Diac, triac and thyristor applications

Thyristors and their relatives are semiconductor switches designed to control substantial AC and DC power. A thyristor, also called a silicon-controlled rectifier (SCR), conducts in one direction once a small current is applied to its gate, and then keeps conducting until the current through it falls to zero. This latching behaviour makes it well suited to switching power at the AC line, where the current naturally passes through zero twice each cycle, allowing the device to turn off.

A triac behaves like two thyristors in opposite directions combined in one device, so it can conduct on both halves of an AC waveform. This makes it the natural choice for controlling AC loads such as lamps and heaters. The diac is a small two-terminal device that does not conduct until the voltage across it reaches a breakover point, after which it switches on sharply; it is commonly used to trigger a triac reliably and symmetrically.

The most important application is phase control, the basis of light dimmers and motor speed controllers. By delaying the moment within each AC half-cycle at which the triac or thyristor is triggered, the circuit lets through only part of each cycle, varying the average power delivered to the load. Thyristors are also arranged into single- and three-phase bridge rectifiers, including fully controlled bridges, to convert AC to adjustable DC. The unijunction transistor (UJT) is another device historically used to generate the timed trigger pulses.

Frequently asked questions

How does a thyristor (SCR) turn off?

An SCR latches on once triggered and only turns off when the current through it falls to zero. On an AC supply this happens naturally at each zero crossing.

What is the role of a diac with a triac?

A diac does not conduct until its breakover voltage is reached, then switches sharply, providing a clean, symmetrical trigger pulse to fire the triac at a consistent point in each half-cycle.

What is phase control?

Phase control varies power by delaying when the triac or thyristor is triggered within each AC half-cycle, passing only part of the waveform. It is the principle behind light dimmers and motor speed controls.