WO2002071800A1

WO2002071800A1 - Thick film heaters and resistances

Info

Publication number: WO2002071800A1
Application number: PCT/GB2002/000951
Authority: WO
Inventors: Vernon James Donnelly
Original assignee: Strix Limited
Priority date: 2001-03-05
Filing date: 2002-03-05
Publication date: 2002-09-12
Also published as: WO2002071800A8; DE60224013D1; GB2373157B; ES2298343T3; ATE381240T1; GB2373157A; GB0105384D0; DE60224013T2; EP1366640B1; EP1366640A1

Abstract

A thick film heating element or resistance comprises a main track (4) arranged to carry electric current during normal operation and an auxiliary track portion (26a, 26b) electrically coupled to said main track (4) by a bridge (28) of glass, glass-ceramic or ceramic material such that it does not carry current during normal operation, but wherein the configuration of the auxiliary track (26a, 26b) and the bridge (28) of glass, glass-ceramic or ceramic material is chosen such that at a predetermined temperature a leakage current between the main (4) and auxiliary (26a, 26b) tracks rises to such an extent that a failure current flows through a section of the main track (4).

Description

Thick Film Heaters and Resistances

This invention relates to thick film heaters and resistances, particularly, but not exclusively, to thick film heaters which are suitable for use in liquid heating vessels.

The use of thick film heaters for liquid heating vessels has now become well known in the art . When using a thick film heater, it is necessary to provide protection in the event that it overheats - e.g. due to being energised without being in contact with water. It is also usual to provide some secondary protection in case the primary protection should fail for any reason. One extremely successful method of providing such protection is effectively to provide dual primary protectors in the form of a pair of independent snap-acting bimetals. This arrangement is described more fully in W095/34187 and is evident in the Applicant's U series of controls.

More recently a practical way incorporating secondary protection into the element itself, by making the element fail-safe in the event of serious overheating has been proposed in WO 97/39603. In this arrangement a current sufficient to cause failure of a track on the element is made to flow by providing a glass bridge across two adjacent track portions. Under normal operating conditions this glass bridge is electrically insulating and thus has no effect. Under serious overheat conditions however - e.g. those that would be experienced if the element should be switched on dry and the primary overheat protector should fail - the temperature of the glass bridge rises to such a level that it becomes conductive. This short-circuits the two adjacent track portion causing an increased current to flow which causes one of the track sections to fail .

This idea is developed further in our co-pending application GB-A-2353457 whereby in an arrangement of the sort described above, a notch is provided in one of the track sections to concentrate locally the current flowing through it. This allows the position at which the track will fail to be predetermined by placing the notch at the desired point along the track.

It is an object of the present invention to enhance the flexibility, particularly in element track layouts, of arrangements of the sort set out above . The invention provides, a thick film heating element or resistance comprising a main track arranged to carry electric current during normal operation and an auxiliary track portion electrically coupled to said main track by a bridge of glass, glass-ceramic or ceramic material such that it does not carry current during normal operation, but wherein the configuration of the auxiliary track and the bridge of glass, glass- ceramic or ceramic material is chosen such that at a predetermined temperature a leakage current between the main and auxiliary tracks rises to such an extent that a failure current flows through a section of the main track. Thus it will be seen that the present invention provides an arrangement whereby instead of short- circuiting two physically adjacent portions of track in the event of a serious overheating, an auxiliary track portion may be used effectively to provide a short circuit between two physically separate parts of the main circuit, thereby bypassing the part of the circuit between these two points. This- affords greater flexibility when designing the track layout.

Although not essential, preferably a predetermined region of the part of the main track which is left in circuit after the short-circuit is designed preferentially to rupture. It has been found that- in general, the track which is left in circuit could fuse at any point along its length depending, for example on how accurately the track has been laid down and so on. Furthermore, when the track fails, an arc is generated. This arc is highly conductive and is mobile in a magnetic field and may, therefore, be drawn to other components such as control components. The arc may even damage the insulating layer on which the track is provided, leading potentially to the element being live after failure. This is potentially dangerous.

It is therefore highly desirable to make the failure occur in a predetermined region of the track so that this could, for example be arranged in a position away from other components such as control components. Preferably the predetermined region is provided with means for concentrating locally the current flowing therethrough.

Preferably the means for concentrating the current locally comprises a local reduction in the width of the track. This may be achieved by waisting the track in, but more preferably the narrowing comprises a hole, e.g. a circular hole, through the track. This hole will then act as a focus for the failure of the track.

The auxiliary track could comprise resistive material, e.g. the same resistive material as is used for typically the majority of the main track. Preferably however, the majority of the auxiliary track comprises a low resistive material, e.g. silver. This gives maximum flexibility in locating the points on the main track which are bypassed by the auxiliary track since the latter is effectively not then limited in length and may easily be routed around obstacles, parts of the main track etc . without incurring any problems associated with current crowding, further details of which are given in WO 98/36618.

The auxiliary track portion could be coupled directly to the main track - e.g. the coupling could comprise a gap between the main track and an end of the auxiliary track, the gap being bridged by the bridge of glass etc. material.

More preferably the auxiliary track portion is coupled to the main track via a second auxiliary track portion which is in turn connected to the main track. In other words, in this case the coupling comprises a gap in the auxiliary track itself which is bridged by the glass, glass-ceramic or ceramic material. This means that the point at which the serious overheat is detected - i.e. when the glass etc . bridge becomes sufficiently conductive - need not necessarily be immediately adjacent the main track. This could, for example, allow a sufficient delay to ensure that the primary protector has time to operate before the track blows .

Both of the above arrangements enable current flow through the auxiliary track only under serious overheat conditions as required in accordance with the invention. In the preferred embodiment, a short section of preferably low resistance - e.g. silver - track is provided laterally across the main track, with the gap to the rest of the auxiliary track being in the vicinity of the main track. This allows the glass etc. bridge over the gap to be provided in the vicinity of the main track and thus react as rapidly as possible in the event of a serious overheat condition arising from failure of the primary protector.

A single auxiliary track may be provided, but it is preferred in some circumstances to have two or more. This will, inter alia, give an even further level of protection since either may render the e.lement supply inoperative in the event of a serious overheat . Multiple auxiliary tracks may short-circuit completely different parts of the main track, but preferably they are substantially coterminous at one end thereof respectively. Preferably, these ends terminate in the vicinity of a common point on the main track, most preferably on either side respectively thereof. The other ends of the -two auxiliary tracks may then be connected to different points of the main circuit. Preferably the common point is approximately half-way around the circuit - i.e. the main track has approximately the same resistance on either side thereof . This allows an arrangement whereby the two auxiliary tracks are connected at their other respective ends to substantially the two ends of the main track - i.e. to where the power supply is applied. At one end of the main track in this preferred embodiment however, the portion designed preferentially to rupture is located between the power supply and the connection to the auxiliary track. This ensures that it remains in- circuit and so interrupt the power supply if it should be needed to rupture .

This arrangement has the advantage that the same piece of glass, glass-ceramic or ceramic material may bridge the ends of both auxiliary tracks to the main track, or more preferably to a short section of track provided laterally across the main track.

Where two or more auxiliary tracks are provided, leakage current flow through them respectively may cause failure of different sections of the main track. More preferably each auxiliary track is so arranged that it will cause a failure current to flow in the same, preferably predetermined, section of the main track. This facilitates the desirable result that the ultimate failure of the element may be controlled so as to be as safe as possible. It is also preferred that multiple auxiliary tracks are arranged such that if a leakage current path should be formed between the or any pair of them, a failure current will still flow through a or the predetermined portion of the main track. Put another way, in preferred embodiments of the invention, the auxiliary track or tracks is or are arranged so that part of the main track which includes a weak portion, is always kept in-circuit.

In a particularly preferred version of the arrangements set out above, two auxiliary tracks are arranged as above such that in the event of a serious overheat, the glass etc. will become sufficiently conductive to connect the two tracks together, thereby forming a single composite auxiliary track. In the preferred arrangement, set out above, where the two tracks terminate substantially at the ends of the main track, this will result in all of the main track except the weak portion being bypassed. The very high resulting currently causes the weak portion to fail very quickly - before an ordinary domestic circuit breaker can operate in the preferred embodiment .

When viewed from a further aspect the present invention provides a thick film electric heater comprising a heating track, a low resistance track terminating in the vicinity thereof and a bridge of glass, glass-ceramic or ceramic material coupling said low resistance track to the heating track, the low resistance track and the bridge being arranged such that at a predetermined temperature the bridge becomes sufficiently conductive that the low resistance track provides a short-circuit across part of the main track, thereby causing a portion of the remainder of the heating track to fail.

A preferred embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Fig. 1 is a view of a thick film heating element showing the flow of electric current during normal operation;

Fig. 2 is a schematic block circuit diagram showing the main and auxiliary tracks;

Fig. 3 is an enlarged view of region A marked on Fig. l; Fig. 4 is a view similar to Fig. 1 showing the current flow during a serious overheat situation;

Fig. 5 is a view similar to Fig. 4 showing an alternative current path; and Figs. 6 and 6b are views before and after failure respectively of the weak portion of the main track. Turning firstly to Fig. 1, there may be seen a thick film printed element 2 for use in a coffee maker. As is well known in the art, this comprises a stainless steel substrate layer onto which is fired an electrically insulating glass layer. Electrically resistive ink and silver are laid onto the insulating layer to form the heating tracks.

As will be seen from Fig. 1 and the schematic block circuit diagram of Fig. 2, there are three separate heating tracks 4, 6, 8 which may be selectively energised, to give different total power outputs, by selectively applying mains power to the line contact pads 10, 12, 14. The control arrangement is such that the first portion of track 4 will always have current flowing through it during normal operation, regardless of whether either of the other two tracks 6, 8 are energised. It is thus only necessary to consider the first portion of track 4 for the purposes of explaining the present invention.

The path of the current through the track 4 during normal operation is marked on Fig. 1 as a solid black line. From the line terminal pad 10, this path extends in a serpentine manner in both horizontal and vertical directions around the upper left corner region (as viewed from Fig. 1) 4a of the element. The arrangement of the path in this region 4a is intended to give reasonably even heating for a snap-acting bimetallic actuator 16 (shown in transparent outline) . This bimetallic actuator is intended to give primary protection against overheating - e.g. in the event that the element is switched on dry or boils dry. The movement of the bimetallic actuator 16 opens a pair of contacts (not shown) to interrupt the supply of electrical power to the element as is well known in the art . The path continues in an arcuate portion circumventing the neutral supply contact pad 20, to a further horizontal serpentine region 4b. There is a kink 18 in the path in this region which may be seen more clearly in the enlarged scrap section of Fig. 3. This part of the track is explained in greater detail later herein.

After the horizontal serpentine region 4b is a vertical serpentine region 4c, the leftmost leg 4d of which extends down to the lower end of the heater. The lower two thirds or so of the leftmost leg 4d comprises silver track. The track continues upward in a widened portion 22. The width of the widened portion 22 is approximately 3 mm wide compared to the rest of the track which is approximately 1 mm wide . Halfway along the widened portion 22 is a hole 24 in the centre of the track, approximately 1 mm in diameter. Beyond the widened portion 22, the track continues 4e to a contact pad 10 to which the neutral side of the mains power supply is connected. The majority of the main track 4 comprises a resistive ink such as a blend of Dupont F/612/F629 screen printed onto an insulating substrate layer - e.g. of Dupont 3500 ink. This is well known to those skilled in the thick film art. The contact pads 10, 12, 14, 20, connecting portions of track, the turns in the serpentine regions 4a, 4b, 4c, the arcuate portion of track circumventing the neutral pad 20; and the track 4d, 4e on either side of the widened portion 22 however are all printed with a silver ink having a significantly lower resistivity than the rest of the track. As is explained in greater detail in WO 98/36618, this arrangement avoids the problems of current crowding where the track undergoes sharp changes of direction or width.

In addition to the main track 4, there is provided a pair of auxiliary tracks 26a, 26b both of which are printed in silver. These are shown cross-hatched in

Fig. 1. The first auxiliary track 26a is connected at one end to a silver portion 4f of the main track connecting the line contact pad 10 to the first serpentine region 4a. The auxiliary track extends • around the neutral contact pad 20 and stops just short of the kinked portion 18 of the main track as may be seen more clearly in Fig. 3. Similarly one end of the second auxiliary track 26b is connected to the silver portion of the extended leg 4d of the vertical serpentine portion 4c, with the other end stopping just short of the kink 18 in the main track.

Considering Fig. 3, there may be seen the kinked portion 18 of the main track as well as the two silver auxiliary track portions 26a, 26b. A further short length of silver track 28 is printed over the kinked portion 18 of the main track at right angles to it to form small gaps 30a, 30b between it and the two respective auxiliary tracks 26a, 26b. Furthermore, a disc of low temperature overglaze 32 in the form of a • layer of ES 4771G material is printed so as to cover the two gaps 30a, 30b.

In ordinary use of the element, when main power is applied to the line and neutral contact pads 10, 20, current flows along the path indicated in Fig. 1 so that the track 4 generates heat which heats water on the other side of the insulating substrate. The overall resistance of the main track 4 at normal operating temperature is approximately 151 ohms giving a steady state current of 1.52 amps and thus an overall power of 350 W assuming a supply voltage of 230 N RMS.

In the event that the element should be switched on dry or allowed to boil dry, its temperature will rise to such an extent that the bimetallic actuator 16 will operate, opening a pair of contacts (not shown) to interrupt the power supply to the element, thus allowing it to cool. In the unlikely event that the bimetallic actuator 16 fails to operate or the associated contacts fail to open, the temperature will continue to rise to such an extent that the low temperature overglaze 32 begins to break down and allow a leakage current to flow across one or other of the gaps 30a, 30b.

If the lower gap 30a is breached first, the subsequent current flow will be as depicted by the solid black path in Fig. 4. Thus, rather than flowing around the rightmost portion (as viewed- from Fig. 4) of the track (4a and part of 4b) , the current preferentially flows through the auxiliary track 26a, across the gap 30a bridged by the now conductive overglaze 32 and to the silver track portion 28 where^~±tr^~ then continues along the ordinary current path formed by the main track 4 to the right of the kink 18 (when viewed from Fig. 3) . The auxiliary track 26a therefore acts as a short- circuit, effectively bypassing the rightmost portion of track. Since the kinked portion 18 is approximately half-way around the track 4, this means that the total resistance is approximately halved so that the current drawn is doubled and the power dissipated is quadrupled.

Alternatively, if the upper gap 30b is breached first the current glow will be as depicted in Fig. 5. In this case it is the rightmost part of the track 4a, 4b which remains in circuit, bypassing the serpentine portion 4b between the kink 18 and the extended leg 4d where the auxiliary track 26b rejoins the main track. The effect, however is the same in that the overall resistance is approximately halved and so the overall power dissipation is quadrupled.

In both cases it will be seen that an increased current is drawn through the widened portion 22. The hole 24 in this portion of track serves to concentrate the current locally and therefore cause a local hot ^• spot. The mere^' doubli-ng of current through this section 22 of track is not sufficient to cause it to fuse. However, whichever of the gaps 30a, 30b is breached first, the other will be breached very soon thereafter in view of the proximity of the two gaps 30a, 30b. The increased power dissipation from the track remaining in- circuit will when only one gap is breached will of course accelerate this.

Once both gaps 30a, 30b have been breached, current will simply flow through both auxiliary tracks 26a, 26b and thus the only resistive part of the track to remain in-circuit is the widened portion 22. This reduces the overall resistance to 2.88 ohms and thus a current of 80 amps to be drawn. In a matter of milliseconds this very high current causes the track 22 to blow where it is concentrated by the hole 24. Figs. 6a and 6b show the widened portion 22 before and after it has blown. The current is drawn for a sufficiently short period that it will not trip ordinary domestic circuit-breakers or fuses. Once the track has blown, no further current can flow and the heater is safely and permanently disabled. Since the hole 24 is well away from the bimetallic actuator 16 etc., potentially dangerous arcs will be prevented.

It will be appreciated by those skilled in the art that many modifications and variations to the arrangement described are possible within the scope of the invention. For example just a single auxiliary track may be provided which short-circuits a sufficient proportion of the track that the weak spot will blow. Also, it is not essential that the heater be used to heat a liquid - the teachings of the present invention are applicable equally to heaters which may overheat for reasons other than operating dry - e.g. excessive applied voltage. Furthermore the principles of the invention may also be applied to other electrical resistances whose prime function is not to generate heat - e.g. controllers for electrical motors etc.

It will also be appreciated that the values of resistance etc. given are purely exemplary and other values may be chosen as appropriate. For example the relative widths of the weak portion of track and the hole may be varied in order to vary the ordinary and failure currents.

Claims

Claims :

1. A thick film heating element or resistance comprising a main track arranged to carry electric current during normal operation and an auxiliary track portion electrically coupled to said main track by a bridge of glass, glass-ceramic or ceramic material such that it does not carry current during normal operation, but wherein the configuration of the auxiliary track and the bridge of glass, glass-ceramic or ceramic material is chosen such that at a predetermined temperature a leakage current between the main and auxiliary tracks rises to such an extent that a failure current flows through a section of the main track.

2. A heating element or resistance as claimed in claim

1 wherein a predetermined region of the part of the main track which is left in circuit after the short-circuit, is designed preferentially to rupture.

3. A heating element or resistance as claimed in claim

2 wherein the predetermined region is provided with means for concentrating locally the current flowing therethrough.

. A heating element or resistance as claimed in claim

3 wherein the means for concentrating the current locally comprises a local reduction in the width of the track.

5. A heating element or resistance as claimed in claim 5 wherein said local reduction of width comprises a hole through the track.

6. A heating element or resistance as claimed in any preceding claim wherein a majority of the auxiliary track comprises a low resistive material, e.g. silver.

7. A heating element or resistance as claimed in any preceding claim wherein the auxiliary track portion is coupled to the main track via a second auxiliary track portion which is in turn connected to the main track.

8. A heating element or resistance as claimed in claim 7 comprising a short section of low resistance track provided laterally across the main track.

9. A heating element or resistance as claimed in any preceding claim comprising two or more auxiliary tracks.

10. A heating element or resistance as claimed in claim

9 wherein said auxiliary tracks are substantially coterminous at one end thereof respectively.

11. A heating element or resistance as claimed in claim

10 wherein the ends of said auxiliary tracks terminate in the vicinity of a common point on the main track.

12. A heating element or resistance as claimed in claim

11 wherein said common point is approximately half-way around the circuit .

13. A heating element or resistance as claimed in claim

12 wherein said auxiliary tracks are connected at their other respective ends to substantially the two ends of the main track.

14. A heating element or resistance as claimed in claim

13 wherein a or the portion designed preferentially to rupture is located between one end of the main track and the connection to the auxiliary track so as to ensure that said portion remains in-circuit and so is able to interrupt the power supply to the main track.

15. A heating element or resistance as claimed in any of claims 9 to 14 wherein each auxiliary track is so arranged that it will cause a failure current to flow in the same section of the main track.

16. A heating element or resistance as claimed in any of claims.9 to 15 wherein said auxiliary tracks are arranged such that if a leakage current path should be formed between the or any pair of them, a failure current will still flow through a or the predetermined portion of the main track.

17. A heating element or resistance as claimed in any of claims 9 to 16 comprising two auxiliary tracks arranged such that in the event of a serious overheat, the glass, glaβs-ceramic or ceramic bridge etc. will become sufficiently conductive to connect the two tracks together, thereby forming a single composite auxiliary track.

18. A thick film electric heater comprising a heating track, a low resistance track terminating in the vicinity thereof and a bridge of glass, glass-ceramic or ceramic material coupling said low resistance track to the heating track, the low resistance track and the bridge being arranged such that at a predetermined temperature the bridge becomes sufficiently conductive that the low resistance track provides a short-circuit across part of the main track, thereby causing a portion of the remainder of the heating track to fail .