DIACs

A DIAC, short for Diode for Alternating Current, is a bidirectional trigger diode used primarily to gate TRIACs in alternating-current circuits. It is a two-terminal device with a four-layer PNPN structure and is designed to conduct in either direction only after a characteristic breakdown (breakover) voltage is reached. Once the breakover voltage is exceeded, the DIAC conducts a sharp, short current pulse and then returns to a high-impedance state when the current falls below its holding value.

In typical use, a DIAC is part of a triggering network for a TRIAC in phase-control applications such as light dimmers and motor speed controllers. A resistor and capacitor form an RC network that charges during each half-cycle. When the capacitor voltage reaches the DIAC’s breakover threshold, the DIAC conducts and delivers a gate pulse to the TRIAC, triggering it. As the current in the circuit drops below the holding current, the DIAC ceases conduction, allowing the cycle to reset for the next half-cycle. The symmetry of the DIAC means the triggering behavior is similar for positive and negative half-cycles.

Key characteristics include a breakover voltage that varies by type (commonly around 28–32 V for standard DB3 types, with other variants offering higher voltages), a relatively high dynamic impedance before triggering, and a rapid, short conduction once triggered. DIACs are designed for low-current triggering and are not used as rectifiers or for continuous conduction.

Common types include DB3, DB4, and DB5, offered in small epoxy or glass packages. They are widely used in mains AC circuits to provide clean, predictable triggering of TRIACs while maintaining simple, cost-effective design.