GB2399702A

GB2399702A - Fail-safe electric heating controller

Info

Publication number: GB2399702A
Application number: GB0405748A
Authority: GB
Inventors: Charles Borrill; Barry Ward
Original assignee: BURCO DEAN APPLIANCES Ltd
Current assignee: BURCO DEAN APPLIANCES Ltd
Priority date: 2003-03-15
Filing date: 2004-03-15
Publication date: 2004-09-22
Anticipated expiration: 2024-03-15
Also published as: GB0305981D0; GB2399702B; GB0405748D0

Abstract

A heating controller 3 supplies a heating panel 4, such as an electric blanket, with pulsed AC power, skipping at least one full cycle of power between successive ON pulses, thereby reducing the power supplied to the element 4. The controller has a fail-safe device such as a thermal fuse (fig 4, F) which breaks the supply to the element 4 if full power is supplied, possibly as a result of an ON state fault in controller 3. Each ON pulse may be a single AC cycle and may be fo every other cycle of the AC supply, the controller 3 may comprise a triac (fig 4,T1) and the fail-safe device may comprise a thermal fuse and associated heating resistor (fig 4, R15, R28) or a time-delay fuse. Controller 3 controls the temperature of heater 4 from a feedback signal from a sensing element 6 which may be separated from heating conductor 7 by a sheath (fig 2, 8) of material such as doped PVC which leaks a current to heating element 7 according to the temperature. Controller 3 may sense the current leaked to conductor 6 during OFF periods of the current. Conductor 6 may be connected in series with element 7 via a diode 21.

Description

ELECTRICALLY POWERED HEATING PANELS

The present invention relates to electrically powered heating panels such as, for instance, electric blankets. In particular, the present invention relates to control circuitry for controlling the supply of current to an electrically powered heating panel.

Electric blankets generally comprise a heater consisting of at least one heating element which follows a tortuous path over the area of the blanket, and a control circuit for controlling the current delivered to the heating element. It is a safety requirement of such electric blankets that the current supplied to the heating element is automatically reduced or cut-off in the event of over heating. In addition, it is generally desirable to construct the control circuit so that the current supply to the heating element can be controlled to provide a predetermined temperature which is preferably selectable by the user.

An example of a known electric blanket is described in British patent application number GB 2178201A. This discloses an electric blanket comprising a dual element heating/temperature sensing cable in which the two elements are separated by an insulating layer of PVC. The PVC insulating layer is constructed from a doped PVC with a resistance which decreases with increasing temperature.

Only one of the elements serves as a heating element and the other forms part of a sensing circuit which picks up any current which leaks across the PVC insulating layer from the heating coil. Power is supplied to the heating coil by a control circuit in pulses. The duration of the ON pulse time is controllable by the user as a means for regulating the temperature of the blanket. Should the heating element begin to overheat, the current leaking across the PVC insulating layer increases with a resulting increase in current through the sensing coil which causes the control circuit to reduce the pulse ON time, hence reducing the temperature of the heating element.

Further safety circuitry is provided to cut power to the heating coil in the event that the ON pulse time exceeds a predetermined maximum limit.

British patent number GB2287591 seeks to improve on the above by providing an electrically powered heating panel which comprises two dual element : : : : À : : : : À 1 À À À À . cables. A first dual element cable comprises two heating elements separated by a polyethylene insulating layer, and a second dual element cable comprises a third heating element and a sensing element (and as such corresponds to the dual element of GB 2178201 mentioned above). Supply current is provided to the heating elements by a triac under control of a burst control switch which varies the ON pulse duration in response to monitored levels of leakage current in the sensing element which varies with the temperature of the third heating element. Whereas the insulating layer between the heating element and sensing element of the second dual heating/sensing cable is constructed from doped PVC with temperature dependent resistance, the insulating layer between the heating elements of the first dual element cable is constructed from polyethylene which melts at a predetermined temperature to provide fail-safe protection. Specifically, the polyethylene is designed to melt when the blanket reaches a potentially dangerous temperature (if for instance the normal control circuitry fails) causing thereby a short circuit between the heating elements of the first dual element cable which blows a fuse in the control circuitry to cut off the mains supply.

Another known electric blanket is disclosed in PCT patent specification WO99/30535. Here a single dual sensing/heating element cable is employed to provide a sensing current for normal temperature control and also failsafe protection.

A doped PVC insulating layer is used between the heating and sensing elements which both has a temperature dependent resistance and which is also designed to melt as the temperature of the heating element approaches an unsafe temperature. Whilst this arrangement has the advantage of requiring only a single dual element cable, there is some concern as to whether the melting temperature of PVC is reliable, particularly as the PVC insulating layer ages over a number of years.

It is an object of the present invention to obviate or mitigate the above disadvantages. .

: : : : À : : : : . e ce Be. cae e e e À ee Àe According to a first aspect of the present invention there is provided a controller for controlling the supply of heating current to an electric resistance heater; the controller comprising: AC power supply terminals for connection to an AC power supply; a microprocessor; a heating current supply circuit connected to said AC power supply terminals and to said microprocessor, the heating current supply circuit comprising means for supplying pulses of AC heating current in response to a control signal provided to the heating current supply circuit by the microprocessor; wherein each ON pulse comprises at one or more full cycles of the AC supply, successive ON pulses being separated by at least one full cycle of AC supply such that the maximum power of the heating current is less than the full power of the AC supply; and fail-safe protection means are provided which operates to break the connection between the AC supply terminals and the heating current supply circuit in the event that the heating current rises to full power of the AC supply indicating an ON state failure of the controller.

For instance, each ON pulse may comprise a single cycle of the AC supply and the heating current supply may be pulsed on a maximum of once every other cycle of AC power supply. This will mean that failure of the control componetary leading to an ON state condition will instantly double the power of the AC signal drawn by the current supply circuit. Such a dramatic difference in power can readily be monitored, for example using a fuse arrangement such as a thermal fuse in combination with a heating resistor which rapidly heats up with the increase in power.

The controller of the present invention thus provides fail-safe operation without the need for fail-safe features to be built into the resistance heater itself.

Thus, in accordance with the present invention there is provided in combination a resistance heater and a controller according to any preceding claim, wherein the resistance heater comprises a first elongate conductor and a second elongate conductor, the first elongate conductor forming a heating element and the c c c c c e c c e c c c e e cc À e ec ace c c À c ace c e c À C À c e e ee À c e second elongate conductor forming a sensing element, the heating element and sensing element being separated by a layer of material with an electrical resistance dependent upon temperature, wherein the sensing element conducts a sense current to the controller indicative of the temperature of the heating element.

The heating element and the sensing element may be connected electrically in series. This reduces the RF emissions of the resistance heater, by allowing substantially the same AC supply to pass along the resistance heater in opposite directions. This reduces the overall RF emissions of the cable as the RF emissions are in opposite phase along the heating element and the sensing element. The sensing element conducts a sensing current to the controller during each OFF pulse of the AC supply.

A diode may be connected in series with the heating element and the sensing element. This provides additional over-current protection.

Further preferred and advantageous features of the invention will become

apparent from the following description.

Specific embodiments of the present invention will now be described, with reference to the accompanying drawings, in which: Figure 1 is a schematic illustration of an electrically powered heating panel and controller in accordance with the present invention; Figure 2 is a part sectioned illustration of a dual heating/sensing cable of the electric heating panel of Figure 1; Figure 3 is a block diagram of one embodiment of a controller for an electrically powered heating panel in accordance with the present invention; Figure 4 is a circuit diagram showing components of the controller of Figure 3; and Figure 5 is a schematic illustration of an alternative arrangement of the dual heating/sensing cable, in combination with the controller, in accordance with the present invention.

Referring to Figure 1, this schematically illustrates an electrical heating panel, in this case electric blanket 1, which comprises an electrical resistance heater 2 I t À 1 disposed within the fabric of the blanket I, and a controller 3. The resistance heater 2 comprises a single dual element cable 4 which follows a tortuous path covering the area of the electric blanket 1. The electric blanket 1 connects to the controller 2 via a blanket socket 5, and heating current is supplied to the electric heater 2 from the controller 3 which is connected to a mains AC source 6.

The controller 3 is a microprocessor based circuit which controls the temperature of the electric blanket by appropriate control of the heating current supplied to the heater 2 and will be described in more detail further below.

Referring now to Figure 2, this illustrates the construction of the dual element heating/sensing cable 4. The cable 3 comprises a single sensing element 6 (conductor wire) separated from a coiled heating element 7 (conductor wire) by a doped PVC insulating sheath 8 which has a temperature dependent resistance. The heating element 7 is itself insulated from the surrounding environment by outer PVC insulating sheath 9. This structure is essentially known and provides for a certain degree of current leakage across the doped PVC insulating layer 8 which produces a current in the sensing element 6, the magnitude of which is dependent upon the temperature of the heating element 7. The cable 4 is not however designed to provide any protection against overheating due to a failure in the control. In other words, cable 4 is not provided with a fail-safe meltdown layer as found in the prior art electric blankets mentioned above. Rather, this protection is provided by novel control circuitry of the controller.

Referring to Figure 3, this is a block diagram illustrating the circuit elements of a first embodiment of the controller 3 in accordance with the present invention.

Heating current for the resistance heater 2 is derived from the AC mains supply 20 and supplied to the heating element 7 of the heating/sensing cable 4 via a drive circuit in response to control signals provided by a microprocessor 1 1. The microprocessor 11 is powered from a 5V regulated supply circuit 12 which is connected to the AC mains supply 20.

The temperature dependent sensing current from the sensing element 6 is supplied to a feedback circuit 13 which provides a feedback signal to the r C CC C C C r C t À C C c C C C C r C C C C À C. À C C C À C microprocessor 11. The microprocessor 11 is programmed to control operation of the drive circuit 10 in response to changes in the feedback signal supplied to the microprocessor 11 which are indicative of the temperature of the heating element 7.

A mains voltage feedback circuit l 4 is included to provide the microprocessor 11 with a reference signal indicative of the precise mains voltage being supplied to the blanket drive circuit 10 by AC mains source 5. This enables the microprocessor to take account of any variations in the voltage of the mains supply signal when controlling operation of the drive circuit 10, thus improving the accuracy of control of the temperature of the electric blanket 1.

A mains synchronization circuit 15 is included to enable the microprocessor 11 to synchronise operation of the drive circuit with the AC mains signal to reduce unwanted RF emissions (explained in more detail further below).

Time and temperature switches 16 and 17 are provided to allow user control of the operating parameters of the microprocessor 11, i.e. to set desired electric blanket temperatures and/or operating times.

Referring now to Figure 4, this is a circuit diagram illustrating one implementation of the controller circuitry in accordance with the block diagram of Figure 3. Microprocessor 11 receives a 5V regulated power supply derived from a mains AC supply input at mains terminals L (live) and N (neutral), and mains supply switch MS, via power supply circuit 12. The power supply circuit 12 is conventional and comprises two 15kQ dropping resistors Rl and R2 (to drop the mains voltage down to the required 5V) a 5V zenner diode ZDI (operating as a 5V stabilizer), and capacitors C2 and C5. C2 is a smoothing capacity to smooth the rectified signal and C5 is provided to filter out transient spikes in the mains supply.

A neon Nl and associated dropping resistor Rl are provided in series between the main switch MS and the power supply circuit 12 to provide a power on indication and a power switch illumination.

The panel drive circuit 10 comprises a triac Tl which provides a heating current to the panel (not illustrated in Figure 4) which is connected to the controller via drive circuit live terminal Ll and power supply neutral terminal N2. The triac Tl Act: 1 1 1 supplies pulses of heating current to the panel in response to a pulsed control signal received from the microprocessor 11 via resistor R9 and capacitor C4. The resistor R9is a current limit resistor for the triac T1 and the capacitor C4 ensures that only the SV pulses are supplied to the triac T1 from the microprocessor 11, in other words operating as a DC blocker preventing a constant 5V signal from being supplied to the triac T1 in the event of microprocessor failure. A resistor R7is included to provide a DC path for the charge/discharge of the capacitor C4.

For convenience, the connection between the panel drive circuit 10 and microprocessor 11 is not shown but it will be appreciated that the connection is made between the resistor R9 and the microprocessor at the connection labelled "triac".

The current supply to the blanket via the drive circuit 10 is monitored by resistors R15 and R28 connected in series in the power line between the drive circuit and the mains terminals together with a thermal fuse F. These components provide fail-safe operation which will be described in further detail below.

The mains voltage feedback circuit 14, which provides a mains reference voltage to the microprocessor, comprises resistors R3 and R4 and capacitor C1. R3 and R4 are mains dropping resistors (required since the mains reference voltage is taken before the mains supply dropping resistors) and C1 is provided to smooth the reference signal. The reference signal is supplied to the microprocessor pin labelled Vref via reference signal circuit output similarly labelled Vref.

The mains synchronization circuit 15 comprises diode D1 and resistor R2. A synchronization signal, identified by the arrow "CLK" connects to microprocessor pin similarly labelled CLK.

The temperature switch 17 comprises a network of resistors R17 to R27 and associated switch terminals WT28 to WT35 (WT is an abbreviation of"wire take off"). A further resistor, R30is provided to ensure continuity of the DC temperature control signal provided to the microprocessor in the event of switch failure (for instance if one switch terminal is connected to another) to ensure that the microprocessor does not interpret such a situation as an instruction to power the panel. te ce

À ces ace À A capacitor C8 is provided to suppress transients in the temperature switch signal provided to the microprocessor 11.

The time switch circuit 16 comprises a second network of resistors Rl I to R14 which provide a switched reference voltage via associated switches WT28 to WT31.

A further resistor R31 is included to provide DC continuity in the event of switch failure (so that the microprocessor will not default to an "ON" condition if the switch circuitry fails) and a capacitor C6 is provided for smoothing transients.

The feedback circuit 13 connects to the panel at contact terminal FB which receives sensing current from the sensing element of the panel sensing/heating cable 4. The feedback circuit 13 comprises sensor resistor 32 (which may be located in the controller 3 or in the connection socket 5 of the panel) which is connected between the feedback circuit and the panel drive circuit (the resistor terminal marked fail connecting to the triac terminal similarly marked fail) to convert leakage current from the panel sensing wire to a voltage which is then rectified by a diode D4. A portion of the rectified voltage is supplied to the microprocessor terminal marked VFB via feedback resistors R16 and R8, a capacitor C7 being provided to smooth transients.

Finally, capacitor C3 provides further suppression of transients in the supply signal.

Operation of the controller and heating panel will now be described.

Desired temperature and operating time instructions are provided to the microprocessor 11 via appropriate operation of the temperature and time switches 17 and 16 and power is supplied to the controller 3 and blanket 1 by closing the mains switch MS. Taking into account the mains reference voltage signal supplied to the microprocessor 11 via the mains voltage feedback circuit 14, the microprocessor supplies a pulsing SV command signal to the triac T1 of the panel drive circuit 10.

The command signal pulses are synchronised to the AC supply signal by way of the clock signal provided to the microprocessor via the mains synchronization circuit 15.

The triac T1 is thereby switched at, or close to, the zero crossing point of the AC supply signal to reduce RF emissions in a known way. À c À c À À

In accordance with the present invention, the microprocessor 11 is programmed to fire the triac T1 at most every other cycle of the mains supply voltage.

In other words, the triac T1 fires for a full supply cycle then skips at least one subsequent supply cycle before firing again. The maximum power supply to the blanket is therefore half of that available from the mains supply. Initially, the triac T1 will be fired every other cycle of the mains supply to provide rapid heating until the desired blanket temperature is reached. Thereafter, the triac T1 may only need to fire once every second or so in order to maintain the blanket at the required temperature.

As mentioned above, the microprocessor 11 monitors the temperature of the blanket 1 via the feedback signal provided from the feedback circuit 13 connected to the sensing element 6 of the dual heating/sensing cable 4.

If for any reason the temperature of the blanket exceeds the predetermined maximum safe level, for instance if the blanket is folded or creased, this will be reflected in the magnitude of the sensing current and thus feedback signal provided to the microprocessor. The microprocessor 11 is therefore programmed to switch off when the feedback signal exceeds a predetermined level, and remain inoperative until the fault is removed and the controller is re-set. Similarly, the microprocessor is programmed to switch off if the connection to the sensor wire becomes open circuit (in the event of disconnection of the controller and blanket) and remain off until the connection is re-made.

Accordingly, under normal operating conditions the microprocessor l l will control firing of the triac T1 to ensure that the blanket temperature remains at that required by the user (as set by the temperature switch 17) and for the time required by the user (as set by the time switch 16) and will shut down if it detects an uncontrollable rise in temperature. However, if for any reason the control circuit fails, for instance if the triac fails or there is a short circuit within the controller 3, the full mains supply signal will be supplied to the blanket 1. In other words, the controller circuit will go full power. In particular, the current flow through the resistors R15 and R28 will immediately double with a rapid increase in temperature sufficient to blow the thermal fuse F. . À 1 c c c À 1 1 À 4

C C

Thus, the controller of the present invention fails safe in the event of any component failure, and in particular in the event of failure of the triac T1, to prevent overheating of the electric blanket. Moreover, this is achieved without damage to the blanket or other circuitry of the controller. Achieving such fail safe operation without relying on melt down of an insulating layer within the heater 2 is a significant

advantage over the prior art.

It will be appreciated that the value of the sense resistor R32 must be matched to the design of electric blanket or other heating panel since the exact relationship between the magnitude of the sense current and temperature of the heating element may vary from one design of panel to the next. This is not a problem if the controller is only ever going to be used with one form of panel, or if the controller and the panel are integral (inseparable). However, a further advantageous feature of the present invention is that the sense resistor may be located in the connecting socket 5 of electric blanket 2 (or similar heating panel) so that a standard controller 3 can be used to operate a variety of different forms (e.g. sizes and shape) of panel.

To take account of the fact that it can take a certain amount of time for the temperature of the heating element to actually transfer to the external temperature of the heating panel (e.g. a blanket), the microprocessor may be programmed with a "warm up" feature so that on start up an instruction to heat the blanket to a given temperature is interpreted as an instruction to heat it to a temperature somewhat higher for a predetermined start up period. Both the start up temperature and period may be predetermined for any particular selected temperature.

Referring now to Figure 5, this is a schematic illustration illustrating an alternative arrangement of the dual heating/sensing cable, in combination with the controller, in accordance with the present invention. Controller 3 remains as described above and illustrated in Figure 4. Dual element cable 4 retains the same physical construction as described as above and illustrated in Figure 2. In this alternative arrangement, coiled heating element 7 remains connected at one end to drive circuit live terminal L1. However, the other end of heating element 7 is electrically connected in series with sensing element 6. Sensing element 6 is formed by the core l. c À -

of the dual element cable 4. In series with heating element 7 and sensing element 6 is a diode 21. Sensing element 6 is connected to the power supply neutral terminal N2.

Additionally, it is also connected to the feedback circuit 13 via contact terminal FB.

Sensing element 6 and heating element 7 remain separated from each other, except at the end connected via diode 21, by insulation layer 8.

It will be appreciated that the diode 21 may be located at any point along the path of heating element 7 and sensing element 6. Conveniently, diode 21 may be located in the connection socket 5 of the heating panel. The effects of diode 21 is that the heating element and sensing element will only conduct for half of each full cycle of the AC power supply 20. This reduces the average power output of the heating element 7. However, the average power output of heating element 7 may be kept at the previous value for the above arrangement of the dual element cable 4 by halving the resistance value of the heating element 7. The resistance of heating element 7 is large relative to the resistance of sensing element 6. Consequently, only the heating element 7 exhibits significant heating effects.

The effect of connecting the heating element 7 and the sensing element 6 in series is to reduce the RF emissions from the heating panel. RF emissions from the heating panel are due to the fluctuation of the voltage within the heating element between the positive and negative peaks of the AC supply cycle. By connecting the heating element 7 and the sensing element 6 in series arranged within the same dual element cable, substantially the same AC supply current passes along the cable in opposite directions. This reduces the overall RF emissions of the cable as the RF emissions are in opposite phase along the heating element 7 and the sensing element 6.

Additionally, the diode 21 gives the resistance heater further overcurrent protection. Should there be a breakdown in the insulation between the heating element 7 and sensing element 6 the diode 21 will be shorted out. This effectively doubles the power output of the resistance heater as the heating will now flow for the whole of each ON cycle, causing the fuse F to blow. 1

À 8 1 8 À À À 1 11 8 8 e ( À 1 8 1 / 8 8 À 8 I e À 8 e À e. e e e As the sensing element 6 is now used to carry the returned heating current from the heating element 7 the method of measuring the feedback temperature of the heating panel must be altered. The temperature sensing via feedback network 13 is arranged to occur on OFF cycles of the supply to the heating cable. During OFF cycles the resistance between the heating element 7 and the sensing element 6 is measured. This is achieved using the same feedback network 13 as is described above and illustrated in Figure 4, in combination with diode 21.

An advantage of this alternative arrangement is that the RF emission of the resistance heater is reduced for any power setting, while retaining over current protection.

Many other modifications could be made to the embodiment of the invention described in detail above. For instance, the temperature and time switches could be combined in a single switch. Alternatively, temperature control can be omitted entirely if the heating panel is intended to be operated at a single predetermined temperature. Similarly, a timer switch may not be required and could be replaced by a simple on/off switch in many applications.

With the above described embodiment of the invention the triac is fired at most every other supply cycle so that the nominal maximum power to be supplied to the blanket is half that available from the power supply. The same effect could be achieved by firing the triac on two or more successive cycles provided that on average over a certain period of time the triac fires for no more than 50% of the supply cycles.

Whether or not this will be possible in any particular embodiment of the invention will depend upon the reaction time of the resistors R15 and R28 and thermal fuse F (or other fuse arrangement used). Similarly, it is not absolutely necessary that the nominal maximum power of the blanket is only half of that of the power supply, this figure is chosen to ensure that there is a very large difference between normal full power operation of the circuit and the full power available from the mains supply. In practice, the triac could be controlled to fire on more than 50 /O of the power supply cycles, again to some extent dependent upon the speed of reaction of the fuse arrangement.

À ce À ca e a ce Àe a À À Àcat À a À À À À À . ..

It will also be appreciated that fuse arrangements other than the thermal fuse arrangement described above might be employed. For instance a time delay fuse could be used which blows when the supply current increases beyond a predetemmined level. This may need to be matched to each individual application of the invention whereas the themmal fuse has the advantage that it may be a standard component for controllers intended for different heating panels.

It will be appreciated that the nature of the heating panel may vary, and that the invention is not limited to electric blankets or the like. For instance, alternative forms of heating panel include under-carpet heating pads and heated mattresses etc. Similarly, the panel may comprise more than one dual heating/sensing cable, for instance two such cables coveringdifferent areas of the panel. It will further be appreciated that the detailed structure of the cable 4 could be modified. For instance, alternate material to the doped PVC could be used as the temperature dependent insulating layer with temperature dependent resistance. For example, undoped PVC also has a negative co-efficient resistance (resistance which drops with increasing temperature) although the resistance is somewhat higher than that of doped PVC and thus some amplification of a leakage current might be required.

With the above described embodiment of the invention the controller and heating blanket are separable components. In other embodiments a controller may be built into the heating panel.

Other possible modifications of the embodiments described above will be apparent to the readily skilled person.

Claims

À c. es. e e co. À À . À . . . À À . . . CLAIMS

1. A controller for controlling the supply of heating current to an electric resistance heater; the controller comprising: AC power supply terminals for connection to an AC power supply; a microprocessor; a heating current supply circuit connected to said AC power supply terminals and to said microprocessor, the heating current supply circuit comprising means for supplying pulses of AC heating current in response to a control signal provided to the heating current supply circuit by the microprocessor; wherein each ON pulse comprises one or more full cycles of the AC supply, successive ON pulses being separated by at least one full cycle of AC supply such that the maximum power of the heating current is less than the full power of the AC supply; and fail-safe protection means are provided which operates to break the connection between the AC supply terminals and the heating current supply circuit in the event that the heating current rises to full power of the AC supply indicating an ON state failure of the controller.

2. A controller according to claim 1, wherein each ON pulse comprises a single cycle of the AC supply.

3. A controller according to claim 1 or claim 2, wherein when said heating current supply circuit is operating to provide a maximum heating current, the current supply means is pulsing ON every other cycle of the AC power supply.

4. A controller according to any preceding claim, wherein the heating current supply means comprises a triac. À

: : se: c:. :e: : :: À e.

5. A controller according to any preceding claim, wherein said fail-safe protection means comprises a thermal fuse and associated heating resistor connected between the heating current supply circuit and the AC power supply terminals.

6. A controller according to any one of claims I to 4, wherein said failsafe protection means comprises a time delay fuse.

7. A controller according to any preceding claim, wherein the microprocessor is programmed to control the heating current supply circuit to provide sufficient current to maintain the resistance heater at a predetermined temperature, and wherein the controller includes a feedback circuit for providing a feedback signal to the microprocessor which is dependent upon the instantaneous temperature of resistance heater, said microprocessor operating to vary control of the heating current supply circuit in response to changes in the feedback signal.

8. A controller according to any preceding claim, wherein the microprocessor is programmed to stop the heating current supply circuit in the event of an uncontrollable temperature rise in the resistance heater or disconnection of the resistance heater from the controller.

9. In combination a resistance heater and a controller according to any preceding claim, wherein the resistance heater comprises a first elongate conductor and a second elongate conductor, the first elongate conductor forming a heating element and the second elongate conductor forming a sensing element, the heating element and sensing element being separated by a layer of material with an electrical resistance dependent upon temperature, wherein the sensing element conducts a sense current to the controller indicative of the temperature of the heating element.

10. A controller according to claim 9, wherein the heating element and sensing element are combined in a single dual core cable.

: e e. c À e À À À C C À

11. A controller according to claim 9 or claim 10, wherein the resistance heater is provided with a sense resistor, which attenuates the sensing current to a control level appropriate to the controller.

12. A controller according to any one of claims 9 to 11, wherein the heating element and the sensing element are connected electrically in series, and the sensing element conducts the sensing current to the controller during each OFF pulse of the AC supply.

13. A controller according to claim 12, wherein a diode is connected in series with the heating element and the sensing element.