Patent ID: 7701089

Claim:
A power supply circuit comprising: a first power supply for supplying a first power voltage to a load; a first constant voltage circuit part for changing the first power voltage input from the first power supply to a first constant voltage and outputting the first constant voltage to the load; a first switch part for controlling the input of the first constant voltage to the load; a second power supply for supplying a second power voltage to the load; a second constant voltage circuit part for changing the second power voltage input from the second power supply to a second constant voltage and outputting the second constant voltage to the load; a second switch part for controlling the input of the second constant voltage to the load; and a control circuit part for detecting at least one of the input voltage and the output constant voltage of the first and second constant voltage circuit parts and controlling the first and second switch parts according to the result of the detection, wherein the first power voltage is a DC voltage, wherein the first constant voltage circuit part is configured to change the DC first power voltage input from the first power supply to the first constant voltage; wherein the second power voltage is a DC voltage, wherein the second constant voltage circuit part is configured to change the DC second power voltage input from the second power supply to the second constant voltage, wherein the first switch part is switched off when the first power voltage is less than a first predetermined voltage value, and the second switch part is switched off when the second power voltage is less than a third predetermined voltage value, wherein the control circuit part switches off the first switch part when the input voltage of the first constant voltage circuit part is less than said first predetermined voltage, wherein the control circuit part switches off the first switch pail when the output voltage of the first constant voltage circuit is less than the second predetermined voltage, where the second predetermined voltage is less than the rated output voltage of the first constant voltage circuit, and wherein the second switch part is switched off when the input voltage of the first constant voltage circuit part is equal to or greater than the first predetermined voltage and the output voltage of the first constant voltage circuit part is equal to or greater than a second predetermined voltage.