PHASE SHIFTING THE TRIAC
The triac, like the SCR, must be phase shifted if complete voltage control is to be obtained. There are several methods that can be used to phase shift a triac, but only one will be covered in this unit. In this example, a diac will be used to phase shift the triac, Figure 56–6. In this circuit, resistors R1 and R2 are connected in series with capacitor C1. Resistor R1 is a variable resistor and is used to control the charge time of capacitor C1. Resistor R2 is used to limit current if resistor R1 should be adjusted to 0 ohms. Assume the diac connected in series with the gate of the triac will turn on when capacitor C1 has been charged to 15 volts. When the diac turns on, capacitor C1 will discharge through the gate of the triac. This permits the triac to fire or turn on.
Once the triac has fired, there will be a voltage drop of about 1 volt across MT2 and MT1. The triac will remain turned on until the AC voltage drops to a low enough value to permit the triac to turn off. Since the phase shift circuit is connected in parallel with the triac, once the triac turns on capacitor C1 cannot begin charging again until the triac turns off at the end of the AC cycle. The diac, being a bidirectional device, will permit a positive or negative pulse to trigger the gate of the triac.
Notice that the pulse applied to the gate is con- trolled by the charging of capacitor C1 and not the amplitude of voltage. If the correct values are chosen, the triac can be fired at any point in the AC cycle applied to it. The triac can now control the AC voltage from 0 to the full voltage of the circuit.
A common example of this type of triac circuit is the light dimmer control used in many homes.