Current source limitation for thick film heating elements

Abstract: A heating element (1) comprises a plurality of thick film electrically resistive tracks (2, 4, 6), of which at least one has a temperature coefficient of resistance in excess of 0.006 per degree C. A first track (2) is for electrical connection to a power supply. Means (12, 14) for electrically connecting the first track (2) to a second track (4, 6) comprise a component which depends on temperature such that, in use, the current flowing in the second track (4, 6) increases with the temperature of the first track (2).

Description: This invention relates to a heating element comprising a thick film electrically resistive track, the thick film being formed of a base metal and a glass. It has been proposed that such a heating element be formed by depositing one or more thick film tracks on a glass ceramic surface of a composite support member, the track or tracks then being overglazed with a glass ceramic material for protection and to allow high temperature stable operation. One or a number of such heating elements may provide one or a number of hot-plate elements in a hob-top or alternatively a heating element may be mounted closely adjacent the underside of a glass ceramic cooktop to provide a heated area on the cooktop. More than one such heating element or a unitary support member bearing more than one heating element can be used to provide more than one heated area on the glass ceramic cooktop. A particularly suitable thick film for such applications includes nickel as base metal and will operate at the necessary power densities. An advantage of a heating element made of such a thick film, as disclosed in our copending EP 0286215A, is its high temperature coefficient of resistance (TCR), i.e. in excess of 0.006 per degree C in the temperature range of from 0°C to 550°C, which provides for rapid heat-up and effective self-regulation. However, the associated problem of the high TCR is that when the heating element is activated, there is a high current surge which may be up to five times the operating current of the heating element and may be sufficient to blow the fuse in many high power appliances. Similarly, a cooker hob consisting of e.g. four such heating elements would have to be designed so that the elements could not be switched on within a few seconds of each other. Such control is expensive and could offset the low cost advantages of the heating element itself. A range of thick films have been developed which utilise a base metal and have a low TCR. Such low TCR thick films may be used for heating elements and will meet the requirements for most applications. However their power handling characteristics are significantly worse than those for a thick film heating element in which the base metal is nickel. EP-A-0,227,405 discloses a heating element comprising a plurality of thick film resistive which include separate conducting tracks for electrical connection to a power supply and which are also connected to the resistive tracks via components having a negative temperature coefficient of resistance. However, as the tracks in which the current flows are resistive, then the current flowing will increase with temperature. It is an object of the present invention to at least alleviate some of the problems discussed hereinbefore. According to the present invention there is provided a heating element comprising a plurality of thick film electrically resistive tracks, the plurality of tracks including a first track for electrical connection to a power supply and a second track, at least one of said first and said second tracks being made of a thick film characterised by having in the temperature range of from 0°C to 550°C a temperature coefficient of resistance in excess of 0.006 per degree C, wherein a means for electrically connecting said first and said second tracks in parallel comprises a component which depends on temperature such that, in use, the current flowing in said second track increases with the temperature of said first track. When such a heating element is activated, current flows in the first track. As the temperature of the first track increases, the first and second tracks become electrically connected in parallel, allowing current to flow in the second track. The first track has a resistance which is higher than that of the first and second tracks electrically connected in parallel. Accordingly, the current initially drawn, and any current surge, which is characteristic of the resistance of the first track because of the temperature dependence of the means for electrically connecting, is less than would be the case if the first and second tracks were electrically connected in parallel when the heating element was initially connected to the power supply. The first track which is electrically connected to the power supply when the heating element is activated may have a high TCR. In this case, the heat up of the heating element would be more rapid when first activated. Preferably the temperature-dependent component is provided by a material having a negative temperature coefficient of resistance (NTC) and so is self-regulating with temperature. Embodiments of the invention will now be described, by way of example only, and with reference to the accompanying drawings in which: Figure 1 shows a plan view of a heating element provided in accordance with the present invention; Figure 2 is a graph showing schematically the principle of the present invention; and Figure 3 shows, in section, a temperature-dependent connection for a heating element provided in accordance with the present invention. Referring to Figure 1, a heating element 1 comprises a plurality of thick film tracks 2, 4, 6 applied to a substrate 7. The tracks 2, 4, 6 are made of a nickel thick film having a high TCR as described in our copending EP 0286215. Thick films including cobalt or iron as base metal have similarly high TCRs and may also be used to produce tracks for heating elements. The track 2 has terminals 8, 10 for connection to an external power supply via electrical connectors (not shown). The track 2 is connected to the other tracks 4, 6 and they to each other at each end by components formed as bridges 12, 14 of a thick film material containing a composition which has an NTC of resistance. Suitable compositions include the following: a) Vanadium oxide (V₂O₃) b) Nickel-cobalt ferrites with small amounts of barium oxide (Ba₂O₃) and silicon dioxide (SiO₂) added. The amounts of (Ba₂O₃ and SiO₂ added affect the NTC of the composition. c) Nickel oxide/Manganese (III) oxide system (mole ratio of NiO: Mn₂O₃ = 1:2). Addition of copper (II) oxide (CuO) dopant varies the NTC of the composition. d) Nickel oxide/lithium carbonate system. In a specific example, a composition which had a mole ratio between the two compounds of 1:1 had an NTC of 1.05 per degree C. e) Chromium (III) oxide/titanium (IV) oxide with chromium metal dopant. In a specific example, a composition with a mole ratio of Cr₂O₃: TiO₂:Cr of 1:4:5 had an NTC of 1.002 per degree C. At room temperature, the NTC bridges 12, 14 have a high resistance compared to the tracks 2, 4, 6 and thus the tracks 2, 4, 6 are effectively insulated from each other. When power is applied to the heating element there is a small current surge characteristic of the track 2 connected to the power supply. As the temperature increases, the current in this track 2 decreases (due to its increasing resistance) but at the same time the resistance of the NTC bridges 12, 14 decreases, thereby allowing increased current flow in the other tracks 4, 6. The net result is an overall current flow to the heating element 1 which shows no large increase or decrease with change in time or temperature. The principle of this is illustrated schematically in Figure 2 which shows variation of current with temperature for the following heating elements: A: a first electrically sensitive track having a positive TCR; B: the first electrically resistive track and a second electrically sensitive track connected in parallel, all the tracks having a positive TCR; C: the first and second electrically tracks having positive TCR and connected in parallel by a bridge of a material having an NTC of resistance. It is envisaged that control of the NTC material characteristics and the track geometry would allow design of a heating element with a virtually flat current response with temperature whatever the TCR value of the track material. Referring to Figure 3, a suitable configuration for connection of tracks 16, 18 and an NTC bridge 20 is shown in which the NTC bridge 20 is sandwiched between the tracks 16, 18 on a substrate 22. The tracks and NTC bridges are protected from oxidation by an overglaze layer (not shown), allowing high temperature stable operation. Variations in the configuration of a heating element provided in accordance with the present invention as defined by the claims may be envisaged. For example, NTC bridges need only be provided at one end of the thick film tracks, at the other end the tracks being electrically connected by conventional means. An NTC bridge could be provided only between the track 2 and its adjacent track 4 to limit current surge. A plurality of tracks could be provided between the connections to the external power supply and the first NTC bridge to determine the extent of current surge. Instead of being connected in series as shown, each NTC bridge could form part of a track and be connectable in parallel which would allow the selective connection of the tracks to the power supply to vary the operating temperature of the heating element. Other variations will be apparent to those skilled in the art.

Claims: A heating element (1) comprising a plurality of thick film electrically resistive tracks (2,4,6), the plurality of tracks (2,4,6) including a first track (2) for electrical connection to a power supply and a second track (4), at least one of said first and said second tracks being made of a thick film characterised by having in the temperature range of from 0°C to 550°C a temperature coefficient of resistance in excess of 0.006 per degree C, wherein a means (12) for electrically connecting said first and said second tracks in parallel comprises a component which depends on temperature such that, in use, the current flowing in said second track (4) increases with the temperature of said first track (2). A heating element according to claim 1 wherein said first track (2) is made of a thick film having in the temperature range of from 0°C to 550°C a temperature coefficient of resistance in excess of 0.006 per degree C. A heating element according to claims 1 or 2 wherein said second track (4) comprises a plurality of other tracks (6) and a means (12,14) for connecting said plurality of other tracks (6) in parallel comprises a component which depends on temperature such that, in use, the current flowing in each of said other tracks increases with the temperature of said first track. A heating element according to claim 3 wherein a said component is provided between adjacent ones of said plurality of other tracks. A heating element (1) according to any one of the preceding claims wherein the temperature-dependent component is provided by a material having a negative temperature coefficient of resistance. A heating element (1) according to claim 5 dependent on claim 4 wherein each one of said temperature-dependent components comprises first and second said bridges at respective first and second ends of the tracks.

Inventors: BALDERSON SIMON NEVILLE, BALDERSON, SIMON NEVILLE
Applicants: EMI PLC THORN, THORN EMI PLC
Application year: 1989
Main classification: H05B3
IPC codes: H05B3