TEMPERATURE SENSOR FOR RADIANT ELECTRIC HEATERS

This invention relates to a temperature sensor for a radiant electric heater.

Temperature sensors for radiant electric heaters, especially those used in cooking hobs, generally comprise a differential expansion member which is connected to a housing in such a manner that an element of the differential expansion member is adapted to operate a snap switch as a result of expansion and contraction of the differential expansion member.

Such a temperature sensor is described in US-A-20050184849 in which the expansion member comprises a metal tube enclosing a rod of ceramic material, the two elements being secured together in the region of the ends thereof remote from a housing. The tube is secured in the housing such that expansion and contraction of the expansion member results in axial movement of the rod within the housing. An arm of a snap switch is clamped between the end of the rod and a resilient assembly so as to reduce the risk of early failure of the snap switch as a result of fatigue. The resilient assembly acts in the axial direction of the rod and comprises a coil spring which extends around a support which acts on the arm of the snap switch by way of a spring plate.

A disadvantage of this arrangement is that the coil spring, the support and the spring plate are all guided by a buttress within the housing or by a carrier for a further arm of the snap switch. In each case, the need to support the components of the resilient assembly gives rise to friction between the resilient assembly and its support. Such friction can give rise to unreliable switching.

It is therefore an object of the present invention to provide a temperature sensor which overcomes or at least ameliorates the above disadvantage.

According to the present invention there is provided a temperature sensor for a radiant electric heater, the sensor comprising:

a switch housing;

a first expansion element secured at one end thereof to the housing;

a second expansion element mounted at its free end with a free end of the first expansion element such that the free ends of the two elements are immovable relative to each other, the first and second expansion elements having different coefficients of thermal expansion;

a snap switch disposed within the housing and including a switch arm having an articulation point in the form of a deformation extending towards the other end of the second expansion element so as to contact the same; and

a resilient assembly disposed in the housing and engaging with the switch arm in the region of the deformation, the resilient assembly including means protruding towards and engaging within the deformation of the switch arm so as to urge the articulation point against the end of the second expansion element.

The first expansion element may be in the form of a tube and the second expansion element may be in the form of a rod arranged within the tube.

The first expansion element may be made of a metallic material. The second expansion element may be made of a ceramic, glass or metal having lower thermal expansion that the first expansion element.

The deformation may be substantially V-shaped with the apex thereof extending towards and engaging with the end of the second expansion element.

The resilient assembly may include a spring element having a portion extending substantially in the axial direction of the second expansion element. The spring element may be substantially C-shaped. The spring may be a flat spring with the axial portion thereof being narrower than the remainder thereof. The resilient

assembly may include a spring element mounted with the switch arm of the snap switch and with a carrier for a reaction arm of the snap switch such that the three components may be co-located in the housing. The three components may be formed as a separate assembly for mounting in the housing. The spring element may be located primarily on that side of the reaction arm of the snap switch remote from the switch arm, an end portion of the spring element passing through the carrier for engagement in the articulation point. Alternatively, the spring element may be located intermediate the carrier and the switch arm. As a further alternative, the spring element of the resilient assembly may be in the form of a V with free ends of the spring element engaging in the carrier, for example in a rectangular formation of the carrier, and an apex engaging in the deformation of the articulation point.

Alternatively, the resilient assembly may comprise a push pin, for example of ceramic material and a spring element urging the push pin towards the end of the second expansion element.

The push pin may include a portion having a relatively large cross-sectional area for receiving the spring element and a portion of relatively small cross-section for engaging within the deformation. The portion of relatively small cross-sectional area may taper towards the end of the second expansion element. The spring element may be a coiled spring arranged coaxially with the second expansion element. The push pin may include a cylindrical portion extending towards the coiled spring and dimensioned to fit therewithin.

Means may be provided within the housing for supporting that end of the spring remote from the push pin against lateral movement. The spring supporting means may support the spring internally and/or externally.

The spring element may alternatively comprise a concavely curved spring element. The spring element may include legs for engaging directly or indirectly with the housing. The legs may engage in a recess formed in a wall of the

housing. The push pin may include a tab engaging with an aperture formed in the spring element. The push pin may taper towards the end of the second expansion element.

For a better understanding of the present invention and to show more clearly how it may be carried into effect reference will now be made, by way of example, to the accompanying drawings in which:

Figure 1 is a diagrammatic illustration of one embodiment of a temperature sensor according to the present invention;

Figure 2 is a diagrammatic illustration of part of a modification of the temperature sensor of Figure 1 ;

Figure 3 is a perspective view of a resilient assembly forming part of the temperature sensor of Figure 2;

Figure 4 is a diagrammatic illustration of part of another embodiment of a temperature sensor according to the present invention;

Figures 5 and 6 show a modification of part of the temperature sensor shown in Figure 4;

Figure 7 shows an alternative modification of part of the temperature sensor shown in Figure 4;

Figures 8 and 9 show a further alternative modification of part of the temperature sensor shown in Figure 4;

Figure 10 is a diagrammatic illustration of part of a further modification of the temperature sensor of Figure 1 ;

Figure 11 is a diagrammatic illustration of part of another modification of the temperature sensor of Figure 1 ; and

Figure 12 is a diagrammatic illustration of part of a yet another modification of the temperature sensor of Figure 1.

The temperature sensor shown in Figure 1 comprises a differential expansion member 1 in the form if two elongate expansion elements 3, 5 which have significantly different coefficients of thermal expansion. In particular, expansion element 3 may comprise a tube of metallic material of relatively high coefficient of thermal expansion, while expansion element 5 may comprise a rod of ceramic material of relatively low coefficient of thermal expansion.

The free ends of the expansion elements 3 and 5 are mounted together in such a manner that they cannot move relative to each other. The other end of the expansion element 3 is secured within a housing 7, while the other end of the expansion element 5 is free to move. Consequently the ends of the differential expansion member within the housing 7 move relative to each other, in the axial direction of the member 1 , as the expansion member is heated and cooled, with the result that the end of the rod-form expansion element 5 moves outwardly relative to the housing as the expansion member is heated and moves inwardly relative to the housing as the expansion member is cooled.

The end of the rod-form expansion element 5 within the housing 7 bears against an actuating arm 9 of a snap switch 11 , the snap switch also including a reaction arm 13 which creates the snap effect of a contact 15. Contact 15 makes or breaks with a counter contact 17 so as to control the supply of electrical power to a radiant electric heater of which the temperature sensor forms a part in a manner well known to the skilled person.

At the point where the actuating arm 9 of the snap switch bears against the end of the rod-form expansion element 5, the actuating arm is formed with an

articulation point 19 in the form of a substantially V-shaped deformation with the apex of the deformation being directed towards and bearing against the end of the rod-form expansion element 5.

The actuating arm 9 of the snap switch is urged against the end of the rod-form expansion element 5 by means of a resilient assembly 21 in the form of a generally C-shaped spring element 23 which incorporates a switch-engaging portion 25 extending substantially in the axial direction of the differential expansion member 1 and engaging in the substantially V-shaped deformation so as to clamp the actuating arm of the snap switch between the end of the rod- form expansion element 5 and the resilient assembly 21.

Thus, the free end of the switch-engaging portion 25 of the resilient assembly 21 is restrained from movement by the substantially V-shaped deformation of the articulation point 19 and requires no further support which could introduce friction and render the switching operation of the snap switch 11 unreliable.

Figures 2 and 3 show a modification of the temperature sensor of Figure 1 , in particular in relation to the configuration of the resilient assembly 21. The same references are used in Figures 2 and 3 to denote the same or similar components. The C-shaped spring 23 in Figures 2 and 3 is reversed compared with that in Figure 1 , but Figure 3 shows that the switch-engaging portion of the spring which engages with the substantially V-shaped deformation of the articulation point is narrowed at each side compared with the width of the flat spring 23.

Figure 4 shows an alternative embodiment of a temperature sensor and the same references are used to denote the same or similar components as those shown in Figure 1. In Figure 4, the resilient assembly 21 comprises two separate components, a push pin 27 and a helically coiled spring 29. The helically coiled spring 29 engages directly or indirectly with the housing 7 and urges the push pin 27 towards the actuating arm 9 of the snap switch 11. The

helically coiled spring 29 has an outer diameter sufficient for the spring to bear against the housing 7 while remaining substantially stable against lateral deflections, while the push pin 27 has a head having a diameter at least as large as that of the spring 29 and a pin which is tapered at the free end thereof to engage in the substantially V-shaped deformation forming the articulation point

19 of the actuating arm 9 of the snap switch 11. The push pin may be made of any suitable material, such as a ceramic.

Figures 5 and 6 show a modification of the resilient assembly 21 of Figure 4 and the manner in which the helically coiled spring 29 is mounted in part of the housing 7.

The push pin 27 shown in Figures 5 and 6 includes a cylindrical portion which extends within the spring 29 in order to increase the stability of the junction between the spring and the push pin, the cylindrical portion having a diameter less than the internal diameter of the spring. Moreover, the end of the spring 29 remote from the push pin 27 is received in a shallow cup which has a diameter slightly larger than the external diameter of the spring. The shallow cup serves to improve the lateral stability of the spring and may be formed directly in the housing 7 or in a separate member, such as a pressed metal member, provided within and secured to the housing. Stability of the spring where it bears against either the housing (directly or indirectly) and/or against the actuating arm 9 of the switch can be improved by grinding or otherwise forming at lest one end of the spring to lie in a single plane.

In Figure 7, the push pin 27 is provided with a cylindrical portion extending within the helically coiled spring 29 as in Figures 5 and 6, but the length of the cylinder is somewhat shorter than that of the cylindrical portion shown in Figures 5 and 6. In addition the end of the spring 29 which bears against the housing 7 actually bears indirectly against the housing, being supported by a carrier 31 for the reaction arm 13 of the snap switch 11. The carrier 31 is formed with a hollow tubular projection 33 which has a diameter a little less than the internal diameter

of the coil spring 29 and which extends a short distance within the spring in order to provide lateral stabilisation for the spring. The carrier 31 may be a pressed metal component.

Figures 8 and 9 show a further alternative modification to the temperature sensor shown in Figure 4. The resilient assembly 21 in Figures 8 and 9 comprises a concavely curved spring element 35 provided with legs for mounting the spring element in a recess formed in a side wall of the housing and with a push pin 37 which is formed with a tab adapted to engage with an aperture formed in the spring element and with a tapered free end for engaging in the substantially V- shaped deformation forming the articulation point 19 of the actuating arm 9 of the snap switch 11.

Figure 10 shows a modification of the temperature sensor of Figure 1 , in particular in relation to the configuration of the resilient assembly 21. The same references are used in Figure 10 to denote the same or similar components.

The spring 39 in Figure 10 is mounted in the housing 7 at the same location as the switch actuating arm 9 and the carrier 31 for the reaction arm 13. Such an arrangement permits pre-assembly of the snap switch together with the resilient assembly 21 in a single step rather than assembling the snap switch and the resilient assembly in the housing in two steps.

Figure 11 shows a further modification of the temperature sensor of Figure 1 , in particular in relation to the configuration of the resilient assembly 21. The same references are used in Figure 11 to denote the same or similar components.

The modification of Figure 11 is similar to that of Figure 10 in that the spring 39 is mounted in the housing 7 at the same location as the switch actuating arm 9 and the carrier 31 for the reaction arm 13. However, in the case of Figure 11 , the carrier 31 for the reaction arm 13 is located outside the spring 39 forming the resilient assembly, whereas in Figure 10 the spring 39 is primarily positioned externally of the carrier 31 and passes through the carrier to engage with the actuating arm 9.

Figure 12 shows another modification of the temperature sensor of Figure 1 , in particular in relation to the configuration of the resilient assembly 21 , and incorporates some of the concepts described in relation to Figure 8. The spring 41 in Figure 12 is mounted in the housing 7 within the carrier 31 for the reaction arm 13, the spring being in the form of a broad V-shape with the free ends 43 of the spring engaging in corners of a rectangular recess formed in the carrier 31 and the apex 45 engaging in the substantially V-shaped depression of the articulation point 19 so as to urge the actuating arm 9 of the snap switch against the end of the rod-form expansion element 5. As with the embodiments of Figures 10 and 11 , the arrangement permits pre-assembly of the snap switch together with the resilient assembly 21 in a single step rather than assembling the snap switch and the resilient assembly in the housing in two steps.