Modes of operation of a TRIAC:
There are four different operating modes of TRIAC:
1) MT2 and gate are positive with respect to terminal MT1(Mode1) : Here terminal MT2 is positive with respect to terminal MT1 current flows through path P1-N1-P2-N2.The two junctions P1-N1 and P2-N2 are forward biased whereas junction N1-P2 is blocked. The TRIAC is now said to be positively biased. A positive gate with respect to terminal MT1 forward biases the junction P2-N2 and the breakdown occurs as in a normal SCR.
2) MT2 is positive but gate is negative with respect to terminal MT1(Mode2): Though the flow path of current remains the same as in mode 1 but now junction P2-N3 is forward biased and current carriers injected into P2 turn on the TRIAC.
3) MT2 and gate are negative with respect to terminal MT1(Mode4) : When terminal MT2 is negative with respect to terminal MT1, the current flow path is P2-N1-P1-N4. The two junctions P2-N1 and P1 – N4 are forward biased whereas junction N1-P1 is blocked. The TRIAC is now said to be negatively biased. A negative gate with respect to terminal MT1 injects current carriers by forward biasing junction P2-N3 and thus initiates the conduction.
4) MT2 is negative but gate is positive with respect to terminal MT1(Mode3) : Though the flow path of current remains the same as in mode 3 but now junction P2-N2 is forward biased, current carriers are injected and therefore, the TRIAC is turned on.