Proportional temperature controller

Abstract

A temperature controller for use with an electrical heater for precisely maintaining the temperature within a system at a selected set point, and employing an improved bridge circuit. A temperature sensitive resistor forms one of the two variable arms of the bridge circuit. The resistor varies in value as a function of temperature exhibiting a temperature coefficient of resistance which is not constant but which varies, decreasing with increasing temperature. The other variable arm, the setting arm, is in the form of a network, including a linear setting resistor and a pair of fixed resistors arranged in a series-shunt circuit. The setting resistor is linearly calibrated in terms of temperature, the series-shunt arrangement providing a composite resistance-set point temperature characteristic which closely corresponds to that of the sensing arm. The temperature controller further includes an operational amplifier which provides an output signal dependent upon the degree of bridge imbalance and a zero voltage switch responsive to the operational amplifier for providing trigger pulses to a triac coupled in the load circuit of the heater. A ramp generator is preferably included as a supplemental input to the zero voltage switch thereby to produce a control system in which the heat released is proportional to the degree the bridge is unbalanced.