Abstract: An inventive series-regulator-type direct current power supply includes a power transformer of which a secondary winding has a plurality of output terminals, a plurality of corresponding rectifier systems connected to the output terminals, an input voltage-detecting circuit detecting a predetermined level of an input alternating current voltage, and a switching circuit opening and closing a line of the rectifying circuit systems. The inventive system selectively switches the output terminals of the secondary winding of the power supply transformer and switches the rectifying circuit, in response to a change in the amplitude of a primary input voltage in order to suppress a variation of a direct current output voltage.