Abstract:
The invention provides a method for producing a connection grid with integrated fuse includes the steps of forming a concave portion that is suitable for forming a cradle for receiving one of the longitudinal ends of a fuse on each of the arms of a pair of arms of a connection grid in order to fix the fuse to the grid via these longitudinal ends, and molding a support formed by molding plastic onto the connection grid after the concave portions of the arms have been produced, wherein the support has a housing that is suitable for receiving a portion of the fuse in a space formed between the at least one pair of arms.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional application of U.S. patent application Ser. No. 11/066,914, filed Feb. 25, 2005 now U.S. Pat. No. 7,453,345, which claims priority under 35 U.S.C. §119 to French Application No. 0402037, filed Feb. 27, 2004. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to fuses and the use thereof on connection grids that are intended, for example, to be used in protective switch boxes for motor vehicles. 
     BACKGROUND OF THE INVENTION 
     As is known, a connection grid is a cut-out metal grid that forms the pins of specific component cases and the extension thereof molded into the insulating case, at the end of which a component connection is welded. 
     Fuses that are suitable for use on connection grids are known, as are methods for connecting these fuses to these connection grids so as to form connection grids with an integrated fuse or fuses. 
     U.S. Pat. No. 5,011,067 describes, for example, a connection grid comprising a series of projections, each having at its end two strips that are intended to be connected by a fuse. 
     Strands made of a fusible material are placed transversely to the strips on the connection grid. Each strand comprises two elements that are capable of forming an alloy if sufficient energy is supplied to the strand. 
     Each strand is then welded to the strips. For this purpose, in the region of each projection, the ends of an energy source are brought into contact at specific points with the fusible material strand to be combined. The energy provided by the energy source allows the elements of the strand to combine, forming a fuse connecting the two strips. 
     Known fuses integrated into a connection grid require fairly precise and complex assembly, and hence considerable production times resulting in high production costs. 
     Moreover, these fuses are subjected not only to the current that passes through them, but also to all of the stresses that are exerted in the connection grid and transmitted to them. 
     A problem of performance is therefore posed, since these fuses are liable to break, not only under the effect of the current that passes through them, but also under the effect of these stresses. 
     Moreover, the surfaces of contact between these fuses and the ends of the connection grids on which they are mounted are arbitrary, which means that the individual performances are difficult to repeat and limit the latitude available to designers. 
     SUMMARY 
     The present invention relates in general to a connection grid allowing a fuse to be fixed to the grid more easily and more quickly, and also resulting in other advantages. 
     In an exemplary embodiment, the present invention provides a connection grid, comprising at least a pair of arms for fixing a fuse to the grid via its longitudinal ends. Each of the arms comprises a concave portion forming a cradle for receiving one of the longitudinal ends of the fuse. 
     This results in simpler installation, with the fuse on the correct part of the connection grid, and also in a substantial contact zone allowing optimal connection by welding. 
     The invention also relates to a method for producing a connection grid with an integrated fuse, characterized in that it comprises a step of forming, on each of the arms of a pair of arms of a connection grid, a concave portion that is suitable as a cradle for receiving one of the longitudinal ends of a fuse, in order to fix the fuse to the grid via these ends. 
     The invention also relates to a device for implementing this method that comprises means for supporting the connection grid, a laser head and a nozzle for supplying fuse wire, which are preferably mechanically connected to a system of robotized shafts. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further characteristics and advantages of the invention will become apparent in the light of the following description, given by way of a non-limiting example, of a preferred embodiment. The description refers to the accompanying drawings, in which: 
         FIG. 1  is a perspective view of a connection grid according to an exemplary embodiment of the invention; 
         FIG. 2  is a larger-scale perspective view of one end of the grid illustrated in  FIG. 1 , with a support molded onto this grid; 
         FIG. 3  is a perspective view of the assembly illustrated in  FIG. 2 , after the addition of a fuse; 
         FIG. 4  is a perspective view of the assembly illustrated in  FIG. 3 , covered with a hot-melt adhesive; and 
         FIG. 5 to 12  are views illustrating the installation and the welding of a fuse onto a grid molded, in order to obtain the assembly of  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As illustrated in  FIG. 1 , a metal connection grid  1  according to an exemplary embodiment of the invention comprises three pairs of arms  2 , each intended to receive a fuse in the form of a wire made of a fusible material  3  ( FIG. 3 ), and arranged one behind another along one side of the connection grid  1 . 
     In the embodiment described here, the arms  2  are in the form of strips of material and are integral with the rest of the connection grid  1 , forming a single piece with the rest of the grid by means of the cutting-out of a metal plate. 
     The arms  2  of each pair extend parallel to each other and protrude from the rest of the grid  1  into openings  4  formed in this grid  1 . They form a plane of symmetry M 1 , cutting the fuse transversely through its middle and indicated by projected axes in  FIG. 3 . 
     As is most apparent from  FIG. 2 , each arm  2  comprises a free end  21 , a resilient portion  22  and an attachment portion  23  ( FIG. 1 ). 
     The free end  21  is concave, being in the shape of a U, the hollow of which is suitable for receiving the fuse  3  and for holding it easily in the correct place. 
     This free end  21  exhibits a curvature that is suitable for providing a maximum contact surface for effective welding with the fuse  3 , in particular without having a detrimental effect on the shape of the fuse  3 . 
     The free end  21  is extended by the resilient portion  22 . This portion  22  is in the overall shape of an inverted U, forming with the U-shaped end an S-shape, and its dimensions are such as to allow it resiliently to absorb axial stresses. It is substantially less wide than the free end  21 . The arm  2  is thus capable of withstanding axial stresses, as indicated by the double-headed arrow F, when the fuse  3  is subjected to stresses resulting, for example, from the effects of heat on a support  5  of the fuse  3 , which will be described in greater detail below. 
     The resilient portion  22  is extended by the attachment portion  23 , the dimensions of which are such as to give the fuse  3  the desired strength. It extends in the same plane as that of the rest of the connection grid  1 , whereas the free end  21  and the resilient portion  22  partially protrude therefrom vertically. 
     As is particularly apparent from  FIG. 3 , a support  5  made of plastic material is molded onto the connection grid  1 . 
     This support  5  comprises, in the zone of each pair of arms  2 , two end portions  51  and  52 , which extend transversely beyond the arms  2  on either side of the corresponding free ends  21  and resilient portions  22 , and respectively cover a part of the opening  4  and the arms  23 . They are formed in a single piece with a central portion  53 , which acts as a support for the fuse  3  ( FIG. 3 ) between the two corresponding arms  2 . 
     The central portion  53  is in the overall shape of a hollow half-cylinder formed by two ribs  54  for holding the fuse  3  in place, which ribs project vertically from the end portions  51  and  52 , and a curved base  56 . These ribs  54  are slightly thicker than the free ends  21  of the arms  2 , and have upper surfaces that are turned down on the inner side of the half-cylinder by chamfers  55 . 
     A fuse  3  placed on the upper part of one of the ribs  54  thus tends to fall inside the central portion  53 , onto the base  56  thereof. 
     The base  56  comprises at its center a well  57 , parallelepiped in shape and aligned along the axis of the half-cylinder. This recess allows a small amount of material, such as a portion of fusible material melted after shorting of the fuse  3  or a material for preventing the formation of electric arcs during this shorting, to be accommodated. 
     The two lateral openings of the half-cylinder  58 , which are located at the axial ends of the central portion  53 , are each provided opposite the hollow of one of the U-shaped free ends  21  of the arms  2 . 
     Moreover, these lateral openings expand outwardly inside the central portion  53 , in the manner of a diffuser. For this purpose, broadening sections  59 , which increase in width toward the outside of the central portion  53 , are cut into the ribs  54 . Thus, when the fuse  3  dilates, it is not subjected to any stress and may spread into the space freed by the lateral openings  58  and the broadening sections  59  thereof. 
     In practice, the support  5  is molded onto the connection grid  1  so as to ensure that the central portion  53  is placed opposite the hollow forming each free end  21 , as a result of which this central portion  53  forms for the fuse  3  a cradle that is complementary to that formed by the free ends  21  of the arms  2 . 
     As is most apparent from  FIG. 3 , the plastics material of the support  5  penetrates the slots  60  in the connection grid  1  in the immediate vicinity of the fuse, promoting optimal adjustment of the expansion coefficient of the grid  1 /support  5  sandwich to that of the fuse  3 , and reduction of the effects of differential heat stresses thereon. 
     This sandwich is also constructed so as to limit the mechanical effects of the other components fixed to the connection grid  1  on the fuse  3 . 
     In practice, the fuse  3  is therefore partially decoupled from the part of the connection grid  1  that holds the other components. 
     In practice, creating a metal and plastics material sandwich allows the real expansion coefficient thereof to be adjusted to that of the fuse  3 , by defining the dimensions of each of the layers of this sandwich. 
     It will also be noted ( FIGS. 3 and 4 ) that the support  5  is fitted to the bus bar  61  of the connection grid  1 , which, in practice, holds the arms  2 , and in which, in particular, the openings  4  and the slots  60  are formed. 
     As is apparent from  FIG. 4 , once the fuse  3  is in place, it is covered with a hot-melt adhesive  6 . In practice, when the fuse melts (shorting), the portion of melted fuse may become lodged in the well  57  and the adhesive  6 , which melted at the same time as the portion of melted fuse, takes the place thereof. 
     This adhesive  6  therefore not only allows the fuse  3  to be held in place between the ribs  54 , but also ensures that no electric arcs may reach the elements surrounding the fuse when it melts. 
     Moreover, this adhesive  6  advantageously absorbs any vibration energy and protects the fuse  3  against any ionizable material by filling the gaps surrounding the fuse  3 . 
     The fuse  3  is integrated into a connection grid  1  in the following manner. 
     The first step consists in obtaining a connection grid  1  comprising arms  2 , as described above and produced by a forming process. The connection grid  1  is, in this case, provided with these arms during production. In a variant, the arms are produced independently, then welded to the grid. 
     The second step consists in molding the support  5  onto the grid  2 . This step involves the selection of a suitable material for the support and also the placement thereof relative to the arms such that the central portion, as described above, is located opposite the hollows of the arms  2 . 
     The third step consists in placing and welding the fuse  3  using an entirely automatic method, which will now be described with reference to  FIG. 5 to 12 . 
     This method utilizes a nozzle for supplying fusible wire and a laser head for welding, both of which are mechanically connected to a system of automated shafts. Moreover, the desired fusible wire length is controlled by means of a step motor and drive roller, and the molded circuit  1 ,  5  of  FIG. 2  is positioned on a support (not illustrated in the figures), which ensures its position relative to the laser head/nozzle for supplying fusible wire pair. 
     In an exemplary embodiment, the alloy used for the fusible wire is a lead-free alloy, in this case of the SnCu or SnAgCu type. These alloys exhibit, in particular, good hot flow properties. 
     More generally, any alloy with a low melting point may be used that allows a cold fuse to be obtained, i.e. a fuse of which the melting temperature is sufficiently low that, in the case of slow shorting, the fuse is able to break the circuit before the temperature of the assembly comprising all of the components rises to the extent that it results in the deterioration of these same components. 
     It will be noted in this regard, that slow shorting is characterized by an intermittent short-circuit with insufficient shorting energy for each short-circuit. As a result, the fuse is not shorted. 
     The corresponding shorting energy is dissipated toward the assembly of the components and the temperature of the entire system increases until the melting temperature of the fusible alloy is reached. The fuse is then shorted and the circuit turned off or opened. 
     It is therefore important to select the melting temperature of the fuse carefully. In practice, this melting point is, in this case, selected so as to be less than or equal to 250° C., preferably less than or equal to 232° C. The aforementioned alloys are suitable for temperatures of this type. 
     In practice, the lower the melting temperature, the more the fuse is what is known as “cold” and the more effectively it protects the other components in the event of slow shorting, but the less effective are its mechanical properties. This is why the expansion of the metal  1 /plastics material  5  sandwich must also be controlled and the fusible entity  3  decoupled from the rest of the product. 
     Before the head and the nozzle are positioned above the circuit, the welding zones (U-shaped concave portions of the circuit  1 ) are subjected to fluxing and the nozzle for supplying fusible wire is then preheated. 
     As is apparent from  FIG. 5 , the laser head  100  and the preheated nozzle  200  for supplying fusible wire are positioned above the circuit of  FIG. 2 , illustrated schematically in  FIG. 5  by a cross-section of two U-shaped concave portions  21  that are intended to receive a fuse  3 , and of two parts that are adjacent to the bus bar  61  delimiting the opening  4  in which these two concave portions  21  extend. 
     As is apparent from  FIG. 5 , fusible wire  300 , which is intended to form the fuse  3 , is then advanced above the connection grid or circuit  1  using the supply nozzle  200 , and the head  100 /nozzle  200  pair is then lowered in order to place the aforementioned wire  300  into its housings, i.e. the U-shaped concave portions  21  ( FIG. 6 ). 
     It will be noted, in this regard, that the wire  300  projects beyond each side of the concave portions  21 : 
     on the left side, in order to have a reserve of fusible material, which will fill the welding zone by capillary action during the fusion. This justifies the projection  62  into the bus bar  61  on the side thereof that is adjacent to the portion  21 , which firstly receives the fusible wire  300 , and this prevents thermal contact between the wire and the circuit. If thermal contact did take place, the welding energy would emanate by conduction into the circuit and the quality of the weld would be unstable; and 
     on the right side, by virtue of the method used. 
     Laser welding is then carried out ( FIG. 7 ), using the laser head  100 , in the U-shaped concave portion  21 , having firstly received some fusible wire  300 . The use of the laser causes fusion of the material to be welded and the soldering wire fills the mould by means of capillary action. 
     The laser is then cut and the laser head  100 /supply nozzle  200  pair displaced, with simultaneous unwinding of the fusible wire  300  beyond the inner part of the bus bar  61  (arrow G in  FIG. 8 ). 
     The laser welding of the second point (second concave portion  21  located on the inner side of the connection grid  1  relative to the concave portion  21 , having firstly received some fusible wire  300 ) is then carried out. 
     During the welding process, once the fusion of the alloy has been attained, there is also a controlled complementary supply of fusible wire  300  (arrow H in  FIG. 9 ) in order to fill the welding zone. 
     The fusible wire  300  is then extracted (arrow I in  FIG. 10 ) by withdrawing the supply nozzle  200 , in order to separate the fusible wire  300  from the welding zone. It will be noted, in this regard, that the laser is still activated. 
     The laser shot is then stopped and the laser head  100 /supply nozzle  200  pair raised (arrow J in  FIG. 11 ). 
     Finally, the fusible wire  300  is withdrawn into the supply nozzle  200  (arrow K in  FIG. 12 ), while the fusible wire is hot, to straighten the bend that formed previously in the fusible wire  300 , in order to produce the fuse. It will be understood that the use of a laser source allows optimal control of the welding zone that is thermally affected, by modifying as little as possible the metallurgic structure of the fuse in each of the two welding zones that are thermally affected. 
     In practice, this automatic process and the S-shape that receives the fusible wire make it conceivable to weld a plurality of fusible wires, one beside another, and consequently to produce different shorting ranges, as desired. 
     Finally, the last step consists in covering the assembly formed in the fourth step with an adhesive  6 . As indicated above, this is a hot-melt adhesive, which penetrates the open zones of the central portion  53 , promoting optimum sealing of the fuse  3 . 
     The invention is not limited to the embodiment described above. 
     In particular, fuses requiring different wires, exhibiting varying melting points, may be envisaged. Similarly, the arms may be made of various materials and may, for example, be printed circuits. 
     A support that is not molded onto the grid, but produced separately and gauged for optimal interaction, is also conceivable.