Q. Explain the working principle of a TRIAC.

"Triac" is an abbreviation for three terminal ac switch. 'Tri'-indicates that the device has three terminals and 'ac' indicates that the device controls alternating current or can conduct in either direction. The triac is a device that is triggered into conduction when a low energy signal is applied to its gate terminal. The triac conducts in either direction when turned on. Either a positive or negative gate signal triggers it into conduction. Thus the triac is a three terminal, four layer bidirectional semiconductor device that controls ac power.

Because of its bidirectional conduction property, the triac is widely used in the field of power electronics for control purposes.

Operation:  Though the triac can be turned on without any gate current provided the supply voltage becomes equal to the breakover voltage of the triac but the normal way to turn on the triac is by applying a proper gate current. The larger the gate current, the smaller the supply voltage at which the triac is turned on..Triac can conduct current irrespective of the voltage polarity of terminals 

     MT1 and MT2 with respect to each other and that of gate and terminal MT2.