PURPOSE: To prevent the destruction of a circuit element by a rush current by surely limiting the rush current even when AC power supply is restarted after the power supply is once stopped for a short time.

CONSTITUTION: This power unit is provided with a rush current limiting resistor 24 connected between a diode bridge circuit 23 and smoothing capacitor 25 and the photothyristor 30a of a photothyrsitor/coupler connected in parallel with the resistor 24. This power unit is also provided with a charge/discharge circuit 38 which is connected to a DC power source Vo through a PNP transistor 37 and has a relatively small electric-discharge time constant, photocoupler 36 which is connected to an AC power source 21 and controls the turning on/off of the transistor 37, and comparator 44 which controls the turning on/off of the light emitting diode 30b of the photothyristor/coupler by detecting the voltage across the terminals of the capacitor 40 of the circuit 38.