The Triac Used for AC Voltage Control
The triac can be used to control AC voltage (Figure 63– 7). If a variable resistor is connected in series with the gate, the point at which the gate current is high enough to fire the triac can be adjusted. The resistance can be adjusted to permit the triac to fire when the AC waveform reaches its peak value. This will cause half of the AC voltage to be dropped across the triac and half to be dropped across the load.
If the gate resistance is reduced, the amount of gate current needed to fire the triac will be obtained before the AC waveform reaches its peak value. This means that less voltage will be dropped across the triac and more voltage will be dropped across the load. This circuit permits the triac to control only one half of the AC waveform applied to it. If a lamp is used as the load, it can be controlled from half brightness to full bright- ness. If an attempt is made to adjust the lamp to operate at less than half brightness, it will turn off.