THE TRIAC USED AS AN AC SWITCH
The triac is a member of the thyristor family and has only two states of operation, on or off. When the triac is turned off it will drop the full applied voltage of the circuit at 0 amps of current flow. When the triac is turned on, it has a voltage drop of about 1 volt and circuit current must be limited by the load connected to the circuit. The triac has become very popular in industrial circuits as an AC switch. Because it is a thyristor, it has the ability to control a large amount of voltage and current. There are no contacts to wear out, it is sealed against dirt and moisture, and it can operate thousands of times per second. The triac is used as the output of many solid-state relays that will be covered later.
THE TRIAC USED FOR AC VOLTAGE CONTROL
The triac can be used to control an AC voltage, Figure 56–5. If a variable resistor is connected in series with the gate, the point at which the gate current will reach a high enough level 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 brightness. If an attempt is made to adjust the lamp to operate at less than half bright- ness, it will turn off.