GB2154817A

GB2154817A - Electrical heating circuits

Info

Publication number: GB2154817A
Application number: GB8404662A
Authority: GB
Inventors: Robert Hollis Walker
Original assignee: NORTHERN BLANKETS Ltd
Current assignee: NORTHERN BLANKETS Ltd
Priority date: 1984-02-22
Filing date: 1984-02-22
Publication date: 1985-09-11
Also published as: NL8500481A; DE3505908A1; GB8404662D0; GB2154817B

Abstract

A heating element 29 for an electric blanket comprises a heating conductor 14 wound concentrically with a sensing conductor 22 but separated from it by a temperature- sensitive insulating layer 28, current flowing through resistor 20 in the event of overheating tripping a thermal switch 16. In the event of local overheat, however, the magnitude of the current through resistor 20 will depend on the position of the local overheat, and will be insufficient to trip switch 16 if the overheat occurs in a first portion AC of the element 29. To overcome this, the element 29 is physically arranged in doubled formation (Fig. 4) so that the unprotected length AC of conductor 14 is physically adjacent and alongside the protected length CB thereof. <IMAGE>

Description

SPECIFICATION Electrical heating circuit arrangements The invention relates to electrical heating circuit arrangements. Such electrical heating circuit arrangements may be used to provide heating over a distributed area such as, for example, in electrical heating pads and in electric blankets and the like, though is not limited to such examples.

It is known to provide an electrical heating circuit in which a resistive heating wire is supplied with an alternating electrical current through a suitable form of circuit interrupter, and a sensing wire is physically associated with and placed adjacent to the heating wire but electrically insulated from it by suitable temperature-sensitive material. In the event of overheat, the electrical impedance of the temperature-sensitive material, which is normally relatively high, falls, either generally or at a particular point along its length. Various forms of circuit means are known which are connected to the sensing and heating wires and respond to the change in electrical conditions caused by this fall in impedance and operate the circuit interrupter to switch off the heating current.

According to the invention, there is provided an electrical heating circuit arrangement, comprising an electrical heating conductor arranged over at least a predetermined portion of its total length to be physically adjacent to an electrical sensing conductor but separated therefrom by temperature-sensitive material which under normal operating temperature has such electrical impedance as effectively to insulate the two conductors from each other but whose impedance falls substantially at any position where it becomes overheated, means for connecting the ends of the heating conductor to an electrical power supply through interrupter means for resistively generating heat by virtue of the flow of current through the heating conductor, means for connecting the supply to one end of the sensing conductor whereby a sufficient general fall in the impedance of the temperaturesensitive material causes at least a predetermined current to flow in the sensing conductor, and means responsive to the predetermined current to operate the interrupter means to disconnect the heating conductor from the power supply, the said predetermined portion of the length of the heating conductor being of such length and so positioned in relation to the total length of the heating conductor that local overheat at any point therealong causes such fall in impedance of the temperature-sensitive material as to cause at least the predetermined current to flow, at least part of the remainder of the length of the heating conductor being laid alongside the predetermined portion thereof, whereby local overheat of the said part of the remainder of the length of the heating conductor will tend to cause said overheat in the immediately adjacent temperature-sensitive material.

According to the invention, there is also provided an electrical heating circuit arrangement, comprising an electrical heating conductor having a predetermined length, means for connecting the heating conductor to an electrical power supply through interrupter means whereby the flow of current through the heating conductor produces resistive electrical heating, an electrical sensing conductor running alongside the heating conductor so as to form a unit therewith of the predetermined length and which includes temperature-sensitive material separating the heating and sensing conductors and having under normal operating temperature such electrical impedance as effectively to insulate the two conductors from each other but whose impedance falls substantially at any position where it becomes overheated, the said unit being laid out to follow a predetermined path such that the two ends of the unit are physically adjacent to each other whereby over at least some of the path different parts of the unit are immediately adjacent to each other, an electrical connecting circuit for connecting the supply to one end of the sensing conductor, and means responsive to at least a predetermined current flow in the connecting circuit to operate the interrupter means to disconnect the heating conductor from the power supply, the arrangement being such that the sensing conductor in one of the said parts of the unit is so positioned in relation to the ends of the heating conductor that occurrence of at least a predetermined overheat condition at any point along that said part causes at least a said predetermined current flow in the said connecting circuit, and occurrence of at least a said predetermined overheat condition in the or each other said part of the unit also produces a said predetermined overheat condition in the said one part of the unit.

According to the invention, there is still further provided an electric blanket having an electrical heating conductor arranged to and fro over the area of the blanket, the heating conductor being constructed as a unit with an electrical sensing conductor arranged alongside it but separated from it by temperaturesensitive material which under normal operating temperature has such electrical impedance as effectively to insulate the two conductors from each other but whose impedance falls substantially at any position where it becomes subjected to at least a predetermined overheat condition, means for connecting the heating conductor to an electrical power supply through interrupter means for resistively heating the blanket, an electrical connecting circuit for connecting one pole of the supply to one end of the sensing conductor whereby the existence of a said predetermined overheat condition in the temperature-sensitive material at any position within a predetermined portion, only, thereof will cause such fall in the impedance of the temperature-sensitive material at that position as to produce at least a predetermined current flow in the electrical connecting circuit, and means responsive to the said predetermined current flow in the electrical connecting circuit to operate the interrupter means to disconnect the heating conductor from the power supply, the said unit being so physically arranged in the blanket that the said predetermined portion of the temperature-sensitive material is also physically positioned immediately adjacent to a substantial part of the remainder of the said unit.

Electrical heating circuit arrangements embodying the invention and electric blankets incorporating such circuit arrangements will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a circuit diagram of one of the heating circuit arrangements; Figure 2 illustrates a heating and sensing element used in the circuit arrangements; Figure 3 is a graph showing the variation of the current in a fault-responsive heater resistor with the position of a fault in the heating circuit arrangement of Figure 1; Figure 4 is a diagrammatic illustration showing one physical layout of a heating and sensing element in one of the circuit arrangements; and Figure 5 corresponds to Figure 4 but shows another physical arrangement.

As shown in Figure 1, a heating circuit is energised from an AC power source 10 through an on-off switch 1 2. When closed, the switch 12 connects the alternating supply across an electrical heating conductor 1 4 through a thermal switch 1 6. The thermal switch may be of any suitable form, for example of a type which contains a fusible link which becomes broken in response to being heated above a predetermined temperature.

A protective circuit comprises a resistor 20 and a sensing conductor 22. The sensing conductor 22 is laid physically alongside the heater conductor 14 but is electrically insulated from it by an insulating layer 28. The resistor 20 is physically mounted so as to be in close thermal relationship with the thermal switch 16.

The heater conductor 14, the sensing conductor 22 and the insulating layer 28 preferably form a composite cable or element 29 such as shown in Figure 2. As there shown, the element 29 can comprise a textile core 30 around which the heater conductor 14 is tautly wound. The insulating layer 28 surrounds the heater conductor 14 and has the sensing conductor 22 wound tautly around it in turn in the opposite direction. An insulating jacket 31 is provided. The heater conductor 14 will have a certain resistance per unit length so as to dissipate electrical power at a given rate to provide the necessary heat for the blanket. The sensing conductor 22 may be made of conventional resistance wire having a resistance per unit length of the same order.

The insulating layer 28 is made of material whose electrical impedance is temperaturesensitive. At temperatures at least up to and including the normal operating temperatures of the electric blanket, the electrical impedance is very high. However, in response to overheat of the blanket, whether generally or locally, the electrical impedance of the layer 28 drops sharply. The layer 28 may in fact fuse at a particular point along its length where overheat occurs and this will cause the impedance of the layer 8 to drop so much at that particular point as effectively to bring the heater conductor 14 and the sensing conductor 22 into electrical contact.

The operation of the circuit of Figure 1 will now be considered in more detail.

During normal operation of the blanket, that is, when there is no overheat condition, the heater conductor 14 is energised through the thermal switch 16.

No current flows through the resistor 20 and the sensing conductor 22. This is because any such flow of current could only pass through the insulating layer 28 which, under the conditions being assumed, has a high electrical impedance.

If the blanket becomes overheated, however, the impedance of the layer 28 will fall and the conditions in the circuit will change as will now be described.

If the blanket becomes generally overheated, the impedance of the material 28 will drop significantly along the whole mutual length of the conductors 14 and 22. Effectively, therefore, the sensing conductor 22 is now connected to the heater conductor 14 by a distributed relatively low impedance. Current will now flow through the resistor 20. The amount of current flowing through the resistor 20 will of course depend on the amount by which the impedance of the layer 28 has been reduced. However, the layer 28 is arranged so that, when overheat becomes unacceptably high, the impedance of the layer 28 drops to such a level that the current through resistor 20 produces sufficient heating power in the resistor as to trip the thermal switch 1 6 and thus disconnect the heating conductor from the mains supply 10.

The condition will now be considered where the layer 28 is not generally overheated but becomes overheated at a local point ("X") so as to provide a virtual short circuit or very low resistance between the conductors 14 and 22 at this point.

It will be apparent that the heating power ("W") in resistor 20 will vary according to the position of the point X along the physical length of the element 29. If point X is at the end A (see Fig. 1) of the element 29, it will be apparent that there will be no current at all flowing through resistor 20-because there will not be any potential difference (during either half cycle of the supply) across the short circuit at this point. If, however, point X occurs at the end B of the element 29, there will be maximum power dissipated in resistor 20-because, during each half cycle of the supply, the potential difference across the short circuit at this point will be the full instantaneous supply voltage.

Figure 3 shows a graph showing how the power W in resistor 20 varies between zero and maximum (indicated as 100%) as the position of point X varies from the end A to the end B, the power clearly being proportional to the square of the distance of point X from the end A.

As explained, the circuit can provide no protection against a fault occurring at the end A of the element 29, that is, when point X is at end A. If point X moves towards end B, the power W in resistor 20 will eventually reach such a level as to trip the thermal switch 1 6.

The position of point X at which such tripping occurs obviously depends on the characteristics of the thermal switch 1 6 and the thermal transfer relationship between this switch and resistor 20. If Wt, representing the minimum value of W for tripping of the thermal switch 16, is 25% of the maximum power or more, it will be apparent that the circuit will only respond to short circuits occurring when point X lies between point C and the end B of the element 29, where point C lies exactly half way between the ends A and B of the element. Only half the element is therefore capable of detecting a local overheat condition.

If Wt is 58%, the element 29 will only detect local overheating when point X lies between point D and the end B of the dual element, where point D lies 78.7% along the length of the element from the end A. Thus, only about 38% of the element is capable of detecting local over heating.

In accordance with the invention, therefore, the element 29 is physically arranged so as physically to distribute the "protecting" proportion of the element over as much of the physical length of the heating conductor 14 as possible. This is achieved by physically arranging the element 29 so that the "protecting" proportion of it runs physically adjacent to as much as possible of the unprotected proportion.

Figure 4 shows one such arrangement, for use where Wt is 25% of maximum power, so that only half the element 29 is capable of detecting a local overheat condition.

In Figure 4, the blanket itself is shown as a dotted rectangle 30. The element 29 is shown physically arranged in a zig-zag or to and fro manner over this rectangle so as to provide reasonably uniform heating. As shown, the two ends A and B of the dual element are physically arranged adjacent to each other and the element 29 is thus doubled up on itself with the first half (the unprotected half between end A and point C) arranged physically adjacent to the protected half, that is, the half from point C to the end B. The cross-hatching emphasises the protected half. It will therefore be apparent that, should a ruck or fold occur such that any particular area of the blanket overheats, it is likely to affect the insulating layer 28 at some particular position along the protected length of the element between the end B and the centre point C.More specifically, it is unlikely that such overheating could occur at a point along the unprotected length of the dual element, between end A and centre point C, without affecting the insulating layer 28 (and reducing its impedance) over the protected length between the centre point C and the end B.

In this way, therefore, the relatively simple circuit shown in Figure 1 can be used and yet can give effective protection against local over heating along substantially the total length of the element 29.

If Wt is set at 50% of the maximum power dissipation, instead of 25%, the protected length of the element 29 is between point D and end B. With a physical layout such as shown in Figure 4, the result is that the element is in fact protected over approximately only half its length (although this is significantly better than only about 30% which would apply if the element were laid in single formation to and fro across the area of the blanket 30).

Figure 5 shows another physical arrangement however, which can provide effective protection against local overheat along substantially the full length of the element 29 for the case when Wt is 50% of the maximum power dissipation. Here the element is laid out in the blanket so that the protected length between the point D and the end B lies physically alongside a large proportion of the remainder of the length of the element.

It will be appreciated that the physical layouts shown in Figures 4 and 5 are merely examples of other such arrangements.

It will also be appreciated that the element 29 may itself be modified so that the sensing conductor 22 is omitted over the unprotected part of the length of the element. Thus, for example, for the arrangement shown in Figure 4 where Wt is 25% of maximum power dissipation, the sensing conductor 22 could be fed into the dual element at the centre point C and then laid alongside the heating conductor 14 from that point to the end B (being of course separated from the heating conductor by the insulating layer 28). The heating conductor 14, alone, would run from the end A to the centre point C (though would of course have to be covered with electrical insulation but this would not need to have the special impedance/temperature characteristics of the material of the layer 28).

Claims

1. An electrical heating circuit arrangement, comprising an electrical heating conductor arranged over at least a predetermined portion of its total length to be physically adjacent to an electrical sensing conductor but separated therefrom by temperature-sensitive material which under normal operating temperature has such electrical impedance as effectively to insulate the two conductors from each other but whose impedance falls substantially at any position where it becomes overheated, means for connecting the ends of the heating conductor to an electrical power supply through interrupter means for resistively generating heat by virtue of the flow of current through the heating conductor, means for connecting the supply to one end of the sensing conductor whereby a sufficient general fall in the impedance of the temperature-sensitive material causes at least a predetermined current to flow in the sensing conductor, and means responsive to the predetermined current to operate the interrupter means to disconnect the heating conductor from the power supply, the said predetermined portion of the length of the heating conductor being of such length and so positioned in relation to the total length of the heating conductor that local overheat at any point therealong causes such fall in impedance of the temperature-sensitive material as to cause at least the predetermined current to flow, at least part of the remainder of the length of the heating conductor being laid alongside the predetermined portion thereof, whereby local overheat of the said part of the remainder of the length of the heating conductor will tend to cause said overheat in the immediately adjacent temperature-sensitive material.

2. A circuit arrangement according to claim 1, in which the said part of the remainder of the length of the heating conductor is arranged physically adjacent to an extension of the electrical sensing conductor but separated therefrom by further said temperaturesensitive material.

3. A circuit arrangement according to claim 1 or 2, in which the means for connecting the supply to one end of the sensing conductor includes resistance means.

4. A circuit arrangement according to claim 3, in which the said interrupter means is thermally responsive switch means and is arranged physically adjacent to the said resistance means, the arrangement being such that a flow of at least the said predetermined current in the sensing conductor causes the resistance means to generate sufficient heat to cause the thermally responsive switch means to disconnect the supply from the heating conductor.

5. An electrical heating circuit arrangement, comprising an electrical heating conductor having a predetermined length, means for connecting the heating conductor to an electrical power supply through interrupter means whereby the flow of current through the heating conductor produces resistive electrical heating, an electrical sensing conductor running alongside the heating conductor so as to form a unit therewith of the predetermined length and which includes temperature-sensitive material separating the heating and sensing conductors and having under normal operating temperature such electrical impedance as effectively to insulate the two conductors from each other but whose impedance falls substantially at any position where it becomes overheated, the said unit being laid out to follow a predetermined path such that the two ends of the unit are physically adjacent to each other whereby over at least some of the path different parts of the unit are immediately adjacent to each other, an electrical connecting circuit for connecting the supply to one end of the sensing conductor, and means responsive to at least a predetermined current flow in the connecting circuit to operate the interrupter means to disconnect the heating conductor from the power supply, the arrangement being such that the sensing conductor in one of the said parts of the unit is so positioned in relation to the ends of the heating conductor that occurrence of at least a predetermined overheat condition at any point along that said part causes at least a said predetermined current flow in the said connecting circuit, and occurrence of at least a said predetermined overheat condition in the or each other said part of the unit also produces a said predetermined overheat condition in the said one part of the unit.

6. A circuit arrangement according to claim 5, in which the interrupter means is a thermally-responsive switch and the connecting circuit includes resistance means which generates sufficient heat in response to the said predetermined flow of current to cause the thermally-responsive switch to disconnect the power supply from the heating conductor.

7. An electric blanket incorporating an electrical heating circuit arrangement according to any preceding claim.

8. An electric blanket having an electrical heating conductor arranged to and fro over the area of the blanket, the heating conductor being constructed as a unit with an electrical sensing conductor arranged alongside it but separated from it by temperature-sensitive material which under normal operating temperature has such electrical impedance as effectively to insulate the two conductors from each other but whose impedance falls substantially at any position where it becomes subjected to at least a predetermined overheat condition, means for connecting the heating conductor to an electrical power supply through interrupter means for resistively heating the blanket, an electrical connecting circuit for connecting one pole of the supply to one end of the sensing conductor whereby the existence of a said predetermined overheat condition in the temperaturesensitive material at any position within a predetermined portion, only, thereof will cause such fall in the impedance of the temperature-sensitive material at that position as to produce at least a predetermined current flow in the electrical connecting circuit, and means responsive to the said predetermined current flow in the electrical connecting circuit to operate the interrupter means to disconnect the heating conductor from the power supply, the said unit being so physically arranged in the blanket that the said predetermined portion of the temperature-sensitive material is also physically positioned immediately adjacent to a substantial part of the remainder of the said unit.

9. An electric blanket according to claim 8, in which the said unit is arranged with the two ends of the heating conductor physically adjacent to each other and so that the unit follows a predetermined path over the area of the blanket along substantially all of which path at least two parts of the unit run alongside each other.

10. An electric blanket according to claim 8 or 9, in which the interrupter means is a thermal switch and the electrical connecting circuit includes resistance means which generates sufficient heat in response to the predetermined current flow therethrough as to cause the thermal switch to disconnect the heating conductor from the power supply.

11. An electric blanket, substantially as described with reference to Figures 1 and 4 of the accompanying drawings.

1 2. An electric blanket substantially as described with reference to Figures 1 and 5 of the accompanying drawings.

1 3. An electric blanket substantially as described with reference to Figures 1,2 and 4 of the accompanying drawings.

14. An electric blanket substantially as described with reference to Figures 1,2 and 5 of the accompanying drawings.