GB2154816A

GB2154816A - Electrical heating circuits

Info

Publication number: GB2154816A
Application number: GB08404544A
Authority: GB
Inventors: Robert Hollis Walker
Original assignee: NORTHERN BLANKETS Ltd
Current assignee: NORTHERN BLANKETS Ltd
Priority date: 1984-02-21
Filing date: 1984-02-21
Publication date: 1985-09-11
Also published as: GB8404544D0; GB2154816B

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 and is fed from the a.c. supply via a bridge rectifier 32 to 38. In the event of local or general overheat, the insulation of the layer 28 drops significantly, so that current now flows through resistor 20 and the resulting heat trips a thermal switch 16. The bridge rectifier ensures that sufficient current will flow through resistor 20 even with a local overheat close to one end of the blanket. <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 electrical blankets and the like, though it 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 though 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 circuit.

According to the invention, there is provided an electrical heating circuit arrangement, comprising an electrical heating conductor separated from an electrical sensing conductor 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 and full wave rectifying 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 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.

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; and 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 Fig. 1.

As shown in Fig. 1, a heating circuit is energised from an AC power source 10 through an on-off switch 1 2 and via terminals 50 and 52. When closed, the switch 1 2 connects the alternating supply across an electrical heating conductor 14 through a thermal switch 1 6 and via a full wave bridge rectifier circuit consisting of diodes 32 to 38. 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 1 4 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 Fig. 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 1 4 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 1 4 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 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 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.

In operation, and during positive half-cycles of the supply, that is, when terminal 50 is positive with respect to terminal 52, the heating current through heating conductor 14 passes through diodes 32 and 34. During the negative half cycles, the heating current passes through diodes 36 and 38.

No current flows through the resistor 20 and the sensing conductor 22. This is be cause 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.

In the case of general overheat (so that the impedance of the material 28 drops significantly overall), the sensing conductor 22 is connected to the heating conductor 14 by a distributed relatively low impedance. During positive half cycles of the supply, the current which flows through the resistor 20 and thence through the low impedance of the layer 28 will flow through diode 34. During negative half cycles of the supply, the current through the resistor 20 will flow through diode 36. The circuit parameters are designed so that in the event of an unacceptable degree of overheat, the heat dissipated by resistor 20 is such as to trip the thermal switch 1 6.

The operation of the circuit will now be considered where the layer 28 becomes overheated at a local point X rather than generally overheated. Curve P in Fig. 3 shows the variation of power W dissipated in resistor 20 during positive half-cycles of the supply as the position of point X varies from the end A of the element 29 and the end B thereof. Thus, it is clear that during such positive half-cycles a short-circuit at the end A of the dual element results in no current flowing through resistor 20 because there is no potential difference developed across such short circuit.

On the other hand, if the short circuit occurs at the end B, there is maximum power dissipation.

Curve Q in Fig. 3 corresponds to curve P but shows the situation during the negative half-cycles: that is, it shows how the current through resistor 20 during such half-cycles varies as point X varies between the end A of the element 29 and the end B thereof.

Clearly, curve Q is the mirror-image of curve P.

The overall result (that is, the power dissipated in resistor 20 during a whole cycle) is shown by curve R, which is the sum of the powers in each half cycle as shown by curves P and 0 respectively.

In practice, the arrangement will be designed so that a certain minimum power dissipation is required in resistor 20 in order to trip thermal switch 1 6. Curve R in Fig. 3 shows that, provided this minimum power dissipation (Wt) is at most 50% of the maximum power in resistor 20, the thermal switch 1 6 will be tripped for any position of short circuit along the element 29 (that is, wherever point X is). If the heater conductor 14 were not fed through the full wave bridge rectifier but simply connected directly across terminals 50, 52 via switch 16, only curve P would apply and, if Wt was 50%, short circuits occurring at any position within the first 71% of the length of the element 29 would not trip the switch 16.

Claims

1. An electrical heating circuit arrangment, comprising an electrical heating conductor separated from an electrical sensing conductor 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 and full wave rectifying 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 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.

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

3. A circuit arrangement according to claim 2, 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.

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

5. An electric blanket substantially as described with reference to the accompanying drawings.