PURPOSE: To prevent an AC 100V input power supply from being damaged by detecting whether input voltage is AC 100V or AC 200V and then switching the input relay.

CONSTITUTION: For example, when an NPN transistor Q1 is turned on, voltage at a tantalum capacitor C1 is discharged from a resistor R5 before it reaches a total voltage value of a Zener voltage of a Zener diode Z2 and a voltage between a base and an emitter of an NPN transistor Q2. Therefore, no base current flows to the NPN transistor Q2 and the NPN transistors Q2 stays off. If the NPN transistor Q2 is not turned on, no voltage is applied to a coil of a 2-contact terminal relay RL1 and contact terminals rl1a and rl1b are not closed. Therefore, even if AC 200V is connected to the AC input terminals 1 and 2, no AC 200V is output to AC output terminals 3 and 4 and no AC 200V is applied to an AC 100V input power supply 5.