GB2363046A

GB2363046A - Thick film element protection arrangement

Info

Publication number: GB2363046A
Application number: GB0013116A
Authority: GB
Inventors: Robert Andrew O'neil; David Andrew Smith
Original assignee: Otter Controls Ltd
Current assignee: Otter Controls Ltd
Priority date: 2000-05-30
Filing date: 2000-05-30
Publication date: 2001-12-05
Anticipated expiration: 2020-05-30
Also published as: GB2363046B; GB0013116D0; AU5865101A; WO2001093638A1

Abstract

A thick film heating element for an electric kettle has a main track section 2 and a track section 5 connected in parallel therewith and the parallel track section has a gap which is bridged by a glass, ceramics or glass ceramics material 6 which exhibits a marked fall in electrical resistance with increasing temperature above a certain level. A current-sensitive cut-out monitors the current through the heating element and cuts-out when, in response to overheating of the element its current rises as a result of the resistance of the material bridging the gap in the parallel track falling. An alternative arrangement employs an NTC material being provided between two elongate parallel conductors which are connected to respective ones of the power supply terminals of the thick film heating element. PTC material may be encorporated in the main element (Fig. 2).

Description

2363046 i IMPROVEMENTS RELATING TO ELECTRIC HEATING ELEMENTS

Field of the Invention:

This invention concerns improvements relating to electric heating elements and more particularly relates to the protection of thick film heating elements against overheating. The invention will be described in the following by reference to heating elements for liquid heating vessels such as kettles and hot water jugs for example, but it is to be appreciated that the invention is susceptible of wider application.

Background of the Invention:

In the field of kettles and hot water jugs it is well known to provide heating elements with overtemperature protection devices designed to disconnect the heating element from its power supply in the event of the element temperature rising above a predetermined level, for example as a result of the kettle or jug being switched on without first being filled with water. Such overtemperature protection devices have commonly comprised bimetallic elements which are juxtaposed with the heating element so as to be responsive to its operating temperature, the bimetal causing a switch to operate to disconnect the heating element from its power supply when the 2 heating element temperature as sensed by the bimetal rises to a predetermined level deemed to be representative of an oVertemperature situation.

It is also well known to utilise fusible materials to protect heating elements against overtemperatures, the fusible material melting at a predetermined temperature and causing a set of contacts to open to disrupt the supply of electricity to the heating element. Proposals have also been made to incorporate a current carrying fusible component into the manufacture of a heating element, the fusible component melting when exposed to a heating element overtemperature situation and disrupting the current path through the heating element.

The disadvantage arises, however, that if a fusible component is incorporated into the configuration of a heating element, then operation of the fusible component in a heating element overtemperature situation causes destruction of the heating element. The appliance is protected from the disadvantageous effects of an overtemperature heating element but at a cost, namely the sacrifice of the heating element. Given that the overtemperature situation might arise simply on account of forgetfulness, in that a kettle or hot water jug for example is not filled with water before it is switched on, and that a bimetallic overtemperature control can be reset in such a situation simply by filling the kettle with cold water, it might well be thought unnecessary to sacrifice the heating element in such a situation and indeed that has, not surprisingly, been the thought process of appliance manufacturers for many years.

Notwithstanding that it has long been considered both unnecessary and undesirable to sacrifice a perfectly good heating element just because it has been inadvertently switched on dry, proposals to do precisely this continue to be made. See, for example the arrangement proposed in WO-A9739603 in this connection. A thick film heating element is disclosed which has between adjoining track sections a bridging portion formed of a glass, ceramics or glass ceramics material which exhibits a resistivity which is negatively temperature dependent such that above a certain temperature the track portion in question operates as a thermal fuse, the leakage current between the adjoining track sections increasing to such a level as to generate sufficient heat to vaporise the respective parts of the heating element track and thereby disable the heating element. Even when such an arrangement is employed to provide secondary or back-up protection operable only in the event of failure to operate of a bimetallic or other device providing primary protection, the destruction of the heating element seems undesirable.

Obiects and Summary of the Invention:

It is the principal object of the present invention to overcome or at least substantially reduce the abovementioned problem.

4 It is another object of the present invention to achieve the abovernentioned object by use of the kind of glass, ceramics or glass ceramic.material as is employed in the arrangement proposed in WO-A9739603 abovementioned.

According to the present invention in one of its aspects a thick film electric heating element has a portion of such a glass, ceramics, or glass ceramics material included in its track and the track is so configured that, upon overheating of the element causing the electrical resistance of said portion to fall, the current in the track increases to such an extent as to trip an associated current sensitive device without causing destruction of the element.

In an embodiment of the abovementioned aspect of the invention which is described in detail hereinafter, a thick film heating element has a track section extending in parallel with the main heating track and a narrow gap in the parallel track is bridged by a patch of glass, ceramics or glass ceramics material as is employed in the arrangement of WO-A-9739603. The power supply to the heating element is through a current-sensitive thermal cut-out of the type used in DC motor protection, for example a Q or B3 control switch as manufactured and sold by us. In the event of the heating element overheating, the resistance of the patch decreases thus connecting the parallel track section in parallel with the main track section and thereby reducing the overall resistance of the heating element and causing the current through the cut-out to increase to such a level as to cause the cut-out to I operate. By appropriate selection of the cut-out and of the characteristics of the thick film heating element, effective overtemperature protection of tht heating element can be assured which does not result in destruction of the element.

In accordance with a modification of the abovementioned embodiment, in order to ensure rapid heating of the glass, ceramics or glass ceramics patch on overheating of the element, an arrangement of PTC (positive temperature coefficient of resistance) and normal heater tracks may be positioned close to the patch so that as the temperature rises the power density close to the patch increases as compared to the remainder of the heating element. By this means an even more rapid response to an overtemperature situation can be achieved, again without causing damage to the heating element.

The operation of the glass, ceramics or glass ceramics patch in the abovementioned arrangements is similar to that of an NTC (negative temperature coefficient of resistance) resistor except that in an NTC resistor the reduction in resistance occurs more slowly and generally at a lower temperature. For some applications an NTC resistor might thus be used instead of the glass, ceramics or glass ceramics patch abovementioned. NTC materials, however, tend to have a high resistivity which can be a problem.

According to a second aspect of the present invention, however, this problem can be overcome or at least substantially reduced by use of a very 6 short NTC material track of great width which enables the necessary lov values of track resistance to be obtained.

The above and further features of the present invention are set forth with particularity in the appended claims and will be explained in the following by reference to exemplary embodiments which are illustrated in the accompanying drawings.

Description of the Drawings:

Figure 1 is a schematic showing of a thick film heater track including a parallel track section having a gap bridged by a patch of glass, ceramics or glass ceramics material of the kind described in WO-A-9739603; Figure 2 shows how the responsiveness of the Figure 1 arrangement can be enhanced by means of a PTC track section; Figure 3 is a graph showing power output as a function of temperature for the PTC and normal track sections of Figure 2; Figure 4 is a schematic showing of an alternative thick film heater track employing a short and wide NTC track section; Figure 5 is a circuit diagram corresponding to the arrangement of Figure 4; and Figure 6 shows a graph of circuit resistance and current as a function of temperature for an exemplary thick film heating element according to Figure 4.

7 Detailed Description of the Embodiments:

Referring to Figure 1, the track layout of a thick film heating element I is shown which has a main track section 2, which extends between conductors 3 and 4, and a parallel track section 5 having a small gap therein which is bridged by a patch 6 of glaze of a glass, ceramics or glass ceramics material of the kind described in WO-A-9739603.

The whole heating element 1 is supplied through a thermal cut-out (not shown) of the type used in DC motor protection - for example a Q or B3 control. In the event of the heating element overheating, the glaze patch 6 will be heated by the surrounding tracks and will start to conduct electricity. This will effectively connect the track section 5 in parallel with the main track section 2 and lead to a reduction in the overall resistance of the element and a rapid increase in the current used by the element which causes the generation of more heat. If the thermal cut-out has been calibrated to just carry the normal operating current of the heating element, this increase in current will cause the cut-out to operate, thereby disconnecting the power supply ftom the heating element.

In order to ensure a rapid heating of the glaze patch 6, an arrangement of PTC and normal heater tracks may be positioned close to the glaze patch 6, so that as the temperature rises so the power density close to the glaze patch 6 increases as compared to the rest of the element. The accompanying drawings 8 illustrate this proposal. Figure I shows a basic track layout incorporating the parallel sensor track 5 with a break, with conductive glaze 6 covering tne break, and bunching of the main heater tracks 2 to raise the local power density is also shown. The track material of the sensor track 5 is selected to give a suitable resistance which will cause the cut-out to operate, but will not cause any mains protection to operate. Fig 2 shows the addition of a PTC branch 10 to the main heater track 2 to cause a raised power density at high temperatures, and Fig 3 shows a graph of power output against temperature for each of the branches, showing the power increase in the non PTC branch.

Any type of current sensitive cut-out could be used, and it may provide for manual reset, either mechanical or electrical.

Referring now to Figures 4 to 6, these show a further embodiment which takes an alternative approach. The reduction in resistance of the glaze in the first embodiment is very similar to the natural behaviour of an NTC resistor, except that in an NTC resistor it occurs more slowly and generally at a lower temperature. For a primary system of protection, a glaze which needs to exceed 400 or 500'C in order to operate could be unsuitable, as such high temperatures would possibly overstrain the rest of the heating element and furthermore there would be a need for considerable thermal insulation of the heating element from the appliance body in which the heating element was mounted and there would be similar implications for any seal that was provided between the heating element and the appliance body. Figures 4 to 6 snow a proposal to use two concentric ring conductors joined by an NTC areL.

NTC materials tend to have a very high resistivity, so having a short track (defined by the spacing between the conductors) of great width (defined by th,:

length of the conductors) will allow the necessary low values of track resistance to be obtained.

As shown in Figure 4, the thick film heating element 20 has a schematically illustrated main track portion 21 which may take any convenient form and there is additionally provided a short and wide NTC track section 22 consisting of parallel arcuate conductors 23, 24 which are bridged by NTC material 25. The NTC track section 22 is connected in parallel with the main track portion 21 across power supply terminals 26 and 27 with a current sensitive thermal cut-out 28 provided in series with the NTC track section 22 as shown in Fig 4 or more preferably in series with the overall heating element as shown in Figure 5. A ballast resistor 29 may, if necessary, be provided. A boil control 30, for switching off the heating element in response to boiling of liquid in an associated vessel may also be provided as shown in Figure 5.

The precise layout of the NTC track section in this embodiment will depend on the materials used. It may be formed as a single almost complete circuit, or it may be divided into a number of shorter arcs, to give the necessary total width and length. For typical NTC materials, a fall in resistance of 11 times may be expected with a 1500C temperature rise, increasing to a fall of 26 times at 250'C rise. Such a material also has a fall in resistance of about 4 - 5 times at boiling point. By the use of suitable values a stable heating element current may be obtained up to boiling point, with a rapid increase in current thereafter. For example, using a PTC material for the track with a coefficient of 3000 ppm, and a room temperature resistance of 20.7 Ohms, together with an NTC material having a Beta of -2076, and a room temperature resistance of 225 Ohms, the overall track resistance of these two in parallel will rise from 19 Ohms at 20T to 19.7 Ohms at boiling, but will fall above boiling to 7.8 Ohms at 250T. Such a heating element would have a nominal power of RW, and could be controlled by a B3 current sensitive cut-out set around 500C, which would break the resulting 30 Amps in less than 8 seconds. Of course the current would rise very rapidly in practice, due to the unstable nature of the NTC resistance, and it would be expected that the cut-out would trip much more quickly than this, thus avoiding the tripping of any mains circuit protection.

Fig 6 shows a graph of circuit resistance and current versus temperature for the embodiment of Figure 4 for a particular set of values suitable for a 2.4kW heating element. Rm, Rmain, is the main track resistance, Rs, Rsensor, is the sensor (NTC) track resistance, and R is the overall element resistance. The temperatures in the graph work backwards from 250T for convenience of calculation, so 230 is 200C and 150 is 100T.

Such an arrangement would allow a section of the NTC part to be exposed before the main track, thus giving a measure of protection against operation of I I a water heating appliance for example on a slope. Note that the circuit resistance falls and the current increases sharply above a predeterinmed temperature.

A significant advantage of the present invention is not only that a thick film heating element can easily be protected by means of a simple and relatively inexpensive current sensitive cut-out, but also that the current sensitive cut-out does not have to be mounted on the heating element or, indeed, anywhere near it if necessary. The ability to provide slope protection in a kettle is an incidental benefit. The ability to operate with a remote current sensitive device would allow the use of a thick film heating element in appliances other than kettles, where a complex control/connector such as our X4 is not needed, and allow the thermal control to be mounted elsewhere, such as on a control panel or circuit board. In addition, if the cut-out were a manual reset, then the reset button may be placed at some convenient place, where it can be made accessible. Thus the invention can be applied to appliances such as washing machines and tumble dryers, where the heating elements are generally mounted in inaccessible locations. Note that, although a tumble drier does not (as a rule) contain water, the heating element is still subject to overheating as a result of blocked airflow filters.

The invention having been described in the foregoing by reference to specific embodiments, it is to be appreciated that the embodiments are in all respects exemplary and that modifications and variations thereto are possible 12 without departure from the spirit and scope of the invention as set forth in the appended claims. For example, the NTC material 25 in the embodiment of Figure 4 could possibly be constituted by a glass, ceramics or glass ceramics material as employed in the embodiments of Figures I and 2.

1 '1,

Claims

Claims:

A thick film electric heating element having included in its track a portion formed of a glass, ceramics or glass ceramics material selected to exhibit a decrease in its electrical resistance upon an increase in temperature above the temperature it would be expected to experience in normal operation of the heating element, the track being so configured that, upon overheating of the heating element, the consequent decrease in the electrical resistance of said portion will cause the heating element current to increase to such an extent as to enable an associated current-sensitive cut-out to operate without causing destruction of the heating element.

2. A thick film. heating element as claimed in claim 1 wherein said portion formed of said glass, ceramics or glass ceramics material is provided so as to bridge a gap in a track section of the heating element which is connected in parallel with the main heating track of the element.

3. A thick film heating element as claimed in claim 1 or 2 wherein the track layout of the heating element is such as preferentially to expose the portion formed of glass, ceramics or glass ceramics material to the heating element temperature.

4.

14 A thick film heating element as claimed in claim 3 wherein heating element tracks are bunched more closely together in the region of said portion than elsewhere on the heating element.

5. A thick film heating element as claimed in claim 3 including a PTC track section near to said portion.

6. A thick film heating element substantially as herein described with reference to Figure 1 or Figure 2 of the accompanying drawings.

7. A thick film heating element as claimed in any of the preceding claims in combination with a current-sensitive cut-out arranged to disconnect the heating element from its power supply in the event of the heating element overheating and drawing a current above the maximum current rating of the cut-out.

8. A thick film electric heating element comprising elongate parallel conductors bridged by an NTC material so as to constitute a temperature dependent resistance exhibiting a substantial decrease in resistance above a predetermined temperature corresponding to the normal operating temperature of the heating element.

9. A thick film heating element as claimed in claim 8 comprising a mai---. heating track having said temperature dependent resistance connected in parallel therewith.

10. A thick film heating element substantially as herein described with reference to Figure 4 or Figure 5 of the accompanying drawings.

i 11. A thick film heating element as claimed in claim 8 or 9 or 10 in combination with a current-sensitive cut-out, the arrangement being such that, n response to the heating element overheating so as to cause the resistance of said temperature dependent resistance to fall, the current through said cut-out increases to such an extent as to cause the cut-out to operate.

12. A thick film heating element and current-sensitive cut-out combination as claimed in claim 11 wherein the cut-out is arranged to be responsive to the total current in the heating element, including any current flowing through said temperature dependent resistance.

13. A thick film heating element and current-sensitive cut-out combination as claimed in claim 7 or 10 or 11 wherein the cut-out is located remotely from the heating element.

16 14. A thick film heating element incorporating, as a temperature responsive component, a portion formed of a glass, ceramics or glass ceramics material exhibiting a decrease in electrical resistance with increasing temperature above a predetermined temperature level, the arrangement enabling disconnection of the heating element from its power supply in an element overtemperature situation without causing destruction of any part of the heating element.

15. An electrical appliance incorporating a thick film heating element and a current-sensitive cut-out responsive to the current flowing in part, at least, of the heating element, the arrangement being such that in response to the heating element overheating above its normal operating temperature there is a substantial change in said current which causes said cut-out to operate before the heating element is damaged by the excess temperature to which it is subjected.