Abstract
In order to provide a method for isolating a circuit and a thermal link, wherein the link has a very low resistance and is suitable for high currents, in particular very high short load currents, and also has a high degree of reliability, in particular under difficult conditions, such as thermal and mechanical loading which lasts for a relatively long time, for example, the invention proposes that, during the phase transition of the material of the fusible element (10) from the solid to the liquid state, the volume of the fusible element (10) increases and the pressure increases and, owing to the increase in volume and the increase in pressure, the fusible element (10) is dislodged so as to break the electrical connection.
Claims
1. A thermal safety device, which executes the disconnection of an electrical circuit by the melting of a fusible element, the thermal safety device comprising: at least two electrically conductive terminals; a fusible element located in a gap between the at least two electrically conductive terminals creating an electrical circuit therebetween; an encasement spanning the gap between the terminals, a coating located between the encasement and at least one of the terminals and being in direct contact with the at least one of the terminals on one side and with the encasement on the opposite side, the coating liquefying at a temperature in the range of a melting temperature of the fusible element; and wherein the melting temperature of the coating is equal or higher than the melting temperature of the fusible element; wherein the fusible element is completely encapsulated by at least the encasement, the coating and the terminals when the coating is in a solid state; wherein when the coating is in the solid state, the coating blocks the melted fusible element such that the melted fusible element cannot flow out of the thermal safety device; and wherein when the coating liquefies, a capillary is formed in a space previously occupied by the coating in the solid state which allows the melted fusible element to flow from the gap between the terminals and out from the thermal safety device to disconnect the electrical circuit.
2. The thermal safety device in accordance with claim 1, characterised in that, the fusible element is in direct contact with the terminals and the encasement.
3. The thermal safety device in accordance with claims 1, characterised in that, the encasement has a layer of lacquer on the inner face towards the fusible element.
4. The thermal safety device in accordance with claim 1, characterised in that, the thermal safety device has a flux.
5. The thermal safety device in accordance with claim 1, characterised in that, the fusible element is located between the two terminals.
6. The thermal safety device in accordance with claim 1, characterised in that, the coating between the terminals and the encasement contains tin, indium, bismuth, or an alloy of tin, indium, or bismuth.
7. The thermal safety device in accordance with claims 1, characterised in that, the coating between the terminals and the encasement has a thickness of between 1 m and 50 m.
8. The thermal safety device in accordance with claim 1, characterised in that, the fusible element comprises a low melting point metal, an alloy containing a low melting point metal, or a lead solder.
9. The thermal safety device in accordance with claim 1, characterised in that, the fusible element comprises a tin-silver alloy.
10. The thermal safety device in accordance with claim 1, characterised in that, the terminals have the form of caps.
11. The thermal safety device in accordance with claim 1, characterised in that, the terminals have the form of a cuboid, or a form similar to that of a cuboid.
12. The thermal safety device in accordance with claim 1, characterised in that, the thermal safety device has at least one electrically non-conductive body, wherein the said at least one electrically non-conductive body serves to hold the terminals.
13. The thermal safety device in accordance with claim 12, characterised in that, the at least one electrically non-conductive body comprises ceramic, glass, plastic, or another organic material.
14. The thermal safety device in accordance with claim 1, characterised in that, the fusible element has the form of a ring.
15. The thermal safety device in accordance with claim 1, characterised in that, an electrical conductor is connected to each of the terminals.
16. The thermal safety device in accordance with claim 15, characterised in that, the electrical conductor has the form of a wire, or a form that is similar to that of a wire.
17. The thermal safety device in accordance with claim 1, characterised in that, the thermal safety device has a lacquer covering, or a lacquer encasement.
18. An application of a thermal safety device in accordance with claim 1 as a fusible safety device, for purposes of protecting solar cells, high energy battery cells, ancillary heating systems, electrical loads, in particular in vehicles, and also for purposes of protection from excess temperature, and fire protection.
Description
BRIEF DESCRIPTION OF THE FIGURES
(1) The invention is now elucidated in an exemplary manner with reference to the accompanying drawings with the aid of preferred forms of embodiment. In a purely schematic representation:
(2) FIG. 1 shows a schematic representation of the inventive thermal safety device (100),
(3) FIG. 2 shows a schematic representation of the inventive thermal safety device (200),
(4) FIG. 3 shows a schematic representation of the switching principle of the inventive thermal safety device (100, 200, 300) before it is activated,
(5) FIG. 4 shows a schematic representation of the switching principle of the inventive thermal safety device (100, 200, 300) on attainment of the melting temperature,
(6) FIG. 5 shows a schematic representation of the switching principle of the inventive thermal safety device (100, 200, 300) after the activation operation,
(7) FIG. 6 shows a schematic representation of the inventive thermal safety device (300), and
(8) FIG. 7 shows a further schematic representation of the inventive thermal safety device (300).
PREFERRED FORM OF EMBODIMENT OF THE INVENTION
(9) FIG. 1 shows a schematic representation of an inventive thermal safety device 100. The inventive thermal safety device 100 consists of two caps 11 and 12 with a centrally connected wire 14 and 15, a ceramic body 13, and also a fusible element 10. In order to ensure a very good electrical conductivity the two caps 11, 12 consist of copper. Alternatively the caps 11, 12 can also consist of another material with a low specific resistance. The caps 11, 12 and the wires 14, 15 are covered with a coating 23, preferably of a layer of tin. The coating could also contain another material, e.g. indium, bismuth, or silver, or an alloy consisting of tin, indium, bismuth or silver. A fusible element 10 is arranged between the two caps 11, 12; this is held by means of a ceramic body 13. The fusible element 10 has the form of a ring, and consists of a tin-silver alloy (e.g. Sn97 Ag3, with a melting point of 217 C.). The alloy could also have another composition with a lower or a higher melting point depending upon the activation temperature required for the safety device. On the fusible element 10 is located a flux 16 with long-term stability, which during the activation of the safety device serves to activate the surface and to reduce the surface tension. The encapsulation or encasement of the safety device, here consisting of a lacquer 17 that can be UV-hardened, and a moulding material 18 manufactured on the basis of an epoxy resin, serves to increase the mechanical stability of the safety device. Moreover the encapsulation or encasement 17, 18 offers both mechanical and oxidation protection. The encasement 18 only encloses the thermal safety device in certain regions. In particular the encasement 18 encloses the thermal safety device in the region in which the fusible element 10 is arranged. The ends of the caps 11, 12, in particular in the region of the terminal connection points, e.g. for the wires 14, 15, are hereby not enclosed by the encasement 18.
(10) FIG. 2 shows a schematic representation of an inventive thermal safety device 200. The thermal safety device 200 consists essentially of the components of the thermal safety device 100 described in FIG. 1. A significant difference from the structure described in FIG. 1 is reflected in the fact that the thermal safety device 200 in FIG. 2 does not have any application of flux on the fusible element 10.
(11) FIGS. 3 to 5 show schematic representations of the switching principle of the inventive thermal safety device 100, 200, 300 before attainment of the melting temperature, on attainment of the melting temperature, and also after attainment of the melting temperature.
(12) FIG. 3 shows the state before the activation of the inventive thermal safety device 100, 200, 300, i.e. before attainment of the melting temperature. Before attainment of the melting temperature the fusible element 10 is located in a solid state in the gap 24 between the terminals 11, 12 with the coating 23 and the encapsulation or encasement 18. For the activation of the thermal safety device 100, 200, 300 the pressure gradient as a result of a volume increase on the one hand, and also a step change in volume during the transition from the solid into the fluid phase, is of particular significance, as is the capillary action.
(13) FIG. 4 shows the state of the inventive thermal safety device 100, 200, 300 on attainment of the melting temperature. On attainment of the melting temperature the fusible element 10 starts to melt. As the fusible element melts the coating 23 in the region of the encapsulation or encasement also melts, as a result of which the fusible element 10 and coating 23 mix together at least partially. The displacement into and through the capillary is essentially caused by the pressure rise during the phase change of the fusible element 10 from a solid to a fluid, and the step change in volume that accompanies this. FIGS. 4 and 5 show the migration of the fusible element 10 as it melts and after the activation. To visualise the process more clearly the flow direction 22 of the fusible element during migration is shown in FIG. 4. Here it should be noted that the fusible element 10 migrates completely out of the gap 24.
(14) FIG. 5 shows the switched state of the thermal safety device 100, 200, 300 after the activation operation and the complete migration of the fusible element 10 out of the gap 24. After the activation operation is complete the coating 23 that is mixed together with the fusible element solidifies and deposits itself on the terminals, i.e. in the original location of the coating 23 before attainment of the melting temperature. After completion of the activation operation and the outflow of the fusible element 10 the current flow through the thermal safety device 100, 200, 300 is interrupted by the interruption at the gap between the two terminals 11, 12 or base bodies 19.
(15) FIGS. 6 and 7 show schematic representations of an inventive thermal safety device 300. The inventive thermal safety device 300 is designed as a flat safety device for surface mounting. The inventive thermal safety device 300 includes two cuboid terminals 19 spaced apart from one another, which are applied on a non-conductive body 13, e.g. a ceramic body. In order to ensure a very good electrical conductivity the two base bodies 19 (terminals) consist of copper, or another material with a low specific resistance. The two base bodies 19 (terminals) are covered with a coating 23, preferably as a layer of tin. The coating could also contain another material, e.g. indium, bismuth, silver, or an alloy consisting of tin, indium, bismuth or silver. Furthermore the thermal safety device 300 has a fusible element 10 between the two base bodies 19 (terminals) and also in the region around the buffer space (gap 24) between the two base bodies 19 (terminals). As shown in FIG. 6, the thermal safety device 300 has two fusible elements 10. The safety device could however also have one, or more than two, fusible elements 10. On the fusible element 10 is located a flux 16 with long-term stability, which during the activation of the safety device serves to activate the surface and to reduce the surface tension. An additional layer of lacquer 17 is located between the encapsulation or encasement 18 of the safety device and the flux. The encapsulation or encasement 18 can only be applied on the upper face of the thermal safety device. The encapsulation or encasement 18 and also the additional paint layer 17 serve to increase the stability of the safety device and also its oxidation protection. The layer of lacquer 17 is in direct contact with the flux 16 without leaving free any buffer space. The thermal safety device 300 could also be designed such that it has no flux 16 on the fusible element 10. In this case the layer of lacquer 17, or, in the event that no additional layer of lacquer 17 is present, the encapsulation 18, would be in direct contact with the fusible element 10 without leaving free any buffer volume.
REFERENCE SYMBOLS
(16) 100 Thermal safety device 200 Thermal safety device 300 Thermal safety device 10 Fusible element 11, 12 Terminals/caps 13 Electrically non-conductive body 14, 15 Wire 16 Flux 17 Lacquer covering/lacquer encasement 18 Encasement/encapsulation 19 Base body 22 Flow direction 23 Coating/layer of tin 23 Coating (melted) 23 Coating/(solidified layer of tin with melted solder material) 24 Gap
