The DC power supply disclosed herein is adapted to be energized from AC supply mains and incorporates a DC to DC switching inverter provided with an energy storage or filter capacitor for supplying an input voltage to the inverter. The capacitor is charged from the AC supply mains through a rectifier. A semiconductor switching device is interposed between the AC supply mains and the capacitor. A sensing circuit actuates the switching device to disconnect the capacitor from the supply mains when the voltage applied to the capacitor exceeds a pre-selected level.