GB2278498A

GB2278498A - Thermally actuated switches

Info

Publication number: GB2278498A
Application number: GB9409529A
Authority: GB
Inventors: John Crawshaw Taylor; Dominic Michael Anthon Oughton
Original assignee: Strix Ltd
Current assignee: Strix Ltd
Priority date: 1993-05-12
Filing date: 1994-05-12
Publication date: 1994-11-30
Anticipated expiration: 2014-05-12
Also published as: GB2278498B; GB9409529D0; GB9309805D0

Abstract

An energy regulator for controlling the flow of electric current to a load includes a bimetallic actuator 10 against which is held an electric ceramic substrate heater 60. The heater is connected in series or parallel with the load so that when current is flowing to the load, the heater heats the bimetallic actuator 10. The bimetallic actuator is arranged to cycle the supply of current on and off in response to being heated. It is provided with a fulcrum 70 about which the heater is held so that it may pivot. One end of the heater is held in place by a tongue 72 and the other is pressed against the bimetallic actuator by a leaf spring 74. Electrical contact to the heater is made via the tongue and the leaf spring. <IMAGE>

Description

Thermallv-Actuated Switches and Thermal Actuators The present invention relates to thermally actuated switches and thermal actuators comprising a bimetallic actuator and an electrically energisable substrate heater mounted to the bimetallic actuator. Such switches and actuators are commonly used in energy regulators for controlling the supply of electrical energy to an electrical load such as the heating element of a domestic cooking appliance, for example. Other applications include thermal relays such as those used in street lamps.

In such constructions, electrical power is initially supplied to the load and to the heater which may be connected either in parallel or in series with the load. The heater heats the bimetallic actuator, causing it to deform to the point where it causes a contact carrying arm associated with the bimetallic actuator to open a set of switch contacts and interrupt the power supply to both the load and the heater. The bimetallic actuator then cools and deforms in the opposite direction to the point where the contact arm closes the contacts once more, whereupon the cycle recommences.

Substrate heaters normally comprise a ceramic substrate on which is printed an electrically resistive heater having terminals at both ends for connection to the power supply. Since the substrate is ceramic, it is relatively rigid and brittle. Accordingly the mounting of the heater to the bimetallic actuator must be such as to allow relative movement between the two as the bimetallic actuator flexes under the heating effect of the heater. GB 1201537 discloses one form of mounting in which one end of the heater is mounted resiliently to the bimetallic actuator by a spring washer, a leaf spring or spring clip. The other end of the heater is free to move with respect to the bimetallic actuator.

In this arrangement, the mounting also conducts electrical energy to one terminal of the heater. It is necessary, however, to solder a flying lead to the other terminal of the heater to complete the electrical circuit thereto. This is time consuming in practice and therefore expensive.

GB 1515356 discloses a further arrangement in which, again, one end of a ceramic substrate heater is resiliently mounted to a bimetal blade using a spring/rivet arrangement. This not only mounts the heater to the blade but ensures electrical connection between the resistor of the heater and the bimetal blade, through which electrical energy may be supplied to one terminal of the heater. A spring loaded contact is urged against the other terminal of the heater to complete the heater supply circuit. This arrangement is however complicated and expensive, requiring the use of two biasing components - one for retaining the heater on the blade and the other for providing an electrical connection to the heater.

The present invention seeks to provide a simpler form of mounting which will facilitate the making of electrical connections to the heater and which will retain the heater on the bimetallic actuator so as to accommodate relative movements therebetween, and press the active heated portion of the substrate below the heater into direct thermal contact with the bimetal.

From a first aspect, the invention provides a thermally actuated switch or thermal actuator comprising a bimetallic actuator and an electrically energisable substrate heater mounted to said bimetallic actuator, characterised in that it comprises a fulcrum cooperating between said bimetallic actuator and the heater and about which the heater may pivot with respect to the actuator, resilient biasing means engaging a first terminal of said heater on one side of said fulcrum, and reaction means provided on said bimetallic actuator and engaging a second terminal of the heater on the other side of said fulcrum.

In accordance with the invention therefore, a single biasing means not only maintains an electrical contact with the first terminal of the heater but also, by virtue of the couple generated around a fulcrum, biases the second terminal of the other end of the heater into contact with the reaction means of the bimetallic actuator. The electrical supply circuit to the heater can thus be made through the biasing means and the bimetallic actuator without the need for soldered joints to the terminals. The heater will effectively rock relative to the actuator about the fulcrum as the bimetallic actuator flexes during heating and cooling, the rocking being accommodated by the resilience of the biasing means. The fulcrum arrangement also results in a greater holding force acting on the heater than the spring force at one end due to the reaction moment generated about the fulcrum.

A further advantage of the fulcrum is that the heated portion of the substrate may be biased directly into contact with the bimetallic actuator so as to improve heat transfer thereto. This differs from prior art arrangements described wherein the force retaining the heater on the bimetallic actuator was provided at one or both ends of the heated portion, i.e. at a or the terminal(s) of the heater.

From a second aspect therefore, the invention provides a thermally actuated switch or thermal actuator comprising a bimetallic actuator and an electrically energisable substrate heater mounted to said bimetallic actuator, characterised in that it comprises a means for applying a force at a directly heated portion of said substrate which biases said portion into thermal contact with said bimetallic actuator. Such means may comprise a fulcrum about which the heater pivots, the force being generated by a reaction to a force applied to one or both sides of the fulcrum, for example by a resilient biasing means arranged at one side of the fulcrum and a reaction surface at the other.

Preferably the heater is arranged at the fixed end of the bimetallic actuator, so as to obtain a "root" heating effect. This produces a larger movement of the free end of the bimetallic actuator than would be obtained if the heater were to be mounted nearer that free end.

In a preferred embodiment the heating means of the heater and the first and second terminals are arranged on the surface of the substrate facing away from the bimetallic actuator. In such an arrangement, the biasing means and the reaction means may both act on this surface of the heater. In a preferred arrangement, the reaction means comprises at least one integral tongue of said bimetallic actuator extending around an edge of said heater and engaging the upper surface thereof. Whilst, for example, a pair of tongues could be provided extending around the side edges of the heater, in a particularly preferred arrangement, a single tongue extends around an end edge of the heater, preferably engaging a laterally central portion thereof.

This tongue will then act as a longitudinal location means for the heater. By using the tongue or tongues merely as a reaction surface, construction of the device can be simplified. A relatively thick bimetallic actuator may be used, which is desirable from the point of view of strength, and the bimetallic material may easily be formed to provide a tongue, since the latter is not required to provide resilience, merely a reaction surface.

In the arrangement described above, where the heater is mounted adjacent the fixed end of the bimetallic actuator, the tongue may be released from a longitudinally intermediate portion of the actuator.

This is an economical use of the bimetal material. To facilitate such an arrangement, the second terminal of the heater may be formed substantially adjacent the end of heater. In an arrangement as described above the biasing means will be arranged nearer to the fixed end of the bimetallic actuator, on the other side of the fulcrum.

The fulcrum may be formed as part of, or mounted to, the heater, or mounted to the bimetallic actuator.

In a particularly preferred embodiment however, it is formed preferably as an integral part of the bimetallic actuator. Preferably the actuator is formed with a shallow inverted V or smooth arc so as to constitute a fulcrum, although a step could also be used. A smooth arc is to be preferred, since it has been found that the heater will rock more easily about such a fulcrum.

The bimetallic actuator may also be provided with location means to locate the heater in the desired position with respect to the fulcrum.

The biasing means is preferably in the form of a leaf spring, having an electrical contact provided on the heater-contacting end thereof. Since the currents carried will be small, the contact may be provided by an end portion of the spring itself. In a particularly preferred embodiment, the other end of the spring is connected to or is formed with a tab for connection into the heater power supply circuit.

The biasing means may be biased into contact with the heater by a part of the housing of the switch after the latter is assembled, but preferably this is achieved by means of its own resilience.

In one embodiment, the bimetallic actuator is pivoted at its "fixed" end. In such an arrangement, it is preferred that the line of action of the resilient biasing means passes through the pivotal axis of the bimetallic actuator. This is advantageous when it comes to calibration of the device, since the biasing force will not create a moment about the axis, irrespective of its size. Thus, for example, where the biasing force is applied by fitting the lid of the housing as described above, calibration may be effected before applying the lid, since the biasing force applied upon fitting will not affect the calibration.

Preferably the switch or actuator of the invention is used in an energy regulator. From a third aspect, therefore the invention provides an energy regulator comprising a thermally actuated switch or thermal actuator in accordance with the first or second aspect of the invention.

A preferred embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings in which: Fig. 1 is a perspective view of an energy regulator incorporating a thermally actuated switch in accordance with the invention; Fig. 2 is a schematic side elevation of energy regulator of Fig. 1, showing the condition in which the switch contacts are shut; and Fig. 3 is a view similar to Fig. 2 except showing the contacts open.

The energy regulator comprises a moulded housing comprising a lower moulding 2 and an upper moulding (not shown). A control knob spindle 4 is rotatably journaled in the housing and extends through an opening in the upper housing moulding, for operation by a user. The control knob spindle 4 is provided at its lower end with a cam body 6 having a number of cam surfaces, for purposes to be described below. In use, a knob (not shown) is fitted to the spindle 4.

The regulator is placed in the electrical supply circuit to the load being regulated, for example a cooker hot plate. The current supply to the load is controlled by a snap-action microswitch 8 operated by a bimetallic actuator 10 which is heated by a heater 60.

The microswitch 8 is arranged in the line side of the supply to the load. A generally U-shaped double line-in tab 12 mounts a fixed contact 14 which co-operates with a movable contact 16 mounted on one end of a switch contact arm 18.

The contact arm 18 is a generally rectangular member provided, along part of its length with downwardly extending lateral flanges 20 for rigidity. A generally U-shaped cut-out in the contact arm 18 releases a central tongue 22 which is bowed and acts both as a compression member and an over-centre spring.

It is integral at one end with the portion of the contact arm 18 mounting the contact 16 and at the other end engages in a notch 24 formed in an upwardly extending support pillar 26. The pillar 26 is an upper portion of a line out tab 28.

The end 29 of the microswitch arm 18 remote from the contact 16 is coupled to a free end 30 of the bimetallic actuator 10, which itself forms one arm of a generally U-shaped bimetallic element 32. The other arm 34 of the element 32 acts as an ambient temperature compensator in a manner known in the art, and is connected to the actuator 10 by a connecting limb in the form of a flanged web 36. The arm 34 carries a cam follower 38 which is biased into engagement with a first cam surface 40 provided on the upper surface of the cam body 6 of the spindle 4 in a manner to be described below. Rotation of the spindle 4, causes the cam follower 38 to move up or down, which in turn causes the free end 30 of the bimetallic actuator 10 to move up or down thus varying the distance through which its free end 10 must move to cause operation of the switch.

The cross-web 29 of contact arm 10 is provided with laterally extending lugs 42 which engage with notches 44 formed in downturned end portions 46 of the actuator 10.

The lugs 42 are biased into the notches 44 by the spring tongue 22, which places the contact arm 18 in tension.

The bimetallic actuator 10 is generally rectangular and formed with a rectangular cut-out 48 which allows access to the tongue 22 during assembly, so that the tongue may be pushed into the notch 24 to 'cock' the switch.

The bimetallic element 32 is pivotally mounted in V notches 52,54 provided in the housing 2 by lugs 56. The lugs 56 are biased into the notches 52,54 by the spring tongue 22, which via the contact arm 18 places the bimetallic actuator 10 in compression. The notch 54 adjacent the base of the compensating limb 34 is moulded into the housing 2. However, the notch 52 is formed in a stainless steel insert 58. This is preferred since the adjacent region of the actuator 10 will become quite hot in use, and also since the reaction force in this notch will be greater than at the other notch 54.

As mentioned above, the bimetallic actuator 10 is heated at its root end by a ceramic substrate heater 60 mounted at the end of the actuator adjacent the pivotal mounting. The heater comprises a ceramic substrate 62, on which is printed a resist heating element 64. First and second electrical terminals 66,68 are provided at opposite ends of the element 64. The heater 60 rests on an arched fulcrum 70 formed across a root portion of the actuator 10. The second terminal 68 of the heater 60 engages under a substantially rigid tongue 72 released and folded back from the cut out 48 of the actuator 10.

To facilitate this engagement, the second terminal 68 of the heater 60 is located adjacent the edge of the heater 60. The tongue 72 locates the heater 60 longitudinally in one direction on the bimetallic actuator 10. The heater is located laterally by tangs 73,75 upstanding from the actuator 10, and at the rear by the flanged web 36.

The first terminal 66 of the heater is contacted and biased downwardly by an end portion of an electrically conductive spring arm 74. The arm 74 may be biased downwardly by the upper moulding of the housing or by its own resilience when the housing is assembled. The downward biasing force of the arm 74 not only ensures a good electrical connection with the first terminal 66, but also causes the heater 60 to pivot on the fulcrum 70, so that the second terminal 68 of the heater 60 engages with the tongue 72. The directly heated portion of the substrate 62 between the first and second terminals 66,68 is biased into direct thermal contact with the bimetallic actuator 10, so as to provide good thermal transfer thereto. This will be the case irrespective of the temperature of the bimetallic actuator 10, since the heater 60 will always remain in contact with the fulcrum 70. Furthermore the line of action of the force of the spring arm 74 passes through the pivot axis 52. Thus the spring force has no moment about the axis. This means that the device can be calibrated before the lid is fitted, since any change in the biasing force will not affect the calibration of the device.

The resilience of the spring arm 74 accommodates movement between the heater 60 and the bimetallic actuator 10 as the latter heats up and cools down. The other end of the arm 74 is formed integrally with a tab 80 extending through the housing 2 for connection into the heater supply circuit.

The electrical circuit to the heater 60 is completed through the tongue 72, the bimetallic actuator 10, the contact arm 18, the contacts 14,16 and the line in tab 12. This circuit is in parallel with the supply to the load being controlled, which passes through the line in tab 12, the contacts 14,16, the central tongue 22 of the actuator arm 18, the fulcrum 24 and the line out tab 30.

Operation of the regulator will now be described.

The desired power setting of the load is set by turning a knob connected to spindle 4 to a suitable position against a scale, not shown. Through deflection of the cam follower 38, this sets an initial position of the free end 30 of the bimetallic actuator 10. The regulator is shown in Fig. 8 on the condition where the contacts 14,16 are closed and thus power is being supplied both to the load and to the heater 60 via the circuits described above. As the heater heats the bimetallic actuator 10, the free end 30 of the latter deflects to the point where the spring tongue 22 moves over-centre with respect to the line extending between the pivots 44 on the bimetal and the fulcrum 24 on the post 28, at which point the contact arm 18 moves upwardly (in the sense of Fig. 8) with a snap action, to break the contacts 14,16, as shown in Fig. 9.

With the contacts 14,16 opened, both the power supply to the load and to the heater 60 is interrupted.

The heater 60 and bimetallic actuator 10 then cool to the point where the spring tongue 22 moves overcentre in the other direction to close the contacts 14,16 with a snap action to reconnect the supply to the load and to the heater 60. The cycle can be varied by turning the spindle 4 and thus changing the initial position of the free end 30 of the bimetallic actuator 10.

The regulator also comprises means for supplying energy to a two-part load such as a split-grill. A first, fixed, contact 80 is mounted on an extension 82 of the line-out terminal 28, and a second, movable contact 84 mounted on a resilient electrically conductive member 86 connected to a secondary line-out terminal 88. The resilient member 86 has a bent down cam follower 90 which engages an inner cam surface on the upper surface of the cam body 6. When the contacts 80,84 are closed, electrical energy is supplied to both parts of the load, through the respective line-out terminals 28 and 88. When power is to be supplied to one part of the load only, the knob is twisted to such a position that the cam follower 86 rides up the inner cam surface to open the contacts 80,84, whereby power is supplied only through the main line-out terminal 28.

If required an on-off switch (not shown) may be provided in the neutral side of the supply.

Certain other features of the energy regulator described herein form the subject of our copending applications references 9309788.9, 9309804.4 and 9309807.7, filed on the same day as 9309805.1 from which this application claims priority, and entitled "Energy Regulators".

Claims

1. A thermally actuated switch or thermal actuator comprising a bimetallic actuator and an electrically energisable substrate heater mounted to said bimetallic actuator, characterised in that it comprises a fulcrum co-operating between said bimetallic actuator and the heater and about which the heater may pivot with respect to the actuator, resilient biasing means engaging a first terminal of said heater on one side of said fulcrum, and reaction means provided on said bimetallic actuator and engaging a second terminal of the heater on the other side of said fulcrum.

2. A thermally actuated switch or thermal actuator as claimed in claim 1, wherein the heated portion of the substrate is biased directly into contact with the bimetallic actuator.

3. A thermally actuated switch or thermal actuator as claimed in claim 1 or 2, wherein the heater is arranged at a fixed end of the bimetallic actuator.

4. A thermally actuated switch or thermal actuator as claimed in claim 1, 2 or 3, wherein the heating means of the heater and the first and second terminals are arranged on the surface of the substrate facing away from the bimetallic actuator.

5. A thermally actuated switch or thermal actuator as claimed in any preceding claim, wherein the reaction means comprises at least one integral tongue of said bimetallic actuator extending around an edge of said heater and engaging the upper surface thereof.

6. A thermally actuated switch or thermal actuator as claimed in claim 5, wherein a single tongue extends around an end edge of the heater and engages a laterally central portion thereof.

7. A thermally actuated switch or thermal actuator as claimed in claim 5 or 6, wherein the tongue is released from a longitudinally intermediate portion of the actuator.

8. A thermally actuated switch or thermal actuator as claimed in any preceding claim, wherein the second terminal of the heater is formed substantially adjacent the end of heater.

9. A thermally actuated switch or thermal actuator as claimed in any preceding claim, wherein the fulcrum is formed as an integral part of the bimetallic actuator.

10. A thermally actuated switch or thermal actuator as claimed in claim 9, wherein the fulcrum is formed as a smooth arc.

11. A thermally actuated switch or thermal actuator as claimed in any preceding claim, wherein the bimetallic actuator is provided with location means to locate the heater in the desired position with respect to the fulcrum.

12. A thermally actuated switch or thermal actuator as claimed in any preceding claim, wherein the biasing means is in the form of a leaf spring having an electrical contact provided on the heater-contacting end thereof.

13. A thermally actuated switch or thermal actuator as claimed in claim 12, wherein the electrical contact is provided on an end portion of the spring itself and the other end of the spring is connected to or is formed with a tab for connection into the heater power supply circuit.

14. A thermally actuated switch or thermal actuator as claimed in any preceding claim, wherein the bimetallic actuator has a fixed end at which it is pivoted.

15. A thermally actuated switch or thermal actuator comprising a bimetallic actuator and an electrically energisable substrate heater mounted to said bimetallic actuator, characterised in that it comprises a means for applying a force at a directly heated portion of said substrate which biases said portion into thermal contact with said bimetallic actuator.

16. An energy regulator comprising a thermally actuated switch or thermal actuator as claimed in any preceding claim.

17. A thermally actuated switch or thermal actuator substantially as hereinbefore described with reference to the accompanying drawings.