PURPOSE: To protect an AC 100V input power supply from being destroyed without applying AC 200V to the AC 100V input power supply even if the AC 200V is connected by mistake to the AC 100V input apparatus by detecting whether the input voltage is AC 100V or AC 200V to switch an input relay.

CONSTITUTION: When an AC 200V is connected to an AC 200V input apparatus, a voltage V2 reduced by rectification exceeds the value of voltage which is a sum of a zener voltage of zener diode Z1 and a voltage between the base and emitter of PNP transistor(Tr) Q1, a base current flows into the TrQ1 to turn it on. Then, voltage VC2 which a capacitor C2 has been charged with is discharged by a resistor R3 and no base current flows in a TRQ2 and the TRQ2 is left turned off. Then, no voltage is applied to the coil RL1 of a two- contact terminal relay, so that contact point terminal rl1 is not closed. Thus, no AC 200V is applied to the AC 100V input power supply.