IE47536B1 - Heating circuits - Google Patents

Description

This invention relates to heating circuits.

It is known to incorporate in an electric blanket a cable comprising a heating conductor, a sensor conductor and separating means that separates the two conductors and 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. An external power supply, generally an AC supply, is connected across the heating conductor to heat it. Keans is provided responsive to the impedance of the temperature sensitive means dropping to a value indicating overheating of the cable (which value will be zero in the event of melting resulting in contact of the conductors) to prevent current flow15 ing through the heating conductor. A disadvantage of such an arrangement is that the reliable detection of overheating becomes difficult due to the voltage gradient occurring along the heating conductor due to the fact that the supply voltage is applied across its ends. While this does not create a problem if the overheating is general, i.e. if it is present along the whole length of the cable, a difficulty arises if the overheating is localised at a position along the length of the cable, because the voltage at the position where the two conductors are connected by the reduced or zero impedance will be anywhere between the full supply voltage and zero, depending on the location of the position. Thus, for example, in the case where the separating means has a temperature sensitive impedance, one could not simply detect overheating hy connecting a resistor to the sensor conductor to constitute a potential divider with the reduced impedance of the sensor conductor, since in the event of localised overheating the voltage across the resistor would vary between a maximum value and zero, depending on the location of the overheat position. As a further example, in the case when melting of the separating means is relied upon one could not simply detect overheating by connecting together the conductors at one end of the cable and detecting the increase in current through the heating conductor, since if melting and contact occurs at or near said end of the cable the increase in current will be very small. Various circuits overcoming this disadvantage have been evolved, but all of them necessitate the use of fairly complex circuits configuration.

According to the present invention there is provided a heating circuit comprising a cable that comprises a heating conductor, a sensor conductor, and separating means that separates said two conductors and 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, the sensor conductor being interrupted at a position along its length to define two sensor conductor portions, a pair of input terminals for connection to a power supply, the heating conductor being connected between the input terminals, a first resistor connected between one end of the heating conductor and the sensor conductor portion adjacent the other end of the cable, a second resistor connected between the other end of the heating conductor and the sensor conductor portion adjacent said one end of the cable, and means responsive to a current of more than a predetermined magnitude flowing through at least one of the first and second resistors to prevent current flowing through the heating conductor.

In a circuit in accordance with the invention, it is the current flowing through either one of the resistors and the impedance (finite or zero) af that part of the temperature sensitive means adjacent the sensor conductor portion to which such resistor is connected that signals overheating. The voltage across the combination of such resistor and impedance, in the event of a localised overheat, will vary, in accordance with the location of the overheat, between the full voltage applied across the heating conductor and a minimum value determined by the position along the cable at which the sensor conductor is interrupted, (if, as is preferred, the position of interruption is halfway along the cable, the said minimum value is equal (neglecting the shunting effect of the said combination) to half the full voltage applied across the heating conductor, assuming the resistance thereof is uniform along its length). Consequently, in the event of a localised overheat at any position along the length of the cable, there will always be at least a minimum current flowing through one or the other of the resistors to signal that overheating has occurred and to cause current to stop flowing through the heating conductor.

In a so-called 'crowbar* system in which a short circuit between the two conductors produces a virtual short circuit across the power supply input terminals, the cable often disintegrates in the event of a short circuit. This is because the short circuit is often lossy, particularly after a long period due to oxidation or the like of the conductors, whereby a large amount of power is dissipated across the short circuit. The fact that, in a circuit in accordance with the invention, one of the said resistors (each of which is typically around 10K in value) is in series with the impedance of a part of the temperature sensitive means results in the possibility of cable disintegration being greatly reduced or eliminated, since the resistor is in series with any short circuit and limits the current flowing to a value which, while greater than the said predetermined magnitude, is several orders of magnitude less than in a crowbar system whereby the dissipation at the site of the short circuit is minimal.

The separating means, if it has a temperature sensitive impedance, is preferably such that its impedance falls substantially logarithmically with increasing temperature, which greatly facilitates detection of overheating since a small increase in temperature j.rom a safe operating level to a dangerously high level will give rise to a correspondingly large increase in the current through one or both of the resistors. The separating means, when it is required to have a temperature sensitive impedance, may be polyvinyl chloride, which may or may not be doped with a substance that increases its conductivity. Preferably, the impedance of the temperature sensitive means drops by a factor of . ten for an increase in temperature of 25 deg. C.

When melting of the separating means (i.e. an abrupt reduction to zero of the impedance thereof) is deemed sufficient to signal overheating, the separating means may be of a material that has an impedance that dues not vary substantially with temperatures below its melting point, for example, polyethylene.

The current respnnsive means may be arranged to resettably or non-resettably prevent current flow through the heating conductor. In the former case, the means may, for example, be arranged to inhibit the closure of switch means (e.g. electronic switch means) arranged in series with the heating conductor to control the flow of current therethrough. Such an arrangement is useful, for example, in a heating circuit incorporated in an electrically heated overblanket and provided with means for adjusting the heat output. In the latter case, according to a preferred embodiment of the invention described below, the current responsive means may be a thermal fuse connected in series with the heating conductor and thermally coupled to both resistors such that it is blown by the heat generated in either or both resistors as a result of current flowing therethrough in the event of an overheat.

Each resistor is preferably connected to both ends of the associated sensor conductor portion, which has the advantage that the circuit will continue to function satisfactorily in the event of a single break in either of the sensor conductor portions.

Heating circuits in accordance with the invention are applicable to the heating of a variety of objects or media. They nay 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 is, however, especially suited to the heating of electric blankets, which term is to be deemed to encompass not only electrically heated overblankets but also electrically heated underblankets, and electrically heated pads.

The invention will now be iirther described, by way of example, with reference to the accompanying drawing, the sole figure of which is a circuit diagram of a heating circuit embodying the invention for an electric blanket or pad.

The illustrated heating circuit comprises a cable 1 which is incorporated in an electric blanket or pad (not shown) in a manner known in the art. The cable 1 comprises a heating conductor 2 and a sensor conductor 3 separated by a material 4 shown in the drawing by cross-hatching.

The heating conductor 2 is of resistance wire and is therefore represented as a resistor. The sensor conductor 3 is preferably a low resistance conductor, for instance of copper. The cable 1 is preferably so constructed that the conductors 2 and 3 are coaxial: the heating conductor 2 is the inner one of the conductors and is wound on an electrically insulative core, the material 4 surrounds the conductor 2, the sensor conductor 3 is wrapped or wound around the material 4, and an outer sheath covers the sensor conductor 3· The material 4 is of such a nature that its impedance falls logarithmically with an increas in temperature. The material 4,may, for instance, be polyvinyl chloride that is doped, in a manner known in the art, with a material that increases its conductivity.

The cable 1 may in fact be constructed along the lines described in UK Patent Specification No. 841,604 and the impedance/temperature characteristic of the material 4 is preferably such that its impedance drops by a factor of ten for every increase in temperature by 25 deg. C. lhe sensor conductor 3 is separated or interrupted substantially half way along the cable 1, at a position 5, to define two sensor conductor portions 3’, 5’· (Alternatively, 4?536 the cable 1 could be constituted by two like cable portions of which the heating conductors are joined end-to end to forn a structure exactly the sane as that just described).

The ends of the sensor conductor portion 3' are connected together and connected via a resistor 6 to the end of the heating conductor 2 remote from the portion 3'. The ends of the sensor conductor portion 3 are connected together and connected via a resistor 7 to the end of the heating conductor 2 remote from the portion 3, The heating conductor 2 is connected -in series with a thermal fuse 8 between a pair of input terminals 9, 10 for connection to the live (L) and neutral (K) conductors of an AC power supply (not shown), the thermal fuse 8 being adjacent the live terminal 9· As is known to those skilled in the art, the thermal fuse 8 is a non-resettable thermal link and comprises a current carrying device (generally incorporating a low melting point alloy) responsive to the application of external heat to nonresettably stop the passage of current therethrough. Each of the resistors 6, 7 is thermally coupled to the thermal fuse whereby the generation of a predetermined amount of beat by virtue of sufficient current flowing through either or both of the resistors will cause the fuse to blow.

The above-described heating circuit operates in the following manner. When the terminals 9, 10 are connected to the power supply, current flows through the heating 7536 conductor 2 and warms the blanket or pad. The current flowing through the heating conductor 2 establishes a voltage gradient across it. Referenced to supplyneutral, the upper end of the conductor 2 is at the supply voltage, which by way of example is taken to be 240 V r.m.s., the centre of the conductor 2 (position 5) is at 120V r.m.s., and the lower end of the conductor 2 is at OV. The resistor 6 is therefore connected, in series with the impedance of that part of the material 4 coextensive with the sensor conductor portion 3’, between OV and the upper half of the conductor 2 which has thereon a voltage varying between 240 V and 120 V. Consequently, a current flows through the resistor 6. The resistor 7 is connected, in series with that part of the material 4 coextensive with the sensor conductor portion 3, between 240 V and the lower half of the conducter 2 which has thereon a voltage varying between 120 V and OV. Consequently, a current flows through the resistor 7, such current being the same as that through the resistor 6 if the resistance of the resistors 6 and 7 are the same and the cable 1 is uniform along its length. As current continues to flow through the heating conductor 2, the material 4 warms up and its impedance drops. The currents flowing through the resistors 6 and 7 thus increase, the currents increasing by an equal extent if the cable 1 is heated uniformly along its length. The circuit is designed so that, up to a temperature of, say, 100° C, the currents through the resistors 6 and 7 are sufficiently small to ensure that the resistors do not generate enough beat to blow the thermal fuse. Consider now wha+ happens if general overheating of the cable 1 occurs. If, for example, the temperature along the length of the cable 1 increases from 100° C to 125° C, the current through each of the resistors 6, 7 increases by a factor of ter. whereby the power they dissipate increases by a factor of one hundred. In other words, there would be a very large increase in the amount of heat generated by the resistors 6, 7, the heat generated being amply sufficient to ceuse the thermal fuse 8 to blow to disconnect the heating circuit from the supply.

Suppose now that instead of the cable 1 being overheated along its whole length, it is overheated at a localised position somewhere along its length, for example due to a ruck in the blanket or pad or a twist in the cable. Suppose, for example, that the position of the overheat is somewhere along the length of the sensor conductor portion 3'. The current through the resistor 7 will remain at its previous value. However, due to the drop .in impedance of the material 4 at the position of the overheat, the current through the resistor 6 will increase. The amount of the increase will depend on the position off the overheat, being at a maximum if the position is at the upper end of the sensor conductor portion 3’» where the voltage across the series combination of the resistor 6 7536 and the reduced impedance of the material 4 at the position of the overheat is 240 V, and at a minimum at the lower end of the sensor conductor portion ϊ, where (neglecting the shunting effect of the said serj.es combination) said voltage is only 120 V- However, even at the minimum voltage position, the logarithmic impedance/ temperature characteristic of the material. 4 will ensure that even a modest temperature rise will be sufficient to increase the heat output of the resistor 6 to'a value that will blow the fuse 8. For example, if only 10% of the length of the cable 1 is overheated and rises in temperature by 50 deg. C. to 150° C, the same amount of heat would be generated as would be the case if the whole length of the cable were heated by 25 deg. C to 125° C.

As will be evident, if the position of the overheat is somewhere along the length of the sensor conductor portion 3, rather than the portion 3*, in this case the resistor 7 will be heated by an amount sufficient to blow the thermal fuse. Thus, not only will the circuit respond to general overheating along the whole length of the cable 1, but it will also respond to localised overheating anywhere along the length of the cable. Due to its responsiveness to localised overheating, the circuit will provide protection against twists or loops in the cable 1. It will also, to some extent, provide protection against arcing in the inner, heating conductor 2 due to a break resulting in a short circuit between the 7536 conductors 2 and 3, which would cause a large amount of dissipation in one or the other of the resistors 6, 7.

The circuit described above could be modified by not connecting together the ends of the sensor conductor portions 3' and 3> i.e. by connecting the resistors 6, 7 only to one end of the portions 3’, 3, respectively. However, the described arrangement is preferred, since connection together of the ends of the- sensor conductor portions 3', 3 ensures that the circuit will continue to function satisfactorily in the event of a single break in either of the portions 3', 3Λ· The circuit described is of simple and cheap construction and is therefore eminently suited for use with an electrically heated underblanket or pad. It could, however, be modified by the provision, in series with the thermal fuse 6, of means to control the current flow through the heating conductor 2 to control the heat output. Such means could comprise electronic switch means (e.g. thyristors or a triac) controlled by manual and/or ambient temperature responsive control means. A more sophisticated arrangement of this nature would be Buited for use with overblankets as well as with underblankets and pads.

Although in the arrangement described above the increased current through one or the other of the resistors 6, 7 in the event of an overheat is employed to heat the resistor 6, 7 to blow the thermal fuse 8 to non-reset13 tably prevent current flowing into the heating conductor 2, the increased current can be employed in other ways to resettably or non-resettably prevent current flowing into the heating conductor. For instance, the voltage appearing across each of the resistors 6, 7 could be detected and used to open or to inhibit the closure of electronic switch means in series with the heating conductor 2 if either voltage rises to a level signifying overheating, Such electronic switch means could, as described in the preceding paragraph, also be used to control the current flowing through the heating conductor 2 to control heating of the blanket or pad.

The circuit described above (and also the modifications of the circuit described above) could be modified by employing undoped polyvinyl chlonie rather than doped polyvinyl chloride for the material 4. In this case, the impedance of the material 4 will rtill decrease logarithmically with temperature thereby the circuit will operate in a similar manner to the case in which undoped polyvinyl chloride is employed. However, due to the absence of the conductivity-enhancing dopant, the material will have to attain a higher temperature for its impedance to drop to a particular value than it would if it were doped. It has been found in practice that if undoped polyvinyl chloride is used for the material 4, the material 4 may in fact have to be heated to a temperature in the vicinity of its melting point, 4-7 5 36 namely 150 to 170° C, before its impedance drops to a sufficient value to ensure that sufficient current flows through one or both of the resistors 6 and 7 to cause blowing of the thermal fuse 8. It has in fact been found that if an overheat is very localised the undoped polyvinyl chloride material 4 may melt, which may cause the outer sensor conductor 3 to collapse ouuO the inner heating conductor 2. The resultant short circuit is equivalent to the impedance of the material 4 dropping to zero at the location of the overheat, whereby the thermal fuse 8 will quickly be blown. In fact, if such an abrupt drop in the impedance occasioned by melting is deemed sufficient for signalling localised overheating, one can employ for the material 4 a substance that has an impedance that does not vary substantially with temperature below its melting point, for example polyethylene.

Claims (15)

CLAIMS:

1. A heating circuit comprising a cable that comprises a heating conductor, a sensor conductor, and separating means that separates said two conductors and has an impedance that 5 falls with increasing temperature, the sensor conductor being interrupted at a. position along its length to define two sensor conductor portions, a .pair of input terminals for connection to a power supply, the heating conductor being connected between the input terminals, a first resistor 10 connected between one end of the heating conductor and the sensor conductor portion adjacent the other end of the cable, a second resistor connected between the other end of the heating conductor and the sensor conductor portion adjacent said one end of the cable, and means responsive to a current 15 of more than a predetermined magnitude flowing through at least one of the first and second resistors to prevent current flowing through the heating conductor.

2. A heating circuit according to claim 1, wherein the impedance of the separating means falls substantially logari20 thmically with increasing temperature.

3. A heating circuit according to claim 2, wherein the impedance of the separating means drops by a factor of ten for an increase in temperature of 25 deg. C.

4. A heating circuit according to claim 1, claim 2 or 25 claim 3, wherein the separating means comprises polyvinyl chloride 4 7536

5. · A heating circuit comprising a cable that comprises a heating conductor, a sensor conductor, and separating means that separates said two conductors and which will melt in the event of overheating to allow contact between the two conductors, 5 the sensor conductor being interrupted at a position along its length to define two sensor conductor portions, a pair of input terminals for connection to a power supply, the heating conductor being connected between the input terminals, a first resistor connected between one end of the heating 1° conductor and the sensor conductor portion adjacent the other end of the cable, a second resistor connected between the other end of the heating conductor and the sensor conductor portion adjacent said one end of the cable,and means responsive to a current of more than a predetermined magnitude flow15 ing through at least one of the first and second resistors to prevent current flowing through the heating conductor.

6. A heating circuit according to claim 5, wherein the separating means comprises polyethylene.

7. A heating circuit according to any one of claims 1 to 6, 20 wherein the current responsive means is operative to nonresettably prevent current flow through the heating conductor.

8. A heating circuit according to claim 7, wherein the current responsive means comprises a thermal fuse connected in series with the heating conductor and thermally coupled to both resistors such that it is blown by the heat generated in either or both resistors as a result of current flowing therethrough in the event of an overheat.

9. A heating circuit according to any one of claims 1 to 6, 5 wherein the current responsive means is operative to resettably prevent current flow through the heating conductor.

10. A heating circuit according to claim 9, wherein the current responsive means is operative to inhibit the closure of switch means arranged in series with the heating conductor 10 to control the flow of current therethrough.

11. A heating circuit according to any one of the preceding claims, wherein the said position of interruption of the sensor conductor is half-way along the length of the cable.

12. A heating circuit according to any one of the preceding 15 claims, wherein the ends of each sensor conductor portion are connected together. IJ.' A heating circuit substantially as herein described with reference to the accompanying drawing.

13. 14. An electric blanket incorporating a heating circuit 2o according to any one of the claims 1 to 13.

14.

15. An electric pad incorporating a heating circuit according to any one of claims 1 to 13.