An interesting and useful configuration for the control of power loads is shown in the figure. In this circuit we use a low voltage transformer to charge the gate circuit when its secondary is shorted. Thus, when the switch is open, the transformer represents a high impedance for the circuit and therefore the voltage applied to the gate does not reach the value necessary for the trip. When the switch is closed, the transformer behaves as a low impedance allowing the passage of the alternating voltage that triggers the TRIAC. The proper functioning of this circuit depends basically on the characteristics of the transformer and the value of resistor R1. Note that the switch will work with very low voltages and currents and will be isolated from the power network that supplies the power circuit.
