GB978750A - Temperature control systems - Google Patents

Abstract

978, 750. Automatic temperature control systems. GENERAL ELECTRIC CO. Aug. 27, 1962, No. 32814/62. Heading G3R. A temperature control system for an electrically heated cooker comprises selector switches whereby the cooking temperature can be selected and a temperature sensing device in contact with the cooking reset by means of which the content in the cooking vessel can be maintained at the selected temperature. A cooking vessel is placed on an electric heating element L so that a temperature sensing device 27 containing a thermistor RT is in heat conducting relationship with the bottom of the vessel. The heating range of the element L is selected by means of a control 21 and the point in this range by a second control 21a. Movement of the control 21 selects the range resistor R3 or R4 or R5 by means of wiper 63 and by means of wipers 61, 62 connects the supply 71, 72 to the primary winding 90 of a transformer Tl. Movement of control 21a modifies the effect of the selected range resistor by placing a portion of resistor RC in series therewith in one arm of a bridge (see Fig. 4). A further arm of the bridge comprises the thermistor RT, shunted by an overload protecting resistor RO, whilst the remaining arms of the bridge are formed by calibrating resistors R1, R2. The bridge is fed with rectified unsmoothed D.C. by means of the transformer T1 and diodes D4, D5. A capacitor C is charged from the transformer T1 over diodes D1, D2 resistors R6, R7 and a Zener diode ZD, the charging circuit including a variable impedance-such as a transistor-the impedance of which is dependent upon the unbalance of the bridge containing the thermistor RT. When the capacitor C reaches the break-down voltage of a device such as a Shockley diode it discharges into a transformer T2 thereby producing firing pulses for backto-back silicon controlled rectifiers SCR1, SCR2 in the supply circuit to the heating element L. Thus when the thermistor RT is cool and a temperature selected by controls 21, 21a the bridge is largely unbalanced, the transistor impedance is low and the capacitor rapidly charges so that the silicon controlled rectifiers fire early in the half cycles and the amount of energy supplied to the element L is large. As the thermistor heats up the bridge tends to approach balance so altering the transistor impedance, lengthening the charging time of the capacitor C and thereby reducing the energy delivered to the heating element L. Equilibrium is reached when the bridge attains a state when the energy delivered to the element L is just sufficient to maintain the cooking vessel and hence the thermistor RT at the temperature selected by the controls 21, 21a. In other embodiments the Zener and Shockley diodes are replaced by neon tubes and the resistors forming the bridge are arranged to form a variable impedance chain to replace the transistor.