Diac

- a diac is a diode AC switch which acts as a bidirectional semiconductor switch

A diac is a full-wave or bi-directional semiconductor switch that can be turned on in both forward and reverse polarities. Indeed the name diac means diode AC switch. The diac is widely used to assist even triggering of a triac when used in AC switches. Diacs are mainly used in dimmer applications and also in starter circuits for florescent lamps.

Circuit symbol for the diac

Typically the diac is placed in series with the gate of a triac. Diacs are often used in conjunction with triacs because these devices do not fire symmetrically as a result of slight differences between the two halves of the device. This results in harmonics being generated, and the less symmetrical the device fires, the greater the level of harmonics produced. It is generally undesirable to have high levels of harmonics in a power system.

To help in overcoming this problem, a diac is often placed in series with the gate. This device helps make the switching more even for both halves of the cycle. This results from the fact that the diac switching characteristic is far more even than that of the triac. Since the diac prevents any gate current flowing until the trigger voltage has reached a certain voltage in either direction, this makes the firing point of the triac more even in both directions.

Structure
The diac can be fabricated as either a two layer or a five layer structure. In the three layer structure the switching occurs when the junction that is reverse biased experiences reverse breakdown. A five layer diac structure is also available. This does not act in quite the same manner, although it produces an I-V curve that is very similar to the three layer diac. It can be considered as two break-over diodes connected back to back.

The structure of a diac