Relay control circuit for domestic electrical appliance - uses switching transistor supplied with control signal dependent on supply voltage amplitude

摘要

The control circuit uses a switching transistor (T) with its collector/emitter path in series with the relay winding (RR) and bridged by a free-running diode (D). The transistor (T) is controlled by a pulse signal which is varied in dependence on the amplitude of the supply voltage (Ub). The control signal for the switching transistor (T) is provided by a microprocessor (MC) including an A/D converter allowing the supply voltage (Ub) to be converted into a pulse width modulated signal (PWM), fed to the base of the switching transistor (T) via a pre-resistance (RV). ADVANTAGE - Reliable operation with widely varying supply voltage.