Abstract: A simplified dual input power supply of the type which automatically responds to either a low level AC supply voltage (e.g. 115 volts) or a high level AC supply voltage (e.g. 230 volts) to provide a single level DC output voltage (e.g. 300 volts). The power supply switches between a voltage doubler operation and a full-wave rectifier operation dependent upon the level of the AC supply voltage. For a low level AC supply voltage an energizing circuit is completed to a relay, or the like, which switches in the voltage doubler circuit configuration. For a high level AC supply voltage, a Zener diode circuit conducts and breaks the energizing circuit to de-energize the relay so as to switch in the full-wave rectifier configuration. A protective circuit is provided to prevent the power supply from switching back to its low input voltage mode less the input voltage drops below a predetermined level for a predetermined time interval.