GB2305585A - Heat source - Google Patents

Abstract

A heat source for a cooker hob, especially a ceramic hob, comprises a quartz tube 1 which contains a filament 4 e.g. of tungsten which is connected to external terminals 8 via foils 7 in the pinch seals of the ends 2. The filament is connected to the foils by springs 26 e.g. of tungsten wire. The spring extension per unit force of the springs 26 is less than that of the filament 4, thereby avoiding distortion of the filament.

Description

HEAT BODRCB The present invention relates to electrically energised heat sources. Embodiments of the invention relate to heat sources for use in cooker hobs, especially ceramic hobs.

Figure 1 is a schematic illustration of a known heat source used in a ceramic hob.

The heat source comprises a quartz tube 1 having end portions 2 closely adjacent and parallel, and a generally circular portion 3 between the ends 2. The tube contains a tungsten filament 4 dimensioned and arranged to produce a color temperature of 2300K. The filament 4 is spaced from the internal wall of the tube 1 by spiral spacers 5, also of tungsten, as well known in the art.

The filament 4 is joined at each end to connectors known in the art as plugs 6. Each plug 6 is joined at one end to the filament 4 each plug 6 having a screw-fit with the filament 4. The plugs are connected at their other ends to metal foil 7 in pinch seals as also well known in the art. The foils 7 have external electrical terminals 8 welded to them. The tube 1 contains a fill of halogen gas e.g. a bromide, as well known in the art.

The known heat source is made in a large variety of sizes and power outputs, each size and power output being made for example in a batch.

Difficulties arise in production of the heat source.

It is very difficult to ensure that the diameter of the circular portion 3 of the source is precisely controlled.

A tolerance of i 2mm is aimed at, but is difficult to achieve. A tolerance of f 2mm on the diameter translates to tolerance in the circumference of the tube of + 2irk, i.e. about 6mm.

It is also difficult to manufacture the filament 4 to a precisely controlled length. A tolerance of f 2 mm is aimed at and is the limit of the current state of the art. Thus, even if the required tolerance is achieved, in a batch of sources, there can be a mismatch of over i 8mm between filaments and tubes. If the tolerances are exceeded, bigger mismatches, e.g. i 10 mm can occur.

If a "short" filament is assembled with a "long" tube, the filament is stretched taut. The spacers 5 may collapse or tilt so that the filament touches the inner wall on the inner circumference of the tube. The tube becomes overheated in use and fails. If a 'long' filament is assembled with a 'short' tube, the foils 7 are not correctly located in the pinch seals and the tube may not seal correctly.

Also it is desirable that the linear spacing of the ends of the filaments is at a predetermined amount + a tolerance. The tolerances on the diameter of the circular portion 3 of the tube and on the length of the filament, make the achievement of the desired spacing difficult.

It is also desirable to keep the filaments 6 central within the tube 4 and out of contact with the tube walls.

It has been proposed to provide a wide variety of plugs 6 of different lengths to compensate for the tolerances in the circumference of the tube 1 and in the length of the filament 4. However, that requires measurements of the lengths of the tubes 1, and of the filaments. Various ways of doing that economically on a production line which mass-produces the heat sources have been investigated.

Ways of more precisely controlling the diameter of the circular tube portion and the filament have also been considered.

The heat sources have been manufactured for nearly five years and the problems created by the tolerances in the lengths of the filaments and in the circumferential length of the tube have been long-standing and unsolved until now.

The present invention provides a heat source comprising a tube of refractory material transparent to infra-red radiation, a filament of refractory electrically conductive material in the tube to produce the infra-red radiation when electrically energised, and electrically conductive connectors connecting the ends of the filaments to electrical terminals extending through sealed ends of the tube, wherein the connectors are springs extendable along the axis of the tube for which the spring extension per unit force is greater than that of the filament. Because the filament is stiffer than the springs at each end thereof, the filament is not unduly stretched if the tube is 'long' compared to the filament. This reduces the problems discussed above of a 'short' filament in a 'long' tube.

If the filament is 'long' compared to the tube, the springs compensate for the additional length, reducing the problems discussed above of a 'long' filament in a 'short' tube.

Preferably each spring is helical and the diameter of the helix thereof is nearly equal to the internal diameter of the tube. The filament which may also have a helical form is coaxial with the spring. Thus the ends of the filament are centred in the tube. Furthermore, the spring is most preferably of a length which extends from within the straight end portion of the tube to the circular portion of the tube.

The spring may be connected to the filament by a portion thereof having a diameter and pitch that allows the spring to be a screw fit with the filament.

The turns of the spring are spaced sufficiently widely to keep the spring cool, if the spring has a diameter equal to the internal diameter of the tube.

For a better understanding of the present invention, reference will now be made by way of example to Figures 2 and 3 of the which: Figure 2 is a schematic illustration of an illustrative heat source according to the invention.

Figure 3A is a side view of a spring plug used in the heat source of Figure 2 and Figure 3B is an end view of the spring plug of Figure 3A.

The reference numerals used in Figure 1 are also used in Figure 2 to represent in Figure 2 the same items as in Figure 1.

The heat source of Figure 2 differs from that of Figure 1 by the plugs 26 which replace the prior art plugs 6 of Figure 1.

The present inventors realised that the prior art plugs 6 are of stiffnesses similar to, or greater than, that of the filament 4. Thus the filament 4 was forced by the plugs 6 to stretch if it was short, compared to the tube 1. If the filament 4 was long compared to the tube 1, the foils 7 were pushed too far out of the tube before the pinch seals were formed.

Referring to Figure 3, the illustrative plugs 26 of the present invention are of tungsten wire formed into a helical spring. The spring comprises a first section 27 having a diameter substantially equal to the internal diameter of the tube 1, and a second section 28 having a diameter and a pitch designed to screw-fit with the filament 4. In the example shown in Figure 2 the filament 4 fits within the second section 28. The end of the spring 26 remote from the second section 28 is a straight tail 29 which connects to the foil 7.

The first section 27 has turns which are widely spaced to keep the first section sufficiently cool to prevent the quartz tube 1 devitrifying where it contacts the spring 26.

The length of the first section is sufficient to extend from within the straight section 2 of the tube 1 to within the circular portion 3 of the tube 1. This ensures that the end of the filament 4 and the tail 29 of the spring are centred in the tube 1.

The stiffness of the spring section 27 is designed to be less than that of the filament. If the filament is short relative to the tube, extension of the spring allows the filament to position correctly in the tube 1, without excessive stretching. If the filament is long relative to the tube, compression of the spring allows the foils 7 to be correctly placed in the ends 2 of the tube 1.

The heat source of the present invention may be manufactured in a variety of different types each of different power and diameter of portion 3 of the tube 1.

Although it is envisaged each type needs a particular design of plug, each type will need only one design of plug. The need for more precise control of tube diameter and filament length is removed by the present invention.

Claims (5)

CLAIMS:

1. A heat source comprising a tube of refractory material transparent to infra-red radiation, a filament of refractory electrically conductive material in the tube to produce the infra-red radiation when electrically energised, and electrically conductive connectors connecting the ends of the filaments to electrical terminals extending through sealed ends of the tube, wherein the connectors are springs extendable along the axis of the tube for which the spring extension per unit force is greater than that of the filament.

2. A heat source according to claim 1, wherein the sealed ends of the tube each external parallel and adjacent to the other, the tube defining a substantially circular arc between the said ends.

3. A heat source substantially as hereinbefore described with reference to Figures 2 and 3 of the drawings.

4. A cooker hob comprising a heat source according to claim 1, 2 or 3.

5. A hob according to claim 4 which is a ceramic hob.