WO1993002533A1

WO1993002533A1 - Electrical heating elements

Info

Publication number: WO1993002533A1
Application number: PCT/GB1992/001348
Authority: WO
Inventors: Stylianos Panayi; Albert Edward Barker
Original assignee: Global Domestic Products Limited; Hoover Limited
Priority date: 1991-07-23
Filing date: 1992-07-22
Publication date: 1993-02-04
Also published as: GB9115902D0; AU2330192A

Abstract

An electrical heating element for use, for example, in a domestic iron and comprising essentially a metallic base plate having a coating of a glass ceramic dielectric upon which is laid an electrically resistive track produced from a platinum-containing material whose electrical conductivity falls as its temperature rises thus providing a self-limiting characteristic for the element. The resistive track is covered with further dielectric material and the element is incorporated along with appropriate control circuitry and equipment, into an appliance to be heated. The resistive track has a temperature coefficient of resistance which is sufficiently positive that at a pre-determined maximum temperature its electrical resistivity is at a level at which the rate of electrical energy absorption is substantially equal to the rate of heat loss.

Description

ELECTRICAL HEATING ELEMENTS

THIS INVENTION relates to electrical heating elements and to appliances, both industrial and domestic, incorporating the elements. The elements are particularly useful for domestic irons. Other typical applications include equipment such as kettles, coffee pots, toasters, panel heaters for space heating, and cooking hobs.

It is well known that most of these appliances incorporate a sheathed element with a separate thermostat to control the maximum temperature, Appliances incorporating these elements often require a separate thermal cut-out device which cuts the power supply to the element at a certain temperature to prevent the appliance from becoming dangerous by over heating and liable to catastrophic failure.

An object of the present invention is to provide an electrical heating element in which a separate thermostatic control is not required to determine its maximum temperature and wherein the need for a thermal cut-out may be avoided.

A further object of the invention is to provide an electrical heating element which will reach its required temperature more rapidly than with the conventional designs and wherein the occurrence of hot spots or cold spots due to external influences is substantially avoided thus ensuring a more uniform distribution of heat across the element.

According to one aspect of the present invention, an electrical heating element comprises a dielectric substrate which carries at least one depos ited res is tive track of a material whose temperature coefficient of resistance is sufficiently positive that at a predetermined maximum temperature its electrical resistivity is at a level at which the rate of electrical energy absorption is substantially equal to the rate of heat loss .

Such a resistive track can be obtained if it comprises platinum or other suitable material having a similar coefficient of electrical resistivity (TCR) to that specified above. Such a track may be produced by applying to a di electri c subs trate a pattern corresponding to the required track, for example by a printing technique, and comprising a paste comprising platinum, and then firing.

There is described in US-A-4689270 the production of an electrical heater element by initially producing a substrate by applying to both sides of a s teel sheet a coating of a BaO-Al ₂θ₃ ~S iθ₂ glass ceramic . There is then applied a resistive heater track . In the example given , the resistive track comprises a layer of ruthenium oxide . This material , as with other materials previously proposed for heater tracks, can continue to absorb electrical energy until, in the event of a control malfunction , the resulting energy absorbed produced a temperature rise causing the element to melt .

The present invention provides a generally similar electrical heater element but uti li ses a resistive track with a particular characteristic which may be obtained by making the track from a material comprising platinum. This has a very substantial and surprising advantage in the context of a heater element . A heater track comprising platinum has the property that its electrical conductivity falls as its temperature rises thus providing a self - l imi ting characteristic to the heater as a whole . This means that it may not be necessary to incorporate a fusible link or other emergency switch-off device by which the overheating leading to melting referred to above has hitherto usually been prevented .

A further advantage of a track comprising platinum is that it can be furnace fired in air without substantial oxidation whereas it has been found necessary to fire some previously proposed track materials in an inert or reducing atmosphere.

The invention may be carried into practice in various ways but a heating element embodying the invention and suitable for use in a domestic iron and the method of production of the heating element will now be described by way of example with reference to the accompanying diagrammatic drawings, in which : -

Fig. 1 is a plan view of the heating element;

Fig. 2 is a typical section to a larger scale through the heating element, for example on the line II-II in Fig. 1.

and Fig. 3 is a circuit diagram of the main components of a control system for operation of the element.

The heating element shown in the drawings comprises a plate-like base 1 of sandwich construction on which the conductive but resistive track 2 is carried .

The base 1 comprises a central layer or lamella 3 of chrome steel 1.6 mm thick on both sides of which there is a dielectric layer 4 , 5 which is 40 μm thick . The dielectric is deposi ted on the steel lamella 3 by a vapour deposition or screen process . It may be possible for the dielectric material to be sucked around the outer edges 6 of the lamella and around the edges of steam outlet holes 7 which extend through the base 1 . Af ter depos i tion of each dielectric , the base is fired . The materials and processes used may be those described in US-A-4689270 to which reference may be made for further details . In the particular example being described, the dielectric material is a glass ceramic of BaO-Al₂ O₃ -Siθ₂ with the approximate composition of 35% of BaO, 10% AI2 O3 and 53% Si0₂ -

The resistive pattern is produced by first applying to one face of the base by a screen-printing process a ribbon 8 of an additional dielectric in the pattern of the required track , the ribbon having a thickness of between 28 and 40 μm and a width of 5 mm. Over the dielectric and in the same pattern there is applied the track 9 of resistive material with a thickness of approximately 0 . 3 μm and a width of approximately 5 mm. The resistive material is in the form of a platinum resinate paste . This is a thin film of paste enabling the printing of platinum on an alumina substrate. The platinum content of the paste is approximately 12.5% by weight and the paste includes a small amount of non-precious metals in the form of soluble organo-metallic compounds. After application the paste must be fired resulting in a film which, due to the simultaneous sintering of the non-precious metal oxides offers high adhesion between the metal film and the substrate.

The paste is applied to the substrate preferably by a screen printing process after which the printed pattern is dried at between 80° and 100° C for between 10 and 20 minutes, the subsequent firing cycle being between 30 and 60 minutes at a temperature of 850° to 950° C.

Several layers of the paste may be applied as required according to the voltage to be applied to the element and each layer must be fired before application of the next. The final layer may be trimmed if necessary using a laser.

The pattern consisting of the dielectric and platinum paste is covered by a layer 11 in the same pattern of an additional dielectric layer of material and to a thickness of some 30 μm, thus to seal the metal layer hermetically within the ceramic. Each layer is sequentially fired at the appropriate sintering temperature.

Spot welded studs (not shown) may be fixed to the base plate 1 so that the latter can be attached to the body of the iron without the need for exposed screws.

Finally, terminals 12 provide electrical connections to the platinum track.

The relative dimensions of the parts shown in Figs. 1 and 2 are somewhat disproportionate for the purpose of clearer illustration.

Referring now to Fig. 3 it will be seen that in line with the mains electrical supply at terminals 12 there is provided a triac 13 which is operable to switch the element 2 on and off. This is connected to an electronic integrated control circuit 14 which, as will be seen, serves largely as a comparator. The circuit 14 is powered by a limiting resistor 15 from the mains supply. Heating element 2 is connected to circuit 14 via an isolating capacitor 16, a coupling transformer 17 and the resistor 18.

A resistance temperature device (RTD) in the form of a track 2a is laid down on the base 1 preferably alongside the heating element 2. The RTD is adapted to monitor the instantaneous temperature of the base 1 , and for this purpose is connected to the control circuit 1 .

A variable potentiometer 19 is connected to control circuit 14 together with an audible alarm 20 and a set of three indicator LED ' s 21 , 22 and 23 coloured respectively yellow, green and red. A touch sensitive switch 24 and a tilt switch 25 are also connected to the control circuit 14.

In the operation of the element as part of an electric iron, triac 13 is in the "on" mode to commence heating up of the element. During the initial period of heating up o f element 2 the s ignal from 2 a representing its resistance is lower than that supplied by the potentiometer and so the triac remains in the "on" mode to continue heating the element 2 and yellow LED 21 is illuminated.

The closer the separate resistive track 2a for temperature sensing is laid to the heating element 2 the faster the thermal response of the temperature sensing element to the heating element becomes .

The signal from the temperature sensor element 2a becomes one arm of a bridge . The temperature set potentiometer 19 offsets the bridge output feeding the circuit 14.

The advantage of such an arrangement is that the lower and upper iron (or any other such device) temperature settings are the anti-clockwise and clockwise limits respectively of the potentiometer thus avoiding the need for an expensive potentiometer.

Power to the bridge is supplied at points 30 and 31 via a voltage regulator (contained in circuit 14) which also supplies the temperature setting potentiometer 19.

Resistances 17 and 18 have values which in conjunction with 2a allow the end stops of the potentiometer to be the lower and upper temperature settings. The whole circuit then tracks with the power supply. A filter network is connected to point 32 to modify the time constant and therefore the response time to temperature variations .

Once the element reaches its pre-set temperature, the green LED 22 is illuminated and the yellow LED 21 is extinguished. Simultaneously, a signal is fed to audible alarm 20 which may be adapted to give a continuous or intermittent bleep to indicate that the element has reached its required temperature.

Should the temperature of the element rise significantly above that required the red LED 23 is illuminated and the green LED 22 is extinguished.

The temperature is continuously monitored and compared with the pre-set temperature of potentiometer 19 and thus the required temperature will be quickly restored from any deviation.

As described previously, the electrical conductivity of element 2 decreases with increasing temperature so that at the maximum temperature setting of potentiometer 19, the resistance in the element will increase to a level at which no further energy will be transferred into the element other than that required to maintain the temperature in response to heat dissipation to the atmosphere or to a surface to which the iron is applied.

In normal operation, only the green LED 22 should remain illuminated unless the potentiometer 19 is set to a lower level in which case, for a short period of time red LED 23 will show. Conversely, if the setting is suddenly increased, then yellow LED will show for a period until the required temperature is reached. The tilt switch 24 is adapted to send a signal to the circuit 14 when the base plate of the iron is placed in a horizontal position.

The touch sensitive switch 25 may be incorporated as a proximity device into the handle of the iron such that when the operator lets go of the handle a signal is fed to circuit 14 to turn triac 13 to the "off" mode and thus allow the element 2 to cool down. If necessary, there may also be incorporated a time delay such that triac 13 is turned off only after a period of say one minute after the user lets go of the iron.

When a signal is received from tilt switch 24, and triac 13 is in the "on" mode it may be turned off and/or a signal may be fed to the audible alarm 20 to alert the user that the iron has been left on its base plate.

It will be appreciated therefore that there are many automatic control and safety aspects of an iron incorporating an element in accordance with the invention. The temperature of the element is substantially uniform and is maintained within a very close tolerance of the required temperature as set by the potentiometer . Wholly electronic circuitry is provided to afford such control and the user is provided with a set of visual indicators and an audible alarm to provide an indication of the instantaneous condition of the heating element . The touch switch 25 and the tilt switch 24 provide further safeguards against the iron being left in a potentially dangerous conditio .

It is not intended to limit the invention to the above example only . There are many variations which could be incorporated . For example, if required, the element 2 itself which is laid on the base plate can act as a temperature sensor and provide a continuous or intermittent signal to the control circuit 14.

Claims

1 . A method of producing an electrical heating element comprising the steps of providing a dielectric layer and applying thereto at least one electrically res i s tive track o f a materi al whos e temperature coefficient of resistance is suff iciently positive that , at a predetermined maximum temperature i ts electrical resistivity is at a level at which the rate of electrical energy absorption is substantially equal to the rate of heat loss .

2. A method according to Claim 1 , wherein said resistive track is applied in the form of a platinum resinate paste which is subsequently fired to provide a platinum-based film bonded to the dielectric layer and having a thickness in the region of 0.1 to 0.3 μm.

3. A method according to Claim 2 , wherein said platinum film is made up from several layers of said platinum paste, each being fired before application of a subsequent layer .

4. A method according to Claim 1 or Claim 2 , wherein the layer is formed as a dielectric substrate providing a continuous coating on a metallic base plate .

5. A method according to any one of Claims 1 to 3, wherein the dielectric layer is formed as a ribbon of dielectric material in a similar shape to that of the track to be deposited, the subsequent pattern of electrically resistive material being superimposed upon said dielectric ribbon, and after application of the resistive track the track being covered, in a similar pattern by an additional layer of dielectric material thus to encapsulate the resistive layer hermetically within dielectric material.

6. A method according to Claim 5, wherein said dielectric layer is initially deposited upon a metallic base plate having an existing dielectric substrate extending over all external surfaces thereof.

7. A method according to Claim 5, wherein the dielectric material used to produce the substrate, the ribbon and the dielectric layer is a glass ceramic material having the formula Ba0-Al2 O3 -Siθ2 with an approximate composition of 35% of BaO, 10% AI2 O3 and 53% Si0₂.

8. A method according to Claim 6, including the step of welding studs to the base plate so that the latter can be attached to the body of an iron without the need for exposed screws.

9. A method according to any preceding claim, including the step of providing terminals conductively connected to said resistive track thus to provide electrical connections thereto.

10. A method according to Claim 6, including the step of laying down a resistance temperature device on the base plate alongside the heating element.

11. A method according to Claim 2, wherein said platinum resinate paste is applied by a screen printing process after which the pattern is dried at between 80° and 100° C for a period of between 10 and 20 minutes, and subsequently fired for between 30 and 60 minutes at a temperature of 850° to 950° C.

12. An electrical heating element comprising a dielectric substrate which carries at least one deposited resistive track of a material whose temperature coefficient of resistance is sufficiently positive that at a predetermined maximum temperature its electrical resistivity is at a level at which the rate of electrical energy absorption is substantially equal to the rate of heat loss.

13. An electrical heating element according to Claim 12, wherein said resistive track comprises platinum or other material having a similar coefficient of electrical resistivity.

14. An electrical heating element according to Claim 12 or Claim 13, wherein said dielectric substrate is a glass ceramic material having the formula Ba0-Al2 O3 -Siθ2 with the approximate composition of 35% of BaO, 10% Al₂ 0₃ and 53% Si0₂.

15. An electrical heating element according to any one of Claims 12 to 14, further comprising a metallic base plate having a coating all around of dielectric material, a superimposed ribbon of dielectric material in the pattern of the required resistive track, several layers of a platinum resinate paste each subsequently fired and made up to form the resistive track and an additional dielectric layer superimposed upon said track thus to encapsulate the latter hermetically within dielectric material.

16. An electrical heating element according to Claim 15, wherein said metallic base plate is in the region of 1.6 mm in thickness having all around a dielectric layer of some 40 μm, the resistive track being laid down to a thickness in the region of 0.1 to 0.3 μm and with a width in the region of 5 mm.

17. An electrical heating element according to Claim 15, wherein said platinum resinate paste contains, in addition to platinum, a small amount of non-precious metal in the form of soluble organo- metallic compounds providing, after firing, oxides offering high adhesion between the platinum and the substrate .

18. An electrical heating element according to Claim 15 or Claim 17, wherein said platinum resinate paste contains platinum in the proportion of 12.5% by weight of the total weight of the paste.

19. An electrical heating element according to Claim 15, including a plurality of studs spot-welded to the metallic base plate to provide means of attachment to an appliance to be heated.

20. An electrical heating element according to Claim 15, including terminals conductively connected to the resistive track.

21. An electrical heating element according to any one of Claims 15 to 20, in combination with a control system for operation of the element, the system comprising a triac for switching the element, an electronic integrated control circuit, a resistive ^• temperature device laid down on the substrate alongside the heating element and connected to the electronic integrated control circuit , and a variable voltage device to select the working temperature of the element .

22. A domes tic iron including an electrical heating element according to Claim 20 , including a touch sens i tive switch and a tilt swi tch , both connected to the electronic integrated control circuit .

23. A domestic iron according to Claim 22 , further comprising a plurality of visual indicators and an audible alarm, said visual indicators being actuated by the control circuit thus to provide an indication of a temperature in the element respectively below and at the temperature selected on the variable voltage device, said tilt switch providing a signal to the integrated control circuit when the base plate of the iron is placed in a horizontal pos ition thus to energise the heating element.

24. A domes tic iron according to Claim 22 , wherein the touch sensitive switch is incorporated as a proximity device into the handle of the iron and is connected to the electronic integrated control circuit thus to de-energise the he.ating element when aii operator lets go of the handle.

25 . A domes ti c iron according to Claim 24 , including a time delay device connected to the touch sensitive switch and operable in conjunction with the control circuit to cause the element to be de-energised only after a pre-determined period after the operator has let go of the handle .

26. A domes ti c iron according to Claim 22 , wherein said tilt switch is connected to said audible alarm adapted to alert an operator when the iron has been left with the base plate in a horizontal position .

27. A domes tic iron according to Claim 22 , wherein the resistive track acts as a temperature sensor and provides a continuous or intermittent signal to the electronic integrated control circuit .

28. A domestic iron having a heating element formed on a heating plate thereof by a method according to any one of Claims 1 to 11 .