PURPOSE: To control an output voltage at a certain value by turning ON and OFF a switching circuit through detecting the double end voltage of a capacitor.

CONSTITUTION: A DC power supply using two power sources jointly is composed of a power switch 12, capacitors 13∼14, a switching circuit 15, a bridge rectifier circuit 16 and a detection and control circuit 17 to take out an output voltage from respective both ends of capacitors 13∼14. Said detection and control circuit 17 detects an output voltage to decide whether said voltage exceeds a certain level. When the output voltage does not exceed said level, the switching circuit 15 is set to ON and to OFF when the voltage is higher than said level. Thus, when the switching circuit 15 is turned OFF, the DC power supply operates by means of a full-wave rectifier circuit so that a given voltage can be obtained from respective both ends of capacitors 13∼14. Said switching circuit 15 also operates to become a voltage doubler rectifier circuit automatically when it is turned ON.