A TRIAC (triode for alternating current) is a silicon semiconductor component with three terminals or legs that allow current to flow in either direction when triggered. TRIACs are a development of a similar family member the thyristor. However, unlike the thyristor which is unidirectional, the TRIAC is able to control current over both halves of the sine wave making TRIACs bidirectional.

How does a TRIAC work?

In basic terms, a TRIAC is a low to medium power semi-latching solid-state switch. The device acts like two standard thyristors inversely connected in parallel or back to back. To activate the component a voltage needs to be applied to the gate terminal. Once the gate current is triggered a TRIAC will continue to conduct even if the supply to the gate is removed.

A TRIAC is a 4 layer silicon component that is PNPN in the positive direction and NPNP in the negative direction. The devices come in panel mount, surface mount and through-hole mounting options in various package types to suit a wide variety of electronics applications and circuits.

On a TRIAC symbol, there are three terminals. These terminals are,

  • MT1 Anode 1 (Main Terminal 1)
  • MT2 Anode 2 (Main Terminal 2)
  • Gate Terminal

What are TRIACs used for?

TRIACs are widely used for switching and power control of AC systems. TRIACS are particularly useful in applications such as lighting dimmers as they enable the utilisation of each half-cycle of the AC wave. The solid-state device can be used in a circuit as a simple on-off device in a wide range of electronic applications. Some of the more common areas are;

  • Temperature control
  • Liquid level control
  • Dimmers for domestic lighting
  • Small motor control
  • Speed control for electric fans


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