AU5961999A

AU5961999A - An electric hob

Info

Publication number: AU5961999A
Application number: AU59619/99A
Authority: AU
Inventors: Albert Kerber
Original assignee: SICERAM GmbH
Current assignee: SICERAM GmbH
Priority date: 1998-12-01
Filing date: 1999-11-24
Publication date: 2001-06-21
Also published as: DE19855481A1; CA2291381A1; JP2000188174A; EP1006755A3; KR20000047736A; EP1006755A2

Description

AUSTRIA

PATENTS ACT 1990 COMPLETE SPECIICATION NAME OF APPLICANT(S): SiCeram GmkbH ADDRESS FOR SERVICE:

S.

.4 a a a. a DAVIES COLLISON CAVE Patent Attorneys 1 Little Collins Street, Melbourne, 3000.

INVENTION T1TLE.- An electric hob The following statement is a full description of this invention, including the best method of performing it known to me/us:a. a a.

la The invention relates to an electric hob for the low-voltage range having a hob plate made of heavy duty ceramics and having a heating conductor which forms a largearea contact with the lower side of the hob plate.

Such a hob for conventional home voltages of 110 OV to 220V is known from DE 297 02 813 U1. In particular, silicone nitrite or silicon carbite is named as the heavy duty S ceramics for the hob plate. For lower temperatures in the range of 250C, other electrically insulating ceramics such as aluminium oxide may also be used. Such .i heavy duty ceramics are particularly suitable for use as hob plates thanks to their thermal conductivity, their thermal coefficient of expansion, their permitted surface strain and their specific electric resistance. The thermal conductivity of these materials is in the medium range between 5-40W/mK. It was found that with such medium thermal conductivity values a good compromise can be reached between the thermal insulation towards the housing and the thermal transmission towards the heating elements. At the same time, the thermal coefficient of expansion of such heavy duty ceramics is very low being in the range of 106 1/K so that the strains and warpage of the hob plate caused by different heating remain low. The installed power per unit area of heavy duty ceramics here can be in the range of other heavy duty hob plates between 4 and 16 W/cm 2 The specific electric resistance of some heavy duty ceramics is in the range of 101 3 ohm/cm and is thus so high that corresponding electrical heating conductors can be affixed directly to the lower side of the hob plate.

If such electric hobs are to be designed for the low-voltage range, special problems arise in the electrical supply as very high currents are required to obtain the same heating power as in the domestic area. If the hob is to be designed for example for a 12V car battery, currents of around 80A flow in the leads while in the domestic area only currents of a few amperes occur. The high currents can lead to the electrical supply leads heating up in an unwanted manner. If, on the other hand, the resistance of the electrical supply leads is kept particularly low, these will as a rule also possess ooooo good thermal conductivity so that in turn there is a risk of the heat being passed oooo through in an unwanted manner from the hob plate to the electrical supply leads.

It is therefore the object of the invention to provide an electric hob for the low-voltage range where it is insured that the electrical supply leads do not heat up excessively.

This object is solved by the features of claim 1. The solution in accordance with the S invention comprises there being interposed in each case between the heating conductor and the electrical supply leads at least one lead element possessing a resistance value lying between the resistance values of the heating conductor and of the relevant electrical supply lead.

The invention is based here on the finding that thanks to the interposition of a further lead element between the heating conductor and the relevant electrical supply lead, a compromise can be found between the heat transmission from the heating conductor and the heating up of the relevant lead element. If an electrical supply lead is connected directly to the heating conductor, there is no need to fear the heating up of the supply lead due to the high currents; however, a heating up of the heating conductor does occur due to the good thermal conductivity of the electrical supply lead. If, in contrast, a lead element is interposed in accordance with the invention between the heating conductor and the relevant electrical supply lead, such lead element having a resistance value being between the resistance values of the heating conductor and the relevant electrical supply lead, then both the heat transmission through the lead element to the heating conductor and the heating up of the lead element due to the high currents can be held in limits.

In accordance with a preferred embodiment it is provided that the lead element consists of a piece of the heating conductor which is led away from the lower side of the hob plate and which possesses a greater cross-section than the heating conductor in large-area contact with the hob plate. In this way it is ensured that no additional contact resistance occurs between the lead element and the heating conductor. At the same time, a particularly low-cost design of the lead element is possible as the lead element can be formed from the material of the heating conductor which is anyway already available. It has been shown here that any excessive heating of the lead element can already be avoided if the cross-section of the lead element is four times that of the heating conductor in large-area contact with the hob plate.

Another problem is the connection of the lead element to the relevant electrical supply lead, as it has to be prevented that the contact resistance between the lead element and the supply lead increases in such a way that an intolerably high heat-up is produced due to the high currents at the contact point. In accordance with a preferred embodiment it is provided that for the almost resistance-free connection between the at least one lead element and a supply lead, a copper block is provided on which the relevant lead ends are applied over the area. A large-area contact to the lead element or the relevant electrical supply lead can be made to the side surfaces of the copper block so that the contact resistance from the lead element to the copper block or from the supply lead to the copper block can be reduced to a minimum. Furthermore, the volume resistivity of the copper block is also extremely low so that any heating due to -4the connection between the lead element and the electrical supply line can be excluded.

In accordance with another preferred embodiment it is provided that the hob plate is made of silicon nitrite, of silicon carbite or of aluminium nitrite. These ceramics possess the above-mentioned advantages of heavy duty ceramics.

With regard to the formation of the heating conductor, various designs can be considered. The design in accordance with the invention with the interposed lead element is particularly simple if the heating conductor consists of a metal foil pressed onto the lower side of the hob plate by means of an insulating film. A meander-like shape of the heating conductor can be formed here by the heating conductor being cut out of a metal foil using a suitable cutting device, for example using a laser cutting device. The corresponding lead element for the supply to the heating conductor can here be cut out in one and the same process.

Further possibilities for the formation of the heating conductor consist of applying resistance films to the lower side of the hob plate in thick film technology or thin film S.technology. Furthermore, flame spraying processes can also be used with which an intermediate film of a material serving as an adhesion agent and/or an electrical insulator can be applied. For example, aluminium oxide can be sprayed on as an electrical insulator, this being particularly necessary if electrically conducting heavy duty ceramics such as silicon carbite are used.

Another possibility consists of affixing the heating conductor to the lower side of the hob plate in the form of a resistance paste. Here, resistance pastes or heating resistances with so-called PTC characteristics, that is with especially positive temperature characteristics of their resistance, are particularly suitable. The PTC characteristics lead to the resistance of the relevant heating resistor increasing suddenly when a certain temperature is reached so that in this range the temperature or the power implemented in the heating resistor remains constant. Preferably, the corresponding jump temperature of the heating resistor is set here to the maximum temperature of the hob plate.

In accordance with another preferred embodiment an insulating film is provided which is pressed against the lower side of the hob plate by means of a press plate. In this way, the insulating film does not have to possess any mechanical stability of its own so that a variety of insulation materials can be used. To lead the lead element to the heating conductor located in the lower side of the hob plate, corresponding passages are provided in the insulating film and the press plate.

In accordance with another preferred embodiment temperature sensors can be o:ooo S* provided in the form of resistance thermometers in direct contact with the ceramic plate in order to the regulate the temperature on the ceramic plate. In this way, an effective overheating protection can be additionally provided.

Further advantages and details of the invention are explained in more detail by means of an embodiment shown in the drawing. The drawing, which consists of one figure, shows a cross-section through the electric hob for the low-voltage range in accordance .ooooi with the invention. The hob plate 1 of the hob consists of a round silicon nitrite disc.

On the outside edge, the silicon nitrite disc possesses a chamfer 10 which is pressed .i against a corresponding edge 11 of the housing wall 7. The hob plate 1 is here held by the press surface 4 which is supported on the housing floor 8 by a screw 9. An insulating layer 3 is located between the press plate 4 and the hob plate 1. To heat the hob plate, a meander-like heating conductor in the form of a metal foil is introduced between the insulating film and the hob plate. For the power supply, passages 14, are provided through the insulating film 3 and through the press plate 4 in the middle and at the edge of the hob plate through which the lead ends 12, 13 of the heating conductor are passed. The ends 12, 13 here possess a cross-section four times that of the cross-sections of the heating conductor 2. On the free ends of the leads 12, 13, copper blocks 5, 6 are provided which create a contact connection between the leads 12, 13 and the electrical supply leads. To better illustrate the position of the press plate 4, the insulating film 3 and the heating conductor 2, the components in question are each shown with corresponding distances to one another, while in operation the individual components are naturally in a secure large-area contact to one another.

The electrical hob shown is preferably operated with a DC voltage of 12V and a current of 80A. The copper blocks 5, 6 ensure that the contact resistance between the electrical supply leads and the free ends of the leads 12, 13 can be neglected so that at this point no unwanted heating up can occur. The leads 12, 13 possess four times the cross-section of the actual heating conductor path 2 so that in relation to the same lead length in the leads only one quarter of the power is implemented over a heating ooooi conductor piece of the same length. In this way, it is ensured that the leads 12, 13 do not heat up in an unwanted manner. At the same time, however, the leads 12, 13 still possess a sufficiently high thermal resistance to prevent any overheating of the copper blocks 5, 6 or the electrical supply line by the heating conductor.

•0o.oi o• o•

Claims

1. An electric hob for the low-voltage range having a hob plate made of heavy duty ceramics and having a heating conductor forming a large-area contract with the lower side of the hob plate, characterised in that between the heating conductor and the electrical supply leads there is interposed in each case at least one lead element possessing a resistance value lying between the resistance values of the heating conductor and the relevant supply lead. -8-

2. A hob in accordance with claim 1, wherein the lead element consists of a piece of the heating conductor which is led away from the lower side of the hob plate and which possesses a greater cross-section than the heating conductor in large- area contact with the hob plate.

3. A hob in accordance with claim 2, wherein the cross-section of the lead element is four times that of the heating conductor in large-area contact with the hob plate.

4. A hob in accordance with any of claims 1 to 3, wherein for the almost resistance-free connection between the at least one lead element and a supply lead a copper block is provided on which the relevant lead ends are affixed •over the area.

5. A hob in accordance with any of claims 1 to 4, wherein the hob plate consists of silicon nitrite, of silicon carbite or of aluminium nitrite.

6. A hob in accordance with any of claims 1 to 5, wherein the heating conductor consists of a metal film evaporated onto the lower side of the hob plate. oeo i

7. A hob in accordance with any of claims 1 to 5, wherein the heating conductor S. consists of a metal foil which is pressed onto the lower side of the hob plate by means of an insulating film.

8. A hob in accordance with any of claims 1 to 5, wherein the heating conductor consists of a resistance paste which is applied to the lower side of the hob plate directly or by means of an adhesion agent. 9

9. A hob in accordance with any of claims 1 to 8, wherein an insulating film is provided which is pressed against the lower side of the hob plate by means of a press plate.

A hob plate in accordance with claim 9, wherein passages are provided in the insulating film and the press plate through which the lead ends of the heating conductor are passed.

11. A hob plate in accordance with any of claims 1 to 10, wherein temperature sensors are provided in director contact with the ceramics plate which are connected to a regulating device for the regulation of the temperature at the S..ceramics plate. S. *S S 10

12. An electric hob or hob plate substantially as hereinbefore described with reference to the drawings.

13. The steps, features, compositions and compounds disclosed herein or referred to or indicated in the specification and/or claims of this application, individually or collectively, and any and all combinations of any two or more of said steps or features. DATED this TWENTY FOURTH day of NOVEMBER 1999 SiCeram GmbH by DAVIES COLLISON CAVE Patent Attorneys for the applicant(s) S @0 S *S 5*r S S. S S 0 S 0 4555 50 0