hot junction (sensing junction) and the leadwire connection to the instrument (cold junction). In typical applications the the hot side is in the end of sensor wire and the cold juction in the temperature measurement device near the sensor connector. In many cheap meters this cold junction is just in the same temperature as the meter itself, and it's temperature is measured in other means to compensate the effect of changes in cold junction temperature. Thermocouple detectors have low impedance.Thermocouples are available in different combinations of metals or calibrations. The four most common calibrations are J, K, T and E. Each calibration has a different temperature range and environment. Propably the most commonly used type is K-type thermocouple, which is a Ni-Cr-sensor very suiable for 0-200 degress celsius temteperature measurements (can be used from -200 to 1250 celsius). The accuracy of any circuit or system that uses a thermocouple to determine the temperature of a process is limited by the accuracy of the method used to perform cold-junction compensation. In a thermocouple measurement, two wires of dissimilar metal join together at the "hot," or measurement, junction. The isothermal termination of the thermocouple wires provides a second "cold," or reference, junction. The potential across the thermocouple is proportional to the temperature difference between the two junctions. Thus, to determine the absolute temperature of the hot junction, you must also know the absolute temperature of the cold junction.

    Resistance Temperature Detector (RTD) is a sensor that uses the resistance temperature characteristic to measure temperature. There are two basic types of RTDs: the wire RTD, which is usually made of platinum, and the thermistor, which is made of a semiconductor material. The wire RTD is a positive temperature coefficient sensor only, while the thermistor can have either a negative (NTC) or positive (PTC) temperature coefficient. A resistive thermal device (RTD) can measure temperatures as high as 850?C over a great distance and without expensive signal conditioning. The most popular RTD is a standardized platinum temperature sensor called the PT100, which exhibits 100 ohms resistance at 0?C and a linear temperature coefficient of 0.38ohms/?C. It also has a nonlinear temperature coefficient that is much smaller, so the /?C characteristic appears almost linear over a narrow range. A PRTD's transfer function of resistance vs. temperature, for temperatures greater then 0?C, are approximated by the equation: RRTD = (100 + 0.39083T ? 0.00005775T2) ohms, where T = temperature (celsius). Unlike thermocouples, which deliver voltages that represent the difference between two temperatures, the resistance of an RTD represents the absolute temperature of that resistance. Measurement is typically accomplished by driving a current of 1mA to 2mA through the sensor and measuring the voltage drop across it.

    Also semiconductors can be used as termperature sensors because semiconductor PN junction characteristics change when temperature changes. This change is well defined and this can be used in some temperature measurement applications. The temperature - versus - bulk resistance characteristics of semiconductor materials allow the manufacture of simple temperature sensors using standard silicon semiconductor fabrication equipment. The ordinary semiconductor diode may be used as a temperature sensor because a forward biased voltage across a silicon diode has a temperature coefficient of about 2.3mV/?C and is reasonably linear. The forward basing can be done with for example around 1 mA current through the diode. To improve the performance of the diode as a temperature sensor, two diode voltages can be measured with two different currents (typically selected to be about 1:10 ratio). The transistor sensor is used in diode mode by connecting the base and collector together or sensor is wired between base and emitter. Semiconductor temperature sensors are available from a number of manufacturers. There are no generic types. The semiconductor (or IC for integrated circuit) temperature sensor is an electronic device fabricated in a similar way to other modern electronic semiconductor components such as microprocessors. These sensors share a number of characteristics - linear outputs, relatively small size, limited temperature range (-40 to +120?C typical), low cost, good accuracy if calibrated but also poor interchangeability. In general, the semiconductor temperature sensor is best suited for embedded applications - that is, for use within equipment. This is because they tend to be electrically and mechanically more delicate than most other temperature sensor types. Semiconductor temeprature sensor typically give you a voltage indication of temperature (for example 10 mV / celsius voltage change) or give you a digital interface to read the actual temperature in digital form (those sensors integrate a sensor and an analog to digital converter to the same chip). The "out of the box" or interchangeability accuracy of most semiconductor temperature sensors is not particularly good. If individual sensors are calibrated, significantly better measurement accuracy is possible (tyically a two point calibration or three point calibration is used). Due to the high sensitivity of some sensors, they can be very good in measuring small temperature changes (as opposed to absolute measurement).

    Bimetallic thermometers are contact temperature sensors found in several forms if you know where to look, e.g. inside simple home heating system thermostats. They typically consist of a strip of bi-metal that has some electrical contacts affixed to it. The temperature changes cause the strip to bend, making or breaking the connection as needed. You will often find long bimetallic strips coiled into spirals. This is the typical layout of a backyard dial thermometer. By coiling a very long strip it becomes much more sensitive to small temperature changes.

    In some applications temperature sensing needs to be done without contact to the measured subject. Those measurements use infrared (IR) techniques. Radiation Thermometers (Pyrometers, if you will) are non-contact temperature sensors that measure temperature from the amount of thermal electromagnetic radiation received from a spot on the object of measurement. There are two types of commonly used sensors for this kind of applicatons: Pyroelectric Infrared Detectors and Thermopile detectors.

    Pyroelectric Infrared Detectors (PIR) convert the changes in incoming infrared light to electric signals. The pyroelectric detectors output is proportional to rate of change of incident radiation. This means that pyroelectric detectors can only be used to detect temperature changes. This kind of PIR sensors are used for example in movement detectors where they sense the moving hot object (like human) through special optics (this optics makes the movement of hot object change the radiation that gets to sensor change sharply when object moves). Pyroelectric detectors have very high impedance requiring an internal impedance converting buffer to make them useable.

    Thermopile detector output is proportional to incident radiation. A thermopile is a number of thermocouples connected in series. The so called cold side of the junction is kept close to ambient temperature by bonding it to a temperature stable mass. The hot side of the junction is exposed to incident radiation. Some thermopile sensors have a built-in thermistor which provides measurement of the ambient temperature thus allowing the temperature of the target to be calculated. A thermopile sensor generates a voltage, which is proportional to the incident infrared (IR) radiation power. Because every object emits IR radiation with a power, which is a strict function of its temperature, one can deduct the object?s temperature from the thermopile signal. This method is called pyrometry. Thermopile-type infrared and thermal detectors are used in a number of applications, including infrared spectroscopy, radiometry, security systems, and many consumer products. Although they do not provide vision-quality images as in the case of quantum detectors, thermopiles are still attractive for many low-cost commercial and industrial applications, mainly because they do not need cooling for operation and the technologies are relativly simple. Thermopile detectors have low impedance.

    In addition to those some applications use Thermal Infrared imaging camera. Thermal Infrared imaging camera is a camera that responds to the infrared signals instead of normal light. Thermal infrared imaging cameras are detector and precision optics platforms that give us a visual representation of infrared energy emitted by all objects. Typical applications for this type of cameras are night vision,wildlife observation, search and rescue, victim location, wild fire recon, predictive maintenance in power transmission and machinery, carona detection, process control and printed Circuit Board evaluation. Depending the applications the IR radiation can be shown as normal grayscale video or through real time color infrared output. The color conversion converts different radiation intensities to different colors for easy visual inspection of picture. With a well calibrated camera, those different colors on picture can be directly mapped to different temperatures. Thermal infrared imaging cameras are expensive devices bceause they need special imaging sensors that needs to be cooled down to make them operate correctly.

      Temperature measuring device circuits

      • Circuit provides cold-junction compensation - The accuracy of any circuit or system that uses a thermocouple to determine the temperature of a process is limited by the accuracy of the method used to perform cold-junction compensation. In a thermocouple measurement, two wires of dissimilar metal join together at the "hot," or measurement, junction. The isothermal termination of the thermocouple wires provides a second "cold," or reference, junction. The potential across the thermocouple is proportional to the temperature difference between the two junctions. Thus, to determine the absolute temperature of the hot junction, you must also know the absolute temperature of the cold junction.    Rate this link
      • Circuit improves on temperature measurement - When current pulses with a stable IHIGH/ILOW ratio modulate a semiconductor junction, the ensuing voltage difference (for example, ?VBE for a bipolar transistor) is a linear function of the absolute (Kelvin) temperature, T. You can use this truism to make accurate temperature measurements.    Rate this link
      • Circuit provides accurate RTD measurements - This circuit is an efficient measuring circuit for PT100 RTD elements. The circuit provides analogue voltage output.    Rate this link
      • 4-20mA Loop Powered Temperature Sensor - A simple circuit that allows a 4-20mA to power an analog temperature sensor.    Rate this link
      • A precision interface for a Resistance Temperature Detector (RTD) - Resistance Temperature Detectors (RTDs) are temperature sensors that make use of the temperature dependence of a metal's resistance. They are used in a wide variety of temperature measurement and control instrumentation. These circuits are based on using a 100 ohm Platinum RTD (PRTD), versions of which are readily available from many sources    Rate this link

      Selecting temperature sensors

    Voltage measurements

      Digital display voltage meters

      • ICL7106, ICL7107, ICL7107S - The Intersil ICL7106 and ICL7107 are high performance, low power, 31/2 digit A/D converters. Included are seven segment decoders, display drivers, a reference, and a clock. The ICL7106 is designed to interface with a liquid crystal display (LCD) and includes a multiplexed backplane drive; the ICL7107 will directly drive an instrument size light emitting diode (LED) display.    Rate this link
      • 3 1/2 Digit Panel Meter - kit from    Rate this link
      • Digital Voltmeter - The ICL7107 is a 3 1/2 digit LED A/D convertor. It contains an internal voltage reference, high isolation analog switches, sequential control logic, and the display drivers. The auto-zero adjust ensures zero reading for 0 volts input. This how this circuit uses that IC to make a voltage meter.    Rate this link
      • Digital Volt meter with video output - This design awarded the third international prize in the Elektor Electronics 1997-98 Microprocessor and Microcontrollers Design Contest.    Rate this link

      RMS measurements

      RMS, or Root Mean Square, is the measurement used for any time varying signal's effective value: It is not an "Average" voltage and its mathematical relationship to peak voltage varies depending on the type of waveform. By definition, RMS Value, also called the effective or heating value of AC, is equivalent to a DC voltage that would provide the same amount of heat generation in a resistor as the AC voltage would if applied to that same resistor. True RMS allows the user to obtain accurate measurements of voltage at any waveform. True RMS meter work for non-sinusoidal AC voltage and current waveform found in controls and circuits. A True RMS meter uses a complex RMS converter to read RMS for any type of AC waveform. Normally True RMS reading meters are very expensive. The typical multi-meter is not a True RMS reading meter. It does this by measuring average voltage and multiplying by 1.11 to find RMS. Trying to use this type of meter with any waveform other than a sine wave will result in erroneous RMS readings. Improper measurement can easily lead someone to believe that a modified sinewave or square wave inverter is not putting out its rated power. A few handy things to keep in mind about RMS values that apply when dealing with a sine wave, are as follows:

      Peak Volts AC x .707= VrmsVrms=1.11 x Vavg1.414 x Vrms= Peak Volts ACVavg= .637 x Peak Volts AC
      For other waveforms these equations do not apply.
      • RMS-to-dc converter is accurate and stable - combining the well-known true rms-to-dc circuit with a simple S/H circuit eliminates offset errors, which increases accuracy and temperature stability    Rate this link
      • RMS Values and Measurement - This document tries to help alleviate confusion about measurement of RMS (Root Mean Square) values of AC voltage. The typical multi-meter is not a True RMS reading meter. As a result it will only produce misleading voltage readings when trying to measure anything other than a DC signal or sine wave.    Rate this link

      High voltage measuring

      Common voltmeters, digital or analog, usually range to some hundred volts maximum. Higher voltages not only cannot be indicated, but will also destroy the instrument. For those high voltage, special techniques and probes are needed.

      • Electronic high voltage meters - Common voltmeters, digital or analog, usually range to some hundred volts maximum. Higher voltages not only cannot be indicated, but will also destroy the instrument. However, the range of any voltmeter can easily be extended using extra series resistance.    Rate this link
      • Measuring high voltages by spark length - The simplest way to get at least a rough value of tension is to measure the maximum distance the voltage can arc over. The maximum spark length is determined by applying the high voltage to a pair of electrodes and bringing the electrodes closer to each other until a spark jumps over.    Rate this link

    Current measurements

    Measuring electrical current can be done using many methods. There are three rival technologies that are typically used for measuring current: sense resistors, Hall effect sensors and current transformers. Each have attributes that differentiate them on a cost versus performance scale.A general characteristics of different current measurement methods:

    • Sense resistor: Usually better than 95% accuracy, no galvanic isolation, usually hig power dissipation, low cost, typically used for less than 20A currents, works from DC easily up to 100 kHz (or even more)
    • Current transformer: Possible to get quite good accuracy (usually 1-5% error), provides galvanic isolation, moderate power dissipation, medium cost, measures easily up to 1000 amperes, works only for AC, usually used for mains frequency AC measurement
    • Open loop hall sensor: Gives around 90-95% accuracy, provides galvanic isolation, low power dissipation, medium cost, works up to 1000 amperes, can be made to work from DC to 20 kHz
    • Closed loop hall sensor: Usually better than 95% accuracy, provides galvanic isolation, moderate to high power consumption, high price, works usually up to 500A, can be mede to work from DC to around 150 kHz
    The most commonly used method for measuring current is to run the current through a know resistor. The voltage drop over thisresistor is determined by the current and the resistor value. If you select a small resistence, you do not cause much voltage drop over it, so measung does not considerably affectthe measured circuit.

    When measuring high currents on mains power 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 to convert 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). When using current transformers on high current circuits, make sure that the current transformer is never run without a proper burden resistor. If there is a high current going on transformer primary and many turns on secondary, the open circuit voltage of current transformer can become very high, even to several kilovolts range that can cause operation danger and damage the current transformer secondary insulation. In current transformer applications where the measuring equipment needs to be service, the output of current transformer should be short circuited before removing the burden resistor load.

    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.

    Some clamp-on multimeters can also measure DC currents. Those application use torid cares, where the Hall generator/sensor is placed within air gap of a magnetic core to measure the current. The hall sensor in the air gap measures the magnetic field cause by the wire running through the toroidal core. There are two techniques for sensing current using Hall effect devices: open loop and closed loop. In an open loop topology, the Hall element output is simply amplified and the output is read as a voltage that represents the measured current through a scaling. In a closed loop topology, the output of the Hall element drives a secondary coil that will generate a magnetic field to cancel the primary current field. The secondary current, scaled proportional to the primary current by the secondary coil ratio, can then be measured as voltage across a sense resistor. By keeping the resultant field at zero, the errors associated with offset drift, sensitivity drift and saturation of the magnetic core will also be effectively canceled. Closed-loop Hall effect current sensors also provide the fastest response times. However, with a secondary coil that may be needed to drive up to several milli-amps of current, power consumption is much higher in closed loop Hall effect devices than open loop topologies.

    Current clamp meters and clamp adapters are especially suited to measure DC and AC currents, in all instruments and systems, without interruption of the circuit. In practice there are many systems where it is not possible or safe to disconnect systems for the purpose of measuring the current flowing. Therefore, with the use of a Clamp Meter the current can be measured without any interruption whatsoever. Generally clamp-on multimeters need the toroidal type core to be closed to get measurements. Lately there has become available "open jaw" style Electrical Tester for measuring current using measurement device which does not need the fully closed core. Clamps and clamp adapters are easy to use. The conductor is completely surrounded by the current clamp. The measurement value appears on the analogue or digital display and can be read immediately. Most cheap clamp meters are designed to measure the current value of current that have sinewave shape. More sophisticated circuitry is required for measuring the True RMS value of AC or currents with complex waveforms. The RMS value is important for all non-sinus shaped currents, e.g. phase-angle control. Generally, the measurement value displayed on conventional clamps is smaller than the actual measurement current present when the waveform is non-sinusoidal. Clamp-on nmultimeters are typically designed to measure currents in range from few amperes to several hundred amperes (some meter go beyond 1000 A). The cheapest clamp-on meters have measuring resolution of around 100 mA. There are more expensive meters that have higher resolution like 10 mA or 1 mA (suitable to measure for example current industrial 4-20 mA current loops). High resolution mini current clamps for leakage currents from 10 ?A are indispensable for troubleshooting and testing of instruments at appliance test systems in compliance with DIN VDE 0701/ 0702 and BGV A2 (appliance tester). Leakage currents which do not return via an electrical conductor (e.g. N, PE) can be quickly, easily and safely measured by surrounding all active conductors (e.g. L1, L2, L3, N or L1, N).

    With traditional clamp-on current meters, measurements can only be made on single conductors. If you need to measure current in multipair cables (for example mains cables), this usually needs covers to be moved to gain access to individual wires. To measure the current consumption of an instrument, the individual wires of a cable had to be opened using e.g. a mains adapter. From the safety aspect, this procedure was not without problems and relatively difficult to perform. Some new special multimeters can measures current in multi-core cables and power cords without the need to split them. This kind of multimeter use techologies which are called (dending on manufacturer) with names like Flexiclamp, multi-core digital clampmeter, duplex clamp and SMF Technology. Different manufacturers use slightly different technologies. Unitest uses in their duplex clamp technology that has several sensor coils are positioned at a certain distance and direction. With this coil layout, the field direction of the current within a lead can be filtered with respect to the neighbouring lead. Thus, current measurement within a multi-lead cable is possible, without opening.

    A new technology for AC current measuring is Rogowski coil. Rogowski coil current transducers can measure alternating currents in a frequency range from less than 0.1Hz to about 1MHz. Their measurement range is impressive ranging from a few milliamps to over 1 million amps. These transducers have an excellent transient response capability and they can be used for measurements on very large or unusually-shaped conductors. A Rogowski coil is an 'air-cored' toroidal coil placed round the conductor. The alternating magnetic field produced by the current induces a voltage in the coil which is proportional to the rate of change of current. The combination of a coil and an integrator provides an exceptionally versatile current-measuring system which can be designed to accommodate a vast range of frequencies, current levels and conductor sizes. The output is independent of frequency, has an accurate phase response and can measure complex current waveforms and transients. One of the most important properties of a Rogowski coil measuring system is that it is inherently linear. The coil contains no saturable components and the output increases linearly in proportion to current right up to the operating limit determined by voltage breakdown. The integrator is also inherently linear up to the point where the electronics saturates. Linearity makes Rogowski coils easy to calibrate because a transducer can be calibrated at any convenient current level and the calibration will be accurate for all currents including very large ones. Also, because of their linearity, the transducers have a very wide dynamic range and an excellent transient response. Some designs of coil can be fitted on the conductor without the need to disconnect the conductor. Most flexible coils can be fitted this way and it is also possible to build split rigid coils.

    When measuring current on mains wires please note that most AC current meters are designed to give right current ratings only when they are connected to pure sinusoidal mains current. Pulse-width motor control systems, SCR and triac controllers and switchmode power supplies, for example, add high frequency (HF) components to the 50Hz mains that can cause false readings on traditional multimeters. Instruments with True RMS employ circuitry that rejects the HF signals and correctly calculate and display the RMS value.

    • Solar Panel Current Meter - This circuit is used to measure the current from a solar panel. It has very low power loss for currents in the 0-10A range. It also works as a general purpose DC current meter. The circuit can be used on either the positive or negative side of a DC circuit. The circuit works with DC circuits at any practical voltage. Accuracy is approximately 2%, depending on the meter movement.    Rate this link
    • Real Time Rotor Bar Current Measurements Using a Rogowski Coil Transmitted Using Wireless Technology - Rotor bar current measurement is a valuable step in verifying the theory of electrical machines design and control.    Rate this link
    • AC Line Current Detector - This circuit will detect AC line currents of about 250 mA or more without making any electrical connections to the line. Current is detected by passing one of the AC lines through an inductive pickup    Rate this link
    • AC Line Current Detector - This circuit will detect AC line currents of about 250 mA or more without making any electrical connections to the line. Current is detected by passing one of the AC lines    Rate this link
    • Current transformers: how to specify them    Rate this link
    • Current transformers: specification errors and solutions    Rate this link
    • Field Adjustment of Current Transformer Ratio    Rate this link
    • How Do Rogowski Coils Work? - A Rogowski coil is an 'air-cored' toroidal coil placed round the conductor. The alternating magnetic field produced by the current induces a voltage in the coil which is proportional to the rate of change of current. The coil output voltage is integrated electronically so that the output from the integrator is a voltage that accurately reproduces the current waveform of the wire going through the coil. Rogowski coil current transducers can measure alternating currents in a frequency range from less than 0.1 Hz to about 1Mhz. Their measurement range is impressive ranging from a few milliamps to over 1 million amps.    Rate this link
    • Integrator forms picoammeter - 41/2-digit picoammeter uses an integrating transimpedance amplifier to achieve a resolution of 0.1 pA    Rate this link
    • Isolated Open Loop Current Sensing Using Hall Effect Technology in an Optimized Magnetic Circuit - With the expected arrival of a 42V parallel bus power supply aboard automobiles and new energy efficiency standards being imposed on most household appliances, there is a growing need for current sensing as a means of monitoring and controlling power consumption. There are three rival technologies that are typically used for measuring current: sense resistors, Hall effect sensors and current transformers. Each have attributes that differentiate them on a cost versus performance scale. This document describes both open loop and closed loop Hall current sensors operation.    Rate this link
    • Measure power-on current transients on ac line - For any electronic or electrical system, you usually determine the ac-line fuse rating based on the steady-state current. However, the power-on current surge is an important parameter in determining the fuse's I 2 t rating. The I 2 t rating is a measure of the energy required to blow a fuse in pulsed conditions.    Rate this link
    • Picoammeter circuit with 4 ranges - This circuit uses a CA3420 BiMOS op amp to form a picoammeter with 4 ranges and exceptionally low input current (typically 0.2pA)    Rate this link
    • Selection Guide to Clamp-On Current Probes - Clamp-on current probes are designed to extend the current measuring capabilities of DMMs, power instruments, oscilloscopes, hand-held scopes recorders or loggers and other diverse instruments. The probe is "clamped" around the current carrying conductor to perform non contact current measurement    Rate this link
    • Selection Guide to Clamp-On Current Probes - Clamp-on current probes are designed to extend the current measuring capabilities of DMMs, power instruments, oscilloscopes, hand-held scopes recorders or loggers and other diverse instruments. The probe is "clamped" around the current carrying conductor to perform non contact current measurement.    Rate this link
    • Simple circuit detects current pulses - provides a visible indication of positive and negative current pulses whose amplitudes can vary from 20 to 150 mA    Rate this link
    • SMF Tecnology - SMF Technology (Suparule Magnetic Field Technology) are based on the measurement of the magnetic field generated from a current carrying conductor. This technology allows "open jaw" and "multi-core digital clampmeter". The key to the performance of this new sensor is a series of planer magnetic coils placed in a specified layout around the conductor. The magnetic field created by the current in the conductor induces a voltage in the magnetic coils, which is proportional to the magnetic field of the conductor.    Rate this link
    • Solar Panel Current Meter - has 0-10A full scale    Rate this link
    • Two sensors measure three line currents - allows you to measure all three line currents in a three-phase system    Rate this link
    • Using Rogowski coils for transient current measurements - In recent years the Rogowski-coil method of measuring electric current has developed from a ?laboratory curiosity to a versatile measuring system with many applications throughout industry and in research.    Rate this link
    • Current Transformer Measurements - This document has information on Current Error (ratio error), Phase Angle error and Basic Calibration Circuits.    Rate this link
    • Field Adjustment of Current Transformer Ratio - The ratio of current transformers can be field adjusted to fulfill the needs of the application. Passing more secondary turns or more primary turns through the window will increase or decrease the turns ratio.    Rate this link
    • Selecting ANSI Class Metering Current Transformers - One of the most common uses of current transformers are in metering and power usage, where a 5 Amp secondary current transformer is applied to a panel meter or a power meter for displaying amperage or recording power. When extremely accurate measurement is required, or when revenue is generated from a power meter, ANSI class current transformers are generally selected.    Rate this link
    • Precision Rectifier Circuit for CT Signal Conditioning - Many times, the designer wishes to generate a DC signal from an AC current transformer for input to a PLC or data acquisition system, or even as part of a current or motor controller. Creating DC from an AC source creates problems with diode voltage drops and the variances over temperature and current. This circuit provides an accurate method for creating this DC signal.    Rate this link
    • Low Cost Fan Control with Hysteresis - This application can be used to control many different devices. In this example, a compressor current is sensed, and when it reaches a selected set point, the circuit turns on a relay, which controls a fan motor. The circuit is generated with a minimum number of parts, and includes hysteresis.    Rate this link
    • An Analysis of Current Transformer Ratio and Phase Angle Error - A technical discussion of the current transformers including equivalent circuits, phasor diagrams and Thevenin equivalents.    Rate this link
    • Application Guide - A general application guide that covers most areas of electrical current monitoring using current and voltage transformers. The primary purpose of this guide is to give the reader a basic understanding of how to apply instrument transformers in a practical way while observing good engineering practice. A special effort will be made to keep to a minimum technical terms and language.    Rate this link
    • Practical Aspects of Rogowski Current Transducer Performance - This paper examines the frequency response for a 10MHz bandwidth Rogowski transducer. The transducer was tested with currents from a tuned LC circuit for frequencies between 0.6 and 13MHz in comparison with a 20MHz coaxial shunt.    Rate this link
    • PCB Rogowski Coils Benefit Relay Protection - Innovative Rogowski coils enable the design of avanced protection systems    Rate this link
    • PCB Rogowski Coils Benefit Relay Protection - Innovative Rogowski coils enable the design of avanced protection systems    Rate this link
    • Current sensing for energy metering - Solid-state electric energy meters contain both voltage and current sensing elements. The current sensing requirement is a more difficult problem. Not only does the current sensor require a wider measurement dynamic range, it also needs to handle a wider frequency range because of the rich harmonic contents in the current waveform. This paper shows how a digital integrator can be used to convert the di/dt signal output from Rogowski coil current sensor to an appropriate signal and how it can be combined for a high-current energy meter.    Rate this link
    • High-Precision Rogowski Coils for Improved Relay Protection, Control and Measurements - Rogowski coils were first introduced to measure magnetic fields. They could not be used for current measurements because coil output voltage and power were not sufficient to drive measuring equipment. However, with today?s microprocessor-based equipment, Rogowski coils are more suitable for such applications. Rogowski coils have many advantages over conventional current transformers.    Rate this link
    • How Do Rogowski Coils Work? - A Rogowski coil is an 'air-cored' toroidal coil placed round the conductor. The alternating magnetic field produced by the current induces a voltage in the coil which is proportional to the rate of change of current. The combination of a coil and an integrator provides an exceptionally versatile current-measuring system which can be designed to accommodate a vast range of frequencies, current levels and conductor sizes. The output is independent of frequency, has an accurate phase response and can measure complex current waveforms and transients.    Rate this link
    • Using Rogowski coils for transient current measurements - In recent years the Rogowski-coil method of measuring electric current has developed from a "laboratory curiosity" to a versatile measuring system with many applications throughout industry and in research. The technique possesses many features which offer an advantage over iron-cored current measuring devices and these are well illustrated by considering how it can be used for measuring transient currents The paper describes the principle of operation of Rogowski coils and the practical aspects of using them and gives several examples of their use in making transient measurements.    Rate this link
    • Current Sensing for Energy Metering - This paper describes the use of low resistance current shunt, current transformer, hall effect sensor and Rogowski coil    Rate this link
    • 21st CENTURY CURRENT SENSORS (feature) - It is in the interests of both provider and consumer that the electrical energy supplied is metered accurately - no-one wants to be overcharged and no utility wants to supply free electricity. But metering technologies have been the Cinderellas of energy infrastructure - they work hard as cash registers for the utilities, using electromechanical designs which have altered little for the past 30 years. However, the utility market is changing, and as the cost of digital microelectronic devices comes down, the opportunities offered by solid-state meters are looking increasingly attractive for more intelligent metering systems.    Rate this link
    • Rogowski Loop Designs for NSTX - The Rogowski Loop is one of the most important diagnostics from a plasma current measurement and control aspect of tokamak operation. On the National Spherical Torus Experiment (NSTX), the plasma current Rogowski Loop had the constraints of the very limited space available on the center stack, 5000 volt isolation and flexibility requirements. This paper tells about the coil design.    Rate this link
    • Sensor Design for Leakage Current Measurement on ADSS Fiber-Optic Cable - All-Dielectric-Self-Supporting (ADSS) fiber-optic cables are installed on high voltage transmission lines for communication purposes. When the cables become polluted and wet, a conductive layer is formed on the cables, and leakage currents are induced on their leading to dry-band arcing. Knowledge of the leakage current levels on the cable surface before dry-band arcing can be used to predict cable failure. Monitoring the leakage current can also yield information on the aging of the cable. This paper presents a review of three different current sensors that can be used on ADSS cables, which include a simple shunt resistor, an active Rogowski coil, and a double core sensor.    Rate this link
    • Rogowski Coils - Rogowski coils are used for measuring alternating current. They work by sensing the magnetic field caused by the current without the need to make an electrical contact with the conductor. These coils have been used in various forms for detecting and measuring electric currents for decades but it is only in recent years that their potential is being realised on a commercial scale. By using the right technique it is now possible to wind both flexible and solid coils with sufficient uniformity for them to be used in a wide range of applications including those demanding precision measurements.    Rate this link
    • Current sensing for energy metering - All energy meters contain advantages comparing with the other current sensing solutions. This is a general introduction to the current measuring topics.    Rate this link
    • Current measurement with electrical isolation - The principal advantage of using current transformers to measure current in electric circuits is that the measurement circuit can be electrically isolated from the circuit under test. Isolation can be a particular benefit where high voltages are present. Current transformers impose a negligible burden on the circuit under investigation and their use has an insignificant effect on the performance of the circuits examined. They are passive devices.    Rate this link
    • Rogowski Coils and Current Transformers - There is considerable confusion over various forms of Rogowski coils and current transformers (CTs). In fact the devices are closely related. Both of them are used to measure or detect currents by mutual coupling the primary circuit to the secondary circuit. The Rogowski coil, rfct (Rogowski coil) and a CT are all basically the same device. The device consisting of a core/ former with a primary and secondary winding, when operating into a open circuit or high impedance it is a Rogowski coil. When it operates to low impedance it is a RFCT (Rogowski coil) or a conventional CT. The terminating impedance controls the frequency where the device changes from a Rogowski coil to current transformer. Rogowski coils tend to be air cored devices and current transformers tend to have magnetic material cores (laminated iron or ferrite), but this is by no means certain as it is possible to design air cored CT's and magnetic cored Rogowski coils.    Rate this link

    Electrical power measurements

    Measuring power is useful when you want to know how many wattscertain electronic device takes power.If the device is powered from DC voltage, determinign the power iseasy: measure the voltage going to the device and the current going to the device (just connect two multimeters to the poweringcircuit). Then calculate the power using formulapower = voltage * current.Measuring AC power is harder. The equation power = voltage * currentdoes still hold, but you can't necessarily do the measurementeasily with two multimeters. If you just measure the current and voltage with two multimeters, you will get the currentand voltage values. You can calculate the power with formulapower = voltage * current (power in VA unit), but remeber that this power is not a real power taken by the device.Depending the phase angel of the current and voltage,the real power taken by the device can be anythign betweenzero and the power calulated with formula power = voltage * currentwhen current and voltage are measured with multimeter. Power meters which measure real power, need to measurethe instantaneous voltage and current many times in a AC power phase, and with every measurement need to do the calculation of voltage * current. The real poweris the sum of those calculations. This more complexpower measurement method works also for non-sine waveforms.Power meters provide an early warning of thermal overload by monitoring power consumption in high-reliability systems. Power monitoring is especially suitable for motor controllers, industrial heating systems, and other systems in which the load voltage and current are both variable.

    Resistance measurements

    The two instruments most commonly used to check the continuity (a complete circuit), or to measure the resistance of a circuit or circuit element, are the ohm meter and the megohm meter. The ohmmeter is widely used to measure resistance and check the continuity of electrical circuits and devices. Typical ohmmeter range usually extends to only a few megohms. When hugher resistances need to be tested, megaohm meter is is used for this.

    There are two basic methods of measuring resistance.

    • One is to apply a known voltage to the unknown and measure the current.
    • The other is to apply a known current and measure the voltage.
    Both of those methods are used in different resistance measurement instruments.

    When you use your ohmmeter to measure the resistance of a wire you touch one meter lead to each end of the wire and you get a resistance measurment. Your ohmmeter forces a current through the wire, measures the voltage that develops, calculates the resistance, and displays the result. To do all this your ohmmeter must have a current source and volt meter. A basic analogue ohmmeter typically consists of a dc ammeter, a dc source of potential (usually a 3-volt battery) and few resistors. Digital multimeters generally measure resistance by applying a known current to the resistor and measuring the voltage drop over it (directly proportional to the resistance value). The measuring current can vary between different ranges (measuring current is usually few milliamperes or less, but can be higher on some low resistance measuring ranges). When you make a 2-wire resistance measurement with your multimeter your meter uses only two leads to connect to the device under test. This setup has the advantage of using just two wires to connect to the DUT but what is the actual resistance it's measuring? Two-wire measurements actually measure the DUT resistance plus the test lead resistance plus contact resistance. You will find that the resistance varies depending on how hard you hold your lest leads to the wire ends. This variation in resistance comes from the point of contact between the device under test and your multimeter measuring lead contacts. This resistance variation from measurement to measurement can add significantly to a learned resistance and will get worse as the mating connectors wear. When resistors have considerable resistance and the resistance of measuring wires are quite low, things work accurately enough with two wire method. The Two-wire technique is typically used in automotive wiring, computer cables and other low specification applications to merely verify that an item is wired correctly and not to verify the integrity or performance of a cable.

    The methods descrubed above are good for measuring resistances that are few ohms or higher, but when measuring very small resistance values, those methods with two measuring wires have their weaknesses. When making resistance measurements for resistances below 1 ohm, the resistance of the multimeter measuring leads plus the resistances on the contacts (banana contacts to multimeter plus contacts to item being measured) can be so high that they cause easily considerable error to resistance readings (those contact resistances can vary easily quite much over time). Some ohmmeters have four connections to overcome the limitations of two wire system. In four wire measuring system two wires come from the current source (sometimes called the "force" leads), and two other come from the voltmeter (usually called the "sense" leads). Four wire measurement using the Kelvin Clip reduces the IR drop in test leads that can cause measurement inaccuracies. With four connections you choose where to connect the voltmeter so you are in control over exactly what resistance you want to measure, and the resistances on the wires and connections do not cause considerable error. The 4-wire testing eliminates the resistance of your interface cabling, which will will greatly improve accuracy if your measuring cable plus contacts resistance is a significant part of the total resistance. It allows you to measure lower resistance values than 2-wire testing (with for example with 1A measuring current you can measure up to milliohms range with soem instruments, one milliom resistance will then show 1 mV voltage). The disadvantage of 4-wire testing is it takes four connections to do the test but it does give you an accurate resistance measurement of the DUT without the resistance of the test leads. Many continuity testers require 4-wire testing to accurately measure resistances under 1 ohm. The Four-wire measurement technique is found in some higher specification ATE, more commonly known as Kelvin measurement. When measuring low resistance measurements using the four-wire technique the test lead or test interface wiring will be automatically nulled out. Most Aerospace and Defence companies have adopted the Four-wire continuity measurement technique as the industry standard, because many of the cables being tested in the aerospace sector are of very low resistance and they need to be measured accurately. Using the four-wire technique you can highlight faults like individual strands of wire broken, wrong gauge wire fitted, poor crimp connection, dry solder joints, squashed wires, dirty connections, poorly mating connectors and bad grounding/shielding.

    Different techniques also come into use when very high resistance values are needed to be measured. The megger is a special instrument that is widely used for measuring insulation resistance, such as between a wire and the outer surface of the insulation, and insulation resistance of cables and insulators. The range of a megger may extend to more than 1000 megohms. Megger has the same operation principle, but it generally uses a much higher measurement voltage, typically 250, 500 or 1000 volts DC. Those high voltage ranges are often used to test the quality of the insulation in electrical cables and equipments (safety checks).

    • Continuity Tester - audible output if resistance is less than 300 ohms    Rate this link
    • Digital position encoder does away with ADC - converts the change in resistance of a potentiometer into a digital value without using an expensive A/D converter    Rate this link
    • Johtavuusilmaisin - A simple resistance measurement circuit which shows if resistance is smaller or higher than the threshold set to circuit. The output is using two LEDs. Test in this circuit is in Finnish. This document is in pdf format.    Rate this link
    • Latching Continuity Tester - A continuity tester is a must on every service bench for testing cables, pcboards, switches, motors, plugs, jacks, relays, and many other kinds of components. But there are times when a simple continuity test (or your multi-meter) doesn't tell the whole story. For example, vibration-induced problems in automobile wiring can be extremely difficult to detect because a short or open is not maintained long enough for a non-latching tester to respond. This Latching Continuity Tester can help you locate those difficult-to-find intermittent short and opens that other testers always seem to miss.    Rate this link
    • Low-Ohm Meter - article which describes few different low resistance measuring circuits    Rate this link
    • Low Resistance Adapter For DMMs - When the resistances to be measured a very low, say 0.1 ohms, analog meters are useless because the reading becomes indistinguishable from zero. A four and a half digit DMM may have 1/100 ohm resolution but the resistance of the connecting leads, the contact resistance where the leads plug into the meter and where the leads clip to the unknown is significant compared to the unknown. This circuit uses four-wire measurement technique to make more accurate measurements.    Rate this link
    • Megger - The megger is a portable instrument used to measure insulation resistance. The megger consists of a hand-driven DC generator and a direct reading ohm meter. A simplified circuit diagram of the instrument is shown in this document.    Rate this link
    • Ohmmeter - Introduction to ohmmeters    Rate this link
    • Accurately measure resistance with less-than-perfect components - For transducers, such as strain gauges or thermistors, you must accurately and inexpensively measure resistance using circuitry built with imperfect components and in which gain and offset errors can significantly limit the accuracy of ohmic measurements. The right circuit topology makes it possible to eliminate most error terms while measuring ohms, leaving the accuracy to be determined by just a single reference resistor.    Rate this link
    • 4-Wire "Kelvin" Testing - If you've used an ohmmeter to make resistance measurements you've probably heard terms such as "2-wire measurement" and "4-wire kelvin measurement." This document explains how ohmmeters measure resistance, how 2-wire resistance measurements work, how 4-wire resistance measurements work, and the special considerations for each measurement type.    Rate this link
    • Kelvin (4-wire) resistance measurement    Rate this link
    • Measurement of Track Resistance - Track resistance is usually less than an ohm, often less than 0.1 ohm. As the connection between a probe and the track surface can easily reach or exceed the track resistance, certain techniques and precautions must be observed in making the measurement. These will involve the use of a "Four Terminal" or "Kelvin Sensing" ohm meter.    Rate this link
    • A precision interface for a Resistance Temperature Detector (RTD) - Resistance Temperature Detectors (RTDs) are temperature sensors that make use of the temperature dependence of a metal's resistance. They are used in a wide variety of temperature measurement and control instrumentation. These circuits are based on using a 100 ohm Platinum RTD (PRTD), versions of which are readily available from many sources.    Rate this link

    Insulation testing

    The importance of sound electrical insulation systems has been acknowledged from the early days of electricity. Insulation failure can cause electrical shocks, creating a real hazard to personnel and machinery. Insulation testing is a common requirement as part of UK, European and International standards. Probably 80% of all testing performed in electrical power systems is related to the verification of insulation quality. Insulation resistance testers can be used to determine the integrityof windings or cables in motors, transformers, switchgear,and electrical installations. The test method is determinedby the type of equipment being tested and the reason for testing.A regular program of testing insulation resistance is strongly recommended to prevent dangers, as well as to allow timely maintenance and repair work to take place before catastrophic failure. All new equipment, motors, transformers, switch gears, and wiring should be tested before being put into service. This test record will be useful for future comparisons in regular maintenance testing.Most electrical equipment in utility, industrial, and commercial power systems uses either 50 or 60 Hz alternating current. Because of this, the use of an alternating current source to test insulation would appear to the logical choice. However, as will be described a little later, insulation systems are extremely capacitive. For this reason, DC has found a large niche in the technology. High potential insulation tests are "go no-go" tests. The cable or equipment is required to withstand the specified voltage for the specified time duration. These tests will normally reveal gross imperfections due to improper handling or construction. DC High-Pot testing is superior to medium voltage testing because it stresses the insulation at or above the working level, providing more information about the condition of the insulation and helping you to predict problems before a breakdown occurs. The tests are more complicated and the instruments more expensive, but the condition of the apparatus in question is better documented and tested and less likely to fail without warning. High voltage AC testing is used to test the dielectric strength of electrical insulation. Sometimes this is called destructive testing in that voltage is increased to some specific point to see if the insulation can withstand that particular voltage. It is a Go/No Go type of test, and can cause deterioration of the insulation, as opposed to the DC non-destructive test megohmmeters. The insulation tester (sometimes called megger) is widely used for measuring insulation resistance, such as between a wire and the outer surface of the insulation, and insulation resistance of cables and insulators. The range of an insulation tester may extend to more than 1,000 megohms. Megger has the same operation principle, but it generally uses a much higher measurement voltage, typically 250, 500 and 1000 volts DC. Those voltage ranges are often used to test the quality of the insulation in electrical cables and equipments (safety checks). The selection of250V,500V and 1000V voltages ensure that the correct test voltagefor fixed installations under test is always available. The 500 V range is suitable for the majority of testing on circuits with a nominal voltage up to 500V. VDC is the most commonly required voltage since it is used to test all circuits except low voltage circuits with a nominal voltage up to and including 500 Volts. The 250V insulation range is necessary where low voltage circuits supplied by an isolating transformer aretested whilst the 1000V range is used for circuits with a nominal voltage exceeding 500V and below 1000V.The 250-1000V insulation ranges are also useful for establishing the integrity of the internal parts such as motors, timers and transformers. 500 VDC is the most commonly required voltage since it is used to test all circuits except low voltage circuits with a nominal voltage up to and including 500 Volts. A 250 VDC test capability is necessary to test low voltage circuits supplied by an isolation transformer. Sometimes lower test voltages are needed. The 50V and 100V test voltages enable testing of circuits and components where higher voltages can not be tolerated whilst the capacitance range can be used on PCB components. The low voltage insulation tests are necessary for the testing of delicate components and equipment found in telecom systems which would be damaged by higher voltages.NOTE: While there are many brands of insulation testers, there is only one "MEGGER"; it is a registered trademark of AVO Biddle.For high voltage testing typically voltages 2500 and 5000 volts DC are used. Those high voltage ranges are often used to test the quality of the insulation in electrical cables.Typically, an insulation resistance test is performed with a megohmmeter (commonly called a megger), which applies a dc voltage and translates the leakage current into ohms. Field acceptance limit is [(kv+1)/L]x1000 megohm where kV is insulation voltage rating and L is cable length in feet.All old wiring and equipment should be carefully checked (for safety), both visually and with an insulation tester. In particular the insulation resistance between liveconnections and any exposed metal parts should be checked with a highvoltage tester at 500V for 230V equipment and 250V for 110V.The highest voltage used for ordinary PAT testing is 500VDC, but you don't use that on electronic or IT equipment unless it conforms to EN 60950. Too high testing voltage can do more damage than good (500V test voltage can damage some equipment). The insulation test performs a measurement of the resistance of a product?s insulation protection by applying a DC voltage between phase and neutral to the earth conductor - forClass I equipment, and between phase and phase and neutral to the outer case - for Class II equipment. The test results in a reading of resistance measured in M ohms. The voltage is applied for 2-3 seconds.The test is designed to ensure that protective insulation is sufficient to form a barrier so that electricity does not come into contact with a user so as to cause injury, or to ensure that other systems are not adversely affected. In the manufacturing environment the advent of legalisation such as the Low Voltage Directive requires evidence of due diligence and the results of this and other tests can be used in this respect.In general, pass/fault limits for Class I equipment is a resistance greater than 2M and for Class II equipment is a resistance greater than 7 M. In both cases an accuracy of 5% is called for.Genral advice is that if there is any leakage worse than about 50 megohms then track it down (it can be a device feature or potential damage). Check your local regulations and product standards for more precise information on this.Please note that the volteges used in insulation testing can be dangerous.Generally the power output in measuring instuments is so limited thatit does not kill you (can give nasty shock though), but the same voltagecharged to a capacitor can be dangerous. For safety reasons theequipment shoudl be discharged after measurement.Equipment should be discharged (shunted or shorted out) for at least as long as the test voltage was applied in order to be absolutely safe for the person conducting the test.HiPot testing is a special insulation testing. Some people refer to this as Insulation Testing but this can lead to the mistake of making a resistance measurement using 500Vdc. While this is good practice and useful (to identify potential failures in filters) it does not test insulation strenghnesss. For production, voltages between 1,500 and 2,500 Volts are necessary to verify that insulation is in place. Anything less may ?Pass? faulty insulation. Some standards allow AC or DC HiPot testers. DC testing should always be the preference because measurements are not affected by filter capacitance. But do make sure there is an indication that the external load is discharged after testing.Relates standards: EN 61557-2 definesprofessional insulation testing and ground connection testing(test voltages 50, 100, 250, 500 and 1000V).

    • Arc Fault Resistance Test for wires for Aircraft Application    Rate this link
    • Basic insulation testing - What does the measurement tell me? Fundamentally, how "good" the insulation is.    Rate this link
    • Cable/Harness Testing Easy - The table in this doocument shows the guidelines for using voltage to detect insulation defects. These tips can assist you in training people for cable and wire harness quality assurance.    Rate this link
    • Insulation Resistance Testing    Rate this link
    • Insulation Resistance Testing - Insulation resistance testing is something that every good electrical technician should know about. It can help ensure public and personal safety by eliminating the possibility of a life-threatening short circuit or short to ground. It can also be helpful in protecting and prolonging the life of electrical systems and motors.    Rate this link
    • Portable Appliance Tester Comparisons    Rate this link
    • Principles of Insulation Testing - Probably 80% of all testing performed in electrical power systems is related to the verification of insulation quality. This Technical Bulletin briefly describes the fundamental concepts of insulation testing including: insulation behavior, types of tests, and some test procedures.    Rate this link
    • Understanding Insulation Resistance Testing - A regular program of testing insulation resistance is strongly recommended to prevent electrical shocks, assure safety of personnel and to reduce or eliminate down time. It helps to detect deterioration of insulation in order to schedule repair work such as: vacuum cleaning, steam cleaning, drying and rewinding. It is also helpful when evaluating the quality of the repairs before the equipment is put back into operation.    Rate this link
    • Understanding insulation resistance testing - How is insulation resistance testing done, in view of the fact that at least 80% of electrical maintenance and testing involves evaluating insulation integrity?    Rate this link
    • Understanding Insulation Measurements on Telephone Cables - Insulation resistance measurement is a non-destructive measurement method when carried out under normal test condi-tions. It is accomplished by applying a DC voltage lower than that used for a dielectric test, and the purpose is to produce a result in k?, M? or G?. This resistance value expresses the quality of insulation between two conductive elements and gives a good indication as to the risk of leakage currents flowing. Insulation measurements are carried out on new cables (not yet installed) at 250V or 500V, then at 50V or 100V for line fault reading on cables already in service. Measurements can be made between pairs of lines and the shield connected to the ground, or between the metal shield and ground.    Rate this link
    • What to look for in a portable appliance tester    Rate this link
    • What impulse testing of transformers tells - Impulse testing simulates a transient surge coming into the transformer terminal from lightning strikes at various distances on the line. The test surge is created by the impulse testing equipment, which includes a group of capacitors that is charged and discharged during the test. The purpose of the impulse test is to show that the insulation of the device being tested can withstand expected transient voltages. And, it is an excellent measure of quality control.    Rate this link
    • Electrical Insulation System Testing - Qualification of an insulation system to either UL 1446 or IEC standards requires testing of complete, assembled insulation systems. Electrical equipment manufacturers are faced therefore with testing the actual equipment; substituting a motorette or transformette that represents their system; or using a recognized EIS through a material supplier like Du Pont; Schenectady; P. Leo; Ripley Resin who have already done the testing. Because final recognition of a system by UL requires that the system go through 5000 hrs of heat aging at the lowest of the three or four temperatures the system is tested at, many manufacturers choose the preapproved system option.    Rate this link
    • Electrical Insulation Systems - Give your electrical insulation systems and components the Recognition they deserve. Today's complex electrical applications demand greater diligence in verifying that individual insulation materials can perform together in shared environments.    Rate this link

    Multimeters

    A multimeter is used to make various electrical measurements, such as AC and DC voltage, AC and DC current, and resistance. It is called a multimeter because it combines the functions of a voltmeter, ammeter, and ohmmeter. Multimeters may also have other functions, such as diode and continuity tests. Multimeters are designed and mass produced for electronics engineers.

    Multimeters are commonly used to measure voltage and resistance between two points. Current is more rarely measured because you must alter the circuit to measure the current (except if you use a clamp type meter which is available for high current measurements). An analogue meter moves a needle along a scale. Digital meters give an output in numbers, usually on a liquid crystal display. Most modern multimeters are digital and traditional analogue types are destined to become obsolete.

    Simple analog meters (with moving coil and pointer) and simple digital meters inherently read the average voltage. Since nobody wants to know the average voltage, the reading is scaled to RMS by multiplying the average reading by 0.707/0.636 . That works when the voltage is a simple sinusoid, but not for other waveforms, including halfwave rectified AC or a sinusoid distorted by harmonics. Scaling average to RMS can be done by converting to peak = average/0.636. Then convert peak to RMS: peak x 0.707 = RMS. Average-scaled-to-RMS = (averge/0.636) x 0.707)

    True RMS digital meters require some sophisticated electronics that does the conversion from all different waveforms to the RMS power. Different meters use different technologies for conversion (anything from measuring heating power of a resistor to sophisticated digital signal processor). RMS digital meters typically have a limit beyond which their accuracy breaks down - like trains of very narrow pulses.

    Here is how a typical measurement are made in typical digital multimeter nowadays:

    • DC voltage: The A/D circuitry in the multimeter is designed to directly show DC voltage values typically in few volts range. For higher voltages the input voltage is divided by a voltage divider network. For lower voltages the voltage is amplified with amplifier.
    • AC voltage: Basically same idea as the DC measurement, except that the input voltage is rectified somewhere in the process.
    • DC current: Input current is run through a known low ohm resistance, which converts the input current to a small voltage drop. This voltage is fed to the DC voltage measurement circuitry.
    • AC current: This is measures in the same way as DC current, except that the voltage is fed to the AC voltage measurement electronics.
    • Diode test: A low current (typically less than 1 mA) is fed to the measurement leads (output voltage limited to few volts). The voltage between measurement leads is measurement with DC voltage measurement electronics.
    • Resistance measurement: An accurately known low current (varied dependign on ohms range) is fed to the measurement leads. The voltage (directly proprortional to the resistance conencted) between measurement leads is measured.

    Some multimeters can have some of the following functionalities in addition to the basic ones described above:

    • Continuity check: Continuity checks with most digital multimeters require the circuit to have low resistance before they'll register (typical sensing range 15..300 ohms depending on multimeter). The resistance is measured typically in the same way as in normal resistance measurements. Continuity check function usually in multimeters includes a "beeper" which makes sound when contact is found (allows quick testing withou need to look at the multimeter reading). Continuity check is a very handy when you test the wiring of different cables.
    • Continuity tester: Works like the resistance measurement measurement, If the voltage between measurement leads is lower than specified value (usually 50 to 300 ohms) would give, make the beeper to signal.
    • Frequency: Input signal is converted to square wave first. The multimeter has either pulse counter (count pulses for one second gifes ouput in Hz) or frequency to voltage converter (output od converter measured with DC voltage measurement circuitry). Many cheap multimeters use the frequency to voltage generator approach and are not very accurate (easily few percent measuring error).
    • Capacitance: Feed known frequency low amplitude signal through the capacitance. Measure the AC current which go through the capacitor. Other option is to measure the capacitor charge and discharge times. The capacitor measurements on multimeters are not generally very accurate, the most common use of capacitance measurement using a meter is during construction to confirm the value of a part before fitting it. Many capacitors hae confusing and/or hard to read markings so the ability to quickly check a value is invaluable.
    • Temperature: Voltage from thermocouple sensor is amplified and processed. Then the result is fed to DC voltage measurement electronics.

    Please note that the information give above are just general statements. The implementation may vary between multimeter brands and models.The electronics inside a typical multimeters typically consist of a measuring IC, LCD display, measuring mode control switch and accessory electronics parts (a set of resistors, some capacitors, usually a rectifier, maybe some extra ICs for extra functions).

    Typical ICs used in cheap digital 3 1/2 digit multimeters with basic functionality are ICL7108 and ICL7109. More expensive multimeters usually have a custom measuring IC designed often by the multimeter manufacturer.If you measure low voltage circuits and do not need very accurate results,some cheap multimeter could be a good choise. You do need to worry much on the meter and measurement wires.

    If you are going to measure mains voltage circuits, then I recommendto get a good reliable multimeter (IEC 1010 and CE compliant) with safe test leads (1000V rated PVC or silicone insulation, safety banana plug connectors, IEC 1010 and CE compliant). If you are going to measure high current circuit (something with high short-circuit current) be sure that you have a properly fused multimeter (all scales fused) and prefereably fused test probes also. And the meter constructed in such way that you cannot easily mix up with the measuring of current and voltage.

    Almost multimeters nowadays have safety banana connecors in then which can accpet both normal banana plugs and safety banana plugs(bananas with plastic "tube" insulation surrounding the plug tip). Please note that there are several different versions of safety banana connectors in use. The banana plug metal tip part inside insulator is similar, but there can be differences in the mechaical construction of the insulation (inner and outer diameter, length of the insulating part etc.). Those differences cause that you might not be able to interchange measuring leads between all different multimeter brands and you can't use all available multimeter measuring leads with your multimeter because of this compatibility issue. The biggest professional multimeter brands (Fluke, better Mastech models etc.) tend to use nowadays the "standard safety banana plugs" that are interchangeable (you can buy leads from many manufacturers and they fit in nicely). Compatibility problems are most often seen on cheapest multimeters.

    Important note: The most common mistake when using a multimeter is not switching the test leads when switching between current sensing and any other type of sensing (voltage, resistance). It is critical that the test leads be in the proper jacks for the measurement you are making.

      General information

      • A DMM Primer - When working with your digital multimeter, there are a number of basic terms and functions every technician should know. Here is a list of the some of the common terms that you'll probably come across when working with DMMs.    Rate this link
      • Check The Specs For Safety - When working with test equipment, it?s important to understand category ratings. The most important single concept to understand about safety standards is the Overvoltage Installation Category, defined as Categories (CAT) I through IV.    Rate this link
      • Cyrustek Multimeter IC datasheets - You can find here datasheets for DMM IC, including ICs similar to ICL7108 and ICL7109.    Rate this link
      • Dictionary of Multimeter terminology    Rate this link
      • Measuring resistance, and current with a nonelectric VOM    Rate this link
      • Playing it safe with your DMM - Taking safe measurements starts with choosing the right meter for the application and the environment in which it will be used. There are a lot of safety issues to think about, from clothing to tools to procedures. Here?s a sampling of just some of the things you should consider on your DMM safety checklist.    Rate this link
      • Selecting the clamp for your job - Choosing the right type of clamp meter is critical when you want to ensure proper power supply to all electrical equipment on a circuit. Current clamps are a simple and reliable means to verify if current is flowing, and if there is continuity between contacts or points of connection. The current clamp has been a mainstay of the electrical technician's toolbox for decades, because it is a cost-effective, simple and accurate means to measure current.    Rate this link
      • The Effect of Meter Resistance - All meters have resistance. The value of this resistance depends upon the voltage range selected. A typical moving coil meter has a SENSITIVITY of 20,000 ohms per volt. Digital multimeter have typically higher resistance (input impedance typically around 10 megaohms on many ranges on good digital multimeters).When the meter is connected to a circuit to measure voltage, this resistance will affect the circuit and therefore the accuracy of the measurement obtained.    Rate this link
      • Using a Multimeter - The descriptions and pictures in this document are specific to the Fluke 73 Series III Multimeter, but other multimeters are similar.    Rate this link
      • Using a Volt Ohm Meter - A very handy tool for trouble shooting problems is a VOM (Volt Ohm Meter) - also called a Multi-Meter. It can be used to test cables, AC power levels and Batteries. You'll often find yourself out on the road with problems that are causing you grief, but you aren't quite sure why.    Rate this link

      Multimeter circuits

      Accessories

      • Build your own Gaussmeter - Have you ever wanted to find out how strong a magnet really was, or how the strength of the magnetic field varied as you changed the distance from the magnet or the temperature of the magnet, or how well a shield placed in front of the magnet worked? This circuit is a hand-held Gaussmeter for measuring the polarity and strength of a magnetic field. This circuit is a very simple, inexpensive Hall effect device Gaussmeter you can build for as little as $6. This circuit uses a normal multimeter as the display device.    Rate this link
      • Inductance Meter Adapter - a circuit that, when connected to a digital multimeter, lets you measure low-value inductances    Rate this link
      • N5FC's Ballpoint RF Probe - small RF probe that connects to a multimeter    Rate this link
      • Power Meter/Dummy Load - adapter to measure small transmitter power with normal multimeter    Rate this link

      Computer software for multimeters

      • JMM (Java Multi Meter) - JMM is data-acquisition software for digital multimeters equipped with a rs-232 port, such as the Metex 3850 and many others. The software is very simple to use and the control is straight forward.    Rate this link

    pH measurements

    • Two Opamp, Temperature Compensated PH Probe Amplifier - The signal from a pH probe has a typical resistance between 10 M?? and 1000 M??. Because of this high value, it is very important that the amplifier input currents be as small as possible. The LMC6001 with less than 25 fA input current is an ideal choice for this application. The theoretical output of the standard Ag/AgCI PH probe is 59.16 mV/pH at 25??C with 0V out at a pH of 7.00. This output is proportional to absolute temperature.    Rate this link
    • Convert your DMM to a pH meter - Even inexpensive pH meters can be relatively costly, and many of the inexpensive models have no output that you can readily connect to a computer interface. A simple solution to this problem is to attach a pH probe to a high-impedance input of an op amp and read the output with a digital voltmeter. Then, convert these readings to pH units using a calculator that can calculate the slope of a line. To calibrate the system, you can use pH standards. This article shows also a circuit for direct pH measurements on DMM.    Rate this link
    • pH Meter Circuit - A pH meter circuit with good circuit introduction document.    Rate this link

    Calibration

      Metal detectors

      There are many techniques for metel detection.One reasonably effective methid is called bfo (bifilar oscillator). During WWII metal detectors based on this principle were utilized my combat engineers of many armies to clear mines. The main idea is really quite simple: build two identical oscillators and adjust them to the same frequency. One of the oscillators uses the search coil while the second one incorporates a variable inductor. When both are operating at the same frequency, the output is zero. If the search coil moves near any metal, however, frequency of the first oscillator shifts and an audible tone is heard in the headphones. Theoretically, this principle works well. In reality, though, it has many weak points. Most home-brew devices of this kind will only detect comparatively large metal objects at a short distance.

    ESD

    • Investigate System-Level ESD Problems - information on ESD measurements    Rate this link
    • Simple ESD gun tests IC - you can use this simple ESD-test gun to test the effects of ESD through sensitive ICs, based in piezo-electric type kitchen gas lighter    Rate this link
    • Simple ESD Gun - You can use the simple ESD-test gun in Figure 1 to test the effects of ESD through sensitive ICs. A piezo-electric type kitchen gas lighter (the so-called ESD gun) is an excellent fast-static-charge generator. This gun can cause ESD through air and also through conducting and semiconducting materials. These devices also meet home-use safety standards.    Rate this link

    Electrical wiring testing

    • Cable Polarity Checker - Check cables for continuity and identify individual wires. LED indication for open circuit, shorts and correct or wrong polarity. Pushbutton operation with auto power off. This is a kit from Velleman. The kit manual has the circuit diagram of this circuit.    Rate this link
    • Ethernet 10BaseT simulator jig yields zero emissions - tool to evaluate emissions from Ethernet unshielded-twisted-pair (UTP) 10BaseT LAN-interface devices without contaminating the measured results with its own RF emissions, this cirucit generates 10BaseT equipment link test pulses without RF emissions so that 10BaseT equipment will keep sending data    Rate this link
    • Fleapower circuit detects short circuits - a short-circuit tester that supplies a low current to the device under test (DUT) and also uses voltages lower than 100 mV to prevent conduction of semiconductors    Rate this link
    • Measure open-circuited cables using a multimeter - You can use a multimeter with capacitance-measurement capability to measure the length of wire or cable to an open circuit. The capacitance of a pair of wires (or a wire to a shield) is directly proportional to the length of the wire. If you know the capacitance per foot of wire, then you can calculate how far it is to the open circuit.    Rate this link
    • Multicore Cable Tracer - unit is designed to help when establishing the connections in multicore cables or when identifying a large number of cables contained in a trunking or conduit, supports up to 63 channels up to 100 meters or more, Originally published in ETI, August 1995    Rate this link
    • RJ45 Network Cable Tester -    Rate this link
    • Test Plug - circuit which indicates whether your mains socket is wired correctly, for 220-240V systems, Originally published in Electronics in Action, March 1994    Rate this link

    Motor measurements

    This section give you idea how to measure properties related to different kind of motors, like for example car motor.

    • Idea for a car tachometer - A tachometer is simply a means of counting the engine revolutions of an automobile engine. In this suggested idea a NE555 timer is configured as a monostable or one shot. The 555 timer receives trigger pulses from the distributor points. Integration of the variable duty cycle by the meter movement produces a visible indication of the automobiles engine speed.    Rate this link

    Instrumention circuits

    Despite the availability of the digital field bus in several versions, industrial control systems continue to employ standard analog signals for transmitting data between the process and the control equipment. Industrial control systems continue to employ standard analog signals for transmitting data between the process and the control equipment. Robust, 4-to-20mA current-loop signals that are easily transmitted over several thousand feet, ?5 and ?10V signals are also very common in industrial systems.

    Differential inputs are used in many intrumentation circuits to get useable reasults on noisy enviroments. The differential inputs used in many automated systems are relatively insensitive to common-mode interference. Process transmitters in a chemical plant, for example, convert low-level temperature and pressure signals into robust, 4-to-20mA current-loop signals that are easily transmitted over several thousand feet. Thermocouples, strain gauges, and other popular sensors deliver low-level nonlinear signals that are sensitive to EMI. Before sending this information to a control system, therefore, a 4-to-20mA transmitter first linearizes and conditions the signal.

      General information on instrumentation

      • A System Designer's Guide to Isolation Devices - Isolation amplifiers provide galvanic isolation of the incoming signal to safeguard equipment and personnel, but the world of isolation, with its own terminology, technologies, and standards, is unfamiliar to many designers. This article reviews the basic concepts and technology of isolation devices and discusses the various options available to the system designer.    Rate this link
      • Beware of under- or overspecifying your next sensor - to choose the best photoelectric sensor for your application, you need to consider a number of criteria, including sensor configuration; environment; and the placement, nature, and speed of the target    Rate this link
      • Circuit makes simple FSK modulator - The need for a compact telemetry system poses a challenge for designing a small, light, low-component-count system. Commercial FSK (frequency-shift-keying) modulators are bulky and need many passive components. This circuit uses a single NOT gate (inverter), an On Semiconductor NL27WZ14 in a surface-mount package, to generate continuous FSK data from TTL-level signals. This circuit is designed to provide 2400 Hz / 1200 Hz FSK, but can be adapted for other frequencies up to an operating frequency of approximately 80 kHz.    Rate this link
      • ECEFast Technical Papers - A selection of temperature measurement information document platinum resistance temperature detectors, thermocouple fundamentals, noncontact thermometers and infrared systems. Also information on water characteristics measurement (conductivity, exygen, pH).    Rate this link
      • Fault protection saves multiplexers, switches, and downstream circuitry - For most situations in which fault conditions are possible, a fault-protected switch, multiplexer, or signal-line circuit protector offers a more practical approach to protection than discrete components.    Rate this link
      • Fight Corruption, perserve purity with ANALOG-SIGNAL isolation - analog-signal isolation can dramatically reduce noise and artifacts that corrupt sensitive measurements    Rate this link
      • Ground Loops and Their Cures - DC power systems used for instrument and loop power are subject to a number of possible ground loops. The method to solving ground loop problems is generally twofold. Remove any extra grounds so that there is one ground in the system. If there must be more than one ground, make sure to isolate each from other(s).    Rate this link
      • Improved amplifier drives differential-input ADCs - ADCs with differential inputs are becoming increasingly popular. This popularity isn't surprising, because differential inputs in the ADC offer several advantages: good common-mode noise rejection, a doubling of the available dynamic range without doubling the supply voltage, and cancellation of even-order harmonics that accrue with a single-ended input. This document shows shows two easy ways to create a differential-input differential-output instrumentation amplifier.    Rate this link
      • Inductive Proximity Switches Introduction - Inductive proximity switches are no-touch, non-interactive devices and sensitive to all metals. They consist of an oscillator, demodulator, level and switching amplifier.    Rate this link
      • Isolation techniques for high-resolution data-acquisition systems - You can implement isolation using optical, digital, and magnetic techniques.    Rate this link
      • Noise and disturbances in process control    Rate this link
      • Testing MEMS: Don't reinvent the wheel - but take little on faith - MEMS, which not only condition signals but also move, require consummate care in handling. But the manufacturers have figured out much of what you must know to successfully apply the devices. So be highly selective in choosing where to independently build up your private body of knowledge.    Rate this link
      • Understanding pH measurement - In the process world, pH is an important parameter to be measured and controlled. The pH of a solution indicates how acidic or basic (alkaline) it is. The formal mathematical definition of pH is the negative logarithm of hydrogen ion activity. A pH measurement loop is made up of three components, the pH sensor, which includes a measuring electrode, a reference electrode, and a temperature sensor; a preamplifier; and an analyzer or transmitter. A pH measurement loop is essentially a battery where the positive terminal is the measuring electrode and the negative terminal is the reference electrode. The measuring electrode, which is sensitive to the hydrogen ion, develops a potential (voltage) directly related to the hydrogen ion concentration of the solution. The reference electrode provides a stable potential against which the measuring electrode can be compared.    Rate this link
      • Understanding pH measurement - In the process world, pH is an important parameter to be measured and controlled. The pH of a solution indicates how acidic or basic (alkaline) it is. A pH measurement loop is essentially a battery where the positive terminal is the measuring electrode and the negative terminal is the reference electrode. The measuring electrode, which is sensitive to the hydrogen ion, develops a potential (voltage) directly related to the hydrogen ion concentration of the solution. The reference electrode provides a stable potential against which the measuring electrode can be compared.    Rate this link
      • Wiring For Trouble Free Signal Conditioning - Signal conditioning equipment for process signals has kept pace with modern technology, but many users never realize the full potential of the equipment because of poor installation and wiring practices. Such practices can degrade equipment performance from a small percentage of error to the point where the equipment is unusable.    Rate this link
      • Wiring For Trouble Free Signal Conditioning - Article published in European Process Engineer Magazine and In-Tech Magazine    Rate this link
      • Analog-Signal Data Acquisition in Industrial Automation Systems - Industrial control systems continue to employ standard analog signals for transmitting data between the process and the control equipment. Robust, 4-to-20mA current-loop signals that are easily transmitted over several thousand feet, ?5 and ?10V signals are also very common in industrial systems. This application note showcases Maxim's integrated Data Acquisition System (DAS) solutions. Maxim's DAS solutions save board space, power, and design time, while requiring minimal external components to convert standard industrial analog signals.    Rate this link
      • Technical Notes: Data Acquisition Techniques - Data acquisition and control systems need to get real-world signals into the computer. These signals come from a diverse range of instruments and sensors, and each type of signal needs special consideration. This page highlights points to think about, and should help you identify the most suitable interface for your measurements.    Rate this link

      Insrumentation amplifiers

      The symbol for an instrumentation amplifier may look similar to that of an opamp and may have a broadly similar function: differential amplification of its inputs, but it is an entirely different creature.An opamp is designed to be used in a negative feedback topology, both to achieve a uniform gain and to compensate for amplifier imperfections. An instrumentation amplifier, on the other hand, is used for open loop differential amplification, and has been designed with this in mind. It provides a smaller gain that is typically set by one external resistor. It is often used as a "pre-amp" for signals that are too low-level for an ordinary opamp buffer. Instrumentation amplifiers can be built out of individual opamps or you can use a single-chip implementation. Typical Instrumentation Amplifier monitors voltages from a few millivolts (DC or AC). It has several switch settings to allow you to select the best gain. It can be used with may measurement devices like A/D converter cards, programmable logic etc.

      Voltage to frequency conversion

      The output of a voltage-to-frequency converter (VFC) is a pulse train at a frequency precisely proportional to the applied input. The output of voltatage to frequency converter is typicallu a train of pulses or square wave. A voltage to frequency converter can be used as a building block in an analog--to-digital (A/D) conversion system. The conversion can be made by counting how many pulses enter the system at any given time frame. This conversion method has monotonicity under all supply and temperature conditions. Other good fact is that the signal from frequency-to-voltage converter can be considered as a serial bit stream, that can be easili transmitted over some suitable medium without loss or measuring errors. Typical application is that VFC is located near the signal source, and there can be quite long distance between the VFC and the pulse counter circuit. VFC is useful in telemetry, where a frequency can get through a link, such as a telephone line, while a direct current cannot. In some cases the conversion on other way is needed. Frequency to voltage converter (FFVC) converts frequency back to voltage. FVC is useful in tachometry, where a voltage proportional to speed is desired, and an alternating voltage can easily be generated proportional to speed.

      Other signal format converters

      • Circuit converts pulse width to voltage - This circuit converts pulse information to a clean dc voltage by the end of a single incoming pulse. In another technique, an RC filter can convert a PWM signal to an averaged dc voltage, but this method is slow in responding. This circuit works better and faster.    Rate this link

      Current loop interfacing

      4-20mA is an analog current loop protocol which has become the defacto U.S. standard for supplying DC power to a field transducer, and receiving a scaled return signal. DC power is typically supplied via an unregulated +10 to +30Vdc supply. Many industrial current-loop data acquisition systems operate on a 24V or 28V single supply. The field transducer controls the current flow, and is often referred to as a 2-wire "transmitter". You can easily receive 4-20 mA signals by passing the current through 100 ohm resistor, so you get 0.4-2V voltage over the resistor (if you select 250 ohm resistor, you will get 1V to 5V reading).

      • Circuit provides 4- to 20-mA loop for microcontrollers - The 4- to 20-mA current loop is ubiquitous in the world of controls in manufacturing plants. Discrete logic, microprocessors, and microcontrollers easily cover the digital portions of control schemes, such as limit switches, pushbuttons, and signal lights. Interfacing a 4- to 20-mA output to a rudimentary microcontroller can be problematic. A built-in A/D converter would be nice, but such a device is sometimes unavailable in the "economy" line of these processors. This circuit is a low-cost alternative that provides not only a 4- to 20-mA output, but also a digital feedback signal that indicates an open wire in the current loop. One output-port pin sets the current, and one input-port pin monitors an open circuit in the loop wire. The circuit derives its drive from a simple timer output in the microcontroller. The duty cycle of the timer determines the output current of the circuit.    Rate this link
      • 4 to 20 mA Analog Current Loop - introduction to current loop technology    Rate this link
      • A 4- to 20-mA loop needs no external power source - This simple circuit uses a low-current-drain MAX4073H amplifier to sense the current flowing through a 4- to 20-mA loop. The circuit senses the current through a 1O resistor with a fixed gain of 100 and uses no battery or dc power supply. The low current drain of the amplifier (0.5 mA) enables the circuit to tap its power from the 4- to 20-mA loop to power the amplifier chip.    Rate this link
      • Convertisseur 4-20 mA vers 0-10 Volts - conversion circuit between 0-20mA and 0-10V interface, text in French    Rate this link
      • Current Loop Interface - very simple circuit to interface a current loop sensor to an input which is designed for a voltage    Rate this link
      • Current Transmitter With Linear Voltage Transfer Rejects Ground Noise    Rate this link
      • Design formulas simplify classic V/I converter - shows a classic voltage-to-current (V/I) converter design suitable for many uses    Rate this link
      • Single Supply 4-20mA Current Loop Receiver - Many industrial current-loop data acquisition systems operate on a 24V or 28V single supply. You can make a single-supply current loop receiver with the RCV420 by using its 10V reference as a pseudo ground. The RCV420 will convert a 4-20mA loop current into a 0 to 5V output voltage with no external components required. The current loop can be sourcing or sinking and can be referenced to either the power-supply V+ or ground.    Rate this link
      • Small circuit forms programmable 4- to 20-mA transmitter - One of the key challenges in the design of 4- to 20-mA current transmitters is the voltage-to-current conversion stage. Conventional transmitters use multiple op amps and transistors to perform the conversion function. An improved Howland current pump can be cost-effective replacement for traditional circuits.    Rate this link
      • 4-20mA Loop Powered Temperature Sensor - This circuit uses an analog temperature sensor, op amp, transistor, and low dropout linear regulator to provide a 4-20mA output over a 3.75 to 28 volt compliance range. The low quiescent current of the devices used permits them to be powered by the loop with the only consequence being a slight offset error.    Rate this link
      • Developing Voltage from 4-20mA Current Loops - Many industrial automation transducers provide a 4 to 20mA current loop to communicate sensed values. These loops can be converted to a voltage for input to a wide variety of instrumentation devices including panel meters, data acquisition systems, and programmable controllers. By adding a precision resistor in series with the loop, a voltage is developed, which can then be inputted to the instrumentation.    Rate this link

      Sensor and measuring circuit ideas

      • Autoreferencing circuit nulls out sensor errors - This autoreferencing circuit nulls out the error of a sensor, such as a pressure transducer, at its reference level, for example, at ambient pressure. The circuit is an analog-digital-feedback control system that uses a digitally programmable potentiometer to provide the variability.    Rate this link
      • Data-acquisition circuit measures almost everything - Using a product developed for PC-motherboard environmental monitoring, you can configure a low-cost, general-purpose DAS (data-acquisition system)    Rate this link
      • Design approach simplifies signal conditioning - low cost and wide availability of 8-bit microcontrollers, such as Motorola's MC68HC11, allow you to easily incorporate intelligence in pressure-measurement systems, your main challenge is to signal-condition the sensor's small, differential bridge signal into a single-ended output voltage that the ?C's A/D converter    Rate this link
      • Dual comparators stabilize proximity detector - circuit transforms distance/capacitance into a proportional voltage    Rate this link
      • Home-brewed circuits tailor sensor outputs to specialized needs - use an untrimmed unit and customize it with a signal conditioner based on two or three op amps to get specially trimmed customized sensors    Rate this link
      • How to build instruments for hang gliders - pressure sensors and altimeter    Rate this link
      • Method offers fail-safe variable-reluctance sensors - Variable-reluctance sensors are preferred for industrial and automotive environments, because they sustain mechanical vibration and operation to 300?C. In most applications, they sense a steel target that is part of a rotating assembly. Because the unprocessed signal amplitude is proportional to target speed, a sensor whose signal-processing circuitry is designed for high speed ceases to function at some lower rate of rotation. Hall-effect sensors are preferable for speeds of several pulses per second, but they require the attachment of a magnet to the rotating assembly. Neither variable-reluctance nor Hall-effect sensors offers fail-safe detection of the processed signal in the event of failure in the cable or sensor. This circuit is a fail-safe variable-reluctance sensor for low- to medium-speed operation.    Rate this link
      • Network imitates thermocouples - Thermocouples find widespread use for temperature measurement in systems. During system design or testing, you must observe the system's response at different temperatures. However, it's inconvenient to heat a thermocouple every time you need to check a system's performance. This simple circuit allows you to set a number of voltages equal to the thermocouples' outputs at given temperatures.    Rate this link
      • Programmable Pressure Transducer    Rate this link
      • ?C uses simple tool for angle measurements - uses a 2V, 2250-Hz resolver as an angle sensor and provides up to 11 bits resolution for angle measurements    Rate this link
      • Current Loop Signal Conditioning: Practical Applications - This paper describes a variety of practical application circuits based on the current loop signal conditioning paradigm. Equations defining the circuit response are also provided. The constant current loop is a fundamental signal conditioning circuit concept that can be implemented in a variety of configurations for resistance-based transducers, such as strain gages and resistance temperature detectors. The circuit features signal conditioning outputs which are unaffected by extremely large variations in lead wire resistance, direct current frequency response, and inherent linearity with respect to resistance change. Sensitivity of this circuit is double that of a Wheatstone bridge circuit. Electrical output is zero for resistance change equals zero. The same excitation and output sense wires can serve multiple transducers. More application arrangements are possible with constant current loop signal conditioning than with the Wheatstone bridge.    Rate this link

    Medical measurements

    The electrical devices used in medical applications have special electrical safety regulations governing them because the potential dangers related to medical electronics. Any electricla device that is in direct contact with a human needs to be carefully designed to be safe to use. The reason for this that even very low voltages and currents can kill a human in certain conditions. Patient treatment areas such as medical and dental surgeries have particular requirements on electrical safety. In Europe the medical devices are covered by Medical Device Directive.It is quite nuts to make medical electronics that connect to a human being yourself unless you have anelectronics engineer and a medical doctor on your development team. Naivety may well get you and others hurt or killed.The basic idea of preserving patientsafety is to analyse your system and the other systems it interfaceswith to try and discover if there is a single fault that would presenta risk. Remember that even a single fault can and most likely willlead to a cascade of secondary failures. In practice this means thattwo safety 'isolation barriers' are necessary the second to protectagains the failure of the first. If it can happen it will happen and usually at the worst possible moment imaginable. Even assuming that you do get the isolation and sigle fault analysisright there is more (as ever)to bear in mind. A couple of millamps applied directly to someone's heart can kill them. Even very small currents (as low as a few uA) can be fatal under certain circumstances. Even a 9v battery can kill - takes a freak accident but, those freak accidents happen. Here are some general safety notices:

    • Under no circumstances connect your self to a mains powered device or to a phone line. A wall brick PSU or battery eliminator is absolutely not safe to use in this kind of application.
    • If you need to build your own in this field, it it must be purely battery powered.
    • To stay safe, even a battery powered device must never be connected to another mains powered device like a PC or a phone line.
    • The device you make must be current limited, even a small current applied through a small electrode may generate sufficent current ensity to cause burns and permanent nerve damage.

    Ultrasonics

    • UltraSonic Radar - This is a very interesting project with many practical applications in security and alarm systems for homes, shops and cars. It consists of a set of ultrasonic receiver and transmitter which operate at the same frequency. When something moves in the area covered by the circuit the circuit's fine balance is disturbed and the alarm is triggered. The circuit is very sensitive and can be adjusted to reset itself automatically or to stay triggered till it is reset manually after an alarm.    Rate this link
    • Sonar A Ultrasonics - sonar circuit, text in French, try to access it using    Rate this link
    • UltraSonic Radar - kit design from Smartkit    Rate this link
    • Ultrasonic Radar 9-12 VDC - movement detector circuit based on ultrasonics    Rate this link
    • UltraSonic Radar - This is a very interesting project with many practical applications in security and alarm systems for homes, shops and cars. It consists of a set of ultrasonic receiver and transmitter which operate at the same frequency. When something moves in the area covered by the circuit the circuit’s fine balance is disturbed and the alarm is triggered. The circuit is very sensitive and can be adjusted to reset itself automatically or to stay triggered till it is reset manually after an alarm.    Rate this link
    • UltraSonic Radar - This is a very interesting project with many practical applications in security and alarm systems for homes, shops and cars. It consists of a set of ultrasonic receiver and transmitter which operate at the same frequency.    Rate this link

    Motor rotation speed

    In some application there is need to measure the rotation speed of electrical motor (or sometimes some other motor). One very commonly used instrument for measuring the rotation speed of large moto is tacho generator. A tacho generator is, essentially, a small p.m. motor which is driven as a generator and which gives an output voltage which is accurately calibrated to be a defined measure of the rotation speed. A tacho generator gives a polarised output voltage: positive voltage for one direction and negative for reverse direction. 'Accurate' usually implies expensive. True tachogenerators are usually expensive - which puts off most potential users. The expense is because they are very accurately manufactured and calibrated. They are also low volume items. However for most hobby uses a small permanent magnet motor is perfectly adequate if you do not need accurate 'volts per rpm'.Other measurement method is to somehow get a know number of pulses per motor rotation and convert this to the information of rotation speed. The pulse information is usually converted to speed using frequency counter or tachometer circuit (a frequency to voltage converter). The pulses for the measuremetn can be usually got using optical sensor (some part of rotating part has somethign which is detectod by optical sensor), mechanical switch (for slow speed rotation) or using magnetic sensor (senses for example permanent magnet on some rotating part of the system).In some DC motor applications the rotation speeds is determined from the current taken by the motor (there are some spikes caused by moto communitation).

    • Idea for a car tachometer - A tachometer is simply a means of counting the engine revolutions of an automobile engine. In this suggested idea a NE555 timer is configured as a monostable or one shot. The 555 timer receives trigger pulses from the distributor points. Integration of the variable duty cycle by the meter movement produces a visible indication of the automobiles engine speed.    Rate this link
    • Tacho generator motor speed feedback - This page is a bit different from most in that it covers the use of tacho generators as feedback elements in motor control systems. This page covers tacho generator rectifier circuit for 4QD Pro-120 electric motor controllers.    Rate this link

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