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Heating circuits

  • US 4,251,718 A
  • Filed: 12/19/1978
  • Issued: 02/17/1981
  • Est. Priority Date: 01/31/1978
  • Status: Expired due to Term
First Claim
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1. In a heating circuit comprising input terminals for connection to an AC supply, a first elongate conductor to effect heating, thermally-operative circuit interruption means, the first conductor and circuit interruption means being connected in series between the input terminals, a second elongate conductor substantially coextensive with the first conductor, separating means which separates the two conductors and which has an impedance that falls with increasing temperature, and a resistor that is thermally coupled to the circuit interruption means, the improvement comprising means electrically connecting said resistor between the second conductor and one of said input terminals such that the resistor and said impedance are connected in series between the input terminals, and half-wave rectifier means connected in series with the first conductor and the circuit interruption means between the first conductor and said one input terminal so that during those half-cycles of the AC supply during which the rectifier means does not conduct the second and first conductors are at the potentials of said one input terminal and the other input terminal, respectively, whereby in the event of general overheating of the separating means or localised overheating of the separating means anywhere along its length the consequent general or localised drop in said impedance will cause the current through the resistor to increase by an amount that will heat the resistor by an amount sufficient to cause the circuit interruption means to disconnect the circuit from the supply.

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