A TRIAC is a bidirectional, three-electrode AC switch that allows electrons to flow in either direction. It is the equivalent of two SCRs connected in a reverse-parallel arrangement with gates connected to each other. A TRIAC is triggered into conduction in both directions by a gate signal like that of an SCR.

TRIACs are a subset of thyristors (analogous to a relay in that a small voltage and current can control a much larger voltage and current) and are related to silicon controlled rectifiers (SCRs). TRIACs differ from SCRs in that they allow current flow in both directions, whereas an SCR can only conduct current in a single direction.

A triac is a structure which can be seen as two transistors intertwined. The transistors act as switches. When the triac is fired (pulse on the gate) one transistor is switched on, which in turn switches the second transistor on. Then the second transistor keeps the first one switched on, even after the pulse was stopped.

A TRIAC acts much like two SCRs connected back-to-back for bidirectional (AC) operation. TRIAC controls are more often seen in simple, low-power circuits than complex, high-power circuits. In large power control circuits, multiple SCRs tend to be favored.

TRIAC dimmer changes the light brightness by controlling the TRIAC switch according to the input of the control signal (dimming knob or smart home control system). Specifically, TRIAC dimmers chop the AC voltage into many small segments, each of which is half a sine wave.