Thyristors, TRIACs and DIACs

Thyristors are a family of four-layer (PNPN) semiconductor switches that, once triggered into conduction, latch on and continue conducting until the current through them falls below a holding level. The basic member is the silicon-controlled rectifier (SCR), a three-terminal device with an anode, a cathode and a gate. A short pulse of gate current turns the SCR on; thereafter it conducts in one direction like a diode and only turns off when the forward current drops near zero, which happens naturally at the end of each half cycle of alternating current.

The TRIAC (triode for alternating current) extends this behaviour to both polarities. Functionally it behaves like two SCRs connected in inverse parallel, so it can conduct in either direction and switch both halves of an AC waveform. This makes the TRIAC a compact device for controlling alternating-current loads such as lamps, heaters and small motors. By varying the point in each half cycle at which the device is triggered, a technique called phase-angle control, the average power delivered to the load can be adjusted, as in light dimmers and speed controllers.

The DIAC (diode for alternating current) is a two-terminal bidirectional trigger device. It remains non-conducting until the voltage across it exceeds a breakover level, at which point it suddenly conducts and its voltage drops. DIACs are commonly placed in the gate circuit of a TRIAC to produce a clean, well-defined trigger pulse and ensure symmetrical firing in both directions. Related protective components, the transient-voltage suppressor and similar clamping devices, guard circuits against voltage spikes but operate on a different, non-latching principle.

Frequently asked questions

What is the difference between an SCR and a TRIAC?

An SCR conducts in only one direction, controlling a single polarity. A TRIAC conducts in both directions, making it suitable for switching full AC waveforms.

How does a thyristor turn off?

It does not turn off via the gate. Conduction stops only when the current through it falls below the holding current, which occurs naturally as an AC waveform crosses zero.

Why use a DIAC with a TRIAC?

The DIAC provides a sharp breakover trigger that fires the TRIAC symmetrically in both half cycles, improving dimming smoothness and reducing flicker.