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.
Circuit action. The operation of triac phase control circuit is as under :
(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.
(ii) During each negative half-cycle of the a.c., a similar action occurs except that now current in the load is in the opposite direction.
Fig. 21.10 shows the waveforms of the line voltage and gate voltage. Only the shaded portion of the positive and negative half-cycles pass through the load. We can change the phase angle of gate voltage by adjusting the variable resistance R1. Thus a triac can control the a.c. power fed to a load. This control of a.c. power is useful in many applications such as industrial heating, lighting etc.