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.
The diac does not conduct except for a small leakage current) until the breakover voltage VS 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 IS in the order of 50 t 200 μA enough to trigger a triac or SCR.
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)
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