GB1380079A - Semi-conductor alternating current regulating means - Google Patents

Abstract

1380079 Automatic control of temperature KALLE AG 20 Dec 1971 [18 Dec 1970] 59086/71 Heading G3R In an automatic control system particularly for temperature control of a plastics extruder, wherein heating coils (13) Fig. I (not shown) are energized by A.C. under the control of a triac 39 Fig. 6, the extruder temperature is detected by a thermocouple and compared in a controller 16 Fig. 2 with a desired value to produce a D.C. error signal. The error signal is converted by a voltage converter 28 Fig. 2 and Fig. 5 (not shown) into a control signal which is a function thereof but is electrically isolated therefrom by virtue of a transformer coupling in the converter. The triac is controlled so that it is fired substantially at zero voltage of the A.C. supply and it is arranged that it will always conduct an even number of successive half-cycles of current (otherwise a D.C. component would arise). Thus the average power supplied to the heating load is varied by changing the on/off duty cycle of the triac, controlled bursts of complete cycles being applied to the load. In Fig. 2 the error signal from a controller 16 is converted at 28 to a control signal in analogue form. This signal is converted to digital form in an amplifier/comparator 25 by means of a sawtooth signal from a generator 27. As shown Fig. 6, the output of amplifier 25 controls an SCR 36 to render it conductive so that C38 is charged each negative half-cycle. Zero cross-overs are "sensed" by transistors 49, 50, the alternating voltage across triac 21 appearing across R46, R47 in the respective base/emitter circuits thereof. Thus 49, 50 conduct to provide gate pulses to SCRs 42, 41 respectively at the beginning of each negative and positive half-cycle respectively. To ensure that triac 21 will conduct for an even number of half-cycles, C38 is charged sufficiently during each negative half-cycle to enable SCR 41 to conduct a gate pulse to the triac during the following positive half-cycle after the control signal has terminated. Alternatively the error signal may be in digital form in which case the sawtooth generator is unnecessary Fig. 3 (not shown). As a further alternative the output stage of controller 16 may be a relay contact which is opened and closed with a mark/space ratio proportional to the error.