Dew point differential power controller
First Claim
1. A control circuit for proportionally controlling and applying an ac current from an available ac electric current source to heater means for heating transparent windows adapted for use as openable walls of a refrigerated cabinet having a surface exposed to ambient air outside the cabinet, said control circuit comprising:
- (a) dew point sensing impedance means coupled to said available ac current source and thermally coupled to said cabinet surface and exposed to ambient air adjacent said surface, for providing a variable impedance to passage of ac current therethrough proportional to the difference between the temperature of said cabinet surface and the dew point temperature of said ambient air;
(b) sensing circuit means powered by said source of ac current, said sensing circuit means being electrically coupled to said dew point sensing means and responsive thereto for producing an intermittent output pulse signal whose on-off duty cycle is within a predetermined range proportional to the impedance of said dew point sensing means, said output pulse signal being initiated only at those times when said ac current is at or close to zero, when the cabinet surface temperature is at or close to the dew point temperature of said ambient air; and
(c) solid state switch means connected in series circuit with said heater means and said available source of ac current, and responsive to said output pulse signal of said sensing circuit means for completing a series circuit of ac current from said source through said heater means during times proportionally determined by said output pulse signal.
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Accused Products
Abstract
A control circuit for proportionally controlling the application of available ac current to thin film heaters and other heating apparatus for transparent windows and other selected surfaces of refrigerated cabinets prevents condensation of moisture thereon. A transducing sensor in contact with the ambient air and thermally coupled to the refrigerated cabinet surfaces provides control signals to an ac heating current control circuit in relation to the difference between cabinet surface temperature and the dew point temperature of the ambient air. As the cabinet surface temperature drops closer to dew point the application of electric power to the transparent window and other surface heating apparatus is proportionally increased by the control circuit by applying heater current during correspondingly increased "on" periods for passage of the available ac heater current. "On" periods of the heater current are initiated during times in a cycle when the ac is at or near its null. At the two extremes of the operating range of the control circuit, heater current may be applied full "on" and full "off" respectively.
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Citations
14 Claims
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1. A control circuit for proportionally controlling and applying an ac current from an available ac electric current source to heater means for heating transparent windows adapted for use as openable walls of a refrigerated cabinet having a surface exposed to ambient air outside the cabinet, said control circuit comprising:
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(a) dew point sensing impedance means coupled to said available ac current source and thermally coupled to said cabinet surface and exposed to ambient air adjacent said surface, for providing a variable impedance to passage of ac current therethrough proportional to the difference between the temperature of said cabinet surface and the dew point temperature of said ambient air; (b) sensing circuit means powered by said source of ac current, said sensing circuit means being electrically coupled to said dew point sensing means and responsive thereto for producing an intermittent output pulse signal whose on-off duty cycle is within a predetermined range proportional to the impedance of said dew point sensing means, said output pulse signal being initiated only at those times when said ac current is at or close to zero, when the cabinet surface temperature is at or close to the dew point temperature of said ambient air; and (c) solid state switch means connected in series circuit with said heater means and said available source of ac current, and responsive to said output pulse signal of said sensing circuit means for completing a series circuit of ac current from said source through said heater means during times proportionally determined by said output pulse signal. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8)
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9. A refrigerated cabinet heating system control circuit for proportionally controlling and applying an ac current from an available ac electric current source to heater means for heating transparent windows adapted for use as openable walls of a refrigerated cabinet having a surface exposed to ambient air outside the cabinet, said control circuit comprising:
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dew point sensing means coupled to said available ac current source and thermally coupled to said cabinet surface and receiving ambient air adjacent said surface, for producing an output signal proportional to the difference between the temperature of said cabinet surface and the dew point temperature of said ambient air; sensing circuit means powered by said source of ac current, said sensing circuit means being electrically coupled to said dew point sensing means and responsive to said output signal for producing an intermittent output pulse signal whose on-off duty cycle is within a predetermined range proportional to the signal produced by said dew point sensing means; and solid state switch means connected in series circuit with said heater means and said available source of ac current, and responsive to said output pulse signal of said sensing circuit means for completing a series circuit of ac current from said source through said heater means during times proportionally determined by said output signal. - View Dependent Claims (10, 11, 12, 13, 14)
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Specification