(i) During each positive half-cycle of the a.c., the triac is off for a certain interval, called firing angle α (measured in degrees) and then it is triggered on and conducts current through the load for the remaining portion of the positive half-cycle, called the conduction angle C . The value of firing angle
(and hence C)can be changed by adjusting the variable resistance R1. If R1 is increased, the capacitor will charge more slowly, resulting in the triac being triggered later in the cycle i.e. firing angle is increased while conduction angle C is decreased. As a result, smaller a.c. power is fed to the load. Reverse happens if the resistance R1 is decreased.