PURPOSE: To automatically discriminate the change in an AC power supply frequency with simple constitution, by dividing the AC power supply voltage with a capacitor and a resistor and using the divided voltage as a compared input voltage for a comparator after rectifying and smoothing.

CONSTITUTION: Between lines 1 and 2 connected to an AC power supply, a series circuit consisting of a capacitor 3 and a resistor 4 is connected. A voltage divided with the capacitor 3 and the resistor 4 is rectified with a diode 7, smoothed at a capacitor 8 and given to a comparator 5 as a compared input voltage. Further, the AC voltage between the lines 1 and 2 is rectified at a diode 13, smoothed at a capacitor 14, divided with voltage dividing resistors 9 and 10 and given to the comparator 5 as a reference voltage. Since the AC impedance of the capacitor 3 is correlated with an AC impedance frequency, the compared input voltage is changed according to the fluctuation of the AC frequency. Then, the reference input voltage is set to a value intermediate between the compared input voltage at, e.g., 60Hz and 50Hz. Thus, the question whether the power frequency is 50Hz or 60Hz can be discriminated.