Diac(Bidirectional Thyristor Diode)
The cross sectional view of a diac showing all its layers and junction is shown above figure.Diac is a two electrode device,it can conduct in either direction.Terminals are denoted by T1 and T2.word diac stands for ‘diode for ac‘.The four layer are pn pn and pn pn’.
Principle of Operation
When T1 is positive with respect to T2,the layers p-n-p-n starts conducting.This happens when voltage of T1 is more than break over voltage VB01.Once the conduction starts,the current through the diac becomes very large and has to be limited by the external resistance in the circuit.When T2 is positive with respect to T1 the layers p-n-p-n’ conducts.This happens when the voltage of T2 exceeds break over voltage VBO2.In both the cases the current during blocking regions are small leakage currents.The behaviour in both the directions are is similar because doping level is same in all the layers in two directions.The break over voltage for commonly used diac is about 30V.
When T1 is positive and voltage is less than VBO1 only a small leakage current flows through the device.When voltage exceeds VBO1 it starts conducting and current becomes large.As the current increases,the voltage drop across diac decreases.Thus it exhibit negative resistance characteristics.The characteristics in the reverse direction(when T2 is positive)lies in the third quadrant and is exactly similar to that in the first quadrant.The breakover voltage VBO1 and VBO2 are exactly equal in magnitude.In both the cases,the device exhibits negative resistance behaviour during conduction region.
Diac is mainly used for triggering triacs.A triac requires either positive or negative gate pulse for turning it on.This is provided by a Diac.Matched Diac-Triac combinations are manufactured for various control circuits.