Patent ID: 7215517

Claim:
A constant-voltage switching power supply, comprising: a smoother, which converts an AC voltage supplied from an AC power source into a first DC voltage; a switching transistor, which converts the first DC voltage into a first cyclic pulse signal; a transformer, which voltage-transforms the first cyclic pulse signal to obtain a second cyclic pulse signal, the transformer comprising a first primary coil to which the first cyclic pulse signal is applied, a second primary coil at which a third cyclic pulse signal corresponding to the first cyclic pulse signal is induced; an overvoltage monitor, which monitors a potential of the third cyclic pulse signal to judge whether a potential of the first cyclic pulse signal is a predetermined value or more, the overvoltage monitor comprising a Zener diode through which a backward current flows in a case where the potential of the third cyclic pulse signal reaches a Zener voltage of the Zener diode; and a deactivator, which turns off the switching transistor when the backward current flows through the Zener diode; wherein: the overvoltage monitor comprises a transistor having a grounded base, an emitter to which the Zener diode is electrically connected, and a collector which is electrically connected to the deactivator; and the deactivator turns off the switching transistor when the backward current turns on the transistor.