TV automatic mains voltage changeover switch - has transistor circuit which detects lower mains voltage and switches in voltage doubler

摘要

The automatic mains voltage supply changeover switch, for television receivers, monitors whether 110V or 220V is present at the supply input. It automatically switches in a voltage doubler if the 110V is detected. The mains input is rectified and applied via a voltage divider (R2,R3) to the base of a transistor (T) and via a resistor (R4) to the gate of a thyristor (TH). If 220V are present, the transistor is on and its collector, coupled to the thyristor gate, prevents the thyristor turning on. The mains input passes through to the output. If 110V are present, the transistor is off and the thyristor is on. The thyristor and a series capacitor (c1) now act as voltage doubler.