A wide range temperature controller using a resistance element
S J Borgars · Journal of Scientific Instruments · 1953
The controller consists primarily of a high gain amplifier with a resistive bridge feedback circuit, the sensitive element being one of the bridge arms. Under certain conditions of sense and degree of bridge unbalance the circuit oscillates. The output is rectified and operates a relay which switches power to a control heater. The circuit sense is such that the mean temperature of the element is kept constant. Adjustment of the desired value is made by varying a bridge arm, which may be directly calibrated in units of temperature. The use of a thermistor for the resistance element enables a high sensitivity to be obtained, particular at low temperatures. For operation at low temperatures the ambient temperature of the controlled unit is maintained below the lowest temperature of control, thus operation can be effected by a supply of heat. A chamber of 570 in 3 was controlled to ± 0.09° C at - 40° C. The intrinsic sensitivity is such that circuit operation will occur with a temperature differential of less than ± 0.01° C. This could be improved by various circuit refinements.