GB2355120A

GB2355120A - Electrically powered heating panel

Info

Publication number: GB2355120A
Application number: GB9918209A
Authority: GB
Inventors: Barry Ward
Original assignee: BURCO DEAN APPLIANCES Ltd
Current assignee: BURCO DEAN APPLIANCES Ltd
Priority date: 1999-08-04
Filing date: 1999-08-04
Publication date: 2001-04-11
Also published as: GB9918209D0

Abstract

An electrically powered heating panel (1), for example an electric blanket, comprises at least two heating elements (2, 3) each disposed in a different part of the panel, with a single control circuit (4) for the supply of current to all elements. The elements may be switched individually, connected in series or connected in parallel. A current reduction means may be employed to reduce the current when a single element is energised. The control circuit may include a single temperature control circuit. The control circuit may be a burst control triac circuit. The invention may be applied to an electric blanket providing more heat to the foot area than to the body area.

Description

2355120 ELECTRICALLY POWERED HEATING PANEL The present invention relates

to an electrically powered heating panel and particularly, but not exclusively, to an electric blanket.

Conventional electric blankets generally comprise a heater consisting of a resistance heating element which is laced in a tortuous pattern over the area of the blanket, and a control circuit for controlling the current delivered to the heating element. Basic control circuitry may provide simple on/off switching but it is more desirable, and quite common, for the control circuit to provide for variable control of the current supplied to the heater, and thus the temperature output. For instance, it may be desirable to set the electric blanket to a relatively high pre-heat temperature, but to set the blanket to a relatively low "over-night" temperature. In addition to on/off switching, and temperature control, it is a safety requirement of electric blankets that the current supplied to the heating element is automatically reduced, or cut-off, in the event of overheating. Thus, the control circuitry will also comprise some form of over-heat protection.

It is recognised that it is desirable to provide more heat to the foot area of an electric blanket than the body area during use, since feet tend to loose heat faster than the rest of the body. Thus, electric blankets are known which have a heater element which is laced in a pattern which gives a higher heat density in the foot region of the blanket than in the body region of the blanket. However, a shortcoming of this arrangement is that although more heat will be supplied to the foot area than the body area, it is necessary to have at least some heat supplied to the body in order to heat the foot 'area. This may be undesirable, for instance, at night time when users may find it is not possible to warm their feet without the body becoming uncomfortably warm.

Swiss patent CH 559495 tackles the above problem by providing an electric blanket with two separately controlled heaters, one covering the lower half of the user and the other covering the upper half. Each heater has its own thermostat and is separately controlled by a respective external control switch. Whilst offering an effective way of overcoming the above problem, the resultant blanket has a disadvantage in that it is relatively expensive to manufacture.

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

According to the present invention there is provided an electrically powered heating panel, comprising an electrical resistance heater and current supply control circuitry for controlling the supply of current to the heater, wherein the heater comprises at least first and second heating elements each disposed in different regions of the panel, and the current supply circuitry comprises a single control circuit for controlling the supply of current to both elements.

The provision of a single control circuit for controlling the current supplied to separate heating elements both simplifies the circuitry reducing manufacturing costs and provides increased ease of use.

Specific embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure I is a schematic illustration of one possible arrangement of an electric blanket in accordance with the present invention; and Figures 2 to 6 are circuit diagrams of five different embodiments of electric blankets according to the present invention having the general arrangement of Figure I.

Referring to Figure 1, this schematically illustrates the basic configuration of an electric blanket I in accordance with the present invention designed to provide more heat to a lower foot region I a than to an upper body region lb. Lower and upper heating elements 2 and 3 are laced over the regions 1 a and lb of the blanket respectively. The current supplied to both elements 1 and 2 is controlled by a single control circuit 4. In the arrangement illustrated in Figure 1, the elements 2 and 3 have the same resistance value and length, giving identical total heat output, but the lower element 2 provides a higher heat density since the area of the foot region I a is smaller than that of the body region lb.

Turning now to Figure 2, this illustrates a circuit diagram of a first embodiment of an electric blanket having the arrangement illustrated in Figure 1. The basic circuit design is that of a burst control triac driver and as such is similar to known control circuitry.

3 The control circuit 4 has live and neutral lines, L and N respectively, for connection to a 240 volt AC mains supply for the supply of current to a burst control switch 5 (which in this example is a Motorola UAA 10 1 6B IC chip) and to a pair of triacs 6 and 7. The triacs 6 and 7 deliver pulses of current to the heating elements 2 and 3 respectively in response to trigger signals supplied by the IC 5 via diode 8. The amount of current supplied to the heating elements 2 and 3, and thus the heat generated thereby, depends upon the ON pulse time. This is controlled by potential divider circuitry, including a user adjustable variable resistor 9. As mentioned above, the trigger signals and the circuitry for controlling the current pulses provided by the triacs 6 and 7 are relatively conventional and thus further details will not be given here.

In the illustrated embodiment, the heating elements 2 and 3 are well known dual core element cables in which the heating elements 2 and 3 are wound together with respective sensing elements 10 and 11, the heating elements and sensor elements being separated by an intermediate insulating sheath indicated by references 12 and 13 respectively. The two sensing elements 10 and I I are connected together and in series with heater resistors 14 and 15 associated with a thermal fuse 16 located in the live power supply line L. The insulating layers 11 and 12 may comprise a relatively low melting point polyethylene designed to melt below a dangerous temperature. Melting will cause a short circuit between the respective heating element and sensing element thereby delivering current to the heating resistors 13 and 14 and tripping the thermal fuse 15. As mentioned above, this type of over-heat protection circuitry is conventional.

The over-heat protection system described is relatively simple, and more sophisticated adaptations may be made. For instance, in place of a simple polyethylene sheath a doped PVC material could be used which has a negative coefficient of resistance allowing a current to leak between the heating elements 2 and 3 and the respective sensing elements 9 and 10 which increases with increased temperature. This current can be used to control the supply of current to the heating elements to provide overheat protection without relying on complete failure of the heating elements. Again, such control circuitry is well known and will not be described in detail here.

4 In accordance with the present invention, the control circuit further comprises a selector 17, which includes switches 17a and l7b, for controlling the supply of trigger signals to the triacs 6 and 7 respectively. The selector will have three positions: a first position in which both switches 17a and l7b are closed and thus trigger signals are provided to both triacs 6 and 7 so that both heating elements 2 and 3 receive current; a second position in which switch 17a is closed and switch l7b is open so that trigger signals are provided to triac 7 only so that only the heater element 3 receives current; and a third position in which the switch 17a is open but the switch l7b is closed so that only the triac 6 receives trigger signals and thus only the heating element 2 will receive the heating current.

Thus, the present invention provides an electric blanket having a heater comprising two separate heating elements but in which there is a single selector for determining which of the elements is energised, a single temperature adjustment circuit, and a single over-heat sensing circuit. Similarly there will be only a single mains plug etc. required. Thus, the present invention provides advantages over the prior art exemplified by the Swiss patent mentioned above in that the number of components is substantially reduced, with a reduction in the complexity and manufacturing cost of the blanket, and that control is more convenient to the user.

Although in the above described embodiment of the invention each resistance element has the same total heat output, differences in heat density between the regions I a and I b of the blanket being provided by concentrating the lacing of the element 2 in a relatively small area, it will be possible to provide differential heat outputs by using elements of different resistance/heating characteristics.

It will be appreciated that the invention is not limited to blankets in which the elements are distributed over foot and body regions. For instance, the invention has similar application to "double" electric blankets having left and right heating elements. Each heating element would generally have the same resistance per unit length, and be laced over the same area, so that the temperature of each element will be the same if they are both energised. However, it would equally be possible to provide elements which give a different heat density, so that one side of the bed may be heated more than the other, which may suit some users.

For an electric blanket in which the two heating elements are located in foot and body regions, it will generally be the case that the user will not want to have the heating element of the body region on but the foot region off, and thus would only want the body heater on together with the foot heater, or not at all. A modification of the control circuit of Figure 2 which allows heating current to be supplied to either both the heating elements 2 and 3, or to the heating element 2 only (but not the heating element 3 only) is illustrated in Figure 3. The control circuit is again a burst control triac driver circuit. The current supply control circuitry, and over-heat protection circuitry, are essentially the same as described above in relation to Figure I (as indicated by the use of common reference numerals). The differences in the two arrangements are that in the embodiment of Figure 3: the elements 2 and 3 are connected in series rather than in parallel; the current is supplied to the elements by a single triac 18 rather than the two triacs 6 and 7; and a simplified selector 19 is used.

The selector 19 is connected in series between the triac 18 and the element 3 and comprises a single switch moveable from a first position (shown in dotted line) in which current is supplied to both elements 2 and 3, and a second position (shown with a solid line) in which current bypasses the heating element 3 and is supplied to the element 2 only. In the latter case, the current supply to the element 2 is via a diode 20 providing a half wave supply to the heater element 2, so that when switching to the foot only position the temperature of the foot region will remain unchanged unless an adjustment is made to the variable resistor 8. Although this arrangement provides slightly less flinctionality than that of Figure 2, it has still fewer components, requiring only a single triac and a simpler selector, and thus has the advantage of being even cheaper to manufacture.

As mentioned above, rather than using a simple polyethylene insulating layer to separate the heating and sensing elements, it will be possible to use known doped PVC layers with a negative temperature coefficient providing a leakage current which allows control of the current supply to the heating elements. Two examples of control circuits which provide this facility are illustrated in Figures 4 and 5. It will be seen that the circuit of Figure 4 corresponds closely with that of Figure 2 and the circuit of Figure 5 corresponds closely with that of Figure 3 (indicated again by the use of common reference numerals). However, IC chip 5 is a TEA 1124 chip, and a 6 modified potential divider circuitry and additional componentry is provided to vary the on pulse duration provided by the chip in response to changes in the leakage current (which changes as a function of the temperature of the elements). The circuitry is based on well known control circuitry and thus no further details will be given here.

Figure 6 illustrates an alternative embodiment of the invention in which a programmable control chip 20 is used to control two triacs 21 and 22 which in turn control the supply of current to heating elements 2 and 3. This provides a similar selection choice (via selector 23) to that of the embodiments of Figures 2 and 4 but achieved in a different manner and is included to give an indication of the wide variety of control circuits that may be designed for electric blankets in accordance with the present invention. Other possible modifications will be readily apparent to the skilled person.

Although the present invention has been described in relation to electric blankets, it will be appreciated that the invention is not limited to electric blankets and may be applied to a variety of different forms of electric heating panels comprising resistance heating elements to provide differential heating to different areas of the panel. It will further be appreciated that the invention is not limited to heating panels incorporating only two heating elements and may extend to panels comprising more than two heating elements.

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Claims

1. An electrically powered heating panel, comprising an electrical resistance heater and current supply control circuitry for controlling the supply of current to the heater, wherein the heater comprises at least first and second heating elements each disposed in different regions of the panel, and the current supply circuitry comprises a single control circuit for controlling the supply of current to both elements.

2. An electrically powered heating panel according to claim 1, wherein the control circuit comprises a single temperature control circuit for determining the level of current supplied to both said first and second heating elements.

3. An electrically powered heating panel according to claim I or claim 2, wherein the control circuitry comprises a single selector switch arrangement which can be selectively set in one of two configurations, a first configuration in which current is supplied to both first and second elements simultaneously, and a second configuration in which current is supplied to the first element only.

4. An electrically powered heating panel according to claim 3, wherein the selector switch arrangement may be selectively set to a third configuration in which current is supplied to the second element only.

5. An electrically powered heating panel according to any preceding claim, wherein the first and second heating elements are connected in series or in parallel.

6. An electrically powered heating panel according to claim 3, wherein the first and second heating elements are connected together in series, and said selector comprises a single switch moveable between a first position in which current is supplied to both the first and second elements and a second position is which the current supply bypasses the first element and is supplied to the second element only.

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7. An electrically powered heating panel according to claim 6, wherein when in said second position the current is supplied to the second heating element via current reduction means so that the second element receives substantially the same current when the switch is in said second position as it does when the switch is in said first position.

8. An electrically powered heating panel according to claim 7, wherein said current reduction means comprises a diode which half wave rectifies the current.

9. An electrically powered heating panel according to anyone of claims 6 to 8, wherein the control circuit is a burst control triac circuit in which current is supplied to the heater in pulses via a single triac.

10. An electrically powered heating panel according to claim 9, wherein said selector is connected in series between said triac and said heater.

11. An electrically powered heating panel according to claim 4, wherein the first and second heating elements are connected together in parallel, and said selector comprises two switches, a first switch for enabling or disabling the current supply to the first element and a second switch for enabling or disabling current supply to the second element, such that both switches are closed when the selector is in said first configuration, the first switch is closed and the second switch is open when the selector is in said second configuration, and said first switch is open and said second switch is closed when the selector is in said third position.

12. An electrically powered heating panel according to claim 11, wherein the control circuit is a burst control triac circuit comprising a first triac which supplies current to the first element and second triac which supplies current to the second element.

13. An electrically powered heating panel according to claim 12, wherein said first triac is connected in series between said first switch and said first element, and said 9 second triac is connected in series between said second switch and said second element.

14. An electrically powered heating panel according to claim 13, wherein the first and second switches enable/disable the supply of trigger signals to the respective triacs from a control switch.

15. An electrically powered heating panel according to any preceding claim, wherein each of said heating elements provides substantially the same total heat output.

16. An electrically powered heating panel according to any preceding claim, wherein said first and second heating elements have substantially the same resistance per unit length.

17. An electrically powered heating panel according to any preceding claim, wherein said first and second heating elements are of substantially the same length.

18. An electrically powered heating panel according to any preceding claim, wherein the first heating element is arranged to provide a greater heat density than the second heating element.

19. An electrically powered heated panel according to claim 17, wherein the panel is an electric blanket and said first heating element is arranged to heat a lower portion of the blanket than the second heating element.

20. An electrically powered heating panel, substantially as hereinbefore described, with reference to the accompanying drawings.