PURPOSE: To stabilize constant-voltage characteristics by bridge-rectifier operating a rectifier circuit when AC input voltage is higher than a reference voltage and operating voltage doubler full-wave rectifier-operating the rectifier circuit when AC input voltage is lower than the reference voltage.
CONSTITUTION: A smoothing circuit to which smoothing capacitors 20, 22 are connected in a series is connected at an output terminal for a single-phase bridge rectifier circuit 18a, while a TRIAC 24 is connected between a node across the capacitors and one AC input terminal for a rectifier circuit 18. Said smoothing capacitors 20, 22 are controlled selectively by turning the TRIAC 24 ON (OFF). An automatic switching circuit changeover-controlling the rectifier circuit 18 is constituted of a diode rectifier 26 for AC input voltage, a capacitor 38 for smoothing, a Zener diode 34 for reference voltage, operational amplifiers 48, 84, etc. at that time. Accordingly, when AC input voltage is of a 115V system, an output from the operational amplifier 84 reaches a high level and the TRIAC 24 is brought to an ON state, and the rectifier circuit 18 conducts voltage doubler full-wave rectifier.