In principle, A TRIAC (triode for alternating current) represents an inverse-parallel connection of two thyristors. This allows for switching of alternating current, while a single thyristor can switch in only one direction and thus acts - in its on-state - like a diode.

A TRIAC has a control terminal G (gate) and two main terminals MT1 and MT2, where MT2 is usually directly connected to the casing. In order to be able to use a single control terminal for both thyristors, TRIACs contain two firing, or helper, thyristor junctions, so that it can be flipped into the low-impedance state by using a positive and negative control pulse.

See also