GB1310129A - Overload protection circuit - Google Patents

Abstract

1310129 Overload protective arrangements ALLEN-BRADLEY CO 3 June 1970 26747/70 Heading H2K An overload protective arrangement for a motor or other device comprises a first bidirectional A.C. switch 8 in series with a relay coil 6 or other relay device, a second bidirectional A.C. switch 9 in series with a first resistor 10 and connected to the gating circuit of the switch 8, and a second resistor 11 connected to the gating circuit of the second switch 9. A heater 7 responsive to overload conditions may heat the resistor 10 which would have a positive temperature coefficient of resistance, or the resistor 11 which would then have a negative temperature coefficient. The circuit constants are such that when the temperature of the heated resistor is low the gating current to the first switch 8 is greater than that to the second switch 9 so that the switch 8 conducts on each half cycle. When the temperature of the heated resistor becomes high, the gating current to the second switch is the greater so that the switch 9 conducts on each half cycle to short circuit the switch 8 and to connect the resistor 10 in series with the coil 6 to cause the relay to drop out. This causes the closing of contacts 19 to connect a resistor 18 in parallel with the resistor 11 to assist in holding the arrangement operated. In addition a latching circuit may be provided, this circuit comprising a silicon controlled rectifier 25 with a D.C. supply 20, 21. This rectifier 25 is triggered by current through a diode 29 when full time voltage across the switch 8 occurs when the switch 8 ceases to conduct. Thereafter the rectifier 25 remains conductive to apply continuing triggering current to the switch 9. The switches 8, 9 may be triacs, or triacs with integral diacs, or other bidirectional A.C. gating devices equivalent to triacs.