Power supplies for electrical and electronic equipment

Claim 1

In a supply path from an electrical supply source to a convertor power supply, said source generating a supply voltage which may be subject to transient excursions, a protective circuit comprising: a transistor having an emitter-collector current path therethrough which constitutes a low resistance current link between the supply source and the converter power supply, the magnitude of the current flowing being controlled by the base current of the transistor; a current link of higher resistance connected in parallel with the low resistance current link; a path having impedance connected in series between the supply source and the parallel-connected links; a bias source for controlling the base current of the transistor; and a diverting circuit connected between the bias source, the transistor base and the said impedance path, said circuit including forwardly biased diode means connected between the transistor base and the said impedance path to change the transistor base current if the sum of the voltages across the base-emitter path and across the impedance path begins to increase in value towards the forward bias of said diode means, thereby to increase the effective resistance of the low resistance current link with increasing current from the supply source and eventually to open circuit the low resistance link by switching off the transistor if the supply voltage rises to a predetermined value during a transient voltage excursion.