Claims Analysis
Total Claims: 11
Claim 1
Independent Claim
212 words
A heating circuit comprising input terminals for connection to an AC supply, an elongate heating conductor and switch means connected in series between the input terminals such that one end only of the heating conductor is connected to the input terminals via the switch means, switch control means operative to close the switch means to cause AC current to flow from the supply through the heating conductor, an elongate sensor conductor spaced from the heating conductor, temperature sensitive means disposed between the heating and sensor conductors and responsive to overheating at any position along the length of the heating conductor to drop substantially in impedance at that position to connect the sensor conductor to the potential of the heating conductor at that position, means to cause the control means, whilst the control means is operative to close the switch means, to periodically open the switch means so that the heating conductor is at substantially the same potential along its length, and sensing means to sense the potential on the sensor conductor during each periodic opening of the switch means and responsive to the potential being such as to indicate that there is an overheat at any position along the length of the heating conductor to prevent subsequent re-closure of the switch means.
Claim 2
Dependent Claim
212 words
A heating circuit comprising input terminals for connection to an AC supply, an elongate heating conductor and switch means connected in series between the input terminals such that one end only of the heating conductor is connected to the input terminals via the switch means, switch control means operative to close the switch means to cause AC current to flow from the supply through the heating conductor, an elongate sensor conductor spaced from the heating conductor, temperature sensitive means disposed between the heating and sensor conductors and responsive to overheating at any position along the length of the heating conductor to drop substantially in impedance at that position to connect the sensor conductor to the potential of the heating conductor at that position, means to cause the control means, whilst the control means is operative to close the switch means, to periodically open the switch means so that the heating conductor is at substantially the same potential along its length, and sensing means to sense the potential on the sensor conductor during each periodic opening of the switch means and responsive to the potential being such as to indicate that there is an overheat at any position along the length of the heating conductor to prevent subsequent re-closure of the switch means.
Claim 3
Dependent Claim
17 words
A heating circuit according to claim 1, wherein the switch means is a solid state switch means.
Claim 4
Dependent Claim
14 words
A heating circuit according to claim 2, wherein the switch means comprises a thyristor.
Claim 5
Dependent Claim
18 words
A heating circuit according to claim 3, wherein the switch means comprises a pair of inversely connected thyristors.
Claim 6
Dependent Claim
66 words
A heating circuit according to claim 4, wherein the pair of thyristors are in a master/slave configuration, the switch control means being arranged to supply firing pulses to one of the thyristors to fire it during supply half-cycles of one polarity and the switch means including means to slave trigger the other thyristor into conduction at the commencement of the subsequent half-cycles of the opposite polarity.
Claim 7
Dependent Claim
122 words
A heating circuit according to claim 1, comprising a shunt switch connected in parallel with the heating conductor and switch means, a voltage detector means arranged to monitor the voltage across the switch means, the voltage detector means being responsive during the periodic opening of the switch means to turn the shunt switch on only for the periodic opening duration to establish a voltage for enabling subsequent operation of the switch control means if the monitored voltage is such as to indicate that the switch means is in fact open and to turn it on continually if the monitored voltage is substantially zero, and disabling means responsive to continual turning on of the shunt switch to disable operation of the heating circuit.
Claim 8
Dependent Claim
14 words
A heating circuit according to claim 6, wherein the shunt switch comprises a thyristor.
Claim 9
Dependent Claim
58 words
A heating circuit according to claim 6, wherein the disabling means comprises a resistor in series with the shunt switch and thermally connected to a thermal fuse in series with the heating conductor, the arrangement being such that the dissipation in the resistor is sufficient to operate the thermal fuse only if the switch is turned on continually.
Claim 10
Dependent Claim
25 words
A heating circuit according to claim 1, wherein the heating and sensor conductors and the temperature sensitive means are constituted by a unitary heating cable.
Claim 11
Dependent Claim
37 words
A heating circuit according to claim 9, wherein the temperature sensitive means comprises a layer disposed around the heating conductor with the sensor conductor wound around the layer so as to be coaxial of the heating conductor.