The Diac Features & Applications

Features of a Diac

The diac is a two-terminal, semiconductor, bi-directional switching device. It can conduct in both directions. The diac closely resembles a PNP transistor without an external base terminal.

Operation of a Diac

The diac does not conduct except for a small leakage current) until the breakover voltage V_S is reached, typically 20 to 40 volts. At that point the diac goes into avalanche conduction, furthermore, at that point the device exhibits a negative resistance characteristic and the voltage across the disc snaps back, typically about 5 volts, creating a breakover current I_S in the order of 50 t 200 μA enough to trigger a triac or SCR.

Diac Applications

The negative resistance characteristic of the diac makes it valuable for very simple relaxation oscillators and pulse generators; however, its key application is in conjunction with a triac to produce AC phase-control circuits helpful for motor-speed control, light dimming and other AC power-control applications.

Also Read: The Junction Field Effect Transistors (JFET)