POWER CIRCUIT

PROBLEM TO BE SOLVED: To surely switch between normal operation and backup operation and to surely execute the backup operation, in a power circuit equipped with a backup function. SOLUTION: The power circuit equipped with a VCC circuit 1 which supplies electric power to a microcomputer 10 in the ignition switch ON state and a VDD circuit 2 which supplies electric power to the microcomputer 10 in the ignition switch OFF state is equipped with a switch BUSW for power source switching which is connected to the power line between the VCC circuit 1 and microcomputer 10 and connects and disconnects the power line, a voltage drop detecting circuit 3 which monitors the output voltage of the VCC circuit 1 and turns off the switch BUSW for power source switching when the voltage of the VCC circuit 1 drops and turns ON when the voltage rises, and a voltage control means which raises the voltage of the VCC circuit 2 when the voltage of VCC circuit 1 falls and lowers the voltage of the VCC circuit 2 when the voltage of the VCC circuit 1 rises in response to the detection output of the voltage drop detecting circuit 3.