ElectronicDesignNet.com

The DIAC, or DIode for Alternating Current, is a bidirectional trigger diode that conducts current only after its breakdown voltage has been exceeded momentarily. When this occurs, the resistance of the diode abruptly decreases, leading to a sharp decrease in the voltage drop across the diode and, usually, a sharp increase in current flow through the diode. The diode remains "in conduction" until the current flow through it drops below a value characteristic for the device, called the holding current. Below this value, the diode switches back to its high-resistance (non-conducting) state.

The behavior is typically the same for both directions of current flow. Most DIACs have a breakdown voltage around 30 V. In this way, their behavior is somewhat similar to (but much more precisely controlled and taking place at lower voltages than) a neon lamp.

DIACs are a form of thyristor. They are typically used for triggering TRIACs, another form of thyristor. Because of this common usage, many TRIACs contain a built-in DIAC in series with the TRIAC's "gate" terminal.

DIACs are also called symmetrical trigger diodes due to the symmetry of their characteristic curve. Because DIACs are bidirectional devices, their terminals are not labeled as anode or cathode but as A1 and A2 or MT1 ("Main Terminal") and MT2.

The SIDAC is a less common electrically equivalent device, the difference in naming being determined by the manufacturer. In general, SIDACs have higher breakover voltages and current handling.