YOUR GUIDE TO DC to AC POWER INVERTERS

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Most standard appliances are designed to accept only AC (alternating current) voltages because that's how electricity is supplied from the grid. To run an electronic device from a DC (direct current) source you obviously need to transform DC into AC. A device that converts electricity from DC form to AC form using electronic circuits is known in power industry as inverter. Note that the same term is used in digital electronics for a circuit that switches the logic level of a signal. To avoid confusion, the device we are talking about is often referred to as power inverter. Its typical application is to convert a 12V battery voltage into conventional residential voltage. Inverters are used in a wide variety of applications from small car adapters to large grid-tie systems that can supply electricity to an entire home.

Note that unlike generators, an inverter does not have any engine, does not burn fuel, and does not emit toxic fumes. There are three basic types of dc-ac converters depending on their output waveform: square wave, modified sinewave, and pure sine wave (see the diagram below). The square wave is the simplest and cheapest type, but nowadays it is practically not used commercially because of low power quality. The modified sine wave topologies (which are actually modified squares) provide square pulses with some dead spots between positive and negative half-cycles. They are suitable for many electronic loads, although their THD is about 25%. Three types of inverter waveformsPriced in the range of $.05-$0.10 per watt, models that employ such a technique are the most popular low-cost inverters on the consumer market today, particularly among car inverters. If you are buying a device whose description does not state that it is a pure sinusoidal type, then most likely it is a square wave or a modified one. Note that output waveform in conventional modified sinewave DC-AC circuits has only two levels: zero or peak voltage of both polarities. By adding another voltage level, a designer can reduce THD typically from 25% to 6.5%. Periodically connecting the output to a specific voltage level with proper timing can produce a multiple-level waveform which is closer to sinusoidal than conventional modified sinewave.
A true sinewave inverter produces output with the lowest total harmonic distortion (normally below 3%). It is the most expensive type of AC source, which is used when there is a need for clean sinusoidal output for some sensitive devices such as medical equipment, laser printers, stereos, etc.

There is a number of topologies used in the power inverter circuits. Cheap circuits suitable primarily for hobbyists projects may use just a push-pull converter with a step-up transformer. This is an example of a single stage design. Most commercially manufactured models use a multi-stage concept. With such technique, first a switching pre-regulator steps up a voltage from an input source to another DC voltage corresponding to the peak value of the desired sinusoid. The output stage then generates an AC. This stage usually uses a full-bridge or half-bridge configuration. If a half-bridge is used, the DC-link voltage should be more than twice the peak of the generated output. Input to output galvanic isolation is provided by either a high-frequency transformer in the SMPS pre-regulator, or by a large low-frequency output transformer. If a low-frequency transformer is used, the sinusoidal voltage is generated on its primary side and transformed to the secondary side. The output can be controlled either in square-wave mode or in pulse width-modulated (PWM) mode. Sine wave circuits operate in PWM mode, in which the output voltage and frequency are controlled by varying the duty cycle of the high frequency pulses. Chopped signal then passes through a low pass LC-filter to supply a clean sinusoidal output. Although such approach is more expensive, it is usually employed in the backup devices for home or business use, which require high quality of AC power.

Power inverters for cars

often come with a jack that can be plugged into the cigarette lighter. Note, however, that the cigarette lighters are protected by a fuse rated typically between 15 and 20 A. This is usually enough to run your laptop or other portable electronics. However, if you need to use an electrical device that consumes more than 12V×(fuse amperage)×0.95 volt-amps, where 0.95 is a typical efficiency of an auto inverter, your unit has to be connected directly to the car battery. That's why the car models above 200 VA usually don't even provide a plug for cigarette lighters, and instead include jumper cables that can be hooked up to the battery terminals.

Note that commercial inverters are usually rated in volt-amps (VA). The real power (watts) they can supply will depend on the power factor of your load: Watts=VA*PF.

Here you will find a theory for designing a power inverter, as well as electronic schematic diagrams for hobbyists, students and engineers.

FIND DC-AC INVERTER BY SPEC








DESIGN THEORY, TEXTBOOKS, APP NOTES, REVIEWS

SCHEMATICS
FOR HOBBYISTS




Inverter- theory of operation and analysis (a power electronics textbook for students)

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Design and analysis of a sine wave PWM AC converter circuit with low harmonic distortion

Switch mode DC-AC converters basics

Pure sinewave inverter design tutorial
2kVA 12VDC to 120VAC sinewave converter for power backup- schematics, PCB and codes

Voltage Inversion Circuits (reversed engineered from commercial products)

A hobbyist 12V to 115/230 VAC inverter for cars or home

Simple 12VDC to 120VAC push-pull inverter circuit with bipolar transistors

A 13W converter for CFL: circuit diagram and PCB

Recommended power electronics books




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