What is a triac diode?

Answer 1

A TRIAC is a solid state AC switch. It has three terminals, MT1, MT2, and Gate. Load current passes through MT1 and MT2. In order for it to turn on, the Gate must be greater than MT1, in the same direction as MT2 is to MT1. One way to turn it on is to connect a resistor from MT2 to Gate. To turn it off, connect MT1 to gate.

Once the TRIAC is turned on in a line cycle, it stays on until the end of that line cycle. This behavior can be used to create a dimmer by delaying the point in the cycle where the Gate is triggered.

Note that the gate trigger voltage is not symmetrical. This can lead to a DC bias in the output. One way to resolve this is with a DIAC, which is like a TRIAC, but with the Gate missing - it will conduct in both directions at the same point - used in series with the Gate, it will help balance the TRIAC's symmetry.