GB1309819A - Thermistor controlled overload protection circuit - Google Patents

Abstract

1309819 Overload protective arrangements ALLEN-BRADLEY CO 3 June 1970 26748/70 Heading H2K An overload protective arrangement for a motor or other device comprises a bidirectional A.C. switch, e.g. a triac 8, in the operating circuit of a relay coil 1 or other device, a D.C. supply 11, 12 to provide a gating signal to the switch 8, a signal gating device, e.g. a silicon controlled rectifier 18 to shunt the gating signal from the switch 8 when rendered conductive by the change in resistance of a thermistor 22 heated, e.g. by a heater 23, in response to over load current. Resetting may be effected by closing a manually operable switch 24 to divert current from the rectifier 18. Alternatively, the primary winding of the transformer 11 may be connected as indicated at 27, 29 so that the opening of holding contacts 7, when the coil 1 is de-energized, effects automatic resetting. The thermistor 22 may have a positive temperature coefficient and be connected as shown or a negative temperature coefficient and connected in place of the resistor 20. The switch 8 may be a quadrac and the device 18 may be a triac, quadrac or an equivalent transistor circuit.