Patent ID: 7697254

Claim:
A power supply protecting circuit configured to protect an electronic device comprising: a normally open switch, a power terminal of the electronic device connected to a power terminal of a power supply, the other power terminal of the electronic device connected to the other power terminal of the power supply via the normally open switch; a voltage dropping circuit; a first rectifier; a relay having a coil and switch, the power terminal of the electronic device connected to the other power terminal of the electronic device via the voltage dropping circuit, the first rectifier, and the coil of the relay in turn, the switch of the relay connected in parallel to the normally open switch; a second rectifier; a first variable resistor, the anode of the second rectifier connected to the power terminal of the electronic device, the cathode of the second rectifier connected to the other power terminal of the electronic device via the first variable resistor; a filter circuit; a first voltage regulator; a photocoupler having a first light-emitting diode (LED) and a phototransistor; a second variable resistor; and a thyristor, the adjusting terminal of the first variable resistor connected to the cathode of the first voltage regulator, the anode of the first voltage regulator connected to the anode of the first LED of the photocoupler, the cathode of the first LED connected to the other power terminal of the electronic device, the collector of the phototransistor of the photocoupler connected to the cathode of the first rectifier, the emitter of the phototransistor connected to the other power terminal of the electronic device via the second variable resistor, the adjusting terminal of the second variable resistor connected to the gate of the thyristor, the anode of the thyristor connected to the cathode of the first rectifier, the cathode of the thyristor connected to the cathode of the other power terminal of the electronic device.