The earliest AC power inverters were electro-mechanical devices. Direct current would flow down one end of a circuit with an electromagnet. As soon as the current hit the magnet, the magnet would activate. This would pull a wire attached to a spring arm, forcing the wire to contact the circuit. This would change the flow of the current to the other side of the circuit, cutting power from the electromagnet. As soon as the magnet released, the spring would snap the wire back, allowing the current to flow on the other side of the circuit, once again activating the magnet. These old inverters were known for making a buzzing sound.
Modern inverters use oscillator circuits to accomplish the same process. They're made with transistors or semiconductors, so there's no longer the need for a spring arm flipping back and forth to alternate the current.
It's not quite as simple as that, however. Alternating current forms a sine wave. The output of an inverter is a very square wave, not like the smooth, round wave of a perfect sine. Some devices are inherently sensitive to the signal produced by an AC wave. Typically, these are devices that receive or broadcast some kind of signal, such as audio or video equipment, navigation devices or sensitive scientific equipment. You can see or hear the square waveform on a television as lines on the screen or a steady buzz or hum.
Cleaning up the sine wave requires a series of filters, inductors and capacitors. Inexpensive inverters have little or no filtering. The alternating current they produce has a very square wave, which is fine if you just want to make coffee or run something with a simple electric motor. If you need a smoother sine wave, you'll need an inverter with better filtering. Of course, better filtering also costs a little more. Inverters can get extremely expensive, even costing thousands of dollars, that is, if you're looking for an inverter with a smooth sine. The good news: Given a large enough budget, you can purchase an AC power inverter that produces virtually perfect AC sines. In fact, some high-end DC to AC inverters can make sine waves that are even smoother than the AC power supplied to your house.
In the next section, we'll learn how to pick the right inverter for the job.