GB1109464A - Improvements in or relating to overheating protection circuits - Google Patents

Abstract

1,109,464. Protective arrangements. STANDARD TELEPHONES & CABLES Ltd. 10 Sept., 1965 [16 Sept., 1964], No. 37810/64. Heading H2K. In a protective arrangement for an electrical machine comprising a positive temperature coefficient thermistor PTC in thermal contact with the machine and a semi-conductor device which, on increase of the resistance of the thermistor, changes its state to control means for interrupting the supply to the machine, a latching circuit is connected to the semiconductor device to prevent its automatic return to its original state when the resistance of the thermistor falls, but which after the interruption and on resumption of the supply permits the semi-conductor device to assume the state appropriate to the resistance of thermistor. In Fig. 2 the semi-conductor device is a transistor T 1 which is normally non-conducting, but conducts when the resistance of the thermistor PTC increases and causes a further transistor T 2 to stop conducting. A relay A is de-energized and opens its contacts to interrupt the supply to the protected machine and at contacts A 3 removes a short-circut from a capacitor C 1 . The capacitor C 1 becomes charged and the potential across it is sufficient to latch the transistor T 1 in the conducting state. When the supply is interrupted the capacitor discharges and on resumption of the supply the resistance of the thermistor determines which transistor conducts. In the arrangement of Fig. 3 (not shown) both transistors are normally conducting and control the firing of a silicon controlled rectifier in series with the coil of a contactor. The latching circuit includes a third transistor.