A power inverter, or inverter, is a power electronic device or circuitry that changes direct current (DC) to alternating current (AC). The inverter does not produce any power; the power is provided by the DC source. This document is collection of information on power inverter technology and gives links to DIY circuits – both good and bad design with comments.

Power inverters are often used to generate mains power from battery power in locations where mains power is not available. Inverters are an important part of conventional and renewable power systems such as solar, fuel cell, electrical energy storage systems, wind power plants and gas turbine power systems. An inverter is required to convert the DC electricity from photovoltaic generation or battery storage, to AC.

There are many different types of power inverters for different needs. The input voltage, output voltage and frequency, and overall power handling depend on the design of the specific device or circuitry. An ideal power inverter will produce same kind of power that is available from mains outlet, which typically means sine waveform 60 Hz 11-120V AC or 50 Hz 220-240V AC. That kind of power source can power all mains powered devices as long as enough power is available. Where power inverter devices substitute for standard line power, a sine wave output is desirable because many electrical products are engineered to work best with a sine wave AC power source. The standard electric utility provides a sine wave, typically with minor imperfections but sometimes with significant distortion. Most inverters are designed to generate single phase mains power like you get from normal mains outlet plug, but there are also power inverters that make three phase power.

Several different main inverter type are available:

• True sine wave inverters produce voltage equal to or better than the grid supply. They may incorporate a battery charger, which allows a generator or CHP unit to be used to charge up the batteries when natural charging conditions are poor.
• Modified sine wave inverters are less complicated but they may not successfully run some appliances, and they may produce a hum. These are not recommended for an average house with many electronic appliances and are not very common now.
• Grid-connected inverters allow for a connection to the grid, they may incorporate a battery charger and they can provide back-up power if the grid power fails.
• AC coupled inverters are designed for use for a micro-grid, i.e. a property with several houses or a remote rural settlement with no national grid connection.
• Also application specific inverters are also available that are designed for a specific uses like to be installed inside equipment. There are small cheap inverter circuit design that output power that is neither sine-wave or at normal mains frequency. They work with some types of loads and not with other.

Because producing sine-wave power is complicated and expensive, there are many simpler power inverter designs that produce output that is “close enough” for some devices to be operated from them. When using a power inverter that has different output than sine-wave AC at mains frequency, you need to be careful to make equipment selection so that your device and power inverter are compatible with each other. Many cheap power inverters output square wave waveform and there are variations to that towards sine-wave, those are compatible with quite many device but not all. In addition there are special inverters that output AC at different than mains frequency, which are useful only in some special applications. There are also devices that are called inverters by sellers, but output DC instead (should be called DC-DC converters) that work only with some devices like some switch mode mains power supplies. When selecting inverter type other an sine-wave, you need to be careful that inverter and intended load are compatible, because if they are are not then the combination does not work and is possible that the device and/or power inverter are damaged in trying.

Smaller popular consumer and commercial devices designed to mimic line power typically range from 150 to 3000 watts. The price of commercial inverter vary a lot depending on inverter power rating and design. Generally higher the power rating and more closely the output is sine-wave, more the inverter costs. Expect to pay about $30 to $50 for a standard 200-watt modified wave inverter and about $150 to $250 for a pure sine wave inverter. It depends on the application if the higher price is justified or not.

A typical power inverter device or circuit requires a relatively stable DC power source capable of supplying enough current for the intended power demands of the system. The input voltage depends on the specific circuits. The most common input voltage is 12 V DC for smaller consumer and commercial inverters that typically run from a rechargeable 12 V lead acid battery or automotive electrical outlet. There are also inverters designed to operate at higher voltages like 24V (trucks), 48V and higher up to few hundred volts. The runtime of an inverter powered by batteries is dependent on the battery power and the amount of power being drawn from the inverter at a given time.

Conversion technologies

A power inverter can be entirely electronic or may be a combination of mechanical effects (such as a rotary apparatus) and electronic circuitry. Static inverters do not use moving parts in the conversion process, and the typical electronic power inverters fall to this category.

An inverter built using electronics components can produce a square wave, modified sine wave, pulsed sine wave, pulse width modulated wave (PWM) or sine wave depending on circuit design. Common types of inverters produce square waves or quasi-square waves.

There are two basic designs for producing household plug-in voltage from a lower-voltage DC source:

• One method is to converts DC to AC at battery level and use a line-frequency transformer to create the output voltage. This is old know technology, but the downside is that the needed transformer is big and expensive.
• Many modern power inverter designs use a switching boost converter to produce a higher-voltage DC (up to needed mains voltage peak voltage) and then converts to AC with suitable circuitry (switching transistors).

The square wave inverter are very simple and easy to make but that is not suitable for sensitive Electric appliances, Modified sine wave inverters are gives output as close as to the sine wave but not pure as much we have receive from wall outlet. PWM (Pulse Width Modulation) signal based inverters are produce output as pure sine wave and it can be used for any electric appliance that meets the inverter output range.

https://www.researchgate.net/profile/Arab_Mustapha2/publication/316040601/figure/fig4/AS:668464880574468@1536385855252/Inverter-control-strategies-a-square-wave-b-modified-square-wave-c-analog-pulse.ppm

nverter control strategies, (a) square wave, (b) modified square wave, (c) analog pulse wave modulation (d) and (d) digital pulse wave modulation.

Electrical safety of inverters

When using power inverter, you expect that inverters and converters are safe, functional and compliant with relevant standards. Power inverters handle large amounts of power and output potentially lethal voltages, so you need to be careful with them.

Galvanic isolation (typically implemented with transformer) is used to protect against electric shock and to suppress electrical noise in sensitive devices.

Inverters produce heat and need ventilation. Many inverters have built-in fans, which in turn consume electricity to keep themselves cool.

Here is a list of some standards related to power inverters safety:

• Electrical safety testing: IEC/EN/UL 62109-1/-2, IEC/EN 62477-1, UL 1741, C22.2 No. 107.1, etc.
• Electromagnetic compatibility testing: IEC/EN 61000-6-1/-2/-3/-4, FCC, etc.
• Grid connection testing: IEC 61727, EN 50438, IEEE 1547.1, UL 1741SA, VDE 0126-1-1, VDE-AR-N4105, VDE 4110/4120/4130, CEI 0-16/21, G83/G59, AS 4777.2, etc.

Inverters should: be mounted above the floor and on a wall or shelf, have ventilation or cooled air flow, be protected from sunlight, be easy to access for emergency operation, have a switch or fuse to isolate the inverter from the electrical systems and be in close proximity to the batteries.

They must also be: protected from dust, protected from overheating, electrically isolated in case of an emergency, protected from damage by lightning and connected to the batteries with large cables (there may be substantial current flows, voltage drops).

Keep in mind that typically inverters are not capable of providing any sustained (more than a second) surge currents, so the rated output current is all that can be delivered. When faced with a short-circuit, the rated output current is all that can be delivered, but more than likely, the reduced line voltage due to the fault will cause the inverter to shut down. Limited output current capability can cause situation that downstream circuit breakers might not trip in case of short circuit like they do with normal mains power. It depends on your electrical system protection design if this is a problem or not.

Be warned on the connection of inverters and some electrical protection devices. The ac output of a utility-interactive inverter should not be connected to a GFCI or AFCI breaker as these devices are not designed to be backfed and will be damaged if backfed. These devices have terminals marked line and load and have not been identified/tested/listed for back feeding.

Use caution when connecting the inverter directly to your battery. Have the unit turned off when making connections. When making connection to the battery, keep in mind that you must have some form of protective device (fuse, breaker) between your battery and the inverter electronics. That protective device can be within the wiring from the battery to the inverter or integrated within the power inverter device. Use correct size breaker and correct size cable.

Keep in mind that in car power inverters can drain your battery quickly unless your engine is running and charging your battery. Most inverters have an audible alarm when they sense a lowered power source and some better ones have protection that shuts them off when it senses a low battery.

You need to consider the temperature. We need to consider the temperature rise to ensure the normal operation of an inverter and will not cause some damage to the operator. Second, it needs to be considered about fire safety performance of the power inverter at the time of production. After all, power inverter is electronic products. So there will be some possibility for failures when it works.

Regardless if the inverter is transformer-based or transformerless, some sort of isolation will be needed. Both for user safety and reliability of electronics. The voltage after conversion is up to 120V or 240V, and this voltage will cause some harm to the operator. To minimize the possibility of damage to the controller in the event of a fault condition, there must be some type of isolation between the power and logic voltage domains.

General Safety Precautions and Installation Tips:

Do not use the inverter near flammable materials. Do not place the inverter in areas such as battery compartments where fumes or gases may accumulate. The inverter should not be installed in the engine compartment, due to possible water/oil/acid contamination, and excessive heat under the hood, as well as potential danger from gasoline fumes and the spark that an inverter can occasionally produce. It’s best to run battery cables to a dry, cool inverter mounting location.

Keep the inverter dry. Do not expose it to rain or moisture. DO NOT operate the inverter if you, the inverter, the device being operated, or any other surfaces that may come in contact with any power source are wet. Water and many other liquids can conduct electricity which may lead to serious injury or death.

In order to properly disperse heat generated while the inverter is in operation, keep it well ventilated. While in use, maintain several inches of clearance around the top and sides of the inverter.

Square wave inverter circuits

Square wave inverters are the simplest quite widely useful power inverter circuits, so there are very many such designs posted on-line. Here are some inverter circuit examples with comments to take a look. A transformer based square wave inverter is basically quite simple circuit. You basically need a center tapped mains transformer that matches the voltage conversion you need (typically from 12V to 230V) and a control circuit that switches power to the either side of the coil on at the right rate (mains frequency 50 or 60 Hz). There are several things that needs to be handled carefully on the circuit: the power on time needs to be same on both sides of transformer (difference causes DC current to transformer that saturates it’s core), the switching needs to be done efficiently (not to much power loss), both sides switch transformer must not be on at the same time, you need to handle the inductive issues of transformer etc… While the basics are simple, a good implementation needs to take account many issues.

The circuits here are posted here just as an example with my comments how I see them with quick circuit overlook – I have not personally tested those circuit and there is no guarantee that they will work as intended.

WARNING: Power inverter circuits are potentially dangerous. Any mistake in the circuit or building them can potentially cause electrocute someone, damage expensive components, damage device being powered, damage the power source, start a fire or cause electromagnetic interference.

This is one of the simplest waveforms an inverter design can produce. It is is best suited to low-sensitivity applications such as lighting and heating. The downside is that square wave output can produce “humming” when connected to audio equipment and is generally unsuitable for sensitive electronics.

Inverter – circuit design Part1- Covering basic function

Inverter – circuit design part2- Covering selection of power semiconductor

Inverter – circuit design Part3 – Covering spikes generated

How NOT to make a Modified Square Wave Inverter

How to make inverter 12V To 220V using TL494 | square wave inverter & 200 Watt

Here is an example of one not very ideal inverter design seen at electronics.stackexchange.com. WARNING While the circuit looks plausible, it may need more development to be a good inverter. The point where conduction switches from Q1 to Q6 is critical. There is no active turnoff for either transistor, and these things tend to turn off slower than they turn on, resulting in a transient short circuit when both are conducting. Often we see a series inductor in the transformer supply to handle this, or more complicated transistor driving.
Unfortunately, a good power inverter is anything but trivial. The above points are just a couple of things that can trip you up, there may be more. FETs generally make for a better design these days than bipolar transistors because with suitable FET you can have much less power losses than with this old 2N3055 transistor. The transistor based astable oscillator does not guarantee 50% drive for both transformer coil halves.

Here is a design from gadgetronicx.com. Similarly we have designed a Square wave Inverter circuit that is capable of driving 220v device and handles 60 Watt load. This circuit is powered from a DC battery and turn it into AC voltage to power some loads such as lights and other AC elements within the limit of 60 Watts. The circuit is designed to be used with 12v Battery.. The working principle of this circuit starts with IC 555 which is wired as Astable Multivibrator that drives a H-bridge driver circuit. The advantage of this H-bridge based design idea is that you you can use a transformer with just single coil, but the disadvantage is that you need more transistors. The potential issues on this design seem to be that using 555 like this does not guarantee accurate 50% drive for both current directions to transformer and this H-drive design has a risk that on the output of the low and high side transformers can be briefly turned both on (increasing power loss).

Engineersgarage.com has also a contributed inverter circuit example of quite similar 555 time based design using center tapped transformer. You can compare this design to the design above (still has many similar design issues). When the circuit is powered up by the 12V DC then the 555-timer starts generating a 50 Hz square wave at its output pin (pin 3 of the 555 IC). The generated square wave is of 12V but the AC appliances require 220 V for their operation. So this 12 V square wave needs to be boost up to a 220 V waveform. For stepping up, first of all, two MOSFET stages are used as switches. One stage is directly connected to the 555 output and another one is connected through a logic inverter designed with the help of switching transistor BC547. A center tap transformer is used to step up the square wave from 12 V to 220 V. At the primary of the transformer, the centre tap wire is directly connected to the 12 V DC source and the remaining two are connected with the drain of both the MOSFET stages.

Eleccircuit has a very basic power inverter design using FETs and 4047:
This is AC Inverter. Input 12VDC from car battery to output 220V AC 50Hz or 60Hz at Square wave signal.The main part is CD4047 (or IC 4047 Series) and output driver FETs. The transformer is 10V-CT-10V, Primary : 220V Secondary. and current 3A up for power output than 100W. The Integrated Circuit (IC) CD4047 is wired as an astable multivibrator. CD4047 is a CMOS Low Power monostable/astable multivibrator that is often used for converting DC current signal to AC signal because it can offer 50% duty cycle of astable output and both direct and inverted polarity outputs (easy direct drive to both side transistors).

Electrocircuits.org has another CD4047 Inverter Circuit Diagram. It is a simple Push Pull CD4047 Inverter circuit diagram which uses only few components. It can output 100W to 150W AC power at 10V to 12V battery and centre taped transformer (12V-0V-12V, 5A). This circuit has a good description of operation: The Integrated Circuit (IC) CD4047 is wired as an astable multivibrator. The two oscillating outputs are out of phase, the frequencies of the oscillations are determined by the 50K variable resistor. The power MOSFETs (IRF3205) amplifies each half of the alternating voltage produced by the oscillator to the output power transformer which steps up the voltage from 12V DC to 230V AC. The documentation says that you can increase the output power to up to 1000W or more by adding more power MOSFETs in parallel, greater power transformer, and larger battery power bank.

Inverter Circuit Using Arduino is a project post to tell how to construct a very simple inverter using Arduino Uno. The advantage of using arduino is we can customize the output parameters, and mainly we can upgrade this square wave inverter to pure sine wave inverter by just writing a new code without any hardware changes (Program only given for Square wave). If you are intermediate in arduino projects, you can easily tweak the hardware and code to add more features like low battery warning, automatic voltage correction, and quick automatic mains changeover and even you may add LCD display to show voltage readings and on-going status. Two MOSFETs are employed which can handle around 300 Watt power with heat sink mounted. If you want to more power you may choose more powerful MOSFET. Keep in mind that the transformer’s voltage and current parameter also decides maximum output power.

More links:
https://electrocircuits.org/simple-power-inverter-circuits-100w-to-1000w/
https://electrocircuits.org/calculating-runtime-of-power-inverter-or-ups/