MXPA98009646A - Resistor protected by fuse that activates termicame - Google Patents

Abstract

The present invention relates to an arrangement or assembly (50) of resistor protected by fuse that is thermally activated, comprising a resistor (58) electrically connected, at one end, to a first terminal (62) of the resistor and at one end opposite a second terminal (64) of the resistor. Uln loop closed solder circuit (66) is provided to make the electrical connection between one end of the resistor and its corresponding resistor terminal. A portion of the solder loop (66) is placed in contact with an electrically insulated portion (74) of the surface of the resistor, which preferably corresponds to the hot point, or of greater heat release, of the resistor, and a medium is provided thermally conductive (76) for thermally and mechanically bonding the solder loop (66) to the electrically insulated portion (74) of the resistor surface. The portion (72) of the solder loop (66) thermally bonded to the resistor (58) can be operated by melting when the temperature of the resistor is increased to within a predefined temperature range, and therefore electrically disconnecting the end of the resistor from its corresponding resistor terminal

Description

RESISTOR PROTECTED BY FUSE THAT IS THERMALLY ACTIVATED FIELD OF THE INVENTION The present invention relates, in general, to techniques for disconnecting an excessively heated resistor, from a set of associated circuits, more specifically to techniques using fuses that are thermally activated.

FIELD OF THE INVENTION Many circuits and electrical systems require the use of a high dissipation resistor to perform various functions such as for example setting the desired voltage and current levels for the associated circuits and / or for diverting electrical energy from another electrical device. An example of the latter use is in the known automotive air conditioning systems that typically use a high dissipation resistor to control the motor speed of a REF blower. : 28936 for air conditioning. In certain operating modes the high dissipation resistor can be used to divert a considerable amount of energy from the blower motor into the incoming air stream. Due to that high energy dissipation, the high dissipation resistor typically operates at temperatures that are approximately in a range of 80 to 150 ° C. In many of the preceding circuits and electrical systems, there are potential failure modes where the high dissipation resistor can become excessively hot due to the large current flow passing through it. That excessive heat can cause thermal damage to surrounding circuits and structures, and possibly result in the generation of fire. To avoid the possibility of dangerous thermal conditions, these high dissipation resistors are typically equipped with a fuse that is thermally activated and is designed to open the resistor when the operating temperature of the resistor increases to a certain temperature range. finido. The designers of systems and electrical circuits have so far devised a variety of approaches to provide electrical components protected by fuses that are thermally activated, particularly they have devised electrical components of the film type, formed on a substrate. One such approach involves the use of a spring-loaded metal bracket, typically connected by a solder, and located between the electrical component and a terminal thereof. When the temperature of the electrical component increases and falls within a predefined temperature range, the solder junction between the component and the bracket is melted, and the spring-loaded bracket is removed from the component to create a condition of open circuit. An example of this approach is shown in U.S. Patent No. 3,638,083 issued to Dornfeld, et al.

Although the preceding approach has been presented successfully, it is inherently unreliable. For example, over time, the temperature cycles in the component, due to normal operation, cause the soldered connections to weaken until the spring-loaded bracket is detached from the component, resulting in an open circuit condition. Another common approach to providing a thermally activated fuse, particularly for use with a film-like electrical component, is shown in Figure 1. Referring to Figure 1, a pair of traces or paths of conductive circuits are formed on one side of a substrate 14, and in accordance with known techniques is formed between them known as thick film electrical component 16, which can be a resistor. A first terminal 18 of the component can be electrically connected to the circuit path 10 and a second terminal 20 of the component to a third conductor circuit 22, formed in a position adjacent to the circuit path 12. A fused element 24 which is activated thermally it is then electrically connected between the circuit paths 12 and 22. As an alternative to the arrangement of Figure 1, Figure 2 shows yet another approach to providing a thermally activated fuse, particularly convenient for use with a film-type electrical component. Referring to Figure 2, a pair of conductor circuit paths 30 and 32 are formed on one side of a substrate 32 and an electrical component of thick film 36 is formed between them. On the opposite side of the substrate 34 is formed. a pair of conductor circuit plots 38 and 40 are formed in alignment with the circuit plots 30 and 32, respectively. A first terminal 42 of the component can be electrically connected to the circuit paths 30 and 38, and a second terminal 44 of the component can be electrically connected to the circuit paths 32 and 40. A fuse element 46 that is thermally activated is then connects electrically between the circuit paths 38 and 40 opposite the electrical component 36. In the thermally activated fuse approaches, shown in Figures 1 and 2, the fuse elements 24 (Figure 1) and 46 (Figure 2) they can typically be meltable wires, connectable connectors that are designed to be detached, or solder paste designed to be able to flow again when the operating temperature of the electrical component increases to a predefined temperature range. However, it is known that in these known fuse structures there are several problems, for example, in the case of a wire that is may melt, the wire may melt and not be removed from the circuit paths 38 and 40 sufficiently to break the electrical connection. In the case of attachable conductive joints, which are typically attached to the circuit plots 38 and 40, via a solder, the solder may melt but the conductive joint may not be removed from the circuit to put the component 36 in open circuit. This problem is compounded if component 36 is not properly oriented. Finally, in the case of the solder paste, that paste tends to lose its liquid component over time, and also due to the temperature cycles, in such a way that it may not melt properly or be removed from the circuit paths 38 and 40 and open circuit the electrical component as desired. Examples of some of the different arrangements of thermal fuses, which are shown and described with respect to Figures 1 and 2, are presented in U.S. Patent No. 4,494,104 issued to Holmes, in U.S. Patent No. 4,533,896 issued to Belopolsky and in U.S. Patent No. 5,084,691 issued to Lester, et al. Another problem associated with each of the known, preceding arrangements of thermal fuses is an inherent lack of accuracy to open the fuse element, and correspondingly open the electrical component, when the operating temperature of the electrical component reaches an excessive temperature range. As shown in Figures 1 and 2, the fuse elements 24 and 46 are located away from the heat generating component. For example, as shown in Figure 1, the fuse element 24 is positioned adjacent the electrical component 16, and as shown in Figure 2, the fuse element 46 and the electrical component 36 are placed in the opposite sides of the substrate 34. In each case, regardless of the type of fuse structure used, the electrical component must heat the entire substrate to an excessive temperature range before the fuse opens. To do so, the operating temperature of the electrical component, typically a resistor, will therefore increase above the temperature at which the fuse opens. This phenomenon is shown in Figure 3 which represents a curve of the temperature 47 of the resistor and the temperature 48 of the fuse, you see the time. As illustrated in Figure 3, if the fuse element is not in intimate contact with the surface of the resistor, the maximum temperature of the resistor, TRfMA ?, is increased to a temperature level above the temperature at which the open the fuse, TF f for a quantity) T before the fuse opens. The foregoing problem related to electrical components protected with fuses that are thermally activated, which is illustrated in Figure 3, can have many undesirable effects. For example, the additional temperature increment T, of the resistor, may be sufficient to cause combustion of the surrounding structures. In addition, excessive heating of the entire substrate can cause damage to unrelated circuits and / or to another structure that is in close proximity to them. Therefore, what is needed is an arrangement or assembly of protected resistor with a fuse that is thermally activated, that reliably puts the heat generating resistor in open circuit when the operating temperature reaches an excessive level. That thermal fuse should ideally be placed in intimate thermal contact with the resistor, so that it opens as soon as the operating temperature of the resistor reaches a predefined temperature range. An optimal location of this thermal fuse would correspond, in fact, to the so-called hot spot of the resistor, a term that is defined in the present as the region of the resistor that generates the maximum heat.

BRIEF DESCRIPTION OF THE INVENTION Many of the disadvantages of what is described in the Background section are addressed by the present invention. In accordance with one aspect of the present invention, a resistor-protected array or assembly with a thermal fuse comprises a resistor having one end thereof electrically connected to a first terminal of the resistor, and a welded loop or circuit, electrically connected an opposite end of the resistor to a second terminal of the resistor. The resistor has an outer surface, and the array includes means for thermally connecting a portion of the solder loop to an electrically isolated portion of the outer surface of the resistor. In accordance with another aspect of the present invention, a method for manufacturing a protected resistor with thermal fuse comprises the steps of providing a resistor having one end thereof electrically connected to a first terminal of the resistor, and having an outer surface, connected electrically to a loop or weld loop, between an opposite end of the resistor and a second terminal of the resistor, and thermally connecting a portion of the weld loop to an electrically isolated portion of the outer surface of the resistor. According to a further aspect of the present invention, a substrate, and a film resistor defined on the substrate, wherein the resistor has one end thereof electrically connected to a first terminal of the resistor and an opposite end electrically connected to a second end. resistor terminal, are combined with a thermally activated fuse array to electrically disconnect the first end of the film resistor from the first terminal, in response to the heat generated by the resistor, within a predefined temperature range. The fuse arrangement comprises an electrical insulation layer in contact with at least a portion of an outer surface of the film resistor, and a fuse that establishes the electrical connection between one end of the film resistor and the first terminal. The fuse further has a portion thereof in thermal contact with a portion of the electrical insulation layer. An object of the present invention is to provide a fuse protected resistor which is thermally activated, wherein the fuse which is thermally activated is placed in intimate thermal contact with a resistor surface. Another aspect of the present invention is to provide a resistor protected by thermal fuse, having the thermally activated fuse placed in thermal contact with the hot spot of the resistor. Yet another aspect of the present invention is to provide a fuse protected resistor that is thermally activated, wherein the fuse which is thermally activated is a loop or welded, soldered, or flux core, attached to a resistor surface. via a thermally conductive epoxy material. These and other objects of the present invention will become apparent from the following description of the preferred modality.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a diagrammatic illustration of a known technique for providing a fuse protected resistor that is thermally activated; Figure 2 is a diagrammatic illustration of another known technique for providing a fuse protected resistor that is thermally activated; Figure 3 is a graph representing the temperature of the resistor compared to the temperature of the thermally activated fuse, for any of the thermally activated fuse protected resistor arrangements of Figures 1 and 2; Figure 4 is a diagrammatic illustration of a preferred embodiment of the thermally activated fuse protected resistor of the present invention; Figure 5 is a cross section of the thermally activated fuse protected resistor of Figure 4, taken along section lines 5-5; Figure 6 is a graph representing the temperature of the thermally activated fuse protected resistor of Figure 4 compared to the temperature of the thermally activated fuse; Y Figure 7 is a diagrammatic illustration of the preferred temperature of multiple resistors protected with fuses that are thermally activated, arranged on a single substrate, in accordance with the present invention.

DESCRIPTION OF THE PREFERRED MODALITY For the purpose of promoting and understanding the principles of the present invention, reference will now be made to the modalities illustrated in the drawings and specific language will be used to describe them. However, it will be understood that this is not intended to limit the scope. of the invention, and as would occur to a person skilled in the art to which the invention relates, alterations and further modifications are envisaged in the illustrated devices as well as additional applications of the principles of the invention as illustrated in FIG. I presented. Referring to Figure 4, a preferred embodiment of the thermally activated fuse-protected resistor 50 is shown in accordance with the present invention. An electrical isolation substrate 52 can be formed from any material known and used in the electronic components industry to print electrical circuit elements thereon, such as for example ceramic alumina. On a surface 53 of the substrate 52 are placed the conductive circuit traces 54, 56 and 60, which may be formed of any known conductive material that is used in the electronic components industry, to provide traces or routes for electrical signals such as example, copper-based compounds and the like. A pair of terminals 62 and 64 of the resistor are joined, according to known techniques, to the circuit paths 60 and 56, respectively. A thick film resistor 58 is placed on the surface 53 of the substrate, preferably by known stenciling or film printing techniques, although the present invention contemplates that other known film deposition techniques can be used to provide the resistor 58. One end of the resistor 58 is electrically connected to the electrically conductive circuit trace 54, and the opposite end of the resistor is electrically connected to the electrically conductive circuit trace 56. Although the thermally activated fuse array of the present invention is shown , and will be discussed in detail below, in cooperative arrangement with a thick film resistor 58, it should be understood that the concepts of the present invention can be used to provide a thermally activated fuse, for other arrangements of known resistors such as for example other film-type resistors and discrete resistors that include electronic chip-type resistors, molded resistors and potentiometers, to name a few. Referring now to Figures 4 and 5, the fuse-protected resistor 50 is preferably provided with an electrical isolation layer 74, on at least a portion of the exposed surface 55 of the resistor. The electrical isolation layer 74 is included to prevent the thermally activated fuse 66, which will be discussed later, from making electrical contact with the active surface 55 of the resistor and causing an electrical short circuit. The layer 74 can therefore cover all the surfaces 55 of the resistor, as shown in Figure 1, or it can cover only one area of the surface 55 of the resistor that could otherwise contact the thermally activated fuse 66. Without However, it should be understood that other types of resistor used with the thermally activated fuse arrangement of the present invention may have an electrical insulation layer that covers the active area of the resistor., such that the layer 74 can be omitted therefrom. Preferably, the electrical insulation layer 74 is a thin layer, and should be formed of a material capable of forming a substantial contact with the surface 55 of the resistor and having a high thermal conductivity so as to be able to efficiently conduct the heat generated by the resistor 58 , through it. The electrical insulation layer 74 is preferably formed of glass (Si02), although the present invention contemplates forming the layer 74 of other known electrical insulation materials having good thermal conductivity, such as for example silicon nitride (SÍ3N4) , polyimide, and coatings known to have good or enhanced thermal conductivity. A thermally activated fuse 66 is electrically connected at one end thereof, to a circuit path 54, and at an end opposite a circuit path 70, and at least a portion 72 therebetween is in contact with the layer. 74. The fuse 6 6 is formed, in a preferred embodiment, of a closed loop or solder circuit having a melting point which is within a first predefined temperature range, which is electrically connected to the circuit paths 54 and 60 via the solder connections 68 and 70, respectively, wherein the solder connections 68 and 70 are formed of a solder having a melting point that is within a second predefined temperature range which is slightly smaller than that of the solder loop 66. Those skilled in the art will recognize, however, that the fuse 66 can be formed of any suitable material having a point d e fusion that is within the first temperature range. In any case, when the temperature of the resistor 58 is increased, in response to the current flowing through it, to a temperature within the first predefined temperature range, the fuse 66 can be operated to melt and thereby electrically disconnect circuit stroke 54 from circuit trace 60. Resistor 50 protected with thermal fuses further includes a thermally conductive means 76 formed in contact with a portion of fuse 6 6 and electrical insulation layer 74. The medium The thermal conductor 76 therefore connects the fuse 66 with a portion of the surface 55 of the resistor, thermally, but not electrically. Preferably, the thermally conductive medium 76 is formed of a known thermally conductive epoxy material, although the present invention contemplates providing the medium 76 as any coating or bonding means having good thermal conductivity or having an enhanced thermal conductivity, according to known techniques. It has been determined, through experimentation, that the thermally conductive epoxy material 76 tends to wet the surfaces of the electrical insulation layer 74 and the solder loop 66, thereby producing a fillet or edge of thermally conductive material between them. . In operation, a portion 72 of the solder loop 66 melts when the temperature of the resistor 58 rises and falls within the first defined temperature range, thereby electrically disconnecting the circuit stroke 54 from the circuit path 60 and setting circuit open to resistor 58 between terminals 62 and 64 of the resistor. Since the portion 72 of the solder loop 66 is enclosed within the thermally conductive medium 76, the molten solder is removed within the medium 76 to a cooler area of the resistor 58, leaving behind a vacuum, or free space, within the medium 76. Preferably the solder loop 66 has a core or flux core 65 which promotes melting of the solders at the appropriate temperature. In addition, the resulting vacuum left by the flux core 65 during melting of the portion 72 of the weld loop 66 creates a significant contraction of the remaining weld metal., which facilitates the breaking of the electrical connection between the circuit paths 54 and 60. It has been determined, through experimentation, that using a solder loop 66 having a flux core 65 results in an empty space between the portions of the melted solder loop, of at least about 0.25 cm (0.1 inches). In applications of an automotive air conditioning system, it is desired that the resistor 58 have a maximum operating temperature of about 220 ° C, which, as has been found, is below the temperature at which they are burned, Under typical conditions, the waste found in air conditioning systems. In those systems, the solder loop 66 is preferably comprised of 95% Tin and 5% Silver, which has a melting point that is within a small temperature range of about 220 ° C. It should be understood, however, that the present invention contemplates that different compositions of the solder loop 66 may be used to vary the range of the melting temperature. For example, commonly available solders have melting points ranging from about 180 to 250 ° C, and other materials can be added thereto to extend this range, which is known in the art. Furthermore, it is preferable that the portion 72 of the solder loop 6 6 is located on the "hot spot" of the resistor 58, which is defined as a region of the surface 55 of the resistor 58 having the maximum operating temperature, if compares with other regions of the surface 55 of the resistor 58. That location of the portion 72 of the solder loop 66 promotes a highly accurate "detection" of the highest operating temperature of the resistor 58. In operation, when the hottest portion of the surface 55 of the resistor 58 reaches an excessive temperature range, the portion 72 of the solder loop 6 6 responds by melting, as discussed above, and therefore puts the resistor 58 open between the terminals 62 and 64 of the resistor. This phenomenon of accurate temperature detection is shown in Figure 6 which shows a curve of the operating temperature 68 of the resistor 58 during an opening event of a thermally activated fuse. In contrast to Figure 3, which shows a similar illustration for known arrangements of resistors protected by thermal fuses, it can be seen, from Figure 6, that the maximum operating temperature, TR, MAX, of the resistor 58 is approximately same temperature, TF, at which the solder loop 66 is opened. The thermally activated fuse array of the present invention therefore minimizes any temperature disparity between the maximum operating temperature of the resistor 58 and the temperature at which the it opens the thermally activated fuse 66, thereby providing a thermal fuse arrangement that has an exact operation with respect to temperature. Referring now to Figure 7, a multiple resistor mode of an array 80 of resistors protected with fuses that are thermally activated is shown in accordance with the present invention. The arrangement or assembly 80 of protected resistor with thermal fuse, includes a substrate 80 on which are formed a number of thick film resistors 84, 86 and 88, in electrical contact with the circuit paths 90, 92, 94, 96 , 98 and 100, respectively. A number of resistor terminals or circuit paths are electrically connected to the circuit paths 90-100 as is known in the art. For example, the terminal 102 of the resistor is connected to the circuit path 90, the terminal 104 of the resistor is connected to the circuit path 94, the terminal 106 of the resistor is connected to the circuit path 98., and resistor terminal 108 is connected to circuit trace 100. Resistors 84-88 are electrically connected in a series arrangement between terminals 102 and 108 of the resistor, with each individual resistor having a pair of terminals of the resistor. resistor extending therefrom, although it should be understood that a number of resistors may also be electrically connected in parallel or in any combination in e / ee / par to elo, as is known in the art. The electrical connections in series between the resistors 84-88 are made using the thermally activated fuse arrangement 66 and thermally conductive medium 76, described above. Preferably, as shown in Figure 7, an electrical insulation layer 74 is formed on all resistors 84-88, although layer 74 can be selectively formed on each resistor 84-88 as previously described. The thermally activated fuses 76 can be operated as previously described to open-circuit the corresponding resistor when the resistor operating temperature rises to within a predefined temperature range. The present invention is illustrated and described in detail in the drawings and the preceding description, it should be considered that it is illustrative and not restrictive, it being understood that only preferred modalities have been presented and described AND that it is desired to protect all changes and modifications that are within the spirit of the invention. It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention. Having described the invention as above, property is claimed as contained in the following:

Claims (21)

1. An arrangement or assembly of resistor protected by fuse that is thermally activated, characterized in that it comprises: a resistor having one end thereof electrically connected to a first terminal of the resistor, and having an outer surface; a loop or welded loop electrically connecting an opposite end of the resistor to a second terminal of the resistor; and means for thermally connecting a portion of the solder loop to an electrically isolated portion of the outer surface of the resistor.

2. The thermally activated fuse-protected resistor according to claim 1, characterized in that the electrically insulated portion of the outer surface of the resistor corresponds to a region of the resistor that generates the maximum amount of heat in response to the current flow that passes By himself.

3. The thermally activated fuse-protected resistor according to claim 1, characterized in that the solder loop includes a core or flux core.

4. The fuse-protected resistor is thermally activated, in accordance with the indication 3, characterized in that the solder loop has a melting point which is within the range of approximately 180 ° C to 250 ° C.

5. The thermally activated fuse-protected resistor according to claim 1, characterized in that the means for thermally connecting the portion of the solder loop to the electrically isolated portion of the outer surface of the resistor is a thermally conductive epoxy material.

6. The thermally activated fuse-protected resistor according to claim 1, characterized in that it includes a means for electrically isolating at least a portion of the outer surface of the resistor.

7. The thermally activated fuse-protected resistor according to claim 6, characterized in that the means for electrically isolating at least a portion of the outer surface of the resistor is an electrically insulating material having a high thermal conductivity.

8. The resistor protected by a fuse that is thermally activated, in accordance with the indication 7, characterized in that the electrically insulating material is glass.

9. The thermally activated fuse-protected resistor according to claim 1, characterized in that it includes a means for electrically connecting the solder loop to the opposite end of the resistor and to the second terminal of the resistor.

10. The fuse-protected resistor which is thermally activated, in accordance with the indication 9, characterized in that the means for electrically connecting the solder loop to the opposite end of the resistor and to the second terminal of the resistor includes a solder having a slightly melting point. less than that of the solder loop.

11. The resistor protected by fuse that is thermally activated, according to claim 1, characterized in that the resistor is a film resistor.

12. A method for manufacturing a fuse protected resistor that is thermally activated, and the method is characterized in that it comprises the steps of: providing a resistor having one end thereof electrically connected to a first terminal of the resistor, and having an outer surface; electrically connecting a loop or welded loop between an opposite end of the resistor and a second terminal of the resistor; and thermally connecting a portion of the solder loop to an electrically isolated portion of the outer surface of the resistor.

13. The method according to claim 12, characterized in that the portion of the solder loop is thermally connected to a portion of the outer surface of the resistor, which corresponds to a region of the resistor that generates a maximum amount of heat in response to current flow. through it.

14. The method according to the indication 12, characterized in that the resistor is a film resistor; and because the method also includes the next step or step, before performing the step of the thermal connection: forming an electrical insulation layer that is in contact with at least a portion of the outer surface of the resistor

15. The method according to claim 14, characterized in that the passage of the thermal connection includes joining the portion of the solder loop to a portion of the electrical insulation layer, through a thermally conductive epoxy material.

16. The method according to claim 12, characterized in that the solder loop is electrically connected to the opposite end of the resistor and to the second terminal of the resistor, through a solder having a melting point slightly less than that of the solder loop.

17. The combination characterized in that it comprises: a substrate; a film resistor defined on the substrate, and having one end thereof electrically connected to a first terminal of the resistor and an opposite end electrically connected to a second terminal of the resistor; and an arrangement or fuse assembly that is thermally activated, to electrically disconnect the first end of the film resistor, from the first terminal, in response to the heat generated by the resistor, within a predefined temperature range, the arrangement or fuse assembly comprises: an electrical insulation layer that is in contact with at least a portion of an exterior surface of the film resistor; and a fuse that establishes the electrical connection between the first end and the film resistor and the first terminal, and that has a portion thereof in thermal contact with a portion of the electrical insulation layer.

18. The combination in accordance with the rei indication 17, characterized in that it also includes a means for thermally connecting the portion of the fuse to the portion of the electrical insulation layer.

19. The combination according to claim 18, characterized in that the means for thermally connecting the fuse portion to the portion of the electrical insulation layer is a thermally conductive epoxy material.

20. The combination in accordance with the indication 19, characterized in that the fuse is a loop or closed circuit of sue Ida.

21. The combination in accordance with the indication rei 20, characterized in that the loop or closed circuit of solder has a core or flux core. SUMMARY OF THE INVENTION The present invention relates to an arrangement or assembly (50) of resistor protected by fuse that is thermally activated, comprising a resistor (58) electrically connected, at one end, to a first terminal (62) of the resistor and at one end opposite a second terminal (64) of the resistor. A loop or weld loop (66) is provided to make the electrical connection between one end of the resistor and its corresponding resistor terminal. A portion of the solder loop (66) is placed in contact with an electrically isolated portion (74) of the surface of the resistor, which preferably corresponds to the hot point, or of the greatest heat release, of the resistor, and a medium is provided. thermally conductive (76) for thermally and mechanically bonding the solder loop (66) to the electrically insulated portion (74) of the resistor surface. The portion (72) of the solder loop (66) thermally bonded to the resistor (58) can be operated by melting when the temperature of the resistor is increased to within a predefined temperature range, and therefore electrically disconnecting the resistor end of the resistor. its corresponding terminal of the resistor.