GB2207014A - Electrically-powered heating panels - Google Patents

Description

1 - Mcrovements in or relatinS to Electricnllp-Powered HeatinG Panelf'

B.kCKPoRO'LT'-,.',ID TO THE I1TITEITIOIN 2207014 This invention relates to electrically-powered heating panels.

As used herein, the term Clectrically-powered heating paneW is intended to include electrically-powered under-carpet heaters, blankets, mattresses and pads. The invention is particularly applicable however, to electrically-powered blankets, oTg more simply, Clectric blankets".

The present invention is concerned with that form of electrically-powered heating panel comprising a tortuously disposed dual coil heating element. As used herein, a Rual coil heating elemenC is defined as an inner heating coil covered with a first insulationy with an outer heating coil wound on the first insulation and insulated with a second insulation.

The invention is an improvement over, for exanple, the invention disclosed in our British Patent Specification No.

2,160,749A, in that overheating protection is provided over the whole length of the heating element in an inexpensive manner.

Electrical connections to the heating blement are also simplified.

STURY OF Til' PIRnSE'IT I7-7U'TIOIT According to the present invention, an electrically-powered heating panel comprises a dual coil heating element (as herein defined) connected to a multi-poRition switch so as to give i differing heat outuuts from the panel accordinG to the chosen position of the switch, the switch beinj operable to connect diode means into respective differing functional positions relative to the coils of the heating element.

The switch preferably has three positions end the diode means is connectable respectively (a) in series with both heating coils of the element, or (b) in series with only one heating coil of the element, or (c) in series with only the other one heatinS coil of the element, the resistance of the heating toils beinj different.

7RIEF DEWRIPTIC7 OF TPI DRATI7JS An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, wherein:- Ficure 1 is a circuit diajram of an electric blanket constructed in accordance with the invention, Figures 1A, 1B and 1C show the circuit of Ficure 1 simplified for each of three respective "on" switch positions, Figure 2 is a circuit diaSram representinG one fault conditiont Land Figure 3 illustrates a modificatipn of the arrangement shown by Figure 1. DETAILED DE5GRIPTION, OF THE, PP7WERIM Tl-,IIBODII=.".T- Mith reference first to Figure 1, an electrically-powered heating panel 20 in the form of an electric blanket comprises a 1 i 1 i 3 - dual coil heating element 21 connected to a rwolti-position switch S so as to give differing heat outputs from the blanket accordin- to the chosen position of the switch. The switch S is U - operable to e"onnect diode means comprising a diode D into L, 3 respective differinG functional -positions relative to the coils of the heating elenent 21.

The dual coil heating. element 21 comprises inner (R1) and outer (R2) coils insulated from each other by insulation I. The heating element 21 is connected into a 240 volt AC power supply circuit (represented. by a "line" terminal L and a "neutral" teri!iinal 11) via a fuse F and the switch S, which has three o-oerative "on" positions and two 11offLI1 positions.

The inner coil R1 is wound cn a- central core of ravon and covered with the insulation I which co:-.iorises a sheath of thermo- -Dlas'uics material. The outer coil R2 is wound on the sheath of insul-- "ioii I and is itself covered with an outer insulating sheath (not shown). A suitable thermoplastics material for insulation I is -polyvinyl chloride (P.V.C.), the insulating resistance of which decreases with increase of te.mperature. Under normal operating conditions, a very small leakage of electrical current takes place between coils Rl.and R2. The outer sheath may also comprise P. V.C. insulating -material.

The switch S has contacts lb and 2b connected to line terminal L and supply contacts 311 and 4b connected to neutral terminal N.

Corres-tionding contects la-, 2a, 3.,-,. and 42, connect with respective W - ends of the coils Rl and R2. Three lines 10/1, 10/2 and 10/3 of the switch S represent how the switch connects the contacts of the switch in the three operative positions of the switch; each operative -position representing, a different heat output of the panel. With switch S in the "off" position, contacts la, 2a upply terminals.

5g, and 4.Eare isolated from the A.C. power s The control circuit incorporates diodes D1 to D5.

In detail, selection of the first or lowest heat position of switch S causes terminal N to be connected to contact 4ja and by way of line 10, and terminal L is connected to contact la.

1 A These connections bring R2 in series with Rl via. diode D3. Fi,pre 1A illustrates.

Selection of the second or medium. heat mosition of switch S causes ter. :iinal N to be connected to both contacts 4a and 3a, and ter-minal L to be connected to contact 1R. This brings Rl alone into circuit via. cli- ole M. Fi.-u-e 13 ill,.ist:.,-'es.

Selection of the third or highest heat position causes terminal N to be connected to contact 4ja and terminal L to be connected to both contacts la and 2a. This brinGs R2 alone into circuit via diode D3. Figure 1C illustrates.

It will be appreciated that diode D3 conducts in all operative uositions of the switch S d (on negative half cycles) but it takes up differing functional -positions dependin. on the position of the switch S.

Diodes D1 and D4 are Drotective diodes to provide an additional protective fusing path in the event that insulation I fails at any point.

Diodes D2 and D5 are supplementarv diod.es connected in parallel with respective protective diodes D1 and D4 so as to provide continual protection if D1 or D4 fails.

The rating of fuse P should be just above the current flowing in the highest heat position of switch S.

In an exam-ole of the circuit of Finre 1, the resistance of L coil Rl is 720 ol-ras; the resistance of coil R is 360 oLrls. This would result in power dissipation of C.0 watts (maximum); 40 watts (M2dium) and 26.7 watts (low) for the three operative positions of switch S. The (half-wave) current at 80 watts is 471 mA. Therefore a suitable value of standard fuse F is 500 mA.

Referrinc_. once agpin to FiEure 1, the ends aiid mid points of coils Rl and R2 have been marked Al. Cl, Bl and A2, C2, 12.

In the event that insulation I fails at wir point between Al and Bl (sa.y at Cl/C2), in addition to the full wave current, conduction can take glace (on vositive half cycles) via diodes D4/D5 and D1/D2 to reduce the overall resistance of the circuit.

This ensures an increase of current such that the fuse F disconnects power supply to the blanket 20.

One fault condition is represented by. Figure 2 in which the insulation I fails at the point Cl/C2 of the heating element with the switch S in the lowest heat uosition. It is seen that under these conditi2ns, effectively the two halves of coil Rl 1 1 1 operate in parallel, and the two halves of coil R2 also operate in parallel, so as to give a resistance in circuit which is lower than any of the normal resistances in circuit, whereby the fuse F operates to disconnect the power supply.

The resistance in circuit under fault conditions varies according to the location of the fault and the operative position i i of the switch S. In most fault conditions the fuse F will operate to open the circuit immediately the fault axises. Marginal conditions can axise when the insulation I fails at or near the centre of the heating element 21. In such a case the fuse F may not operate immediately. In this event, the power input to the heating element 21 is increased. The increases comprise 127 wattsy 133 watts and 149 wattsy at low, medium and maximum heat positions of the switch S. Coils Rl, R2 will then increase in temperature so as to cause further failure of the insulation I so that a fuse operating current level is reached, even where the rating of the fuse F actually installed is somewhat higher than the above- mentioned recommended value. (Say 620 mA instead of 500 IRA).

If diode 13 goes open circuit then no current flows in any i i i position of switch S.

If diode D3 goes short circuit then there is half-wave direct conduction in all switch positions to cause the fuse F to operate.

In a modification of the circuit illustrated by Figure 39 the connections to R2 have been reversed. This modification alters fault current conditions slightly so that operation of fuse F is 1 i i i more specific.

Reversing the connections to coil Rl would have the same effect.

It is observed that overheating protection exists over the whole length of the dual coil heating element 21 of Figures 1 and 3. The circuit components employed axe elementary and few thus providing an inexpensive control and simple electrical connections.

0 1 C L A 1 11 S 1. An electrically-powered heating p,-,.nel comprising a dual coil heatin- element (as herein defined) cor2iceted to a multi position switch so as to give differinS heat outputs from the panel according to the chosen position of the switch, the switch being operable to connect diode means into respective differing functional Dositions relative to the coils of the heating element.

Claims (1)

1. 2. A panel as claimed in Clai-ni 1, in which the switch has three

   -oositions ana the diode means is connectable respectively (a) in series with both heatin., coils of the element, or (b) in series with only one heating, coil of the element, or (c) in series with only the other one heatin: coil of the element, the resistance of the heating coils being different.

   3. A panel as claimed in Claim 1 or 2, havin-- -protective diodes to provide a protective fusing path in the event that insulation between the heating coils of the ele-lient fails.

   4. A panel as claimed in Clair, 3, wherein the protective fusing path is not immediately created but is subsequently created by heating of the coils causing further failure of the insulation.

   5. A panel as claimed in Claim 3 or 49 wherein the protective diodes are supplemented by diodes in barallel to provide continual protection in the event that a protective.diode fails.

   6. An electrically-powered heatinS -Danel, substantially as herein before described with reference to the accompanying drawinrs.

   0 Dublis' c hea lgbB at 171E Patenz Stale Hcuse.6671 High Hc-'born.Londo:. WC1R 4TP. I'arlher copies =ky be obtained from The Patent Offi e, SR-,IeS Brarch. St Mary Cray. Orpington. Zent BR5 35RD P:,inted by MJtr,', e_x ecl,-mTies ltd, St Ala.,7 Cray, Kent. Con. 1 87.

   1 1 1 i 1 1 i