Two wire heater regulator control circuit having continuous temperature sensing excitation independent of the application of heater voltage

Claim 1

A two wire heater regulator control system having continuous temperature sensing excitation independent of the application of heater voltage for controlling the temperature of a heater element; for use in combination with a heater element having only a first and second terminal thereon for the interconnection of said two wire heater regulator control system thereto, the electrical resistance of said element being a function of the temperature of said heater element, said two wire heater regulator control system comprising: means for selectively providing a heater voltage for connection to said first and second heater element terminals, said heater voltage being an AC line voltage which supplies heating power; means for providing a continuous sensing excitation voltage for connection to said first and second heater element terminals, said continuous sensing excitation being simultaneously present with said heater voltage and the means by which the resistance of the heater element is determined and wherein said sensing excitation is a continuous DC excitation voltage floating with respect to said AC line voltage; means for isolating said continuous sensing excitation voltage from said heater voltage when said heater voltage is being selectively provided to said first and second heater element terminals; a four-leg bridge circuit means including said heater element as one leg thereof, responsive to said continuous sensing excitation voltage for providing a continuous reference voltage; means for comparing said sensing excitation voltage at said heater element with said reference voltage for producing a continuous output representing the difference therebetween, and said comparing means rejecting said AC line voltage during the application of said heater voltage to said heater element and wherein said comparing means includes a first amplifier responsive to the reference voltage and a second amplifier responsive to the first amplifier output and the voltage at said heater element, said first and second amplifiers forming a high common mode range instrumentation amplifier; and control means responsive to at least the output from said comparing means for the selective application of said heater voltage to said first and second heater element terminals and wherein the control means includes switch means for the selective application of the heater voltage to the heater element.

Claim 6

The system according to claims 1, or 4 including circuit protection means responsive to the output of the comparing means for the identification of an output therefrom in excess of a predetermined limit, said predetermined limit indicating an electrical short in the heater element, and wherein the control means is responsive to said protection circuit means whereby heater voltage is not applied to the heater element when said predetermined limit is exceeded.