Diodes

A one-way valve for current.

A diode lets current flow in one direction only. It's made from two types of semiconductor material joined together; current can cross the junction one way easily, but is blocked (up to a limit) the other way.

• The "anode" is the positive-in side (the flat end of the triangle); the "cathode" is marked by the bar, often shown as a stripe on the physical part.
• Used to protect circuits from reversed batteries, to convert AC to DC (rectification), and to route power safely.