GB2157514A

GB2157514A - Electrical heating circuits

Info

Publication number: GB2157514A
Application number: GB8409772A
Authority: GB
Inventors: Barry Simpson
Original assignee: NORTHERN BLANKETS Ltd
Current assignee: NORTHERN BLANKETS Ltd
Priority date: 1984-04-13
Filing date: 1984-04-13
Publication date: 1985-10-23

Abstract

A heating conductor 14 for an electric blanket is wound concentrically with a sensing conductor 22 but separated from it by a temperature- sensitive insulating layer 28 to form a composite element 29. In the event of local or general overheat, the insulation of the layer 28 drops significantly and a signal is generated at terminal III of a 3- terminal device 42. Device 42 may be a transistor or a thyristor for example. The device responds to the signal by switching ON so as to provide a virtual short-circuit between its terminals I and II, thus causing open-circuiting of the overcurrent fuse 40. The device 42 is sufficiently sensitive to be switched ON by a short circuit occurring at virtually any position along the element 29, that is, even when the short-circuit occurs so close to one end as to produce a very low potential difference across the fault. In an alternative embodiment (fig. 6, not shown) a heating resistor in series with the thyristor trips the circuit by blowing a thermal fuse. <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 electical 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 electical heating circuit arrangement, comprising an electrical heating circuit arrangement, comprising an electrical heating conductor arranged physically adjacent to an electrical sensing conductor but is 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 heating conductor to an electrical power supply for resistively generating heat, a 3terminal device having a main circuit path and a control input, and means connecting the control input to the sensing conductor to receive therefrom, in response to local overheat at any position along substantially the whole of the length of the heating conductor, a control signal of such magnitude as to trigger the gating device, the device being so connected that such triggering interrupts the power supply to the heating conductor.

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 prodxuces 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 for the predetermined length 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, a 3-terminal electronic switching device having a main circuit path extending between two of its terminals and which is switchable from a high impedence state to a low impedance state by a control signal of at least a predetermined magnitude applied to its third terminal, the third terminal being connected t6 the said sensing conductor and the arrangement being such that local overheat at substantially any position along the said predetermined length produces a said control signal of at least the predetermined magnitude, the device being so connected to the interrupter means that the said switching of the device causes the interrupter means to interrupt the flow of current to the heating conductor.

Electrical heating circuit arrangements embodying the invention and electric blankets incorportating such circuit arrangements will now be described, by way of example only, with reference to the accompanying drawings in which: Figure lisa circuit diagram of a known form of heating circuit arrangement for use in an electric blanket; Figure 2 illustrates a heating and sensing element used in the circuit arrangements; Figure 3 is a graph showing the relationship between the power in a fault-responsive heater resistor in the circuit arrangement of Figure 1 and the position of a fault; Figure 4 is a circuit diagram of one of the circuit arrangements embodying the invention; Figure 5 is a circiut diagram of another of the circuit arrangements embodying the invention; and Figure 6 is a circuit diagram of yet anothger of the circuit arrangements embodying the invention.

As shown in Figure lithe known form of circuit arrangement is energised from an AC power source 10 through an on-off switch 12. When closed, the switch 12 connects the alternating supply across an electrical heating conductor 14through a thermal switch 16.

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 heatforthe 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 temperature-sensitive. 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 impendance 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 28 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 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 16 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 Win 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 16. The position of point X at which such tripping occurs obviously depends on the characteristics of the thermal switch 16 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 halfway between the ends A and B of the element.

Only half the element is therefore capable of detecting a local overheat condition.

It Wt is 50%, 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 70.7% along the length of the element from the end A. Thus, only about 30% of the element is capable of detecting local overheating.

The circuit arrangements to be described with reference to Figures 4 and 5 enable substantially the whole of the element to detect local overheating.

In the circuit arrangement of Figure 4, items corresponding to those in Figure 1 are correspondingly referenced.

The circuit arrangement of figure 4 differs from that of Figure 1 in that the thermal switch 16 is replaced by an over current fuse 40, and the sensing conductor 22 is connected to a terminal III of a 3--terminal device 42 which is connected in parallel with the heater conductor 14 via its terminals I and II. The 3--terminal device 42 is basically a gating device which responds to a very low level of signal on its terminal III by closing the circuit path between its terminals I and II. For example, the device 42 may be a transistor or a thyristor.

The operation will now be considered in more detail.

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 and, therefore, and as explained above, the sensing conductor 22 is now connected to the heater conductor 14 by a distributed relatively low impedance. A signal will now be applied at terminal III of the device 42, this signal being primarily in the form of a voltage or of a current depending on the exact form of the device. Whichever is the case, however, the signal will switch the device ON, that is, will provide a low impedance circuit path between terminals I and II, thus providing a virtual short-circuit across the supply, via the fuse 40. The fuse thus goes open-circuit and the heater conductor 14 is disconnected from the supply.

if local overheat occurs at a particular point along the layer 28, instead of general overheat, there will be a virtual short circuit or very low resistance between the conductors 14 and 22 at the overheat point. Again, therefore, a signal will be applied to the terminal ill. It will be apparent from the explanation given above in conjunction with Figure 1 that the strength of this signal (voltage or current depending on the nature of the device 42) will depend upon the position, along the length of the element 29, of the local overheat. However, in contrast to the arrangement of Figure 1, where a substantial part of the length of the element is "unprotected" (because the potential difference across the short-circuit is too small to heat resistor 20 sufficiently), such a problem does not arise with the circuit arrangement of Figure 4.This is because the device 42 is sufficiently sensitive to be able to respond to the signal produced at terminal Ill even when the fault occurs virtually at the end A of the element 29.

It is found in fact that it is possible to design a circuit arrangement of the form shown in Figure 4 such that local overheat occurring anywhere over about 99.5% of the length of the element 29 will provide sufficient signal at terminal Ill to switch the device 42. Figure 5 shows a modified form of the circuit of Figure 4 (in which corresponding parts are correspondingly referenced) and illustrates how protection against local overheat along the full 100% length of the element 29 may be achieved. As shown in Figure 5, a supplementary heater resistor 14A is connected in series with the heater conductor 14 and is arranged to dissipate just sufficient electrical power that local overheat occurring anywhere along the whole of the length of the heater conductor 14 produces sufficient signal at terminal ill to switch the device 42.In practice, the additional heater 14A will need to dissipate only of the order of one watt and it may, for example, be physically located within the switch 12.

Figure 5 also illustrates the device 42 as a silicon controlled rectifier having an input resistor 43.

The use of the device 42 is advantageous in that it provides very high speed of fault detection, and can be sufficiently fast to detect overheating caused by arcing within the element 29. With the circuit arrangement of Figure 1, such arcing will provide a current flow through the heater resistor 20 which may be insufficient to trip the thermal switch 16 in time to prevent damage.

Figure 6 shows another form of the circuit arrangement of Figure 4 and again parts corresponding to those in Figure 4 are correspondingly referenced.

In the circuit arrangement of Figure 6, a thermal switch 16 (similarto that used in the circuit arrangement of Figure 1) is used, instead of the overcurrentfuse 40 of the circuits of Figures 4 and 5.

The thermal switch 16 is physically associated with a heater resistor 50 through which the main circuit path between terminals I and II of the device 42 is connected in series across the supply.

When overheat occurs within the element 29, the resultant signal produced at terminal lil of device 42 switches the device 42 ON as explained above. This has the effect of allowing heating current to pass through resistor 50 which trips the thermal switch.

It may be advantageous to incorporate modifications, such as suitable filtering or smoothing, so as to minimise the possibility of the device 42 being switched ON by voltage spikes on the mains supply or by a surge current occurring when the blanket is switched on.

Claims

1.An electical heating circuit arrangement, comprising an electrical heating conductor arranged physically adjacent to an electrical sensing conductor but is 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 heating conductor to an electrical power supply for resistively generating heat, a 3-terminal device having a main circuit path and a control input, and means connecting the control input to the sensing conductor to receive therefrom, in response to local overheat at any position along substantially the whole of the length of the heating conductor, a control signal of such magnitude as to triggerthe device, the device being so connected that such triggering interrupts the power supply to the heating conductor.

2. An arrangement according to claim 1, including overcurrent fusing means connected in series with the heating conductor, and means so connecting the circuit path of the device to the fuse that the said triggering of the device causes overcurrent to flow through the fuse and thereby producing the interruption of the power supply to the heating conductor.

3. An arrangement according to claim 1, including thermally responsive switch means connected in series with the heating conductor and electrical heating means juxtaposed with the switch means and connected to the said device so as to be electrically energised by the said triggering of the device and thereby to produce heat which causes the thermal switching means to interrupt the power supply to the heating conductor.

4. An arrangement according to any preceding claim, in which the said device is a thyristor.

5. An arrangement according to any preceding claim, including an auxiliary heater resistor connected in series with the said heating conductor and having such electrical resistance that a said control signal of the said magnitude is produced by local overheat at any position along the entire length of the heating conductor.

6. An electrical heating circuit arrangement, comprising an electrical heating conductor having a predetermined length, means for connecting the heating conductorto an electrical power supply through interrupter means whereby the flow of current through the heating conductor produces resistive electrical heatimg, 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 for the predetermined length 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, a 3-terminal electronic switching device having a main circuit path extending between two of its terminals and which is switchable from a high impedance state to a low impedance state by a control signal of at least a predetermined magnitude applied to its third terminal, the third terminal being connected to the said sensing conductor and the arrangement being such that local overheat at substantially any position along the said predetermined length produces a said control signal of at least the predetermined magnitude, the device being so connected to the interrupter means that the said switching of the device causes the interrupter means to interrupt the flow of current to the heating conductor.

7. A circuit arrangement according to claim 6, in which the main circuit path of the device is connected in parallel with the heating conductor and in which the interrupter device comprises an over current fuse connected in series with that parallel combination.

8. A circuit arrangement according to claim 6 or 7, in which the said device is a silicon controlled rectifier or the like.

9. A circuit arrangement according to any one of claims 6 to 8, including an auxiiiary heating resistor connected in series with the heating conductor and of such electrical resistance that the said control signal of at least the said predetermined magnitude is generated by local overheat at any position along the entire said predetermined length of the firstmentioned heating conductor.

10. An electrical circuit arrangement, substantially as described with reference to Figure 4 of the accompanying drawings.

11. An electrical circuit arrangement, substantially as described with reference to Figure 5 of the accompanying drawings.

12. An electrical circuit, substantially as described with reference to Figure 6 of the accompanying drawings.

13. An electrical blanket or heating pad, incorporating an electrical heating circuit arrangement according to any preceding claim.