PURPOSE: To prevent an AC 100V input power supply from being damaged even if an AC 200V is connected to an AC 100V input device by mistake by detecting whether an input voltage is AC 100V or AC 200V and then switching an input relay.

CONSTITUTION: When an AC 100V is closed, a PNP transistor Q2 is turned on, contact terminals rl1a and rl1b are closed and the AC 100V is closed to an AC 100V input power supply 5. When an AC 200V is closed, a PNP transistor Q2 is turned on and a tantalum capacitor starts charging. When a 200V rectification drop voltage V2 increases, the PNP transistor Q1 is turned on. On the other hand, the transistor Q2 is turned off, thus enabling a charging voltage Vc2 of the tantalum capacitor C2 to be discharged, preventing contact terminals rl1a and rl1b from being closed, and preventing the AC 200V from being closed to the AC 100V input power supply 5.