DIAC

DIAC

DIAC can be expanded to Diode Alternating Current. It is a full-wave or bi-directional switch that can be turned on in both forward and reverse polarities.

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. The three layer version of the device is the more common and can have a break-over voltage of around 30 V. Operation is almost symmetrical owing to the symmetry of the device.

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 version. It can be considered as two break-over diodes connected back to back.

V-I characteristics

DIAC operates in both forward and positive polarities.And DIAC is essentially a diode that conducts after a ‘break-over’ voltage is exceeded. Until the device exceeds the break-over voltage(Vbo), the resistance of it is very high. When it exceeds the break-over voltage, it enters the region of negative dynamic resistance. As a result, a decrease in the voltage drop across the diode takes place with increasing voltage and a sharp increase in the level of current conducted by the device takes place.

And it remains in the conduction state until the value of the current is equal or above to the holding current; designated by the letter I_H. And below the holding current, it reverts back again to non-conducting state as a result of high resistance between two terminals of it. And it is bi-directional, its operation occurs on both positive and negative cycle of an alternating current.

References:

• http://www.radio-electronics.com/info/data/semicond/diac/diac.php
• https://en.wikipedia.org/wiki/DIAC