Abstract: In a power supply system, an inductance L is connected between a rail at a voltage V.sub.1 and a rail at earth through a switch S. When the switch is opened, a current i.sub.2 flowing from the inductance L is directed to a third rail through a diode D, voltage on the third rail rising to V.sub.2. The system may function as a dc to dc converter with an output voltage between the V.sub.1 rail and V.sub.2 rail equal to V.sub.2 -V.sub.1. The system may alternatively be used in, for example, a reluctance motor drive, in which case a further inductance with associated switch and diode is included in the circuit in inverse configuration to prevent excessive voltage rise on the third rail.