Abstract: A circuit for reducing the voltage of a d-c power supply with one grounded terminal comprises a chopping transistor in an ungrounded supply lead, periodically turned on and off by an electronic switch, and an L/C network downstream of that transistor along with a flywheel diode. The switch lies in series with an ancillary voltage generator, having a terminal voltage substantially lower than the supply voltage, in a grounded loop which is reactively coupled to the transistor base via a capacitor or a transformer. A control signal for opening and closing the switch may be obtained from a square-wave generator connected across the transistor output and driven by the stepped-down supply voltage.