(i) During each positive half-cycle of the a.c., the triac is off for a certain interval, called firing angle (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 capaci- tor 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.