PURPOSE: To prevent damage of an AC 100 V input power source even if AC 200 V is erroneously connected to an AC 100 V input unit by detecting whether an input voltage is AC 100 V or AC 200 V, and opening or closing a relay of the input.

CONSTITUTION: When an AC 100 V is applied at a time t5, a base current flows to a P-N-P transistor Q2 by a rectified and step-down voltage V2, the transistor Q2 is turned ON, and a tantalum capacitor C2 starts to be charged. However, If the voltage V2 exceeds a Zener voltage VZ1, a current flows from the emitter of a P-N-P transistor Q1 to the base, and the transistor Q1 is turned ON. Then, no current flows from the emitter to the base in the transistor Q2, and the transistor Q2 is turned OFF. Thus, a charging voltage VC2 is discharged before it reaches an operating lowest voltage value of a 2-contact terminal relay RL1, the contact rl1 of the relay is not closed, and an AC 200 V is not applied to an AC 100 V input power source 5.