Classifications

• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
  • H05B1/00—Details of electric heating devices
  • H05B1/02—Automatic switching arrangements specially adapted to apparatus ; Control of heating devices
  • H05B1/0227—Applications
  • H05B1/0252—Domestic applications
  • H05B1/0258—For cooking
  • H05B1/0269—For heating of fluids
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
  • H05B1/00—Details of electric heating devices
  • H05B1/02—Automatic switching arrangements specially adapted to apparatus ; Control of heating devices
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
  • H05B3/00—Ohmic-resistance heating
  • H05B3/20—Heating elements having extended surface area substantially in a two-dimensional plane, e.g. plate-heater
  • H05B3/22—Heating elements having extended surface area substantially in a two-dimensional plane, e.g. plate-heater non-flexible
  • H05B3/26—Heating elements having extended surface area substantially in a two-dimensional plane, e.g. plate-heater non-flexible heating conductor mounted on insulating base
  • H05B3/262—Heating elements having extended surface area substantially in a two-dimensional plane, e.g. plate-heater non-flexible heating conductor mounted on insulating base the insulating base being an insulated metal plate
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
  • H05B3/00—Ohmic-resistance heating
  • H05B3/20—Heating elements having extended surface area substantially in a two-dimensional plane, e.g. plate-heater
  • H05B3/22—Heating elements having extended surface area substantially in a two-dimensional plane, e.g. plate-heater non-flexible
  • H05B3/26—Heating elements having extended surface area substantially in a two-dimensional plane, e.g. plate-heater non-flexible heating conductor mounted on insulating base
  • H05B3/265—Heating elements having extended surface area substantially in a two-dimensional plane, e.g. plate-heater non-flexible heating conductor mounted on insulating base the insulating base being an inorganic material, e.g. ceramic
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F15—FLUID-PRESSURE ACTUATORS; HYDRAULICS OR PNEUMATICS IN GENERAL
  • F15B—SYSTEMS ACTING BY MEANS OF FLUIDS IN GENERAL; FLUID-PRESSURE ACTUATORS, e.g. SERVOMOTORS; DETAILS OF FLUID-PRESSURE SYSTEMS, NOT OTHERWISE PROVIDED FOR
  • F15B2211/00—Circuits for servomotor systems
  • F15B2211/70—Output members, e.g. hydraulic motors or cylinders or control therefor
  • F15B2211/71—Multiple output members, e.g. multiple hydraulic motors or cylinders
  • F15B2211/7107—Multiple output members, e.g. multiple hydraulic motors or cylinders the output members being mechanically linked
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
  • H05B2203/00—Aspects relating to Ohmic resistive heating covered by group H05B3/00
  • H05B2203/013—Heaters using resistive films or coatings
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
  • H05B2203/00—Aspects relating to Ohmic resistive heating covered by group H05B3/00
  • H05B2203/017—Manufacturing methods or apparatus for heaters
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
  • H05B2203/00—Aspects relating to Ohmic resistive heating covered by group H05B3/00
  • H05B2203/021—Heaters specially adapted for heating liquids

Definitions

• the present invention relates to electric heaters and in particular to electric heaters of the type comprising a resistive track provided on an insulating substrate .
• Such heaters are used or have been proposed for use in a variety of applications, for example in domestic appliances such as water heating vessels, water heaters and irons.
• an insulating layer eg of glass, ceramic, or glass ceramic (hereinafter collectively referred to as "glass") is provided on a metallic base such as a plate (which may for example form a part of the base of a liquid heating vessel) and the resistive track laid down on the insulating layer, usually by a printing technique.
• the base may be a solid ceramic body.
• a further electrical insulating layer may be applied over the track to protect it and prevent corrosion and oxidation.
• Such heaters are termed “thick film” heaters in the art .
• the heater should not be allowed to seriously overheat in a fault condition since this may cause substantial damage not only to the device or appliance in which it is being used, but also, potentially, to users thereof.
• a heater or resistance with built-in protection.
• the Applicant has proposed such an arrangement in O97/39603. According to this proposal, a bridge of a selected glass material is provided between adjacent heating tracks, the configuration of the track and the position and material of the track being chosen such that at a predetermined temperature the glass between the track sections becomes sufficiently conductive such that the track sections short circuit, thereby resulting in a controlled failure of the heater.
• the heater could therefore be said to "self protect", without the need for an external control .
• the present invention in its various aspects builds further on the above proposals.
• the self protecting glass bridge was by preference applied as a layer over the whole heater track.
• the invention provides an electrical resistance or heater of the type comprising a thick film resistive track provided on an insulating substrate, two predetermined sections of said track having a predetermined current carrying capacity being bridged by a discrete bridge of an insulating material which at a predetermined temperature becomes sufficiently conductive to cause a failure current to flow through one or both of said sections, the resistive track being protected from oxidation by a separate overglaze layer.
• the severe overheat protection bridge is also applied as an overglaze bridging the appropriate track sections .
• the bridge is positioned over abutting track section ends .
• a "self protecting" heater will self protect in a severe overheat condition depends on the temperature of the heater in the region of the bridge of material. The hotter the heater becomes, the hotter the bridge becomes and the more quickly it will reach the temperature at which it will pass a failure current. In the context of say a water heating vessel such as a kettle, it is quite clearly important to prevent premature failure of the heater, particularly in a time before any primary overheat protection such as a bimetallic actuator has operated. This problem is alleviated in accordance with a further aspect of the present invention by placing the bridge of material in a region of the heater which has a lower power density than an adjacent region.
• the invention provides an electrical resistance or heater of the type comprising a thick film resistive track provided on an insulating substrate, two predetermined sections of said track having a predetermined current carrying capacity being bridged locally by a bridge of an electrically insulating material which at a predetermined temperature becomes sufficiently conductive to cause a failure current to flow through one or both of said sections, said resistive heating track having a variable power density, the said bridge of material being provided in a region of the heater track having a relatively low power density.
• the eg glass bridge By placing the eg glass bridge in a lower power density area of the heater, the temperature rise of that region in an overheat situation will lag behind the temperature rise in adjacent higher power density areas, thereby providing a longer time to failure.
• the position of the bridge should still be such that the heater will fail in that location rather than elsewhere. Accordingly, if a separate overglaze is provided over the heater track, it should be chosen such that breakdown does not occur elsewhere on the track first.
• the lower power density region is flanked by higher power density areas.
• the lower power density area is provided in a radially intermediate section of the heater track, with at least the radially outer regions of the track having a higher power density.
• the track fails due to a current in excess of the track's current carrying capability passing through a section of track when the bridge becomes sufficiently conductive at elevated temperatures . It has been found that in such a situation, the track section may 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 heater being live after failure. This is potentially dangerous and such a system would not meet present day safety standards .
• the invention achieves this goal by providing means which concentrates locally the current flowing through the section of track which will fail .
• the invention provides an electrical resistance or heater of the type comprising a thick film resistive track provided on an insulating substrate, two predetermined sections of said track having a predetermined current carrying capacity being bridged by an electrically insulating material, eg a glass, which at a predetermined temperature becomes sufficiently conductive to cause a failure current to flow through at least one of said sections, wherein the said track section is provided with means for concentrating locally the current flowing therethrough.
• the current concentrator is preferably arranged away from any current carrying or earthed parts eg of a control associated with the heater.
• the current concentration can be achieved in a number of ways. Preferably, however, it is achieved by reducing locally the width of the track.
• the track may be waisted in to achieved the desired concentration, but preferably the effect is achieved by providing a hole, for example a circular hole, through the track. This hole will then act as a focus for the failure of the track.
• the bridge may be applied locally between adjacent track sections or over the whole track.
• Fig. 1 is a plan view of a heater in accordance with the invention
• Fig. 1A is a section along line A-A of Figure 1; and Fig. 2 is a table giving relevant data for the heater shown in Figure 1.
• a thick film planar heater 2 embodying the present invention.
• the heater comprises a 0.5 mm thick stainless steel substrate 4 on which is deposited in a conventional manner an insulating layer 6 and heating track 8.
• the insulating layer 6 is made from of Dupont 3500 ink and is about 85 microns thick (+/- 10 microns).
• the resistive heating. track 8 laid down on the insulating layer 6 is made from a blend of Dupont F/612/F629 resistive inks and is about 13 microns (+/- 2 microns) thick.
• the resistive heating track 8 is made up of a series of eight concentric arcuate track sections 8a, 8b....8h whose ends are joined by silver links 10. Such a track configuration is disclosed in general terms in Applicant's 098/366182. Track sections 8a, 8b, 8c and 8h extend substantially completely around the heater while the other track portions are subdivided into generally semi-circular portions.
• One end of track 8e is connected by a silver track 14 to a silver pad 16 for receiving a silver contact (not shown)
• one end of track 8d is connected to a further contact receiving pad 18 by a silver link 20.
• the contacts mounted on the pads eg by soldering, receive a 230V (or other voltage) supply.
• the outer diameter of the outermost track 8a is about 60mm, the gap between adjacent tracks being about 0.5 mm.
• the total power of this element is 1000 at 230V AC, the power contribution of each track portion being given in Figure 2.
• the width of the track portions 8a to 8h varies from the edge of the element to its centre.
• the outermost track portion 8a is the narrowest, the track width increasing towards the track portions 8e and 8f in the radially central region, before decreasing again towards the innermost track portion 8h.
• power density is inversely proportional to the track width, the power density decreases from a maximum at track 8a to a minimum at tracks 8e - 8f and then increases to a second, local maximum in track 8h.
• a hole 24 approximately 1mm in diameter is provided in the section of the track 8d between the bridge 22 and the end of the track 8d linked to the contact pad 18.
• the whole element is overprinted with protective glaze eg of Dupont 3500 to a thickness of eg 13 microns +/- 2 microns.
• the heater shown is mounted for example in the base of a liquid heating vessel such as a kettle. Should the kettle boil dry or be switched on without any water in the kettle, then the temperature of the heater will rise very quickly due to the high power rating of the heater and its low thermal mass . Should any primary overheat protection such as a bimetallic actuator fail to operate then the temperature of the heater will continue to rise. However at a certain predetermined temperature, the conductivity of the self protecting overglaze bridge 22 will rise to the point where it effectively short circuits the majority of the track resulting in a very high current passing through the track sections 8d and 8e bridged by the track.
• the resistance of these track sections is about 5 ⁇ , leading to a current flowing through the sections of about 46 amps, which is well above the normal operating current of about 4.35 amps and well in excess of the current carrying capability of the track sections. Accordingly, one or both of these track sections will fail.
• the current is, however, sufficiently low to prevent domestic fuses from blowing or earth leakage trips from operating. In general it has been found that a current at failure of between 10 and 15 A/mm produces a satisfactory failure.
• the track section 8d will in fact fail due to the presence of the hole 24. This acts to concentrate locally the current flowing through the track so that the region around the hole 24 will be much greater than elsewhere in the section 8d causing failure to occur in that region.
• the bridge 22 is provided between relatively wide track sections. This acts to reduce the power density in the region of the bridge which means that the bridge will not get as hot as surrounding regions. This has the effect of increasing the time taken to reach the breakdown temperature of the bridge 22, thereby prolonging the time before the heater fails. This is advantageous in that it reduces the likelihood of the heater failing before a primary overheat protector operates .
• the time in which the heater self protects is about 8 seconds. This is well above the 4 seconds as would be expected for the operating time of a primary overheat protection bimetal so that the heater will not prematurely fail in the event of overheating say in a dry boil or dry switch on situation in a kettle or the like. It is, however below the time at which the insulating substrate of the heater will break down, leading to a safe failure mode.
• the invention in its various aspects allows a controlled failure of the heater in a severe overheat situation in a manner which reduces the likelihood of the heater breaking down to neutral or earth.

Landscapes

• Engineering & Computer Science (AREA)
• Food Science & Technology (AREA)
• Chemical & Material Sciences (AREA)
• Ceramic Engineering (AREA)
• Resistance Heating (AREA)
• Surface Heating Bodies (AREA)
• Control Of Resistance Heating (AREA)
• Measuring Volume Flow (AREA)
• Particle Formation And Scattering Control In Inkjet Printers (AREA)
• Saccharide Compounds (AREA)
• Electrodes For Cathode-Ray Tubes (AREA)
• Design And Manufacture Of Integrated Circuits (AREA)
• Semiconductor Integrated Circuits (AREA)
• Non-Adjustable Resistors (AREA)

Priority Applications (4)

Applications Claiming Priority (2)

Publications (1)

Family

ID=10859151

Family Applications (1)

Country Status (10)

Cited By (1)

Families Citing this family (3)

Citations (1)

Family Cites Families (5)

• 1999
  • 1999-08-13 GB GB9919205A patent/GB2353457B/en not_active Expired - Fee Related
• 2000
  • 2000-08-12 CN CN00255073U patent/CN2461236Y/zh not_active Expired - Fee Related
  • 2000-08-14 ES ES00953316T patent/ES2284516T3/es not_active Expired - Lifetime
  • 2000-08-14 CN CNB00801678XA patent/CN1180661C/zh not_active Expired - Fee Related
  • 2000-08-14 EP EP06013593A patent/EP1713307B1/de not_active Expired - Lifetime
  • 2000-08-14 RU RU2001111004/09A patent/RU2001111004A/ru not_active Application Discontinuation
  • 2000-08-14 DE DE60034019T patent/DE60034019T2/de not_active Expired - Lifetime
  • 2000-08-14 DE DE60045400T patent/DE60045400D1/de not_active Expired - Lifetime
  • 2000-08-14 AT AT06013593T patent/ATE492139T1/de not_active IP Right Cessation
  • 2000-08-14 EP EP00953316A patent/EP1121835B1/de not_active Expired - Lifetime
  • 2000-08-14 AT AT00953316T patent/ATE357832T1/de not_active IP Right Cessation
  • 2000-08-14 WO PCT/GB2000/003136 patent/WO2001013680A1/en active IP Right Grant
  • 2000-08-14 CN CNB031786944A patent/CN100396163C/zh not_active Expired - Fee Related
  • 2000-08-14 AU AU65830/00A patent/AU6583000A/en not_active Abandoned
• 2002
  • 2002-02-08 HK HK02101032.6A patent/HK1040590A1/zh unknown

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