GB2178201A

GB2178201A - Electrically-powered beating panels

Info

Publication number: GB2178201A
Application number: GB08616521A
Authority: GB
Inventors: Leonard Charles Owers
Original assignee: THERMONETTE APPLIANCES Ltd
Current assignee: THERMONETTE APPLIANCES Ltd
Priority date: 1985-07-09
Filing date: 1986-07-07
Publication date: 1987-02-04
Also published as: DE3623087A1; GB8616521D0

Abstract

An electrically-powered heating panel in the form of an electric blanket comprises a dual coil heating element 2, a device 3a for supplying electrical current to the element 2 in pulse form, and a control (4) operable so as to at least reduce current flow to the blanket in the event of undue increases in "on" pulse duration, so as to prevent overheating of the panel. The device may comprise a thermal relay which supplies electrical energy to the blanket cylically, in "on" and "off" pulses of discrete form. The control may comprise a positive temperature coefficient thermistor operable whereby when the current "on" pulses become too long, (and therefore cause the blanket 1 to overheat), the thermistor rapidly increases in temperature and operates to reduce current flow. <IMAGE>

Description

SPECIFIGATION Improvements in or relating to electricallypowered heating panels BACKGROUND TO THE INVENTION This invention relates to electrically-powered heating panels.

As used herein, the term "electrically-powered heating panels" is intended to include electrically-powered under-carpet heaters, blankets, mattresses and pads. The invention is particularly applicable however, to electricallypowered blankets, or, more simply, "electric blankets".

SUMMARIES OF THE INVENTION According to one aspect of the present invention, an electrically-powered heating panel comprises a heating element, means for supplying a pulsed current to the heating element, means sensitive to a predetermined increase in "on" pulse duration, and operable so as to at least reduce the supply of pulsed current to the heating element.

The panel may have a dual coil heating element, wherein one coil of the heating element is used to heat the panel and the other coil of the heating element is used to sense leakage of current from the said one coil, the panel being provided with means sensitive to said leakage and operable so as to regulate the on/off ratio of the pulsed current supply to the heating element.

The means sensitive to a predetermined increase in "on" pulse duration may comprise positive temperature coefficient thermistor means.

Alternatively, the means sensitive to a predetermined increase in "on" pulse duration may comprise thermal fuse means.

The means operable to regulate the on/off ratio of the pulsed current supply to the heating element may comprise thermal relay means, or, alternatively, burst control triac driver means.

The means sensitive to current leakage may comprise capacitor means operable to control the on"off ratio of the pulsed supply so as to stabilize the temperature of the panel at a level selectable by control resistor means.

The invention also comprises any novel subject matter or combination including novel subject matter herein disclosed.

BRIEF DESCRIPTION OF THE DRA WINGS An embodiment of the invention will now be described by way of example only, with reference to the accompanying drawings, wherein: Figures 1 and 2 are circuit diagrams.

In the figures, like reference numerals refer to like components and features.

DETAILED DESCRIPTIONS OF THE PREFERRED EMBODIMENTS With reference first to Fig. 1, an electricallypowered heating panel in the form of an electric blanket 1 has an A.C. mains input 9 with line (L) and neutral (N) connections. The blanket 1 comprises a dual coil heating element 2, means 3 for accepting the A.C. current input and using the input to supply pulsed current to the heating element 2, and means 4 sensitive to a predetermined increase in "on" pulse duration and operable so as to at least reduce the supply of pulsed current to the element 2.

As explained hereinafter, this reduction in pulsed current supply prevents overheating of the blanket.

The dual coil heating element 2 comprises an inner (heating) coil 5 wound on a central core of rayon and covered with an insulating sheath 6 of thermoplastics material. An outer coil 7 is wound on the sheath 6 and is itself covered with an outer insulating sheath. (Not shown). A suitable thermoplastics material for use in constructing sheath 6 is polyvinyl chloride (P.V.C.), the insulating resistance of which decreases with increase of temperature. Under normal operating conditions, a very small leakage of electrical current takes place between coils 5 and 7. The outer sheath may also comprise P.V.C. insulating material. Coil 7 is not used as a heating coil but is used as a sensing element so as to sense leakage of current from the coil 5.

The pulsed current supply means 3 comprises a thermal relay. (Alternatively, an electronic "burst fire" control device may be employed). The means 3 supplies electrical energy to the heating element 2 of the blanket 1 cyclically, in "on" and "off" pulses of discrete form. Typically, the "on" and "off" periods are each of 5 second duration for maximum energy input.

The pulsed current control means 4 is sensitive to a predetermined increase in "on" pulse duration. It comprises a positive temperature coefficient thermistor means operable whereby when the current "on" pulses become too long, (and will therefore cause the blanket 1 to overheat), the thermistor means rapidly increases in resistance so as to reduce current flow to a safe level.

Alternatively, or in addition, the pulsed current control means 4 can comprise thermal fuse means of the form operable to release switch contacts held closed by wax against biasing towards opening. Should the wax be heated sufficiently, the switch contacts will open, and the circuit will be broken. The wax may be heated by a heater resistance connected in series with the circuit. Alternatively, thermal relay means of the type using a bimetal strip may be employed.

A further alternative is wire fuse means.

The coils 5 and 7 are both used to control the pulsed current supply means 3.

Should the on"off ratio of the pulsed cur rent supplied to the heating element 2 change unduly whereby duration of the "on" pulses tends to increase above the predetermined level, (say by 50% or nore), the insulation 6 between the coils 5 and 7 will start to increase in temperature, causing a corresponding decrease in resistance. The reduced resistance of insulation 6 allows the leakage of current between coils 5 and 7 to increase.

The increased leakage current is employed, using an amplifier if necessary, to automatically reduce the "on" pulses and increase the "off" pulses of current supplied to the heating element 2 of the electric blanket 1. The "on" pulse duration can be reduced to zero if required.

Increased leakage of current between coils 5 and 7 can also be used to control the pulsed current supply means 3 should the duration of the "on" pulses tend to decrease below the predetermined level, or should "off" pulse duration change significantly.

The required on/off ratio of the pulsed current supply to the heating element 2 can be varied manually by use of a pulse ratio adjuster 8 connected to the regulating means 3.

Overheating of the electric blanket 1 could occur should the pulsed current supply means 3 develop a fault whereby "on" current pulse duration is increased unduly, i.e. above a safe design criteria. In this event, the pulsed current control means 4 is operable so as to at least reduce current flow to the heating element 2 of the blanket 1.

Fig. 2 illustrates an electric blanket 1 a which makes use of a modified circuit, which is of more sophisticated form than that illustrated by Fig. 1.

In the circuit of Fig. 2, the pulsed current supply means 4a comprises a thermal fuse F and associated heater resistors R5 and R6, all connected in series.

The pulsed current supply means 3a comprises an integrated circuit 20 formed by an l.C. Type 443A marketed by R.S.Components Limited of London, United Kingdom, and described and illustrated by Data Sheet R/3611 issued by that company in July 1979.

The switch 20 comprises a zero voltage "burst control" triac driver integrated circuit in an 8-pin dual in-line package, and is used to fire a triac T2. It also comprises means for generating a constant inernal D.C. supply of 7 volts.

Pins P1, P2 of the switch 20 are connected to a 240 volt A.C. mains supply line L and neutral N respectively. Pin P1 is connected to line L by way of resistors R5, R6, fuse F, and single pole switch SW. Pin P2 is connected to neutral N by way of a mains dropping resistor R3.

A capacitor C4 is connected to pin P3 and to line L. The capacitor C4 provides smoothing of the internal D.C. supply.

Pin P5 is connected to the internal 7 volt supply. Pin P6 is connected to a spike filter and delayed pulse generator capacitor C1, and pin P7, which is connected to line L by way of capacitor C2, controls the timing cycle.

Main interference suppression is provided by two circuits. One of these circuits comprises resistor R4, capacitor C5, diodies D2 and D1, resistor R2 and capacitor C2. The other of the circuits comprises resistor R7 and capacitor C6.

The switch 20 discharges triac gate (G) firing pulses from pin P4, and by way of a lightemitting diode D3 whereby the triac T2 supplies the heating coil 5 with pulses of heating current. The duration of the heating current "on" pulses is determined by the voltage applied to pin P8 of the switch 20. The lower the voltage applied, the longer the duration of the pulse. The voltage applied to pin P8 is controlled by a variable resistor VR 1 and FET transistor T1.

Resistor R1, coil 7, and a pre-set resistor VR2 together provide a potential dividing circuit governing voltage across capacitor C3.

This voltage is also applied between the gate (G) and drain (D) of the transistor T1, and is initially pre-set by the resistor VR2. This voltage is, however, changed by leakage of current between coils 5 and 7, across the P.V.C.

sheath 6. The current leakage increases as the temperature of the sheath 6 increases, and lowers the potential applied to the transistor T1, allowing the impedance between source (S) and (D) of the transistor to go high, whereby voltage applied to the pin P8 increases. Voltage increases at the pin P8 result in the duration of "on" heating current pulses decreasing.

The variable resistor VR 1 can be adjusted to vary the potential at pin P8, whereby the on/off ratio of the pulsed output of heating coil current can be regulated. The resistor VR 1 thus corresponds to the pulse ratio adjuster 8 of Fig. 1. As the "on" pulses increase in duration, those of the "off" pulses decreases correspondingly, and vice versa.

As mentioned above, R5 and R6 are heating resistors associated with the thermal fuse F. If the generation of pulsed current supplied to heating coil 5 should cease, caused for example by the triac T2 staying locked on for some reason, whereby a steady current flows, then heat emitted by the resistors R5 and R6 will cause the thermal fuse F to trip open circuit.

Claims

1. An electrically-powered heating panel comprising a heating element, means for supplying a pulsed current to the heating element and means sensitive to a predetermined increase in "on" pulse duration, and operable so as to at least reduce the supply of pulsed current to the heating element.

2. A heating panel as claimed in Claim 1, having a dual coil heating element, wherein one coil of the heating element is used to heat the panel and the other coil of the heating element is used to sense leakage of current from the said one coil, the panel being provided with means sensitive to said leakage and operable so as to regulate the on/off ratio of the pulsed current supply to the heating element.

3. A heating panel as claimed in Claim 1 or 2, wherein the means sensitive to a predetermined increase in "on" pulse duration comprises positive temperature coefficient thermistor means.

4. A heating panel as claimed in Claim 1 or 2, wherein the means sensitive to a predetermined increase in "on" pulse duration comprises thermal fuse means.

5. A heating panel as claimed in Claim 2, 3 or 4, wherein the means operable to regulate the on/off ratio comprise thermal relay means.

6. A heating panel as claimed in Claim 2, 3 or 4, wherein the means operable to regulate the on/off ratio comprises burst control triac driver means.

7. A heating panel as claimed in any one of Claims 2 to 6, wherein the means sensitive to current leakage comprise capacitor means operable to control the on/off ratio of the pulsed supply so as to stabilize the temperature of the panel at a level- selectable by control resistor means.

8. An electrically-powered heating panel substantially as hereinbefore described with reference to Fig. 1 of the accompanying drawings.

9. An electrically-powered heating panel substantially as hereinbefore described with refeence to Fig. 2 of the accompanying drawings.

10. Any novel subject matter or combination including novel subject matter herein disclosed, whether or not within the scope of or relating to the same invention as any of the preceding claims.