Diode

A diode is the electrical equivalent of a 'check valve', current can flow in only one direction. Diodes effectively turn AC into DC, even if that DC is very lumpy. If you combine a diode with a capacitor of the right size you can make your DC a little better quality.

A set of four diodes can be used to form a 'full wave bridge', this will take both 'halves' of the AC and will produce a DC current that contains as much ''power' as the original AC current. (the single diode solution throws away half of the available power).