GB2332287A - Controlling electrically-powered heating panels - Google Patents

Abstract

An electrically-powered heating panel in the form of an electric blanket 1 comprises a pair of dual coil heating elements E1, E2, current-regulating means 3, 4 for supplying electrical current to the elements E1, E2 in pulse form, and current-limiting means including a resistor R4 operable automatically in accordance with predetermined panel (blanket) temperature to interrupt ON pulse durations for one or more cycles at a time until said predetermined temperature is substantially constant. The current-regulating means 3, 4 comprise an integrated circuit chip 3 in combination with a triac 4. The chip 3 is a monolithic integrated bipolar zero voltage switch which enables the triac 4 to be switched when the AC voltage is zero, and to control the amount of power supplied.

Description

2332287 IMPROVEMENTS IN OR RELATING TO ELECTRICALLY-POWERED HEATING PANELS

This invention relates to electrically-powered heating panels.

As used herein, the term 'electrically -powered heating panels' is intended to include electrical ly-powered under-carpet heaters, under-soil heaters, blankets, mattresses and pads. The invention is particularly applicable however, to electrical ly-powered blankets, or, more simply, electric blankets.

According to the invention, an electrically-powered heating panel comprises at least one heating element, current-regulating means for supplying electrical current to the heating element in pulse form, and current limiting means operable automatically in accordance with a predetermined panel temperature, so as to interrupt 'on' pulse durations for one or more cycles until said panel temperature is substantially constant.

The current-regulating means may comprise a zero voltage switch in combination with a triac device driven thereby.

Embodiments of the invention will now be described by way of example only, with reference to the accompanying drawings wherein:

Figures 1, 2 and 3 are circuit diagrams, Figures 4 and 5 are side views, with parts removed, of heating elements, and Figure 6 is a plan view which illustrates the tortuous manner whereby the heating elements are wound.

In the Figures, like reference numerals refer to like components and features.

4Z With reference to Figure 1, an electrical ly-powered heating panel in the form of an electric blanket I comprises a pair of dual coil heating elements El, E2, current-regulating means 3, 4 for supplying electrical current to the elements El, E2 in pulse form, and current-limiting means including a resistor R4 operable automatically in accordance with predetermined panel (blanket) temperature to interrupt ON pulse durations for one or more cycles at a time until said predetermined temperature is substantially constant.

The elements El, E2 are preferably tortuously wound, as shown in Figure 6, whereby they cover substantially the plan area of the blanket 1.

With reference to Figure 4, the dual coil heating element El comprises an inner resistance coil 5 wound on a central support core 6 of rayon and covered with an insulating sheath 7 of thermoplastics material such as polyethylene. An outer resistance coil 8 is wound on the sheath 7 and is itself covered with an outer insulating sheath 9 of PVC.

Figure 5 illustrates the dual coil heating element E2, which is similar to E I. It has an inner resistance coil 20 wound on a central support core 21 and covered with an insulating sheath 22 of doped PVC. The PVC sheath 22 is doped so as to give it a substantially increased negative- resistance temperature coefficient. An outer resistance coil 23 is wound on the sheath 22 and is itself covered with an outer sheath 24 of non-doped PVC..

Coils 5, 8 of El and coil 20 of element E2 are blanket heating coils. Coil 23 of element E2 is a sensing coil. As already mentioned, sheath 7 of element El is of polyethylene. The coils 5, 8, 20 and 23 have common resistances; each coil having a resistance of 183 ohms.

Line (L) and neutral (N) connections of a 240 volt AC current supply are provided.

The current-regulating means 3, 4 comprise a TEA 1124 integrated circuit chip 3 in combination with a triac 4. The chip 3 is a monolithic integrated bipolar zero voltage switch which enables the triac 4 to be switched when the AC voltage is zero, and to control the amount of power supplied.

The chip 3 has a fixed mains synchronised ramp generator of 1280 ms duration and has eight pins, namely pins 1 to 8.

An alternative chip is available. This comprises the TEA 1024 integrated circuit chip, which has a fixed mains synchronised ramp generator of 640 ms duration.

The provision of the ramp generator enables the chip 3 to switch on the triac 4 for a period, each 1280 ms, providing burst fire power control.

Pin 1 of the chip 3 is not used. The potential on pin 2 determines the length of the duration which the triac 4 is turned on for each ramp cycle. The higher the potential, the shorter the duration, until the point where the triac 4 is fully switched OFF is reached. Alternatively, if the potential is lowered, the ON duration is increased until a point is reached where the triac 4 is fully switched ON.

Pin 3 is not used. Pin 4 is a negative DC supply connection. Pin 5 is the triac gate connection and provides the pulse to turn the triac 4 on. Pin 6, being connected to line L, is a positive DC supply connection. Pin 7 is the zero switching synchronous connection. Pin 8 is not used.

Low voltage (6.2 volt) supply for the chip 3 is provided by the circuit from Line, overload fuse 10, diode D3, pin 6, pin 4, and diode D2, R2 to Neutral. This half wave DC is smoothed by capacitor C2, and the magnitude of the voltage is controlled by the Zener diode D5.

The synchronising voltage to pin 7 is provided via resistance R1 which is connected to Neutral (N). Pin 5 is connected to the gate of the triac 4 via the light emitting diode D4.

Pin 2 is connected to capacitor Cl, the other side of which is connected to the line L and positive of the DC supply. Also connected to pin 2 is a variable DC voltage supply via resistor R5 of potentiometer VR, which is connected in series with resistor R6 across the DC supply. A further connection to pin 2 comes from the sensing element 23 of heating cable E2, via resistor R3 and diode D1. The potentiometer VR, which is used to determine blanket temperature, has a moveable contact I I and associated resistance with ends 12 and 13.

In series with the heating coils 8, 20 and 5 across the AC supply is the triac 4 and fuse 10. These three resistance wire coils which constitute the heating elements are of equal resistance, (183 ohms). The arrangement illustrated by Figure 1, is such that should part of element El become overheated, the polyethylene sheath 7 will melt and allow heating coils 5 and 8 to contact each other. This contact decreases the resistance of E I and E2 from 549 ohms (3 x 183 ohms of coils 5, 8 and 20) to 183 ohms. This 5 large fall in resistance causes fuse 10 to blow.

Using the TEA 1124 chip 3 in the conventional way, ignoring resistance values and their effect on the operation of the circuit, with VR set to maximum heat (i.e. the moving contact 11 at resistance end 12), and the AC supply connected, 6.2 volts is applied across VR and resistance R6 in series. The potential at the moveable contact 11 of VR is therefore zero. Current flows through resistance R5, lowering the potential on pin 2 of chip 3 and the negative side of capacitor Cl. Ignoring any other connections to pin 2 for the moment, pin 2 would become zero potential. This would cause the pulse at pin 5 to keep the triac 4 fully and continuously turned on.

The coil 23 of heating element E2 has both ends connected to Line via R4. Any leakage current through the doped PVC sheath 22 from the heating coil 20 creates an AC voltage drop across R4, one end of which is connected to pin 2 via R3 and diode D1. This voltage provides a half wave DC current which charges up capacitor Cl and pushes the potential positive on pin 2. As the temperature of the heating elements and the doped PVC sheath 22 increases, so the leakage current increases and pushes the pin 2 potential more positive. This reduces the pin 5 pulses and therefore the duration of the ON periods of the triac 4.

If VR is set at minimum heat (the moving contact 11 at end 13 of VR), then the potential on pin 2 is high enough (even without the influence of the doped PVC sensing circuit) to prevent the triac 4 from being switched on. If VR is set half way with contact 11 between ends 12 and 13 the potential reaches a balance controlled by the leakage in the doped PVC. As the leakage depends on the temperature of the panel, this gives the required thermostatic control.

If for instance the blanket is in use at a comfortable setting and the ambient temperature drops, this causes the doped PVC sensing sheath 7 to cool, which lowers the potential across R4 and thus pin 2, so as to increase the duration of the ON periods of the triac 4. If the ambient temperature increases, then the sensing sheath 7 decreases in resistance, increasing the potential drop across resistance R4, and also increasing the potential on pin 2. Thus the duration of the triac's ON pulses is shortened.

An explanation will now be given of the circuit, illustrated by Figure 1, but taking into account the values of resistances R3 and R5. By making the values of R3 and R5 substantially higher than would be the case if the TEA 1124 chip 3 was used in a conventional circuit, (when R3 and R5 would have values of about 1.0 x 10 4 ohms each), the substantially increased resistance results that charging and discharging of capacitor CI is very much slower. The potential on pin 2 therefore charges very slowly, thus causing the chip 3 to operate in a different manner from that it was designed to do.

With the VR at maximum heat setting, i.e. with movable contact 11 at end 12, when an AC supply is connected to the circuit, the doped PVC sensing sheath 22 being cold, has a high resistance.

Therefore the current flowing to capacitor Cl and pin 2 is less than that being drained off slowly through the high resistance R5. The potential thus stays low and the pulses at pin 5 keep the triac 4 switched ON. As the sensing sheath 22 heats up and the leakage current increases,. then the current slowly flowing to capacitor Cl from diode DI, starts to beat the slow drain through resistance R5. The potential on pin 2 gradually increases, and starts the ON/OFF 2380 ms cycle of the triac 4. The temperature of the heating elements El, E2 is still increasing and the sensing current intensifies to a point where it beats the slow drain through resistance R5. The potential on pin 2 eventually gets high enough to cut the pulses from pin 5. The triac 4 is therefore switched OFF. With triac 4 switched OFF, the heating elements El, E2 cool, and the potential on pin 2 slowly decreases, eventually starting the pulses at pin 5 again so as to cause the triac 4 to restart the ON and OFF pulses. At first, the odd 1280 ms cycle may be missed but if the blanket temperature continues to increase, then the triac 4 is switched OFF for more than one cycle at a time.

This continues until a substantially steady blanket temperature is reached.

Thus, by providing resistances R3 and R5 with high values, (3.3 x 10 6 ohms and 4.7 x 105 ohms respectively), a circuit results which not only operates so as to cut the length of ON pulses in each ramp cycle, but also interrupts the ON pulse durations for one or more cycles. This provides the blanket I with excellent temperature control.

Figure 2 shows a fuller version of the circuit of Figure 1, with the addition of interference suppressing circuits including resistance R7, capacitor C3, and a varistor 29, a neon indicator 30. A thermal fuse 31 is used instead of the fuse 10.

With Figure I as an example, polyethylene could be used as an inner sheath between the two resistance coils 5 and 8 of heating element El to give blanket overheat protection, and doped PVC for sensing element E2.

PVC would normally be considered unsuitable for use with heating element El, as PVC age-hardens whereby it fails to melt and allow contact between the inner and outer coils 5, 8. However, now the heating cable manufacturers have produced improved PVC material which does not age harden. This improved PVC material is used in the sheath 7 of the heating element El, instead of polyethylene. It is now possible to use just one length of heating cable instead of two. Figure 3 shows one way of adapting the circuit so as to employ one length of heating cable E3 in which the inner insulating sheath 42 comprises the new type doped PVC, coils 40, 41 are inner and outer resistance coils (each with a resistance of 548 ohms which is suitable for a blanket heating output of 105 watts. In this embodiment, inner and outer coil resistance are the same, although the resistance of coil 41 is not critical). The change to the circuit involves the use of a thermal fuse 45 heated by resistance R4, in which the current is normally very small. However, if overheating occurs, sufficiently to melt sheath 42 and allow the coils 40, 41 to make contact, then a much heavier current would flow in resistance R4, heating up the thermal fuse 45 and tripping it OFF.

Claims (22)

1. An electrically-powered heating panel comprising at least one heating element, current-regulating means for supplying electrical current to the heating element in pulse form, and current limiting means operable automatically in accordance with a predetermined panel temperature, so as to interrupt 'on' pulse duration's for one or more cycles until said panel temperature is substantially constant.

2. A heating panel as claimed in claim 1, wherein the current -regulating means comprise a zero voltage switch in combination with a triac device 10 driven thereby.

3 A heating panel as claimed in claim 2, wherein the zero voltage switch is a monolithic integrated bipolar zero voltage switch.

4. A heating panel as claimed in claim 3 wherein the zero voltage switch comprises a TEA 1124 integrated circuit chip.

5. A heating panel as claimed in claim 3 wherein the zero voltage switch comprises a TEA 1024 integrated circuit chip.

6. A heating panel as claimed in any one of claims 2 to 5, wherein the current limiting means comprise a resistance connected between line and said heating element, and operable by increased panel temperature, to 20 reduce ON periods of the triac device.

7. A heating panel as claimed in any one of claims 1 to 6, wherein said heating element comprises a pair of dual coil heating elements.

8. A heating panel as claimed in claim 7, wherein one of said dual coil heating elements comprises an inner resistance coil wound on a central support core of plastics material and covered with an insulating sheath of thermoplastics material, with an outer resistance coil wound on said sheath 5 and itself covered with an outer insulating sheath of plastics material.

9. A heating panel as claimed in claim 8, wherein the other of said dual coil heating elements comprises an inner resistance coil wound on a central support core of plastics material and covered with an insulating sheath of negative-resistance temperature coefficient material, with an outer resistance coil wound on said sheath and itself covered with an outer insulating sheath of plastics material.

10. A heating panel as claimed in claim 9, wherein said first-mentioned insulating sheath is of polyethylene.

11. A heating panel as claimed in claim 8 or 9, wherein said central support core is of rayon.

12. A heating panel as claimed in claim 8, 10 or 11, wherein said outer insulating sheath is of PVC.

13. A heating panel as claimed in claim 9, wherein said first-mentioned insulating sheath is of PVC which is doped so as to give it a substantially increased negative-resistance temperature coefficient.

14. A heating panel as claimed in claim 13, wherein said other insulating sheath is of non-doped PVC.

15. A heating panel as claimed in any one of claims 1 to 6, wherein said at least one heating element comprises a single heating element of dual coil form.

16. A heating panel as claimed in claim 15 wherein said single heating element comprises inner and outer resistance coils separated by negativeresistance temperature coefficient insulating material.

17. A heating panel as claimed in claim 16 wherein said coil-separating insulating material comprises PVC which is doped so as to give it a substantially increased negative-re s i stance temperature coefficient.

18. A heating panel as claimed in any one of claims 1 to 17 wherein the heating element is tortuously wound so as to cover substantially the plan area of the panel.

19. A heating panel substantially as hereinbefore described with reference to Figures 1, 4, 5 and 6 of the accompanying drawings.

20. A heating panel substantially as hereinbefore described with reference to Figures 2, 4, 5 and 6 of the accompanying drawings.

2 1. A heating panel substantially as hereinbefore described with reference to Figures 3, 4, 5 and 6 of the accompanying drawings.

22. A heating panel as claimed in any one of claims 1 to 21, comprising 20 an electric blanket.