WO1998031033A1

WO1998031033A1 - Improved thermal cut-off device

Info

Publication number: WO1998031033A1
Application number: PCT/US1998/000557
Authority: WO
Inventors: Robert P. Boesel
Original assignee: Valentine Magnetics, Inc.
Priority date: 1997-01-14
Filing date: 1998-01-13
Publication date: 1998-07-16
Also published as: AU5914398A

Abstract

An improved thermal cut-off device is provided. The thermal cut-off device includes a housing defining two apertures. A platform is enclosed within the housing and disposed on a wall of the housing. A terminal pin is disposed through each aperture and adjacent a side of the platform. A fusible link is mounted on the platform and interconnected between the ends of the terminal pins. The thermal cut-off device may be integrally formed with or mounted on or near a product desired to be protected from excessive heat.

Description

IMPROVED THERMAL CUT-OFF DEVICE

Field of the Invention The present invention relates generally to thermal cut-off devices, and more particularly, to products having an integral thermal cut-off device and method of assembling the same.

Background of the Invention Thermal cut-off devices have long been in use as the thermal equivalent of the electrical fuse. Similar to a simple fuse, these devices open an electrical circuit based on the achievement of excessive temperature rather than excessive current. Earlier designs consisted of stored energy springs retained by fusible elements which when melted allowed the spring to open the circuit. The most popular design in use today typically comprises two 0.5 mm tinned brass lead wires, the ends of which are separated by a distance on the order of 4 mm, connected by a low melting temperature alloy or fusible link.. The area surrounding the fusible link assembly is packaged in plastic either within a cylinder in which the leads are axial to the cylinder, or within a rectangular box, in which the leads exit the box on one side paral^' to each other. In both cases, the volume within the container is sufficient to provide clearance around assembly. In normal product operation, under sufficiently high temperature, the fusible link melts and the surface tension of the molten alloy causes it to rapidly retract to the two lead wires •opening the circuit permanently. in some devices wax or other material of a melting. point lower than the fusible link is employed within the plastic housing so as to increase heat transmission and limit arcing as the circuit opens .

These types of thermal cut-off devices are commonly installed on transformers. In a typical assembly- process, the primary winding of the transformer is first wrapped with multiple layers of tape over which barrier insulation, such as fiberboard, is taped to protect the winding from contact with the bare leads of the thermal cut-off device. The thermal cut-off device body is then taped to the barrier insulation. The tape and barrier insulation are used to insulate the terminal pins from the coil winding. Unfortunately the insulation also thermally insulates the fusible link from the heat of the winding and causes inconsistent and often ineffective performance.

^■ .iπ ifirY

Generally, the present invention relates to an improved thermal cut-off device. In accordance with one embodiment of the invention, a thermal cut-off device is provided which includes a housing defining two apertures. A platform is enclosed within the housing and disposed on a wall of the housing. A terminal pin is disposed through each of the apertures and adjacent a side of the platform. A fusible link is mounted on the platform and interconnected between the ends of the terminal pins.

In accordance with one aspect of the invention, the TCO device is integrally formed with a product . For example, the TCO device may be integrally formed with a bobbin of a transformer. In accordance with another aspect of the invention, the TCO device is provided as a standalone unit which may be mounted on or near a product.

The above summary of the present invention is not intended to describe each illustrated embodiment or every implementation of the present invention. The figures and the detailed description which follow more particularly exemplify these embodiments.

Brief Description of the Drawings

The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which: Figure 1 is a perspective view of an exemplary thermal cut-off device in accordance with the present invention;

Figure 2 is a front plan view of the exemplary thermal cut-off device of Figure 1;

Figure 3 is a cross-sectional view of the exemplary thermal cut-off device of Figure 1;

Figure 4 is a perspective view of an exemplary transformer bobbin having a thermal cut-off device in accordance with the present invention;

Figure 5 is a top view of the exemplary transformer bobbin of Figure 4;

Figure 6 is a front view of the exemplary transformer bobbin of Figure 4; and Figure 7 is a side view of the exemplary transformer bobbin of Figure 4.

While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

Detailed Description of the Drawings

The present invention is believed to be applicable to a number of products in which excessive temperatures present problems. These products include electromagnetic devices such as transformers and electrically resistive devices such as heaters. While the present invention is not so limited, an appreciation of various aspects of -the invention will be gained through the discussion provided below.

Figures 1-3 illustrate one embodiment of a thermal-cut off (TCO) device in accordance with the invention. The TCO device 10 generally includes a housing 12 and a platform 14 disposed on a wall of the housing 12, as best illustrated in Figure 1. The housing 12 defines a pair of apertures 16 each used to receive a terminal pin 18. The platform 14 generally provides a mounting structure for interconnecting the two terminal pins IB with a fusible link 20. Generally, the TCO device 10 is positioned near a source of excessive heat. The heat from the heat source is transmitted to the fusible line 20 through the housing 10, platform 14, and terminal pins 18 of the TCO device 10.

Typically, the housing 12 and the platform 14 are integrally formed from a thermoplastic resin which includes a thermally conductive material, such as glass fibers, to increase the thermal conductivity of the resin and facilitate the transfer of heat from the heat source to the fusible link 20. Moreover, the resin forming the housing 12 and platform 14 may be a crystalline material so that these features retain their structural form until the resin reaches its melting point. The housing and platform may however be made separately and/or from other materials. For example, in temperature environments exceeding the melting point of plastic, the housing 12 may be formed from a higher melting point material such as ceramic.

The surface of the platform 14 on which the fusible link 20 is mounted may be recessed with respect to the outer surface of the terminal pins 18, for reasons to be discussed below. In the exemplary embodiment, the platform 14 is provided with a recess 22 (best illustrated in Figures 1 and 3) on which the fusible link 20 is mounted and a pair of recesses 24 (best illustrated in Figures 1 and 2) which receive the terminal pins 18. The pin recesses each typically have a beveled side 25 (best shown. in Figures 1 and 3) . The beveled sides 25 are generally aligned with the apertures 16 such that when a terminal pin 18 is disposed through an aperture 16, the terminal pin 18 firmly presses against the beveled sides 25 of the platform 14.

The fusible link 20 is typically a metal alloy having a melting point temperature below a predetermined threshold temperature dictated by design specifications of the product to be protected. The exemplary fusible link 20 is a eutectic material and melts relatively rapidly on attainment of its melt temperature. The fusible link 20 is generally mounted against the recess 22 of the platform 14. To fill any air pockets between the two components and facilitate the conductance of heat to the fusible link 20, a relatively high thermal conductivity material (as compared to air) , such as paste flux, may be supplied between the fusible link 20 and the platform 14. This thermally conductive material may be an adhesive which securely retain the fusible link against the platform 14 and further facilitates the conductance of heat to the fusible link 20.

The TCO device 10 can be mounted to or integrally formed with a product so that the platform 14 is accurately located near a desired area or part of the product typically at risk of excessive heat. The heat is efficiently transmitted to the fusible link 20 through the various components of the TCO device 10. This provides a highly accurate and consistent TCO device 10. Generally, the platform 14, housing 12 and terminal pins 18 transfer heat to the fusible link 20. Where the platform 14 and housing 12 are integrally formed, thermal resistance between the two structures is reduced (due the lack of surface contact resistance) and heat is more efficiently transferred to the fusible link 20. The provision of thermally conductive material between the fusible link 20 and the platform ,1 further decreases the thermal resistivity of the TCO device 10.

The exposed portions of the terminal pins 18, i.e., the portions outside of the enclosed housing, may be suitably coupled to a circuit in a variety of manners as well known in the art. For example, the terminal pins 18 may be wired in series with a product so as to shut down the product when the TCO device 10 is triggered and/or the terminal pins 18 may be wired to an indicator, such as a light -emitting diode, to indicate a temperature in excess of a threshold. The terminal pins 18 may moreover be substantially rigid, thereby facilitating electrical coupling of the terminal pins. For example, rigid terminal pins 18 may be used to plug the TCO device 10 into circuit boards .

An exemplary assembly method will now be described to illustrate further features of the invention. In the exemplary assembly method, the terminal pins 18 are pressed into the terminal apertures 16 so that the terminal pin ends 19 approach the inside bottom of the housing 12 as shown in Figures 2 and 3. Once inserted, the interior ends 19 of the terminal pins 18 are biased against the platform 14 as a result of the beveled sides 25.

A fusible link 20 having a suitable melting temperature is placed across the lower ends of the terminal pins 18. The melting temperature of the fusible link 20 is generally selected to be lower than the rated temperature of the protected product. A wide variety of materials may be used to form the fusible link 20, including, for example, Tin-Lead and Indium-Tin. The fusible link 20 is generally centered between the two terminal pins 18 and typically has an initial length which exceeds the spacing between the terminal pins 18 in order that the fusible link may be efficiently fused to the terminal pin ends. In one embodiment, the length of the fusible link is about 50% greater than the spacing between the outer surfaces of the terminal pins 18. A pressing tool 14 is used to press the fusible link 20 firmly against the platform 14. Prior to pressing the fusible link against the platform, a relatively high thermal conductivity adhesive material may be supplied between the fusible link 20 and the platform 14 to firmly retain the fusible link 20 against the platform 14 and facilitate the conductance of heat to the fusible link 20, as discussed above.

While the fusible link 20 is pressed by the pressing tool, heated soldering elements are placed in contact with a surface of each terminal pin 18 without directly contacting the fusible link 20. The contact surface of each terminal pin 18 may be within the housing or exterior of the housing.

Generally, heat is transmitted from the soldering element through the terminal pins 18 and to the fusible link 20. As the surface of the platform 14 on which the fusible link 20 is mounted is recessed with respect to the outer surface of the terminal pins 18, the pressing tool also firmly presses the fusible link 20 against the terminal pins 18. This improves the flow of heat to the fusible link 20 and facilitates the fusing of the fusible link 20 to the terminal pins 18.

In particular, when the temperature of the fusible link 20 surface in contact with a terminal pin 18 exceeds the melting point of the fusible link material, the link begins to melt and surface tension draws the link material which is outboard of the terminal pin 18 into the melt plane proximate the terminal pin 18 surface. The molten fusible link material forms a solder bond to the terminal pin end. Melting of the central portion of the fusible link 20, that is the portion between the terminal pins 18, is advantageously prevented by the platform 14 (as well as pressing tool) which act as heat sinks drawing heat from the central portion of the fusible link 20. In addition, the beveled sides 25 of the platform 14 prevent the ends of the terminal pins 18 from bending away from the platform 14 during soldering.

To enclose the housing 12, a cover 30 may then be installed over the housing 12. The cover may be retained by, for example, ultrasonic welding, adhesive, or mechanical methods. The cover 30 may include apertures to prevent overpressurization of the enclosed housing 12 due to arcing of the terminal pins when the TCO device 10 is triggered. The apertures may be provided in multiple directional changes to prevent expulsion of heated material from within the enclosed housing 12.

The TCO device 10 may be integrally made with a particular product or component thereof or may be made as a stand-alone device capable of being mounted on or nearby a product or heat source. It will be readily seen that the TCO device 10 can be used with a multitude of products, including electromagnetic devices, such as transformers, solenoids, alternators, power supplies, etc. and electrically resistive devices, such as heaters, light fixtures, hair dryers, and so on. Higher order assemblies of the above mentioned products such as computers and their peripheral equipment, consumer electronics, transportation vehicles, etc. may also benefit from the TCO device 10. Moreover, the heat source from which the TCO device 10 protects against need not be electrical in nature but can also be the result of combustion, chemical activity, and so forth.

Figures 4-7 illustrate a TCO device which is integrally formed with a bobbin of a transformer. Figure 4 generally illustrates a perspective view of a transformer bobbin 400 having an integrally formed TCO device 410. The bobbin 400 generally includes three flanges 422, 424, and 426 which define zones for a primary winding 442 and a secondary winding 444. The TCO device 410 is typically integrally formed with the outer bobbin flange 422 adjacent the primary winding 442. The TCO device 410 generally includes a housing having a rear wall 411 which comprises an outer face of the flange 422, two side walls 413, and a cover 415. Within the housing 412, there is provided a platform 414, a fusible link 416 and two terminal pins 418, the interconnection and structure of which is similar to that discussed above.

The platform 414 is generally disposed on the rear wall 411 of the housing 412, against the outer face of flange 422. The particular location of the platform 414 and fusible link 416 can be selected to precisely position the fusible link 416 near a desired area typically associated with excessive heat. This location may vary with the particular heat profile of the transformer. Generally, the platform 414 is disposed such that the fusible link 416 rests adjacent the interior of the primary winding 442. In the exemplary embodiment, the fusible link 416 is positioned at a distance h (from the inner perimeter of the coil winding) which measure 1/3 of the thickness t of the coil winding. Through personal research, this area of the coil winding is associated with the highest temperatures and first failure of a bobbin transformer. In this manner, the TCO device 410 is more precisely located near the highest point of heat than conventional TCO devices, which are positioned on the relatively cool exterior surface of the winding.

The position of the TCO device as well as the structure of the bobbin is provided by way of example and not of limitation. Generally, the TCO device 410 may be positioned on the bobbin near any area where a risk of excessive heat is present. For example, the TCO device 410 may be integrally formed with the inner flange 424 or the outer flange 426 associated with the secondary winding 444, or the TCO device 410 may be mounted to the bobbin, for example, by press fitting. The excessive heat may be internally generated from the transformer itself or may be generated by an external heat source. Moreover, multiple TCO devices may be provided on the bobbin 410. The bobbin 410 as well as the housing 412 and platform 414 may be molded from a thermoplastic resin which remains solid until the resin's melt temperature is reached, such as a crystalline or semi-crystalline resin. This prevents low-temperature softening of the bobbin which may cause failure of a bobbin flange resulting from internal pressures of the coil windings. These internal pressures, typically caused by decomposition of coil winding film insulation, can compress adjacent coils windings together and introduce dangerously high voltages into the transformer. Furthermore, by integrally molding the platform 414 with the bobbin flange 422, thermal resistivity between the coil windings and the fusible link 416 is reduced and the heat of the coil winding is efficiently transmitted to the fusible link 416, as discussed above. Furthermore, the coil bobbin resin may be manufactured with a thermally conductive material, such as glass fiber, to more efficiently transmit heat from the heat source, e.g., the coil windings, to the fusible link 416. Glass fiber may be used in amounts of about 30% by weight, for example.

The terminal pins 418 are typically rigid and extend outwardly from the upper portion of the flange 422. In this manner, the terminal pins 418 are thermally isolated from the coil windings without need of tape or barrier insulation. The terminal pins 418 of the TCO device 410 may be wired in a well known variety of manners. For example, the terminal pins 418 may be wired to working terminal pins 432 in series to cut off the transformer when the fusible link 416 is opened. In the exemplary embodiment, the working terminal pins 432 are mounted on the side of the flange 422 opposite the TCO terminal pins 418. To facilitate wiring of the TCO terminal pins to the working terminal pins 432, one or more wire channels 434 are provided about the perimeter of the flange 422 and two wire turning posts 436 are provided for diverting wire 438 from the TCO terminal pins 418 into the wire channel 434.

By mounting the working terminal pin 432 opposite the TCO terminal pins 418, the interconnect wire 438 runs a sufficient length to prevent heat transmission to the TCO terminal pins 418 from the working terminal pins 432 during soldering processes used to terminate wires on the working terminal pins 432. This structure is particularly advantageous in smaller transformers. However, other positions of the working terminal pins 432 and TCO terminal pins 418 are covered by the invention. For example, in a larger transformer, a coil bobbin might employ an integral TCO device on the same side of the coil bobbin as the working terminal pins. An exemplary method of assembling the above- described transformer is provided below. This assembly method is intended to be illustrative only. Many variations are possible. Generally, the coil bobbin 410 along with the platform 416 are formed using an injection molding machine. A number of plastic resins may be used to mold the bobbin, including polyethyleneterephthalate (PET) , polybutyleneterephthalate (PBT) and Nylon™. These plastic resins may be made of a crystalline material and may contain glass fiber, as discussed above. Two TCO terminal pins 418 are inserted through the apertures 417 in the flange 422 and retained against the platform 416 in a manner as described above with respect to Figures 1-3. During the same step, the working terminal pins 432 may also be installed. Exemplary working pins include .025" square tin plated phosphor bronze for transformers up to about 25 Volt-Amp (VA) rating, and larger diameter pins for larger transformers.

The TCO terminal pins 418 are then wired to the working terminal pins 432 using a coil winding machine. Generally, a wire 438 is wrapped, for example, in a dual- spiral pattern, about a first terminal pin 418 and run to a turning post 436 while remaining essentially in contact with the TCO housing 412. The wire 438 is then turned on the turning post 418 and routed into the wire channel 434. The winding machine spindle is then rotated 180° for feeding the wire along the wire channel 434 to an appropriate working terminal pin. The process is repeated in similar fashion to connect the other terminal pin 418 to a working terminal pin 432. Generally the interconnect wiring 438 ,is of the same size as that used for the coil windings. However, an interconnect wire 434 of different size or characteristics than that used for the coil winding may be employed. For example, a larger interconnect wire may be used to reduce the chance of arc-opening of the interconnect runs, while smaller interconnect wire may be used to facilitate arc- opening within the wire channel if desired. Furthermore, a cement over-coated interconnect wire may be used to facilitate bonding of these wire runs to the wire channel and reduce vibration of the interconnect wires. These examples are provided by way of illustration and are not intended to be exhaustive.

The bobbin 410 is then processed through a typical coil finishing machine where the terminal pins are dipped in soldering flux, dipped in a molten solder bath, and checked for electrical continuity between wire run ends. The fusible link 416 is then positioned on the platform 414 and heated to connect the fusible link 416 to the TCO terminal pins 418, as discussed above with respect to Figures 1-3. The cover 416 may then be installed to enclose the housing 412. The assembly process may then continue with well-known assembly steps such as coil winding and so forth.

As noted above, the present invention is applicable to a number of different applications which can benefit from an improved thermal cut-off device. Accordingly, the present invention should not be considered limited to the particular examples described above, but rather should be understood to cover all aspects of the invention as fairly set out in the attached claims.

Various modifications as well as numerous structures to which the present invention may be applicable will be readily apparent to those of skill in the art to which the present invention is directed upon review of the present specification. The claims are intended to cover such modifications and devices.

Claims

ClaimsWHAT IS CLAIMED:

1. A thermal cut-off device, comprising a housing defining two apertures; a platform enclosed within the housing and disposed on a wall of the housing, the platform having two sides; two terminal pins each disposed through one of the two apertures and adjacent one of the two sides of the platform; a fusible link mounted on the platform and interconnected between the ends of the terminal pins.

2. The thermal cut-off device of claim 1, wherein the housing and platform are integrally formed.

3. The thermal cut-off device of claim 2, wherein the housing and platform are integrally molded from a plastic resin.

4. The thermal cut-off device of claim 3, wherein the plastic resin includes a crystalline material.

5. The thermal cut-off device of claim 1, wherein the plastic resin includes glass fiber.

6. The thermal cut-off device of claim 2, wherein the housing and platform are integrally formed from a ceramic material. 1033

7. The thermal cut-off device of claim 1, wherein the platform sides are beveled to facilitate contact between the platform and the terminal pins.

8. The thermal cut-off device of claim 1, further including a thermally conductive adhesive disposed between the platform and the fusible link.

9. The thermal cut-off device of claim 1, wherein the housing includes a body and a cover.

10. The thermal cut-off device of claim -1, wherein the cover includes apertures to prevent overpressurization of the housing.

11. The thermal cut-off device of claim 1, further including a ligh -emitting diode (LED) connected to the terminal pins, the LED being connected so as to emit light when the thermal cut-off device is triggered.

12. The thermal cut-off device of claim 1, wherein the fusible link is mounted on a surface of the platform, the platform surface being recessed with respect to a front surface of the terminal pins.

13. A transformer bobbin having integral thermal cutoff device, comprising: a flange adjacent a winding zone; a housing integrally formed with the flange and defining two apertures through which two terminal pins may be disposed; a platform disposed within the housing adjacent the winding zone, the platform configured to receive and interconnect a fusible link and the two terminal pins.

14. The transformer bobbin of claim 13 , wherein the flange comprises an outer flange of the bobbin.

15. The transformer bobbin of claim 13, wherein the flange comprises an inner flange of the bobbin.

16. The thermal cut-off device of claim 12, wherein the flange and platform are integrally formed.

17. The thermal cut-off device of claim 16, wherein the flange and platform are integrally molded from a plastic resin.

18. A bobbin-assembled transformer having an integral thermal cut-off device, the transformer including a bobbin having a flange adjacent a coil winding, the transformer comprising: a housing formed integrally with the bobbin flange, the housing including a rear wall defined by a surface of the bobbin flange, the housing defining two apertures; a platform disposed on the rear wall of the housing adjacent an interior portion of the coil winding; two terminal pins each disposed through one of the two apertures, each terminal pin being disposed adjacent a side of the platform; a fusible link mounted on the platform and connected at each end to the ends of the terminal pins .

19. The transformer bobbin of claim 18, wherein the flange comprises an outer flange of the bobbin.

20. The transformer bobbin of claim 18, wherein the winding coil comprises a primary winding.

21. The transformer bobbin of claim 18, wherein the flange comprises an inner flange of the bobbin.

22. The thermal cut-off device of claim 18, wherein the flange and platform are integrally formed.

23. The thermal cut-off device of claim 22, wherein the flange and platform are integrally molded from a plastic resin.