GB2275823A - Thermal switches - Google Patents

Abstract

A thermally-responsive switch, designed to go open circuit in response to relatively low overload currents of the order of 2 amps and below, comprises a moulded plastics body 1 capturing therein first and second wire terminals 2, 3 and having an internal chamber 4 wherein a snap-acting bimetal blade 6 of very low thickness is spot-welded to one of the wire terminals 2 and carries a silver contact 12 which cooperates in switching operations with the other wire terminal 3. The moulded plastics body 1 is formed of a polymeric PTC material and acts as a heater in parallel with the bimetallic switch. <IMAGE>

Description

INPROVEXENTS RELATING TO ELECTRIC SWITCHES Field of the Invention: This invention concerns improvements relating to electric switches and more particularly concerns thermally responsive electrical switches employing bimetallic elements as thermal actuators.

Background of the Invention: Many kinds of electrical switches employing bimetallic actuators are known and likewise many different forms of bimetallic switch actuators are known. Early bimetallic switches simply employed a plain bimetal blade which moved relatively slowly in response to temperature changes and gave rise to arcing problems in the switch, and the development of the snapacting bimetallic actuator, constructed as a dished bimetallic element capable of moving between oppositely curved configurations with a snap-action, provided a major advance in the art.

Various forms of snap-acting bimetallic actuators are known, such as those disclosed in GB 600055, GB 657434, GB 1064643, GB 1542252 and GB 2124429 for example, and various forms of electric switches employing such bimetallic actuators are likewise known; GB 2124429 abovementioned for example discloses the utilization of a pear-shaped snap-acting bimetallic actuator in a current-sensitive switch where the heating of the bimetal by electric current flow therethrough is designed to trip the switch in a current overload situation.

It is also known to provide heater elements in bimetallic switches, either in series with or in parallel with the switch contacts. With a heater element provided in series with the switch contacts, it is possible to adjust the switching characteristics of the switch to accommodate special requirements such as those of motor protection switches for small electric motors for example. In GB 2 133 931 there is described a bimetallic switch having terminal parts formed of a relatively high resistance material, such as nickel chromium alloy or stainless steel for example rather than the more commonly used brass, so that heat is generated in normal operation of the switch and affects the switch characteristics; the response of the switch to a high overload current is not significantly affected by the heat generated in the terminal parts since the bimetal response to the high current will be more rapid than the time taken for the heat generated in the terminal parts to transfer to the bimetal, but the responsiveness of the switch to a current barely at an overload level will be enhanced. With a heater element connected in parallel with the switch contacts, the heater passes a current dependent on the relative resistances of the heater and of the bimetal so long as the switch contacts are closed, and the heater current in this situation may or may not be negligible, but once the bimetal operates the heater passes the full load current and generates heat which is transferred to the bimetal so as to delay its reset period or prevent it from resetting altogether.

As well as utilizing switch parts formed of relatively high resistance materials, as in GB 2 133 931 abovementioned, it is also known to form switch heaters as thin film resistors, from conductive inks and as ceramic PTC materials (that is to say ceramic materials having a positive temperature coefficient of resistance). PTC material heaters have the advantage that, when connected in parallel with switch contacts, the self heating caused by through current in the PTC material when the switch contacts open not only provides heat to the bimetallic element of the switch but also increases the resistance of the PTC material thereby effectively reducing the current supplied to the switch load. Examples of bimetallic switches incorporating PTC material heaters are described in GB 2 252 674.

PTC material resistances have also been used by themselves as overload protection devices for electric motors and GB 1 604 111, for example, discloses the use of a PTC material resistance connected in series with the motor windings and operative to reduce the current through the motor to a safe level in the event that a motor overload causes the resistance of the PTC device to increase significantly. Such devices are also useful as thermal relays in starting circuits for electric motors and GB 2 015 823, for example, discloses the provision of a PTC resistor in series with start windings of an electric motor, the PTC resistor permitting a high initial current to flow for starting the motor and thereafter increasing its resistance so as to correspondingly reduce the current in the start winding. Plastics materials having PTC characteristics are also known and in WO 91/07804, for example, such a PTC thermistor is connected in series with the armature windings of a small DC motor and serves a motor protection function.

From the foregoing it will be seen that the art is replete with proposals for bimetallic switches, bimetallic switches incorporating heating elements and including PTC material heating elements, and PTC material protective devices. In motor protection applications, particularly for the protection of small DC motors such as for example automotive window lift motors which are becoming ever smaller in size and more powerful, the use of simple bimetallic switches gives rise to problems in that the cycling of the switch as it intermittently heats and cools can cause jarring of gears in sensitive gear trains, sets lifetime limits on the switch, and must be carefully controlled to ensure that a stalled motor is held at a protected and limited temperature. Furthermore, in some situations the internal resistance of a stalled motor increases rapidly thereby producing a steep decline in through current which exacerbates the difficulty of protecting the motor with a current sensitive bimetallic switch and places rigorous requirements on the design and production of the switch. The incorporation of PTC resistor elements into bimetallic switches has, in some measure, overcome these problems in that the combination of bimetal characteristics and PTC device characteristics provides sufficient additional switch variables to provide for additional design variations to enable particular application requirements to be met, but at a cost of increased switch complexity. PTC devices by themselves have the advantage of simplicity, but have to be individually designed to suit specific applications, are not particularly time stable, and display operating characteristics that are both temperature and current dependent.

Objects and Summarv of the Invention: The object of the present invention is to provide a thermally responsive bimetallic switch capable of overcoming or at least substantially reducing the abovementioned problems, the switch additionally preferably being of inexpensive and uncomplicated construction.

According to one aspect of the present invention there is provided a bimetallic switch enclosed within a moulded plastics switch body and the moulded plastics switch body is formed of a polymeric PTC material.

According to a more particular aspect of the invention there is provided a switch comprising a moulded plastics body portion capturing therein first and second terminal conductors, and a snap-acting bimetallic actuator secured to one of said conductors and carrying a contact which constitutes the moving contact of the switch, such contact being arranged for co-operation in switching operations with the other of the two conductors, and the moulded plastics body portion being formed of a polymeric PTC material.

Any suitable polymeric PTC material can be utilized in the practice of the invention, but it is preferred to utilize a polyolefin material, e.g. polypropylene, incorporating one or more conductive fillers and preferably also incorporating a non-conductive filler such as a fibre to provide stability and reproduceability. Carbon black is the preferred conductive filler and we prefer to utilize a relatively low proportion (0 to 5%) of a high conductivity carbon black (such as Ketjenblack EC 600 from Akzo) and a relatively high proportion (0 to 30%) of a carbon black such as to make a substantial contribution to the PTC effect (e.g. Elflex 120 from Cabot Corporation). The non-conductive filler may for example be from 0 to 40% glass fibre. The percentages quoted are by weight.

The bimetallic actuator of the switch is advantageously of such low thickness as to be responsive to through current as low as 2 amps or less, for example a bimetallic element of the order of 0.076mm (0.003 inch) thickness, and the switch body part preferably provides physical support for the bimetallic element during switching operations.

In an exemplary embodiment which will be described in detail hereinafter the bimetallic actuator is of a kind having a generally U-shaped cut-out defining a tongue between spaced apart leg portions which are bridged adjacent the free end of the tongue. The terminal conductors are formed as simple wires and the tongue of the bimetal is secured to one of the terminal conductors, for example by welding, and the bridging portion carries the contact which co-operates with the other conductor. No discrete contact is provided on the other conductor which however comprises a silver or silver alloy coating, for example a silver antimony coating as described in WO 92/14282. The moulded plastics PTC material body portion of the switch defines an enclosure for the bimetallic actuator and, as will hereinafter be described, also incorporates portions which provide structural support for the bimetal. The resultant switch is of simple and easily manufactured construction which enables small size to be achieved for enhanced sensitivity to very low overload currents.

Furthermore, by variation of the characteristics of the PTC material the switch can readily be customised to suit particular applications.

To further enhance the current sensitivity of the switch, a series-connected heating element could, if desired, be provided for injecting heat into the bimetallic actuator when the switch is in closed condition, and in a particularly convenient arrangement such a series heating element may be constituted by a portion of one or the other or both of the two terminal conductors of the switch which is formed as a resistance heating element. Such an arrangement could be arranged to obtain a more rapid switch response to a current overload situation than would be obtained if the series heating element were not provided.

The above and further features of the invention are set forth with particularity in the appended claims and will best be appreciated from consideration of the following detailed description of an exemplary embodiment given with reference to the accompanying drawings.

Brief Description of the Drawings: Figure 1 is a sectional side elevation view of the subject switch on the line I...I in Figure 2; Figure 2 is a plan view showing the switch of Figure 1 with its top cover removed; and Figure 3 shows an alternative conductor construction which could if desired be employed in the switch of Figure 1 so as to provide a series connected heater within the switch.

Detailed Description of the Embodiment: The switch hereinafter described is in many respects identical to the switch described in WO 92/20086, but differs principally therefrom in that the moulded plastics body portion 1 of the switch is formed of a polymeric PTC material. The accompanying drawings are identical to those in WO 92/20086.

The views in the drawings show the switch to an enlarged scale and the dimensions indicated are the actual dimensions of the switch in millimetres. The moulded plastics body portion 1 of the switch is thus generally rectangular with dimensions of 10.5mm x 6.0mm x 2.7mm and the terminal conductors 2, 3 project outwardly by a further 7.0mm. A top cover for the switch has a thickness of 0.5mm. The switch thus has such small overall size that it may conveniently be supplied in a bandolier suitable for use by automatic component insertion equipment.

Any suitable plastics material exhibiting PTC characteristics could be utilized in the practice of the invention for forming the body portion 1, but it is preferred to utilize a polyolefin material, e.g.

polypropylene, incorporating one or more conductive fillers and preferably also incorporating a nonconductive filler such as a fibre to provide stability and reproduceability. Carbon black is the preferred conductive filler and we prefer to utilize a relatively low proportion (0 to 5%) of a high conductivity carbon black (such as Ketjenblack EC 600 from Akzo) and a relatively high proportion (0 to 30%) of a carbon black such as to make a substantial contribution to the PTC effect (e.g. Elflex 120 from Cabot Corporation). The non-conductive filler may for example be from 0 to 40% glass fibre. The percentages quoted are by weight.

Within the body portion 1 of the switch there is defined a chamber 4 which has dimensions of the order of 8.0mm x 5.0mm x 1.7mm, and an upstand 5 occupies part of this chamber. A simple copper wire conductor 2 has a square or rectangular section and is moulded into the body portion 1 at one end thereof with its forward part received in a recess in the upper surface of the upstand 5, and a simple copper wire conductor 3 has a circular section, though it too could have a square or rectangular section, and is moulded into the opposite end of the body portion 1 so as to be exposed at 7 within the chamber 4. A bimetallic actuator 6 is welded to the forward part of conductor 2. The portion 7 of conductor 3 constitutes a switching contact of the subject switch and accordingly conductor 3 is preferably formed of silver plated copper wire or more preferably silver-antimony plated copper wire as described in WO 92/14282; conductor 2 may be similarly formed though this is not essential.

In accordance with the teachings of WO 92/14282 abovementioned, a preferred form of conductor wire providing excellent electrical characteristics in combination with superior wear characteristics comprises a copper wire, or a wire formed from a copper alloy having a thermal conductivity at least 90% that of copper, and more preferably 95% to 99% that of 99.95% pure copper, provided with a thick plating layer of silver and antimony, and the conductor wire 3 abovedescribed can advantageously have this construction. By the use of a thick plating (e.g. at least 30 microns and preferably 40 microns thickness) comprising fine silver (99.9% purity) with a small amount of antimony, typically about 1% and particularly between 0.3% and 0.7%, on a conductor formed of copper or a high thermal conductivity copper alloy, the formation of silver powder during switching operations is inhibited and a life of about 70,000 switching cycles may be obtained.

Bimetallic actuator 6 is of the Otter Controls type comprising a dished blade of bimetallic material having a generally U-shaped cut-out 8 which defines a tongue 9 between legs 10 which are bridged by a bridging portion 11. The moving contact of the switch is constituted by a silver contact 12 welded to the underside of bridging portion 11 as best shown in Figure 1. The actuator 6 is secured to terminal conductor 2 by virtue of the tongue 9 being welded thereto. The shape of the bimetallic blade is such as to enhance its responsiveness to through currents by increasing the current density in the legs 10 and in the contact-carrying forward region of the blade.

The up stand 5 provides support for the forward portion of conductor 2 which in turn provides support for tongue portion 9 of bimetallic actuator 6, whereas the legs 10 and bridging portion 11 of the bimetal 6 are free to move within the chamber 4. By virtue of this arrangement, the temperature responsive characteristics of the switch can better be predetermined since switching operations are effected substantially exclusively by flexure of the legs 10 about the stable position established for tongue 9 by virtue of its support on conductor 2. Furthermore, by supporting the tongue 9 in this way, the risk of stress cracking at the root of the tongue is reduced and the working stresses in the bimetal are concentrated towards its elongate legs 10.

The switch construction as thus described comprises a simple bimetallic switch, constituted by the conductors 2 and 3 and the bimetal 6, with a PTC heater, constituted by the body portion 1 of the switch, connected in parallel with the bimetallic switch. In operation of the switch a's thus described, contact 12 will move away from portion 7 of conductor 3, with a snap-action, whenever the temperature of the bimetal 6 rises to a certain predetermined level, either as a result of thermal conduction from the switch environment, or as a result of heating of the bimetal by current flow therethrough, or as a result of heating of the bimetal by current flow through the PTC material or as combination of any two or more of these three effects. When the bimetal cools sufficiently the switch will remake. The characteristics of the polymeric PTC material from which the switch body portion 1 is formed do, however, have an effect upon the overall switch characteristics and may be designed to prevent the bimetal from cooling to such a temperature that it resets so as to cause the switch contacts to remake, or to inject such an amount of heat into the bimetal 6 in its open-contacts condition as to modulate the on-off time of the switch cycles to suit a particular application, for example a motor protection application where the cycling of the switch must not result in a stalled motor being subjected to excessive temperatures.

The fact that the PTC characteristics of the switch body portion 1 combine with the switching characteristics inherent in the bimetal 6 provides for the ready design of special switches to suit particular applications.

Further design variables'which can be manipulated in order to adapt a switch according to the present invention to a specific application can be obtained by inclusion of a series-connected heater in the switch as will hereinafter be described.

The closure 15 may conveniently be moulded as an integral part of the switch body which is hingedly coupled thereto and is ultrasonically welded shut after assembly of the bimetal 6 into the chamber 4 and spot welding of the tongue 9 to the forward part of conductor 2. The closure 15 may be formed so as to isolate the chamber 4 from the environment of the switch, or may alternatively be provided with one or more openings 16 as indicated.

The bimetallic material of the actuator 6 has a thickness of only 0.076mm (0.003 inch) and had to be specially manufactured for us. By use of such a thin bimetallic material in a switch construction where the switch body provides physical support to the bimetal and also, by virtue of being formed of polymeric PTC material, contributes to the operational characteristics of the switch, we have been able to obtain repeatable switch action for low current switching below 2 amps.

Furthermore, by virtue of the provision of the PTC switch body in parallel with the bimetal switch elements, the result is obtained that, in the event of failure of the bimetallic switch in a contacts-open condition, overload current protection is maintained by the PTC material.

For certain switch applications the provision of a series-connected heater may be desirable and the switch described in the foregoing could readily be modified to incorporate such a heater. Figure 3 shows an alternative form of conductor which could be used in the switch of Figures 1 and 2 in place of the conductor 2.

As shown in Figure 3, conductor 20 is formed of a resistance heating material and has a forward portion 21 which is adapted to be received within the switch body chamber 4 and is formed generally as a spiral terminating in a pad 22 to which the tongue 9 of the bimetal actuator 6 is spot welded. The dimensions shown in Figure 3 are in millimetres, Figure 3 being an enlarged showing of the conductor.

The described switch is well suited to automatic manufacture and installation, comprises a minimum of parts and can be relatively inexpensive, and is capable of miniaturisation for enhanced current sensitivity.

The switch is, however, but an example of what is achievable by virtue of the invention and modifications and variations are possible without departure from the spirit and scope of the invention. For example, the bimetal could be pear-shaped as described in GB 2124429 aforementioned for enhanced current sensitivity, or could take a variety of alternative shapes.

Furthermore, the wire terminals 2 and 3 could be replaced by appropriate sheet metal parts insert moulded into the polymeric PTC material body portion 1 and having terminal pads accessible in the undersurface of the body portion for surface mounting of the switch to a printed circuit board for example either by soldering or, more preferably having regard to the desirability of avoiding exposure of the switch to extremes of temperature, by means of a mechanical spring fastening arrangement.

Claims (14)

CLAIMS:

1. A bimetallic switch enclosed within a moulded plastics switch body formed of a polymeric PTC material.

2. A thermally-responsive switch comprising a moulded plastics body portion capturing therein first and second terminal conductors, and a snap-acting bimetallic actuator secured to one of said conductors and carrying a contact which constitutes the moving contact of the switch and is arranged for co-operation in switching operations with the other of the two conductors, the moulded plastics body portion being formed of a polymeric PTC material.

3. A switch as claimed in claim 2 wherein the first and second terminal conductors comprise simple conductive wire conductors.

4. A switch as claimed in claim 2 or 3 wherein at least the said other of the two conductors comprises copper with a plating of silver containing antimony.

5. A switch as claimed in claim 4 wherein said contact comprises silver.

6. A switch as claimed in any of claims 2 to 5 wherein the first and second conductors are captured in spaced-apart opposite ends of the moulded plastics body.

7. A switch as claimed in any of claims 2 to 6 wherein that one of the conductors to which the bimetal is secured has a square or rectangular section and the bimetal is welded to a flat surface of the conductor.

8. A switch as claimed in any of claims 2 to 7 wherein the bimetallic actuator comprises a dished bimetallic blade having a generally U-shaped cut-out defining a central tongue extending between a pair of external legs, and the tongue of the bimetal is secured to the said one of the conductors.

9. A switch as claimed in claim 8 wherein the bimetallic blade has a thickness of 0.076mm.

10. A switch as claimed in any of claims 2 to 9 and wherein one and/or the other of said conductors comprises a resistance heating portion adapted to pump heat into the bimetallic actuator.

11. A switch as claimed in any of the preceding claims and wherein the polymeric PTC material incorporates one or more conductive fillers providing the polymeric material with PTC characteristics and a non-conductive filler.

12. A switch as claimed in claim 11 wherein the conductive fillers comprise carbon black.

13. A switch as claimed in claim 11 or 12 wherein the non-conductive filler comprises a fibre material.

14. A thermally-responsive switch substantially as herein described with reference to the accompanying drawings.