Patent ID: 7495356

Claim:
A dual power supply system comprising: a first sensor having a detecting terminal connected to a first power supply and an output terminal to send a first power signal; a second sensor having a detecting terminal connected to a second power supply and an output terminal to send a second power signal; a selection switch having an output terminal to send a selection signal; a control circuit having three input terminals connected to the output terminals of the first sensor, the second sensor, and the selection switch respectively, and an output terminal to send a control signal according to the first power signal, the second power signal, and the selection signal; and a relay switch having two throws connected to the first and the second power supplies respectively, a pole connected to an electronic device, and a control terminal connected to the output terminal of the control circuit for selectively coupling the pole to one of the throws thereby coupling the first or second power supply to the electronic device according to the control signal; wherein upon a condition that one of the first and second power supplies does not work, the control signal is controlled by the first and second power signals and causes the relay switch to couple the other one of the first and second power supplies to the electronic device; upon a condition that the first and second power supplies both work normally, the control signal is controlled by the selection signal and causes the relay switch to couple a selected power supply of the first and second power supplies to the electronic device; the control circuit includes a NOT gate, an OR gate, and an AND gate, two input terminals of the OR gate are connected to the output terminal the second sensor via the NOT gate and the output terminal of the selection switch respectively, the output terminal of the OR gate and the output terminal of the first sensor are connected to two input terminals of the AND gate respectively, an output terminal of the AND gate acting as the output terminal of the control circuit is connected to the relay switch.