DIAC

This thyristor is a two terminal device without a gate electrode, as shown in following fig. The Diac is used as a bi-directional trigger diode. Current can flow either way when enough voltage is supplied for break-over. Typically, the firing potential is about 30 V, in either direction.

The Diac is in its off state until the voltage across MT1 and MT 2 exceeds the break-over voltage. In power control circuits a Diac can be used for more effective control of the turn-on point for the gate electrode of a Triac or the SCR.