PURPOSE: To form a power source circuit of space saving and parts saving by making common the power source circuit to supply a voltage to a main circuit and the power source to supply the voltage to a back-up circuit.
CONSTITUTION: At the time of power on, a boosting circuit action control circuit 2 operates an oscillating circuit 7. By a coil 8, a diode 9, a transistor 10 and a smoothing capacitor 11, the output voltage of a nickel-cadmium battery 1 is boosted up to about 5.3V. The voltage, since it is fed back to the oscillating circuit 7 by a feedback circuit 12, comes to be an approximately constant value. At the time of power off, since the boosting circuit action control circuit 2 does not operate the oscillating circuit 7, the output voltage of the nickel- cadmium battery is outputted to an A point regardless of the voltage value. When a voltage outputted to the A point is higher than 5V, it is stepped down up to 5V, and when the voltage is lower than 5V, the value, in which the voltage drops for the saturated voltage between the collector emitters of a transistor 13, is outputted to a B point and supplied to a back-up circuit.