How is alternating current changed to direct current?
Alternating current, as suggested by its name, flows in both directions. A diode or rectifier allows current flow in only one direction. If you connect a single diode in line with an AC supply, then current can flow during each half cycle of the AC that is in the same polarity as the diode. This is called half-wave rectification and produces DC (since all of the voltage is of one polarity and the current flow, if any, is in a single direction), but it fails to use half of the available input. A full wave rectifier or a full wave bridge provides a means by which both halves of the AC waveform can be used. Essentially it is a little trick that turns half of the AC upside down so that the polarity agrees with the other half and produces a much better quality DC. But even with a full wave rectifier, you need a substantial amount of filtering (capacitors) to fill in the places where the AC input goes to zero. Edit: How does a rectifier actually work? Now we’re talking semiconductor physics.
