GB2149243A - Electrical heating circuits and protective circuits for use therewith - Google Patents

Abstract

A heating circuit, such as for an electric blanket, comprises a heater conductor 14 supplied with AC power through a thermal switch 16 which is in thermal relationship with resistors 20 and 24. Resistors 20 and 24 are connected in series with a sensing conductor 22 through capacitor 18 and rectifier 26. Sensing conductor 22 is wound around and along the heater conductor 14 but separated therefrom by temperature-sensitive insulating material 28. Under normal conditions, rectifier 26 blocks current flow through sensing conductor 22 during alternate half cycles and the charge on capacitor 18 blocks such current flow during the other half cycles. If blanket overheat occurs, however, the impedance of layer 28 falls, either locally or generally depending on the type of overheat, and current can now flow through either or both of resistors 20 and 24, depending on the position of the overheat. Resistors 20 and 24 are positioned in thermal relationship with the thermal switch 16 so that when one or both of them carries sufficient current and becomes sufficiently heated, the thermal switch open- circuits. <IMAGE>

Description

SPECIFICATION Electrical heating circuits and protective circuits for use therewith The invention relates to electrical heating circuits and to protective circuits for use therewith. Such electrical heating circuits 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.

Various novel features of the invention will be apparent from the following description, given by way of example only, of electrical heating circuits embodying the invention and for use in electric blankets, reference being made to the accompanying drawings in which: Figure 1 is a circuit diagram of one form of the circuit; Figure 2 is a perspective view of a cable used in the circuit of Figure 1; and Figure 3 is a circuit diagrm of a modified form of the circuit of Figure 1.

More specifically to be described below is an electrical heating circuit comprising first and second electrical conductors arranged physically adjacent to each other but separated 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 two conductors in parallel with an alternating power supply through respective connecting circuits, the connecting circuit for the first conductor including thermal switch means, and the connecting circuit for the second electrical conductor comprising resistive means, rectifying means and capacitive means, the rectifying means connecting the second conductor to one side of the said supply, the capacitor means connecting the second conductor to the other side of the said supply, and the resistive means being so positioned that any current flowing through the rectifying means or the capacitive means flows through the resistive means, the resistive means being so juxtaposed with the thermal switch means that, when an overheat of at least a predetermined extent occurs, the fall in the said impedance of the temperature sensitive material causes such current to flow through the connecting circuit for the second electrical conductor that the resultant heating of the resistive means causes the thermal switch means to interrupt the connecting circuit for the first electrical conductor.

Advantageously, the resistive means comprises first and second resistors, the rectifying means connecting the second conductor to the said one side of the power supply through the first resistor and the capacitive means connecting the second conductor to the other side of the power supply through the second resistor.

The rectifying means may connect the second conductor to the said one side of the power supply by being connected to one end of the second conductor and the capacitive means may connect the second conductor to the said other side of the power supply by being connected to the opposite end of the second conductor.

Advantageously, the two conductors are wound together concentrically with each other but physically separated by the said temperature-sensitive material.

Advantageously, additional resistive means is connected in parallel with the capacitive means.

In a more specific sense, there will be described below a protective heating circuit such as for an electric blanket or the like, comprising a longitudinal and flexible cable containing a heating conductor and a sensing conductor arranged alongside each other within the cable but physically separated 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 significantly at any position where it becomes overheated to at least a predetermined extent, power supply terminals for connection to an alternating power supply, a first resistor and a rectifier connected in series between an end of the sensing conductor and one said terminal, a second resistor and a capacitor connected in series between an end of the sensing conductor and the other said terminal, and a thermal switch mounted to be thermally responsive to electrical heating of the first and second resistors and connected in series with at least the heating conductor; whereby the electrical impedance of the temperature-sensitive material is under normal operating temperature sufficient to prevent any significant current flow through either the first or the second resistors but when an overheat of at least the said predetermined extent occurs the electical impedance of the temperature sensitive material falls sufficiently to cause a resultant current flow through either or both of the said resistors which heats it or them sufficiently to cause the thermal switch to disconnect the heating conductor from the power supply.

The first resistor and the rectifying means on the one hand and the second resistor and the capacitor on the other hand may be connected to respectively opposite ends of the sensing conductors. Instead they may be connected to the same end thereof.

Advantageously, a third resistor is provided and is connected in parallel with the said capacitor.

The foregoing are exemplary of and not exhaustive of the various features of the heating circuits now to be more specifically described.

As shown in Figure 1, the heating circuit is energised from an AC power source 10 through an on-off switch 1 2. When closed, the switch 1 2 connects the alternating supply across an electrical heater conductor 14 through a thermal switch 1 6. The thermal switch 1 6 may be of any suitable known form and is basically of a type which contains a fusible link which becomes broken in response to being heated above a predetermined temperature.

Connected in parallel with the heater conductor 14 is a protective circuit comprising, in series, a capacitor 18, a resistor 20, a sensing conductor 22, a second resistor 24 and a rectifier 26. The sensing conductor 22 is laid physically alongside the heater conductor 14 but is electrically insulated from it by an insulating layer 28. The resistors 20 and 24 are 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 such as shown in Figure 2. As there shown, the cable 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 32 is provided.

The heater conductor 14 will have a relatively high 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 signaliing wire have a low resistance per unit length.

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 (due, for example, to a ruck or fold in the blanket) and this will cause the impedance of the layer 28 to drop so much at that particular point as to effectively 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 1 6.

No current flows through the parallel circuit connected to the sensing conductor 22. This is because such flow of current is blocked by the capacitor 1 8 and the rectifier 26. When the circuit is first switched on, capacitor 1 8 will be charged up to the peak supply voltage (340 volts for normal 240 volt main supply) by the initial current flow through the rectifier 26 and this will block further current flow in response to such half cycles, while the rectifier itself will block current flow during the half cycles of the other polarity.

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 considered.

First, the case will be considered where the blanket becomes generally overheated so that the impedance of the material 28 drops significantly overall. Effectively therefore, the sensing conductor 22 is now connected to the heater conductor 14 by a distributed relatively low impedance. During those half cycles of the supply which render point A positive with respect to point B, current can now flow through rectifier 26 and resistor 24 and via the distributed resistance provided by the now-overheated layer 28. During the half cycles of the other polarity, the rectifier 26 continues to block the flow of such current through resistor 24. However, during all half cycles, current can now flow through capacitor 18, resistor 20 and the distributed resistance provided by the overheated layer 28.

Resistors 20 and 24 therefore become heated by their respective current flow and if they become heated sufficiently (that is to say, if the blanket has become so overheated as to have reduced the impedance of layer 28 sufficiently), the thermal switch 1 6 will become open-circuit and cut off the current to the heater conductor 14.

The condition will now be considered where the layer 28 is not generally overheated but becomes overheated at a local point, for example point C adjacent the left-hand end of the heater and sensing conductors 14 and 22-so as to provide a virtual short circuit or very low resistance at point C between these two conductors.

Because the left-hand end of the heater conductor 14 will be at virtually the same potential as the potential of point B, this local short circuit will not result in any substantial current flowing through capacitor 1 8 and resistor 20, and the resistor will therefore not be heated. However, during those half cycles of the power supply which make point A positive with respect to point B, current will now flow through rectifier 26, resistor 24, the sensing conductor 22 and the short circuit at point C.

Resistor 24 will thus heat up and, provided that the heating up is sufficient, the thermal switch 1 6 will again become open-circuit, thus interrupting the current to the heater conductor 14.

If local overheat occurs such as to cause a virtual short circuit at point D, at the righthand end of the heater and sensing conductors 14 and 22, the opposite effect will be produced. That is, substantially no current will flow through rectifier 26 but substantial current will flow through capacitor 18, resistor 20, sensing conductor 22 and the shortcircuit at point D and the thermal switch 1 6 therefore becomes heated by resistor 20 and again interrupts the heater current flow. During those half cycles which render point A positive with respect to point B, no current will flow through the rectifier 26 because points A and D are at virtually the same electrical potential. During the other half cycles, the same applies of course, but the rectifier itself blocks current flow in any case.

If a short circuit should occur at a localised point intermediate points C and D, then of course current will flow through both resistors 20 and 24 (through resistor 24 during alternate half cycles only), the amount of current through each depending on the position of the short circuit. Again, the resulting heat in resistors 20 and 24 will cause the thermal switch 1 6 to open-circuit.

It will be clear that the circuit parameters are all so chosen as to produce sufficient heat in one or both of the resistors 20 and 24 so as to cause the thermal switch 1 6 to opencircuit in the required conditions.

Advantageously, the resistances of resistors 20 and 24 are chosen to be equal (R, say) and X, the impedance of the capacitor 18 at the mains frequency is chosen to be R also.

If a short circuit occurs at point C, the heat generated in resistor 20 will be given by +V2/R.

If a short circuit occurs at point D, the heat generated in resistor 24 will also be V2/R.

If the short circuit occurs in the middle of the heater and sensor conductors, the heat generated in each resistor 20 and 24 will be given by +(iV2)/R making a total of iV2/R.

The power in resistors 20 is much more dependent on X than on R. Any relatively small variation (up to about 20%) in X changes the power by about the same percentage. Any variation (up to about 30%) in R changes the power by 6% at the most. Therefore, in selecting circuit values, the capacitance of capacitor 1 8 is chosen to provide the required heating and R is made approximately equal to X. (Figure 3 shows a modification to the circuit in which a resistor 34 is connected in parallel with the capacitor 1 8. The effect of this is to limit the peak voltage developed across the capacitor 18 when the blanket is operating under normal (that is non-overheat) conditions and this can advantageously permit the use of a smaller and cheaper capacitor.

Typical circuit values for the circuit shown in Figure 3 for use with a 240 volt mains supply, are as follows: resistors 20 and 24 : 6.8 kilohm each resistor 34 : 82 kilohm capacitor 1 8 : 0.48 microfarad

Claims (10)

1. An electrical heating circuit, comprising first and second electrical conductors arranged physically adjacent to each other but separated 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 two conductors in parallel with an alternating power supply through respective connecting circuits, the connecting circuit for the first conductor including thermal switch means, and the connecting circuit for the second electrical conductor comprising resistive means, rectifying means and capacitive means, the rectifying means connecting the second conductor to one side of the said supply, the capacitor means connecting the second conductor to the other side of the said supply, and the resistive means being so positioned that any current flowing through the rectifying means or the capacitive means flows through the resistive means, the resistive means being so juxtaposed with the thermal switch means that, when an overheat of at least a predetermined extent occurs, the fall in the said impedance of the temperature sensitive material causes such current to flow through the connecting circuit for the second electrical conductor that the resultant heating of the resistive means causes the thermal switch means to interrupt the connecting circuit for the first electrical conductor.

2. A heating circuit according to claim 1, in which the resistive means comprises first and second resistors, the rectifying means connecting the second conductor to the said one side of the power supply through the first resistor and the capacitive means connecting the second conductor to the other side of the power supply through the second resistor.

3. A heating circuit according to claim 1 or 2, in which the rectifying means connects the second conductor to the said one side of the power supply by being connected to one end of the second conductor and the capacitive means connects the second conductor to the said other side of the power supply by being connected to the opposite end of the second conductor.

4. A heating circuit according to any preceding claim, in which the two conductors are wound together concentrically with each other but physically separated by the said temperature-sensitive material.

5. A heating circuit according to any preceding claim, including additional resistive means connected in parallel with the capacitive means.

6. A protective heating circuit such as for an electric blanket or the like, comprising a longitudinal and flexible cable containing a heating conductor and a sensing conductor arranged alongside each other within the cable but physically separated 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 significantly at any position where it becomes overheated to at least a predetermined extent, power supply terminals for connection to an alternating power supply, a first resistor and a rectifier connected in series between an end of the sensing conductor and one said terminal, a second resistor and a capacitor connected in series between an end of the sensing conductor and theother said terminal, and a thermal switch mounted to be thermally responsive to electrical heating of the first and second resistors and connected in series with at least the heating conductor; whereby the electrical impedance of the tempernture-sensi tive material is under normal operating temperature sufficient to prevent any significant current flow through either the first or the second resistors but when an overheat of at least the said predetermined extent occurs the electrical impedance of the temperature sensitive material falls sufficiently to cause a resultant current flow through either or both of the said resistors which heats it or them sufficiently to cause the thermal switch to disconnect the heating conductor from the power supply.

7. A heating circuit according to claim 6, in which the first resistor and the rectifying means on the one hand and the second resistor and the capacitor on the other hand are connected to respectively opposite ends of the sensing conductor.

8. A heating circuit according to claim 6, in which the first resistor and the rectifying means on the one hand and the second resistor and the capacitor on the other hand are connected to the same end of the sensing conductor.

9. A heating circuit according to any one of claims 6 to 8, including a third resistor connected in parallel with the said capacitor.

10. An electrical heating circuit substantially as described with reference to the accompanying drawings.