GB1400623A - Circuit for determining the condition of a load - Google Patents

Abstract

1400623 Automatic control of temperature WAYNCO Inc 15 June 1972 28096/72 Heading G3R The change in resistance of an electric heater is used as a measure of its temperature which is to be controlled. The heater 20 is fully energized from A.C. supply lines 14, 15 via a triac 50 controlled in dependence upon the output of a bridge circuit 10 which includes the heater 20 and a desired temperature setting potentiometer 17. During periods when the triac 50 is off, the bridge will sense the heater resistance, a current pulse passing through the bridge each half-cycle of the supply when the line 14 is negative with respect to line 15. The bridge unbalance is sensed by a differential amplifier 27. If the heater temperature is too low the amplifier will have a positive output which charges a capacitor 33. When C33 is charged to a predetermined level it turns ON a transistor 34 of a flip-flop circuit and thereby a capacitor 42 is charged. When the charge in C42 reaches the threshold voltage of a U.J. transistor 46, the transistor is fired and a trigger pulse is applied to triac 50 via a transformer 47. As long as C33 is charged, C42 will repeatedly trigger the U.J. 46. When the amplifier output is such that C33 no longer receives a charge and so long as the charge in C33 has not dropped below the critical voltage level of the flip-flop, the U.J. 46 will continue oscillating and the triac will be triggered ON even during the negative half-cycles of the supply when line 14 is negative with respect to line 15. During the time line 14 is positive a transistor 31 will conduct and clamp the amplifier output to the potential of line 15 thus preventing any amplified noise from charging C33. In addition this prevents charging of C33 during positive halfcycles when the triac is conducting. Zener diodes 54, 55 protect the amplifier and diodes 19, 22 protect the bridge when full supply voltage is applied to the load 20. The other bridge components are effectively by-passed when the load 20 receives power via the triac. Alternatively, the load impedance may vary with humidity or pressure.