Heating circuits
First Claim
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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Abstract
A heating circuit, in particular for electric blankets or pads, comprises a first, heating conductor and a second conductor separated by a material which has an impedance that falls with increasing temperature and/or which will melt in the event of overheating to allow contact between the two conductors. A half-wave rectifier and a thermally-operative circuit interruption means are connected in series with the heating conductor across an AC supply. A resistor is electrically connected to the second conductor such that the resistance and the impedance of said material are connected in series across the supply. The resistor is thermally coupled to the circuit interruption means whereby, in the event of overheating of said material, the drop in the impedance of the material to a reduced or zero value will cause the current through the resistor to increase to heat the resistor, which causes the circuit interruption means to disconnect the circuit from the supply.
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Citations
28 Claims
- 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.
- 15. 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, and separating means which separates the two conductors and which will melt in the event of overheating to allow contact between the two conductors, the improvement comprising a resistor, means electrically connecting the resistor between the second conductor and one of said input terminals such that the resistor and the impedance of the separating means 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, the resistor being thermally coupled to the circuit interruption means whereby in the event of overheating of the separating means causing contact between the first and second conductors anywhere along the length of the separating means the consequent drop in said impedance to substantially zero 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.
Specification