TRIAC is a bi-directional solid state device and conducts in both halves of ac supply. During positive half cycle MT2 becomes positive w.r.t MT, and gate becomes positive w.r.t MTr TRIAC conducts when gate pulse is injected at a firing angle a. The current follows TRIAC in the voltage wave shape from angle α to π. At π, the zero crossing of the supply voltage, the TRIAC is switched OFF due to line commutation.