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

• US 4,554,439 A
• Filed: 11/23/1984
• Issued: 11/19/1985
• Est. Priority Date: 10/04/1982
• Status: Expired due to Fees

• Current Assignee: Rosemount Analytical, Inc. (Emerson Electric Company)
• Original Assignee: Westinghouse Electric Company LLC (Brookfield Asset Management Incorporated)
• Inventors: Cross, Michael, Gordon, John T.
• Primary Examiner(s): Paschall, M. H.
• Application Number:
  US06/674,706
• Time in Patent Office: 361 Days
• Field of Search: 219/490, 219/494, 219/491, 219/497, 219/499, 219/518, 219/505, 219/501, 219/212, 219/507, 323/235, 323/365, 307/117
• US Class Current: 219/497
• CPC Class Codes: G05D 23/2401 using a heating element as a sensing element

Abstract

A heater control circuit functions in conjunction with a suitable heating coil to provide precision, automatic temperature regulation. The combination of heating and temperature sensing functions into a single system results in a two-wire heater regulator control circuit in which a continuous excitation voltage and a power voltage can be simultaneously present at the heater coil. Temperature of the heater is established by continuous excitation and sensing independent of the occurrence of power for heating. A short circuit protection circuit is responsive to an electrical short in the heater coil (which significantly changes its resistance with temperature) and disables the power voltage.

Claims

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.

2. The system according to claim 1 wherein the isolation means is an AC line capacitor which blocks the sensing excitation voltage from being short circuited by and current from flowing into the AC line.

3. The system according to claim 1 wherein the isolation means is a bridge rectifier for the full wave rectification of the AC line voltage for the heater element, said bridge rectifier blocking the sensing voltage from being short circuited by the AC line voltage and from flowing into the AC line.

4. The system according to claim 3 including means responsive to the AC line voltage for detecting the zero crossings thereof and means responsive to said zero crossing detecting means for providing a mark pulse at or near said zero crossings and wherein the control means includes gate means responsive to said mark pulse and to the output from the comparing means for providing a control signal for controlling the actuation of the switch means.

5. 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.

6. The system according to claim 5 including means for blocking the heater voltage from the means for providing a continuous sensing excitation voltage.