Abstract: A heating circuit for an electric blanket or the like comprises a cable (1) formed from a heater conductor (2) separated from a sensor conductor (3) by temperature sensitive material (4) and two series resistor-diode pairs (R1-D1 and R2-D2) each connected between ends of the conductors (2, 3) so that each pair conducts in series with the impedance (Z) of the material (4) during respective half-cycles of an AC mains supply, the arrangement being such that general overheating of the cable (1) or localized overheating anywhere along its length reduces the impedance (Z), (possibly to zero if the material (4) melts), such that the current through one or both of the resistors (R1, R2) heats the resistor(s) and blows a thermal fuse (F) to terminate heating. The thermal fuse (F) may be replaced by a resettable interruption element, namely a self-regulating PTC resistive element.

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.

Abstract: A heating circuit, in particular for an electric blanket or pad, comprises an elongate heating conductor and switch connected in series between input terminals for connection to an AC supply. Switch control is operative to close the switch to cause current to flow through the heating conductor. The circuit further comprises a sensor conductor coextensive with the heating conductor, and a temperature sensitive resistance coextensive with the heating and sensor conductors and having an impedance that falls with increasing temperature. A resistor is connected in series with the sensor conductor such that a current dependent on the impedance of the temperature sensitive resistance flows through the resistor, whereby the voltage across the resistor varies with said impedance. Voltage sensing structure is responsive to the voltage across the resistor reaching a predetermined value to inhibit operation of the switch control, thereby to provide normal and/or overtemperature control.

Abstract: A heating circuit comprises input terminals for connection to an AC supply, an elongate heating conductor and a switch connected in series between the input terminals, a switch control operative to close the switch to cause AC current to flow from the supply through the heating conductor, an elongate sensor conductor spaced from the heating conductor, and a temperature sensitive element disposed between the heating and sensor conductors and responsive to over-heating 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.

Abstract: The invention relates to electrical apparatus for detecting a change in the electrical characteristics of a sensor device, such as a device for sensing fire, the apparatus comprising a plurality of electrical circuits arranged in parallel and connected to an output circuit which can activate or initiate operation of a device which is required to be operated when the electrical characteristics of the sensor device change, each parallel circuit being provided with means for automatically indicating that the circuit is functioning correctly, any fault in the circuit being automatically indicated and the circuit preventing the output circuit from activating or initiating operation of said device due to a fault.