On-off heater control

Claim 1

A proportional temperature regulating control circuit which prevents the generation of radio frequency interference, comprising: a. a temperature sensing circuit means having an output voltage which is a function of input temperature, said temperature sensing circuit means comprising: 1. a resistance bridge network, one leg of which is a thermistor which is exposed to temperature changes from said heating element, the output of said bridge network being an error signal, 2. the output of said bridge network being connected to an integrated circuit differential amplifier whose output which is a function of the error signal input is a function of the temperature sensed by said thermistor, b. a triangle voltage function generator for producing a repetitious pulse waveform voltage at its output, said voltage function generator producing a triangular waveform voltage, c. a heating element, d. a comparator and switching means having its input connected to the summed outputs of said temperature sensing circuit means and said voltage function generator, said comparator and switching means comprising high gain voltage amplifier whose output is fed to control transistor means, and feedback network means for slowing down switching transition to avoid generation of radio frequency interference due to fast current transients and insure loop frequency stability, the output of said comparator and switching means connected to said heating element for driving the heating element, and heat generated in said heating element producing input temperature which is sensed by said temperature sensing circuit means, e. said summed outputs of the temperature sensing circuit means and the voltage function generator having a pulse repetition frequency in which significant harmonics fall in the subaudio range and also avoid thermal flicker sufficient to maintain low power dissipation, f. a positive voltage at the summed outputs of said temperature sensing circuit means and said voltage function generator being fed to said high gain voltage amplifier means driving the amplifier positive and saturating said control transistor means to allow current to flow in said heater element, a negative voltage at said summed outputs switching said control transistor means to off condition preventing current from flowing in said heater element, the heater voltage being pulsewidth modulated resulting in the power supplied to said heater element being proportional to the tEmperature deviation sensed by said temperature sensing circuit means.