GB2287591A - Electrically powered heating panel - Google Patents

Abstract

An electrically powered heating panel comprises a resistance heater 1 and a current supply means 2 for supplying current to the heater. The heater comprises a first heating element 6 connected in series with a second conductor 8, which may also be a heating element. The first and second elements lie in close proximity to one another and are separated by a layer of relatively low melting point insulating material such as polyethylene. A third heating element 7, is connected in series between the first and second elements. A sensing element 9 lies in close proximity to the third heating element to detect current leaking therefrom. The sensing element is connected to means sensitive to the leakage current and operable to regulate the current supply to the heater. A fuse F1 is provided to cut off the current supplied to the heater in the event of it exceeding a predetermined limit. The arrangement is such that should the heater begin to overheat the insulating layer between the first and second elements the layer will melt at at least one location along their lengths. This causes a short circuit which bypasses the third heating element 7 so that the current flowing through the heater increases to at least the predetermined limit and blows the fuse. <IMAGE>

Description

ELECTRICALLY POWERED HEAVING PANEL The present invention relates to an electrically powered heating panel and particularly, but not exclusively, to an electric blanket.

Electric blankets generally comprise a heater consisting of at least one heating element which follows a tortuous path over the area of the blanket, and a control circuit for controlling the current delivered to the heating element. It is a safety requirement of such electric blankets that the current supplied to the heating element is automatically reduced or cut-off in the event of overheating. In addition, it is generally desirable to construct the control circuit so that the current supply to the heating element, and thus the temperature of the heating element, can be controlled by the user.

An example of a known electric blanket is described in British Patent Application No. GB2178201A. This discloses an electric blanket comprising a dual coil heating element in which the two coils are separated by an insulating layer of PVC. Only one of the coils serves as a heating element and the other forms part of a sensing circuit which picks up any current which leaks across the PVC insulating layer from the heating coil. Power is supplied to the heating coil by a control circuit in pluses. The duration of the ON pulse time is controllable by the user as a means for regulating the temperature of the blanket. The resistance of the PVC insulating layer between the two coils decreases with increasing temperature.Should the heating coil begin to overheat, the current leaking across the PVC insulating layer increases the resulting increase in current through the sensing coil causes the control circuit to reduce the pulse ON time, hence reducing the temperature of the heating coil. The control circuit has a further safety arrangement which cuts power to the heating coil completely in the event that the ON pulse time exceeds a predetermined maximum limit.

It is an object of the present invention to provide an improved electrically powered heating panel, for example in the form of an electric blanket.

According to the present invention there is provided an electrically powered heating panel, comprising an electrical resistance heater and a current supply means for supplying current to the heater, wherein the heater comprises a first conductor in the form of a heating element connected in series with a second conductor which lies in close proximity to the first conductor and is separated therefrom by a layer of relatively low melting point insulating material, a third conductor in the form of a heating element connected in series between the first and second conductors, a sensing element which lies along the length of the third conductor in close proximity thereto for sensing leakage of current from the third conductor, said sensing element being connected to means sensitive to said leakage current and operable so as to regulate the current supplied to the heater, and means sensitive to an increase in current flowing through the heater and operable to at least reduce the current flow through the heater if said current exceeds a predetermined level, the arrangement being such that should the heater overheat the insulating material between the first and second conductors will melt causing a short circuit between the first and second conductors which bypasses said third conductor and results in an increase in current flowing through the heater to at least said predetermined limit.

Thus the insulating layer may be chosen so as to melt as the heater begins to overheat so that power supply to the heater is reduced before it reaches a potentially dangerous temperature.

The present invention has the advantage that no matter where along the length of the heating element the melting and thus short circuiting occurs, the resistor will always be cut out of the circuit so that the current increases to at least said predetermined limit.

The means sensitive to an increase in current may comprise means for isolating the heating element from the current supply. such as a simple fuse.

Preferably the second conductor is in the form of a heating element.

The third conductor and the sensing element may be combined in a single dual core cable. Similarly the first and second conductors may be combined as a single dual core cable.

The sensing element is preferably separated from the third conductor by a layer of insulating maternal e.g. doped PVC. which has a negative coefficient of resistance so that as the temperature of the third conductor rises the current leaking to the sensing element increases and said means sensitive to leakage current is operable to cause a reduction in the current supplied to the heater in response to an increase in said leakage current thereby stabilising the temperature of the heater at a desired level.

The relatively low melting point insulating material which separates the first and second conductors may, for example, be polyethylene.

Preferably means are provided to at least reduce the current supplied to the current supply means in the event of a short circuit between the third conductor and the sensing element. Such means may for example. be a simple fuse.

Control means for instance a manually variable resistor, are preferably provided for enabling user selection of the heater temperature.

In a preferred embodiment of the invention the current supply means is operable to supply current to the heater in pulses and to regulate the current supplied to the heater by varying the ON-pulse time. The current supply means may for instance be a burst control triac driver means.

Preferably means are provided for isolating the heater from the current supply in the event that the pulse current supply means fails to a permanent ON condition.

Said means may, for example, comprise a thermal fuse. For instance, the thermal fuse may be connected in series between the power supply and the heating element. Said means may further comprise one or more heating resistors located in close proximity to the thermal fuse and connected in series with the heating element such that should the ON-pulse time exceed a certain limit the heat output of the or each heating resistor will be sufficient to blow the thermal fuse.

Preferably further means are provided for limiting the ON-pulse time to a length which will not cause the thermal fuse to blow under normal operating conditions.

The heating panel in accordance with the present invention may have many embodiments and applications such as, for instance, as an electric blanket or an under floor heating panel.

A specific embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings. in which: Figure 1 is a circuit diagram of a heating panel in accordance with one embodiment of the present invention; Figure 2 is a schematic illustration of embodiment of Figure 1; Figure 3 is a part sectioned illustration of a heating element cable of the embodiment of Figure 1; Figure 4 illustrates a modification of the control circuit of Figure 1; Figure 5 illustrates a further modification of the control circuit of Figure 1; and Figure 6 illustrates a still further modification of the control circuit of Figure 1.

Referring to Figs 1 to 3. the illustrated embodiment of the invention is an electric blanket (shown schematically in Fig. 2) which comprises a resistance heater, indicated generally by reference 1, and a control circuit, indicated generally by reference 2.

The control circuit 2 has live and neutral lines, L and N respectively, connected to a 240V AC mains supply via a two pole isolating switch 3. The live line L is connected to a pin P1 of an integrated circuit (IC) 4 via fuses F1 and F2, and the neutral line N is connected to a pin P2 of the IC 4 via a mains dropping resistor Rl. The IC 4 is a readily available eight pin "Burst Control Zero Voltage Switch" and is used to drive a triac 5 which supplies pulses of heating current to the heater 1.

The IC 4 generates an internal regulated DC voltage of 7 volts which is produced at pin P5 and an unregulated DC voltage of 14 volts which is produced at pin P3. The ground or negative for the internal voltages is at pin P1. The pin P3 is connected to a capacitor C1 which smooths the 14 volts DC and pin P5 is connected to a variable resistor VR1. Pin P3 functions as a low voltage detection circuit and impedes the output pulse on pin P4 (see below) in the event that the internal DC supply drops below a predetermined minimum value for any reason.

Pin P4 supplies a trigger pulse for switching on the triac 5 via a diode D1 which can be a light emitting diode to give an indication of ON pulses. Pin P6 is connected to a delayed pulse generator capacitor C2 and delays the pulse on P4 to give zero voltage switching. Pin P6 also acts as a spike filter. Pin 7 is connected to live line L by way of a capacitor C3, the size of which determines the overall pulse cycle time.

The voltage on pin P8 determines the duration of the trigger pulse from pin P4 and therefore the ON pulse duration. The higher the voltage applied to pin P8 the shorter the ON pulse becomes.

Radio frequency radiation and spikes are suppressed by circuits formed with the various resistors and capacitors R2. C5. D3, D4 C1, R3 and C3, and also R4 and C6.

The heater comprises three series connected resistance heating elements 6, 7 and 8. and a sensing element 9. One end of heating element 6 is connected to neutral line N and one end of heating element 8 is connected to the live terminal of the power supply via triac 5 and the fuse F1. The heating element 7 is connected in series between elements 6 and 8. The sensing element 9 is connected to live via a variable resistor VR2 and the fuses F1 and F2. The variable resistor VR2 is connected to both ends of sensing element 9 and also to IC pin P8 (via VR 1). A diode D2 is connected in parallel across VR2 and provides a half wave rectified DC voltage at VR2 which is smoothed by a capacitor C4.

The first and third heating elements 6 and 8 are wound together as a single dual element cable 10 (the structure of which is described below), and the second heating element 7 is wound together with the sensing element 9 as a single dual core cable 11 (the structure of which is also described below). The two cables 10 and 11 follow substantially the same tortuous path covering the area of the electric blanket, as shown in Fig. 2. The heating elements 6 and 8 each have the same resistance per unit length, the value of which is half that of the heating element 7, so that the power output per unit length of cables 10 and 11 is the same.

The structure of the dual core cables 10 and 11 will now be described with reference to Fig. 3 which is a part sectioned illustration of a length of cable 10.

The first heating element 6 is wound onto a central rayon core 12 and is covered by an intermediate insulating sheath 13 of polyethylene, which has a relatively low melting point. The third heating element 8 is wound around the intermediate sheath 13 and is itself covered by an outer insulating sheath 14 of PVC. The structure of cable 11 is the same as that of cable 10 with the important exception that the intermediate insulating sheath 13 is made of doped PVC which has a negative coefficient of resistance, i.e. it's resistance decreases with rising temperature.

As discussed above, the triac 5 supplies the heater 1 with pulses of current in response to trigger pulses from pin P4 of the IC 4. The amount of current supplied to the heater 1, and thus the heat generated by the heater 1, depends upon the ON pulse time, which is determined by the voltage applied to IC pin P8.

The IC pin P5 (+7 volts DC), the variable resistor VR1, the variable resistor VR2 and the IC pin P1 (0 volts DC), together form a potential divider for producing a voltage at pin P8. The overall voltage range is pre-set at the time of manufacture using variable resistor VR2, but within that range the oltage applied to pin P8 (and thus the heat generated by the heater 1) is manually variable by adjustment of the variable resistor VRI. For instance, if the vanable resistor VRl is adjusted so that IC pin P8 is connected to the same end thereof as the pin P5, then the potential applied at pin P8 will be at a maximum and no pulses will be supplied at pin P4.At the opposite extreme, if the moving contact of the variable resistor VR1 is moved to it's opposite end, then pin P8 will be at it's minimum potential causing pin P4 to emit a continuous current turning on the triac 5 and energising the heater 1. At intermediate positions current will be supplied to the heating element in pulses of varying duration.

When the heater 1 is energised a small current leaks across the doped PVC insulating layer of cable 11 from the heating element 7 to the sensing element 9.

This current flows to live line L via the preset variable resistor VR2 and the fuses F1 and F2. The leakage current raises the potential at VR2 and thus at the low voltage end of the variable resistor VRl and IC pin P8.

As mentioned above, the doped PVC insulating layer of cable 11 has a negative coefficient of resistance, as a result of which the leakage current increases as the temperature of the heating element 7 increases. Thus as the temperature of the heater 1 increases the potential at IC pin P8 increases and the ON pulse time is decreased which reduces the current supplied to the heater 1 to lower it's temperature. This will continue until a balance is reached between the power input to the heater 1 and it's heat losses, thereby stabilising the blanket's temperature.

The sensing element 9 and associated leakage current circuit thus provides an automatic mechanism for preventing overheating of the heating element under normal operating c-onditions. Should the heating element 7 and the sensing element 9 short circuit for any reason, the diode D2 provides a low resistance path to fuse F2 which will blow and disconnect the live line L from the IC 4 (to protect the IC).

The arrangement of the heating elements 6 7 and 8, and the structure of cable 10, together with the fuse F1, provides a safety mechanism in the event that the heater 1 begins to overheat, e.g. through failure of the sensing circuit (element 9 etc.) or otherwise. Should the cable 10 begin to overheat (cable 10 will be at the same temperature as cable 11 and so cable 11 cannot overheat without cable 10 overheating) the polyethylene insulating sleeve between heating elements 6 and 8 will melt. The elements 6 and 8 will then make contact and in the resulting short circuit the heating element 7 will be bypassed.As the element 7 has twice the resistance value of each of elements 6 and 8 (which each have the same resistance value). the resistance of the heater 1 will be reduced by 75% by the short circuit no Natter at which location along their lengths the elements 6 and 8 contact. The corresponding increase in current will cause the fuse F1 to blow thus cutting off the mains supply.

The low melting point insulating layer between elements 6 and 8 is chosen so that it will melt before the heater reaches a potentially dangerous temperature.

In one example of an electric blanket as described above, the various components are as follows: IC(4) - Zero Switching Type SL443A TRIAC(S) - 2 Amp 600 PIV VRl/Switch 3 - Combined 470KQPotentiometer & Double - Pole Switch VR2 - Preset to 47K F1 - 1A Fuse F2 - 100 MA Fuse D1 - LEO D2/D3/D4 - Diode IN4005 C1 - 470 mF 16V C2 - 10 nF 35V C3 - lmF 35V C4 - 47mF 35 C5 - 470nF 600V C6 - 100nF 600V R1 - 220 KnlW R2 - 220Q.5W R3 - 2.7 M JL .25W R4 - lKA .25W Cable 10 - Each of the inner and outer heating elements (6 and 8) has a resistance of 100SL Cable 11 - The heating element (7) has a resistance of 200Q, and the intermediate insulating sheath (13) is made from doped PVC type HN2/413 Under most operating conditions should the triac 5 develop a fault such that it produces a continuous ON-pulse the heating elements 6 and 8 will overheat causing the polyethylene insulating sleeve 13 to melt with a resulting short circuit which will cause the fuse F1 to blow, thus cutting off the mains supply from the heating elements. However, under certain conditions of use and ambient temperature it may be possible for the blanket to become dangerously hot, (with the triac failed into a permanently ON-pulse condition), without the elements 6 and 8 overheating.A modification which eliminates this possibility is illustrated in Fig. 4.

Fig. 4 illustrates a modified control circuit in which a thermal fuse F3 is connected in series between switch 3 and fuse F1. Parallelly connected resistance heating elements If and 16 are connected in series between the thermal fuse F3 and fuse F1 and are physically disposed around the thermal fuse F3 in close proximity thereto. A zener diode Z1 is connected across IC pins P8 and P5.

Under normal operating conditions with the triac T5 cycling ON and OFF the resistors 15 and 16 will not get hot enough to blow the thermal fuse F3. However, should triac T2 fail into a permanent ON-pulse condition the resistance heaters 15 and 16 will heat up sufficiently to blow the thermal fuse thus isolating the heating elements from the power supply regardless of whether or not the heating elements themselves overheat sufficiently to cause a short circuit.

The zener diode Z1 is added to the circuit to prevent the thermal fuse F3 from blowing under normal operating conditions. The voltage on IC pin P8 determines the length of the triac T5 ON-pulse period (the lower the voltage the longer the ON-pulse period) and is itself in part dependent upon the level of the leakage current flowing between heating element 7 and the sensing element 9. At very low ambient temperatures the doped PVC insulating sleeve 14 between the heating element 7 and sensing element 9 may be so cold that its resistance is very high and thus the leakage current is very low.In the absence of zener diode Z1, if under such low ambient temperature conditions the variable resistor R1 is set for maximum heating, the potential at pin P8 would be very near to zero which would cause triac T5 to remain in a permanent ON-pulse condition until the blanket warmed up sufficiently to increase the leakage current and thus reduce the voltage at pin P8. This permanent ON-pulse condition could cause the thermal fuse F3 to blow. However, such an eventuality is prevented by zener diode Z1 which limits the minimum potential at pin P8. That is, should the potential on pin P8 drop to the value of the potential at pin P5 less the zener voltage, then the diode Z1 will conduct and hold the pin P8 potential above zero.For instance, if the potential at P5 is 7 volts then a 5 volt zener diode may be used which would prevent the voltage at pin P8 dropping below 2 volts. This in turn will prevent the triac 5 from producing a continuous ON-pulse which would blow fuse F3.

Examples of suitable values for the various components are: Heating elements 15 and 16 3.3 ohms Zener diode 5 volts Fuse F3 tripping temperature 95C In addition to the above. the modified circuit also includes a neon light 17 connected in series with a limiting resistor 18 across the live and neutral lines.

The neon light 17 serves to indicate whether or not current is flowing in the circuit.

In the above embodiments of the invention, the voltage at IC pin P8 (which controls the pulse output of the IC 4) is produced by a combination of the IC pin P5. the manually variable resistor VRI. the pre-set variable resistor VR2 and the IC pin P1 (0 volts DC), which together form a potential divider. The variable resistor VR2 is used to pre-set the overall voltage range at the time of manufacture and VR1 allows the voltage applied to pin P8 (and thus the heat generated by the heater 1) to be manually varied by appropriate adjustment thereof.However, with this arrangement it might be found that with VR1 adjusted to a low temperature setting, i.e. with the movable contact close to the higher potential end of the resistor so that the potential applied to pin P8 is a maximum, that the leakage current potential from VR2 has very little influence on the voltage at IC pin P8 which will be held at a potential very close to that of pin P5 (i.e. 7 volts).

A modified control circuit which overcomes this potential disadvantage is illustrated in Fig. 5. For the sake of simplicity the modification is based on the circuit of Fig. 1. However it will be appreciated that the circuit of Fig. 4 could be similarly modified.

Referring to Fig. 5 the factory pre-set variable resistor VR2 (of Fig. 1) is replaced by a manually variable resistor VR3, and the original manually variable resistor VR1 (of Fig. 1) is replaced by a fixed resistor R5. In addition, resistors R6 and R7 are connected to each end of VR3 respectively. A neon 17 is again added to indicate whether or not current is flowing in the circuit.

In use, with the movable contact of VR3 positioned at the end of the resistor which appears at the bottom of Fig. 5, current flows from IC pin P5 through R5 and R6 producing a minimum potential at pin P8 and therefore a maximum power output. At this setting the operation of the circuit, and particularly the leakage circuit, is essentially unchanged from that of the circuit of Fig. 1.

However, if variable resistor VR3 is adjusted to a "low temperature" setting, i.e. if the contact is moved to the opposite end of the resistor (the upper end of the resistor as shown in Fig. 5) there is an increase in the potential applied to pin 8 but also the potential change at VR3 due to changes in the leakage current will be a maximum.

Resistors R6 and R7 are included because there is no longer a factory set pre-set variable resistor VR2 and instead the values of resistors R6 and R7 are chosen to give the required voltage and thus temperature range. In addition, selection of the values at resistors R5 and R6 can be used to control the desired degree of change in potential at VR3 as a result of changes in the leakage current.

Based on the preferred component values listed above in relation to Fig. 1, the various new conlponents may have the following values: VR3 - 150K 5L potentiometer and double pole switch R5 - 470Kn.25W R6 - 10K5L.'SW R7 - l0KA.25W An alternative modification of the control circuit of Figure 1 which is designed to give an increased improvement in the temperature control at lower power input settings is illustrated in Figure 6. The variable resistor VR2 is moved from the position illustrated in Figure 1 into series with the variable resistor VR1. With this arrangement one end of VR1 is connected to IC pin 5 (+7 volts) and the other end of VR1 is connected to VR2 which is in turn connected to 0 volts.The sensing element 9 is connected to IC pin 8 via a diode D5 (which replaces the diode D2 of Figure 1) and a resistor R8. With this arrangement the change in potential from the sensing element 9 is always fully applied to IC pin 8. thereby giving improved control at lower temperature settings.

In addition to the modifications mentioned above, a neon indicator 17 and limiting resistor 18 are included in the modified circuit. An additional capacitor C7 is also included to suppress possible outgoing radio interference and incoming mains spikes.

It will be appreciated that the present invention is applicable to a wide variety of heating panels, such as, for instance, mattresses and under-carpet heating pads, and is not limited to electric blankets.

Further, it will be understood by the skilled reader that the above described control circuits represent only a few of a large number of ways in which the present invention could be put into effect. For instance, alternate components to the IC 4 and triac 5 could be used to supply current to the heater, for instance a simple thermal relay could be used. Moreover. the current need not be supplied in pulses but could be continuous and of variable m.lënltude.

It will also be appreciated that features such as the manually adjustable variable resistors are advantageous but not essential to the invention.

With regard to the heater 1, it will be understood that the elements 6 and 8 need not necessarily have the same resistance and the resistance of element 7 need not be greater than the resistance of either of the elements 6 and 8. Similarly further heating elements could be added to the heater.

Moreover, only one of elements 6 and 8 need necessarily be a heating element, the other could be a simple low resistance conductor.

Furthermore. the simple fuse F1 could be replaced by other means for cutting off the power supply in response to an increase in the current, or some means which does not cut off the power supply altogether but simply reduces the current to a safe level.

It will further be appreciated that the detailed structure of the cables 10 and 11 could be modified. For instance, alternate materials to polyethylene and doped PVC could be used for the respective intermediate insulating layers 13.

For example, in the case of cable 11, undoped PVC, which also has a negative coefficient of resistance, could be used for the intermediate layer of cable 11.

However, in this case the increases in leakage current may need amplification in order to achieve the desired reduction in supply current (see for example British Patent Application No. GB2178201 A, referred to above).

Claims (18)

1. An electrically powered heating panel, comprising an electrical resistance heater and a current supply means for supplying current to the heater, wherein the heater comprises a first conductor in the form of a heating element connected in series with a second conductor which lies in close proximity to the first conductor and is separated therefrom by a layer of relatively low melting point insulating material, a third conductor in the form of a heating element connected in series between the first and second conductors, a sensing element which lies along the length of the third conductor in close proximity thereto for sensing leakage of current from the third conductor, said sensing element being connected to means sensitive to said leakage current and operable so as to regulate the current supplied to the heater, and means sensitive to an increase in current flowing through the heater and operable to at least reduce the current flow through the heater if said current exceeds a predetermined level, the arrangement being such that should the heater overheat the insulating material between the first and second conductors will melt causing a short circuit between the first and second conductors which bypasses said third conductor and results in an increase in current flowing through the heater to at least said predetermined limit.

2. An electrically powered heating panel according to claim 1, wherein said means sensitive to an increase in current comprise means for isolating the heating element from the current supply.

3. An electrically powered heating panel according to claim 2, wherein said means comprises a fuse.

4. An electrically powered heating panel according to any preceding claim, wherein the second conductor is in the form of a heating element.

5. An electrically powered heating panel according to any preceding claim, wherein the first and second conductors are combined in a single dual core cable.

6. A electrically powered heating panel according to any preceding claim, wherein the third conductor and the sensing element are combined in a single dual core cable.

7. An electrically powered heating panel according to any preceding claim, wherein the sensing element is separated from the third conductor by a layer of insulating material which has a negative coefficient of resistance so that as the temperature of the third conductor rises the current leaking to the sensing element increases. and said means sensitive to leakage current is operable to cause a reduction in the current supplied to the heater in response to an increase in said leakage current thereby stabilising the temperature of the heater.

8. An electrically powered heating panel according to claim 7, wherein the insulating layer separating the third conductor from the sensing element is made of doped PVC.

9. An electrically powered heating panel according to any preceding claim, wherein means are provided to at least reduce the current supplied to the current supply means in the event of a short circuit between the third conductor and the sensing element.

10. An electrically powered heating panel according to any preceding claim, wherein the relatively low melting point insulating layer separating the first and second conductors is made of polyethylene.

11. An electrically powered heating panel according to any preceding claim, wherein control resistor means are provided to enable user selection of the heater temperature.

12. An electrically powered heating panel according to any preceding claim, wherein the current supply means is operable to supply current to the heater in pulses and to regulate the current supplied to the heater by varying the ON-pulse time.

13. An electrically powered heating panel according to claim 12, wherein the current supply means comprises a burst control triac driver means.

14. An electrically powered heating panel according to claim 12 or claim 13. wherein means are provided for isolating the heater from the current supply in the event that the current supply means fails to a permanent On condition.

15. An electrically powered heating panel according to claim 14, wherein said means for isolating the current supply comprises a thermal fuse.

16. An electrically powered heating panel according to claim 15, wherein at least one heating resistor is located in close proximity to the thermal fuse and connected in series with the heater such that should the ON-pulse time exceed a certain limit the heat output of the or each heating resistor will be sufficient to blow the thermal fuse.

17. An electrically powered heating panel according to any preceding claim, wherein the heating panel is an electric blanket.

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