ELECTRONIC CIRCUITS FOR TEMPERATURE CONTROL

1,210,432. 210, 432. Automatic temperature control systems. QUICKFIT & QUARTZ Ltd. AND D. OBERSBY. Feb. 11, 1969 [Feb. 20, 1968], No. 8274/68. Heading G3R. In a temperature control system utilizing a heater as a sensor, a signal is produced indicative of the reciprocal of heater current which is multiplied by signal indicative of heater voltage to produce a signal proportional to heater resistance which is compared with a reference in order to regulate heater current. A heater 1 has its current measured by a winding 5 and a Hall effect device 6 arranged as an analogue inverter, the latter preferably being connected in the feedback loop of a high gain differential amplifier 8 supplying an input winding 11 of a second Hall effect device 12 which is additionally connected across heater 1. The output from device 12 is proportional to the product of heater voltage and reciprocal of heater current. This is applied to a comparator circuit which also receives a reference signal 15, the difference being applied through an amplifier 17 to a pulse generator 7 which varies the firing point of a semi-conductor rectifier network 4 in series with the heater and mains terminals 2, 3 according to the magnitude and sign of the difference. Devices other than of the Hall type may be used in the arrangement which may be utilized to maintain a desired viscosity in molten glass flowing along a canal in the forehearth of a melting furnace, to control the temperature of pipes prior to bending or drawing, or in electro-phoresis instruments. The signal 15 represents the resistance of heater 1 at a desired temperature the value or values being derived from previous calibration, and the error signal on line 16 may be continuous or discontinuous. The heater may be a resistive path formed through a glass part which is being manipulated and/ or shaped, the electrical connection being made to either side of the part by means of flame electrodes playing on the hot glass. The resistive path may also be formed between two probe electrodes which project into a body of molten glass.