In April 2009 I wrote an article on the basics of changing alternating current (AC) into direct current (DC) called “How to Change Alternating Current into Direct Current and Supply DC Electronic Circuitry With Power. . This kind of transformation (Known as rectification.) is extremely common; and is the first thing that happens (Although in much more detail with far more complicated circuits than presented therein.) to the AC Mains supply in some form or another as soon as it enters most modern electronic equipment.

Less common, though just as useful, is the inverter circuit: This circuit (The very basics of which are shown here in this article.) changes straight DC electricity (From a rechargeable lead-acid battery usually. – Because it requires quite a bit of power.) into AC electricity; similar to the AC mains power that runs your computer and pretty much everything else electrical in your house. This type of circuitry appears in units such as a UPS or Uninterruptible Power Source that you may have connected onto your computer’s power lead.

The main components of an inverter are:

1) A DC current source. – Usually a rechargeable lead-acid battery.

2) A timebase: This is a piece of circuitry known as an oscillator. It produces a very specific waveform that is an engineering pattern for the final output’s AC waveform.

3) A mixer:  (see further down.)This is the stage where up to the full-force of the DC current source’s power is mixed with the waveform produced by the timebase to produce a very-precisely-tuned low-voltage sine wave, capable of driving the transformer unit, which is the last stage, to produce a pseudo-AC mains output.

4) A transformer. (see above, 3).)

The oscillator drives the mixer, a piece of circuitry regulated by purpose-built voltage regulators that mirror the control input’s actions precisely; outputting the required sinusoidal waveform. In my representation of the mixer circuit I’ve used a pair of power-NPN/PNP transistors wired as a complimentary pair to achieve this. That is a very basic circuit which will be given to a huge distortion factor on the output. In reality, heavy-duty power-CMOS voltage and current regulator chips using power-MOSFET technologies are commonly used to give a perfect stabilised output. – I’ve used the power-NPN/PNP-transistor-pair in the example-circuit for the sake of simplicity.

The AC output from this circuit is then applied to the coils of a step-up transformer which increases the voltage to the AC mains voltage.

Some of the cheaper inverters, although able to supply the power-input needs of a computer and peripherals, are nevertheless unable to supply the perfect AC waveform required to run a fluorescent or low-energy lamp. – For example, of my 2 UPS units, only one; the more expensive of the two, is able to run a Thorn 2D sidelight and a PL-lamp desk light properly. The cheaper of the two UPS’s just causes the lights to flicker dimly. This is probably due to the quality of components used as well as the type of circuitry present in the unit.

I don’t intend to go into this further and subsequently go off-topic. I hope the above gives you some idea of how DC electricity is changed into its AC counterpart.