GB2047487A - Heating circuits - Google Patents

Abstract

A circuit for an electric blanket or other heater comprises a heating conductor (2) connected between input terminals (5, 6), and a sensor conductor (3) and temperature sensitive medium (4) substantially co- extensive with the heating conductor. In the event of overheating, the impedance (Z) of the medium (4) drops from a relatively high to a relatively low value and/or it melts to permit contact of the conductors (2, 3) whereby the impedance (Z) will drop to substantially zero. A self-regulating PTC resistive element (7) is connected in series with the heating conductor (2) but disposed remotely therefrom. The element (7) is thermally coupled to resistors (9, 11), such that in the event of the medium (4) being overheated anywhere along its length the resultant current through the resistors (9, 11) will heat the element (7) so that it adopts and stays in a high resistance state, terminating the flow of heating current to the conductor (2). <IMAGE>

Description

SPECIFICATION Heating circuits This invention relates to heating circuits. More particularly, but not exclusively, the invention relates to heating circuits for electric blankets or pads.

It is generally desirable in electrically heated articles to provide some form of overheat protection arrangement which, in the event of an overheat being caused, for instance due an abuse condition, acts so as to render the article safe. In the case of electrically heated panels, blankets or pads, the overheat protection arrangement often takes the form of one or more bimetallic thermostats. It is also known to use heating panels incorporating positive temperature coefficient (PTC) heating elements which cause the current supplied to the panel to fall to a low value once the heating element reaches a certain temperature, due to an increase in the temperature of the element.

Protection against overheating can also be provided by the use of a self-regulating PTC resistive element. The expression 'self-regulating PTC resistive element', as used herein, means a resistive element that can be connected in series with an electrical load to control the supply of power to the load, the element having a positive temperature coefficient of resistance and exhibiting an anomaly in its resistance/temperature characteristic in that on the temperature reaching a value known as the switching or anomaly temperature, the resistance is subjected to a large increase whereby the element goes from a low resistance 'on' state into a high resistance 'off' state, the element regulating itself to remain in the off state until deenergised in that the increased dissipation in the device resulting from the increased voltage across it and the device tends to stay at a temperature at which its resistance is high. In other words, the element is triggered into the off state and remains in such state, even in the absence of the condition causing the overheat, until it is de-energised and has had the opportunity to cool down.

Self-regulating PTC resistive elements have now reached the stage where their use is technically feasible. In particular, they can be used as direct replacements for bimetallic thermostats and the like. In this connection, the self-regulating PTC resistive element has the advantages over a bimetallic thermostat of being electrically fail-safe and of having no moving contacts so that there are no pitting or arcing or radio-frequency intereference problems.

Naturally, if a self-regulating PTC resistive element is to be used as a replacement for a thermostat or the like to detect overheating in an electrically heated device, the device must be thermally coupled to the device, that is to say it must be mounted physically adjacent to the heat source. This presents a problem in some applications, particularly when the heat source is distributed, as in the case of an electric blanket or pad or the like. One way of overcoming this disadvantage would be to use a large number of the elements positioned at different points along the heating element of the blanket or the like, but this is obviously an unsatisfactory solution, in particular from the point of view of cost.

We have proposed heating circuits for electric blankets, pads and the like in which heating is effected by a heating condutor, the arrangement including a sensor conductor and temperature sensitive means substantially coextensive with the heating conductor, whereby in the event of overheating of the heating element due to a ruck in the blanket or the like the impedance of the temperature sensitive means is reduced from a high to a low value, or the means melts to allow contact of the conductors, i.e. the impedance effectively drops to zero. A resistor is connected in series with the impedance of the temperature sensitive means and the resistor and impedance have a voltage applied across them in use, whereby in the event of overheating the current through the resistor increases.The resistor is thermally coupled to a thermal fuse whereby, in the event of overheating, the current through the resistor is sufficient to provide enough dissipation to blow the thermal fuse and therefore non-resettably interrupt the supply of heating current to the heating conductor.

A disadvantage of this arrangement is that, in the case of any overheat at all causing operation of the thermal fuse, the blanket, pad or the like must be returned to the manufacturer for servicing. In the case of a severe overheat, the returned product generally has to be discarded.

According to the present invention there is provided a heating circuit comprising input terminals for connection to an electrical power supply, a heating conductor connected between the input terminals, a sensor conductor and temperature sensitive means substantially co-extensive with the heating conductor, the temperature sensitive means being of such a nature that in the event of overheating its impedance drops from a relatively high to a relatively low value and/or it will melt to permit contact of the conductors whereby said impedance will drop to substantially zero, and at least one resistor connected in series with said impedance such that, in use, a voltage is applied across the series combination, wherein a self-regulating PTC resistive element is electrically connected in series with the heating conductor but disposed remotely therefrom, and the PTC resistive element is thermally coupled to said resistor, the arrange ment being such that in the event of the temperature sensitive means being overheated anywhere along its length the resultant current through the resistor will be sufficient to heat the PTC resistive element to a temperature at which it will adopt and stay in a high resistance off state, whereby the flow of heating current to the heating conductor is substantially terminated.

In this way, a heating circuit is provided which resettably stops heating, in the event of overheating, in a simple and reliable manner.

The heating and sensor conductors may form part of a unitary cable, in which case they may be separated by the temperature sensitive means. Alternatively, the heating and sensor conductors may form parts of separate but adjacent cables. In the latter case, the cable containing the sensor conductor may contain a second sensor conductor, the temperature sensitive means separating the two sensor conductors.

The temperature sensitive means is preferably polyvinyl chloride (PVC), which may or may not be doped with a material which enhances its conductivity.

Heating circuits in accordance with the invention are applicable to the heating of a variety of objects or media. They may be used, for example, in pipe heating, soil warming, industrial process heating or in space heating, for instance in ceiling heating or underfloor heating. The invention, however, is especially suited to heating of electric blankets, which term is to be deemed to encompass not only electrically heated overblankets but electrically heated underblankets, and also electrically heated pads, The invention will now be further described, by way of example, with reference to the accompanying drawing, in which: Figure 1 is a circuit diagram of a first heating circuit, embodying the invention, for an electric blanket or pad; Figure 2 is a graph of the resistance/temperature characteristic of a self-regulating PTC resistive element used in the circuit of Fig. 1; and Figure 3 is a circuit diagram, corresponding to Fig. 1, of another embodiment of the invention.

The heating circuit illustrated in Fig. 1 comprises a heating cable 1 which, in a manner known to those skilled in the art, is incorporated in an electric blanket or pad. The cable 1 comprises a heating conductor 2 and a sensor conductor 3 separated by a material 4. The heating conductor 2 is of resistance wire and is therefore represented as a resistor.

Since in this embodiment of the invention the sensor conductor 3 is not used for heating purposes, it can be of a relatively low resistance material such as copper. The material 4 separating the conductors 2, 3 preferably has an impedance which falls with temperature, preferably logarithmically. In this case the material 4 is preferably PVC, which may or may not be doped with a material that exhances its conductivity. At normal temperatures, the PVC or like material 4 acts as an insulator, whereas at elevated temperatures its impedance drops considerably.

The cable 1 is preferably so constructed that the conductors 2 and 3 are coaxial; the heating conductor 2 is the inner one of conductors and is wound on an electrically insulative core, the material 4 surrounds the conductor 2, the conductor 3 is wrapped or wound around the material 4, and an outer sheath covers the sensor conductor 3.

The heating conductor 2 is connected between a pair of input terminals 5, 6 for connection to an AC supply, in series with a self-regulating PTC resistive element 7. The element 7 is preferably of the type disclosed in UK Patent No. 1 529 354 and may be obtained from Raychem Corporation, Menlo Park, California, U.S.A.

The left hand end of the sensor conductor 3, as viewed in Fig. 1, is connected via a diode 8 and a resistor 9 (1 0K) to the corresponding end of the heating conductor 2. In similar manner, the other end of the sensor conductor 3 is connected via a diode 10 and a resistor 11 (10K) to the other end of the heating conductor 2.

The resistors 9 and 11 are thermally coupled to the element 7, as diagrammatically represented by a dotted line 1 2.

The resistance/temperature characteristic of the element 7 is shown in Fig. 2. As can be seen from Fig. 2, for normal temperatures (i.e.

below about 50"C) the resistance of the element 7 is very low (approximately 0.1 ohms).

Accordingly, the element 7 can be considered, at normal temperatures, to act as a switch in an on state and therefore to have substantially no effect on the operation of the circuit.

During positive half-cycles of the supply voltage, current will flow between the supply terminals 5, 6 via the series combination of the resistor 9, the diode 8 and the impendance (Z) of the material 4. During negative halfcycles, current will flow between the supply terminals 5, 6 via the resistor 11, the diode 10, and the impedance Z. Since, undernor- mal temperature conditions, the impedance Z is very high, such currents are very small. If, however, the material 4 becomes generally overheated (i.e. overheated along its whole length) or locally overheated at any point along its length, the impedance Z drops and one or both of the above currents adopts a substantial value. Accordingly, the self-regulating PTC resistive element 7 becomes heated by the dissipation occurring in one or more of the resistors 9 and 11. Depending on the impedance/temperature characteristic of the material 4, which can be selected at will, when the material 4 reaches a particular temperature the heat generated by one or both of the resistors 9 and 11 will be sufficient to heat the element 7 to its switching or anomaly temperature Ts. When the element 7 is heated to its switching or anomaly temperature Ts, which may typically be around 70"C to 1 00 C, the resistance of the element 7 sharply increases, as shown in Fig. 2. The resistance of the element 7 soon reaches a value which, instead of being negligibly smaller than that of the heating conductor 2, is of the same order and eventually much greater than the resistance of the conductor 2, which is typically of the order of hundreds of ohms.Accordingly, a much larger proportion of the mains voltage is dropped across the element 7, whereby the dissipation produced by the current flowing through the element 7, which was previously negligible, starts to become substantial. Very soon, the heating of the element 7 caused by the dissipation therein due to current flowing directly therethrough is sufficient to keep the element 7 in a high resistance off state (around 105 to 106 ohms), this occurring when the element 7 settles down to a steady state temperature TD (Fig. 2) at which the resistance, power dissipation in, and temperature of the element 7 become stabilised. Accordingly, the element 7 will remain in its high resistance off state, even if the cause of the overheating is removed, provided that power is not disconnected from the circuit.Only if the power is disconnected from the circuit and the element 7 allowed to cool down will the element 7 revert to its low resistance on state.

In some instances it is possible that in the event of an overheat the material 4 might melt whereby the outer sensor conductor 3 will collapse onto the inner heating conductor 2. The resultant short circuit is equivalent to the impedance of the material 4 locally dropping to substantially zero, whereby the current flowing through one or both of the resistors 9 and 11 will be ample to trigger the element 7 into its off state. In fact, if such an abrupt drop in the impedance to substantially zero occasioned by melting of the material 4 is deemed sufficient for indicating localised overheating, one can employ for the material 4 a substance that need not have an impedance that varies substantially with temperature below its melting point, for instance a plastics material such as polyethylene.

Fig. 3 shows a circuit which is in many respects similar to that shown in Fig. 1 and will only be described in so far as it differs therefrom. In particular, reference numerals in Fig. 3 corresponding to reference numerals in Fig. 1 indicate the same or similar items.

In this case, the heating conductor 2 is the sole conductor in a cable 1'. The material 4 separates the sensor conductor 3 and a second sensor conductor 3' in a second cable 1". The two cables are substantially coextensively arranged in the blanket or pad whereby, in the event of an overheat, the material 4 is overheated in a similar manner to that of the material 4 in the circuit of Fig. 1. A resistor 9' is connected, as shown, such that it and the impedance Z of the material 4 are connected in series between the supply terminals 5, 6.

The results of overheating are substantially the same as in the circuit of Fig. 1, in this case the resistor 9' supplying the heat to activate triggering of the element 7.

Claims (10)

1. A heating circuit comprising input terminals for connection to an electrical power supply, a heating conductor connected between the input terminals, a sensor conductor and temperature sensitive means substantially co-extensive with the heating conductor, the temperature sensitive means being of such a nature that in the event of overheating its impedance drops from a relatively high to a relative low value and/or it will melt to permit contact of the conductors whereby said impedance will drop to substantially zero, and at least one resistor connected in series with said impedance such that, in use, a voltage is applied across the series combination, wherein a self-regulating PTC resistive element is electrically connected in series with the heating conductor but disposed remotely therefrom, and the PTC resistive element is thermally coupled to said resistor, the arrangement being such that in the event of the temperature sensitive means being overheated anywhere along its length the resultant current through the resistor will be sufficient to heat the PTC resistive element to a temperature at which it will adopt and stay in a high resistance off state, whereby the flow of heating current to the heating conductor is substantially terminated.

2. A heating circuit according to claim 1, wherein the heating and sensor conductors form part of a unitary cable.

3. A heating circuit according to claim 2, wherein the heating and sensor conductors are separated by the temperature sensitive means.

4. A heating circuit according to claim 1, wherein the heating and sensor conductors form parts of separate but adjacent cables.

5. A heating circuit according to claim 4, wherein the cable containing the sensor conductor contains a second sensor conductor, the temperature sensitive means separating the two sensor conductors.

6. A heating circuit according to any one of the preceding claims, wherein the temperature sensitive means comprises polyvinyl chloride.

7. A heating circuit according to claim 6, wherein the polyvinyl chloride is doped with a material which enhances its conductivity.

8. A heating circuit substantially as herein described with reference to Figs. 1 and 2 or Fig. 3 of the accompanying drawings.

9. An electric blanket incorporating a heating circuit according to any one of the preceding claims.

10. An electric pad incorporacting a heat nag circuit according to any one of the preceding claims.