How to Turn D/C Into A/C

Switch the polarity of the D/C source periodically, either through mechanical or electrical means. Imagine a spinning battery. If it stopped every half turn when the poles lined up with contacts, the D/C from the battery would look like A/C to the circuit. This is similar to what happens in automobile alternators. The D/C from the batteries go to two brushes of the constantly spinning shaft of the alternator. The rings that the output brushes contact are split so that half the time one brush is positive and the rest of the time it is negative. Totally electric alternators create this effect with a transistor-controlled oscillator.

Correct the problems with the first stage of an alternator. The current goes from totally positive to totally negative with no in-between state. Pictorially, it looks like a square wave, while generator-produced A/C changes smoothly between the extremes of positive and negative, only spending a brief moment at each extreme before smoothly transitioning to the opposite extreme. Square-wave A/C can actually damage some A/C devices. Knocking off the corners and making the A/C smoother is done in the second stage with a combination of capacitors and coils. These components work together to store electrons during surges and release them during periods of no change, thereby smoothing out the A/C.

Fix the final problem that an alternator might have: the voltage level. Often the input D/C voltage is different from the required voltage level of the A/C output. This last stage of an alternator is a single component -- a transformer. Transformers consist of two coils wound close to each other. The changing magnetic field of one coil induces current into the other coil. The ratio of the number of windings of each coil determines the voltage change between input and output.