A triac can be used to control the average a.c. power to a load by passing a portion of positive and negative half-cycles of input
a.c. This is achieved by changing the conduction angle through the load. Fig. 21.9 shows the basic triac phase control circuit. This circuit uses a capacitor C and variable resistance R1 to shift
the phase angle of the gate signal. Because of this phase shift, the
gate voltage lags the line voltage by an angle between 0° and 90°. By adjusting the variable resistance R1, the conduction angle through the load can be changed. Thus any portion of positive and negative half-cycles of the a.c. can be passed through the load. This action of triac permits it to be used as a controlled
bidirectional switch.