Patent Document

This application is a divisional of application Ser. No. 08/811,167, filed Mar. 4, 1997 now U.S. Pat. No. 5,867,086. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates in general to a resistor unit, and more particularly to a resistor unit installed in a speed controller for controlling the speed of a fan motor employed in an automotive air conditioning device, and a method of producing the resistor unit. More specifically, the present invention is concerned with a resistor unit which generally comprises a resistor block including a flat resistor, a flat insulating plate and a flat heat radiation plate which are respectively positioned against one another, a holder block of molded plastic having the resistor block mounted thereon, a plurality of terminals partially embedded in the holder block and connected to given portions of the flat resistor, and coupling means for coupling the resistor block with the holder block. 
     2. Description of the Prior Art 
     A blower unit of an automotive air conditioning device has a blower installed therein. The blower has a fan held in a fan scroll and driven by an electric motor. The speed of the motor is controlled by a speed controller. The speed controller has a resistor unit including a plurality of resistors connected in series. By varying the total resistance of the resistor unit, the voltage applied to the electric motor is varied, so that the rotation speed of the fan can be controlled to, for example, a high level, a neutral level or a low level. 
     For cooling the speed controller, the speed controller is usually placed in an air flow duct of the air conditioning device through which cool air flows. Thus, it is desired to reduce the size of the speed controller as small as possible for obtaining a satisfactory air flow in the air flow duct. 
     Hitherto, for reducing the size and weight of the speed controller, there has been proposed a compact resistor unit called “flat resistance” which generally comprises a flat insulating base board and a resistor pattern printed on the base board. The compact unit having such a flat resistance type is shown in, for example, Japanese Utility Model First Provisional Publications Nos. 1-125708 &amp; 2-145507. 
     Furthermore, at present, there has been proposed a very compact light weight resistor unit, which is schematically shown in FIG. 8 of the accompanying drawings. 
     As shown in the drawing, the resistor unit comprises a flat resistor  1 , two flat insulating plates  2   a  and  2   b  which are placed in intimate contact against the respective opposite surfaces of the flat resistor  1 , and two flat heat radiation plates  3   a  and  3   b  which are placed in intimate contact against the outer surfaces of the two flat insulating plates  2   a  and  2   b . With the parts thus united, a resistor proper part  4  is created. 
     The resistor block  4  is mounted to a terminal-mounted plastic holder block  6 . That is, the holder block  6  has a plurality of terminals  5  of metal mounted thereto. The holder block  6  comprises a rectangular coupler portion  7  and an elongate flange portion  8  on which the terminals  5  are aligned. Each terminal  5  has a lower part embedded in the plastic holder block  6 . A so-called “insert molding technique” is used for producing the terminal-mounted plastic holder block  6 . 
     The flange portion  8  is integrally formed at both ends thereof with connecting lugs  9   a  and  9   b  through which the resistor block  4  is secured to the holder block  6  with the aid of rivets  10 . That is, each rivet  10  extending between the heat radiation plates  3   a  and  3   b  is tightly received in an opening formed in the connecting lug  9   a  or  9   b . Although not shown in the drawing, upper portions of the heat radiation plates  3   a  and  3   b  are connected through other rivets. Designated by numeral  11  is a projected portion in which a fuse (not shown) is installed. 
     However, due to inherent construction, the above-mentioned resistor unit has failed to achieve a satisfied durability against shocks applied thereto. That is, because the connecting lugs  9   a  and  9   b  are constructed of plastic which is poor in durability, it tends to occur that the lugs  9   a  and  9   b  are broken or at least damaged when a strong shock is applied thereto. In fact, such breakage tends to occur when the rivets  10  are brought into engagement with the lugs  9   a  and  9   b  for fixing the resistor block  4  to the holder block  6 . Such breakage becomes much severe when the plastic lugs  9   a  and  9   b  are deteriorated due to long usage of the resistor unit. Furthermore, the plastic lugs  9   a  and  9   b  have poor dimensional stability, which tends to induce a loose assembly of the resistor unit. 
     Furthermore, hitherto, the arrangement of a fuse for such resistor unit has been given little consideration. In fact, in a conventional arrangement of a fuse, there is a possibility that a burnt out part of the fuse causes a short-circuit of two parts. That is, when the fuse is actually operated, the burnt out part of the fuse tends to dangle from a proper position, thereby increasing the possibility of such undesired short-circuit. In fact, the burnt out part of fuse tends to induce various problems in the circuit. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a resistor unit of an automotive air conditioning device, which is free of the above-mentioned drawbacks. 
     It is a main object of the present invention to provide a resistor unit of an automotive air conditioning device, which has a satisfactory durability against shocks applied thereto. 
     It is another object of the present invention to provide a resistor unit of an automotive air conditioning device, which is equipped with a fuse holding structure by which operation of a fuse is assured. 
     It is still another object of the present invention to provide a resistor unit of an automotive air conditioning device, which is equipped with a fuse holding structure by which a fuse is tightly held in a fuse receiving portion of the resistor unit. 
     It is a further object of the present invention to provide a method of producing the resistor unit. 
     According to a first aspect of the present invention, there is provided a resistor unit which comprises a resistor block including a flat resistor, a flat insulating plate and a flat heat radiation plate which are respectively positioned against one another; a holder block of molded plastic on which the resistor block is mounted; a plurality of terminals of metal partially embedded in the holder block, the terminals being connected to given portions of the flat resistor; connecting lugs partially embedded in the holder block; and securing means for securing the resistor block to the connecting lugs, wherein the connecting lugs are constructed of metal. 
     According to a second aspect of the present invention, there is provided a resistor unit which comprises a resistor block including a flat resistor, a flat insulating plate and a flat heat radiation plate which are respectively positioned against one another; a holder block of molded plastic on which the resistor block is mounted; a plurality of terminals of metal partially embedded in the holder block, the terminals being connected to given portions of the flat resistor; a fuse connected to selected two of the terminals; means for defining in the selected two terminals respective recesses which are positioned below the fuse, the recesses being so sized and shaped so as not to establish an electric connection or short-circuit between the selected two terminals by the fuse once the fuse is actually operated; and means for coupling the resistor block with the holder block. 
     According to a third aspect of the present invention, there is provided a resistor unit which comprises a resistor proper part including a flat resistor, a flat insulating plate and a flat heat radiation plate which are respectively positioned against one another; two projections formed on separated resistor pattern parts of the flat resistor; a fuse connected to the projections to extend therebetween, the fuse and the two projections thus constituting a raised structure which projects toward the flat heat insulating plate; means for defining by the flat insulating plate a hinged tongue portion pressed out therefrom; means for defining by the flat heat radiation plate a fuse receiving portion; a holder block of molded plastic on which the resistor block is mounted; a plurality of terminals of metal partially embedded in the holder block, the terminals being connected to given portions of the flat resistor; and means for coupling the resistor block with the holder block, wherein when the flat resistor, the flat insulating plate and the flat heat radiation plate are assembled, the raised structure presses the tongue portion of the flat insulating plate against an inner wall of the fuse receiving portion. 
     According to a fourth aspect of the present invention, there is provided a method of producing a resistor unit, which comprises the steps of (a) stamping a metal sheet to produce a comb-like single metal sheet, the comb-like single metal sheet having a plurality of semi-finish portions which are integrally connected through thin strip portions; (b) molding a semi-finish product of a molded holder block using the comb-like single metal sheet as an insert, the semi-finish product having the semi-finish portions exposed; (c) removing the thin strip portions from the comb-like single metal sheet to produce mutually isolated terminals and connecting lugs thereby to finally produce the molded holder block; and (d) mounting a resistor block tightly to the molded holder block using the connecting lugs as a structurally basic connector means therebetween. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a schematic side view of a terminal-mounted plastic holder block used in a resistor unit of the present invention; 
     FIG. 2 is a plan view of a comb-like metal sheet which is subjected to an insert molding for producing the resistor unit of the invention; 
     FIG. 3 is a view similar to FIG. 1, but showing a terminal mounted plastic connected used in a first modification of the resistor unit of the invention; 
     FIG. 4 is a plan view of a fuse holding structure possessed by the first modification; 
     FIG. 5 is a view similar to FIG. 4, but showing a condition wherein a fuse is actually operated; 
     FIG. 6 is an exploded view of a second modification of the resistor unit of the invention, showing a fuse holding structure installed in a resistor proper part; 
     FIG. 7 is an enlarged sectional view of the fuse holding structure of the second modification in an assembled condition; and 
     FIG. 8 is an exploded view of a conventional compact resistor unit of flat type. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following, a resistor unit of the present invention will be described in detail with reference to the drawings. 
     It is to be noted that the entire construction of the resistor unit of the invention is similar to that of the above-mentioned conventional resistor unit shown in FIG.  8 . 
     That is, the resistor unit of the invention comprises generally a resistor block which is substantially the same as the resistor block  4  of FIG. 8 and a terminal-mounted plastic holder block  60  (see FIG. 1) which is different from the terminal-mounted plastic holder block  6  of FIG.  8 . 
     As can readily be seen from FIG. 8, the resistor block  4  of the present invention comprises a flat resistor  1 , two flat insulating plates  2   a  and  2   b  which intimately put therebetween the flat resistor  1  and two heat radiation plates  3   a  and  3   b  which intimately put therebetween the insulating plates  2   a  and  2   b . The flat resistor  1  is a stamped resistance plate having a given pattern. The insulating plates  2   a  and  2   b  are made of mica or the like, and the heat radiation plates  3   a  and  3   b  are made of aluminum or the like. One of the heat radiation plates  3   a  and  3   b  is formed with a fuse receiving portion  11  in which an after-mentioned fuse  12  (see FIG. 1) is installed. 
     As is seen from FIG. 1, the terminal-mounted plastic holder block  60  of the present invention comprises a rectangular coupler portion  70  and an elongate flange portion  80  on which a plurality of terminals  50   a ,  50   b ,  50   c ,  50   d  and  50   e  of metal are aligned. Each terminal  50   a ,  50   b ,  50   c ,  50   d  or  50   e  has a lower part embedded in the holder block  60 , as shown. 
     It is to be noted that, in the present invention, connecting lugs  90   a  and  90   b  corresponding to the connecting lugs  9   a  and  9   b  of FIG. 8 are constructed of metal. Each connecting lug  90   a  or  90   b  has a lower part embedded in the flange portion  80 , as shown. Insert molding technique is used for producing the terminal-mounted plastic holder block  60 . 
     As will be described in detail hereinafter, the connecting lugs  90   a  and  90   b  and the terminals  50   a ,  50   b ,  50   c ,  50   d  and  50   e  have been integrally connected before their separation. That is, they are portions which have constitute a comb-like single metal sheet  13  such as that shown in FIG.  2 . 
     Similar to case of the conventional resistor unit of FIG. 8, the resistor proper part  4  is mounted to the plastic holder block  60 . For this mounting, the connecting lugs  90   a  and  90   b  of metal respectively receive the rivets  10  which extend between the heat radiation plates  3   a  and  3   b . Particular portions of the flat resistor  1  are spot-welded to selected ones, for example, the terminals  50   a ,  50   d  and  50   e  of the terminals  50   a  to  50   e.    
     As is seen from FIG. 1, a fuse  12  is arranged to connect the terminals  50   b  and  50   c . When subjected to an excessive current flows, the fuse  12  is melted down to protect an electric circuit of the speed controller. The fuse  12  is received in the fuse receiving portion  11  of the heat radiation plate  3   a  (see FIG.  8 ). 
     In the following, a method of producing the resistor unit according to the present invention will be described with reference to the drawings. 
     Since the method of producing the resistor proper part  4  is known, only the method of producing the holder block  60  will be described in detail in the following. 
     By stamping or punching a metal sheet, a comb-like metal sheet  13  as shown in FIG. 2 is produced. The metal sheet may be of steel, brass or the like. As is seen from the drawing, the shaped metal sheet  13  thus produced has various corresponding portions of the terminals  50   a  to  50   e  and the connecting lugs  90   a  and  90   b , which are integrally connected through thin strip portions  15 . The shaped metal sheet  13  is formed with a plurality of openings  14  at lower parts of the various corresponding portions. The portions corresponding to the connecting lugs  90   a  and  90   b  are formed with circular openings  17   a  and  17   b,  respectively. 
     The shaped metal sheet  13  is then subjected to an insert molding to produce a semi-finish product of the holder block  60 . That is, the semi-finish product has such a construction as is illustrated by the solid line and the phantom line in FIG.  1 . With this molding, the apertured lower parts of the corresponding portions  50   a  to  50   e  and  90   a  and  90   b  of the shaped metal sheet  13  are embedded in the molded plastic holder block  60 , as is seen from FIG.  1 . Then, the thin strip portions  15  are removed from the metal sheet  13  to isolate and produce the terminals  50   a  to  50   e  and the connecting lugs  90   a  and  90   b . Then, a fuse  12  is connected to the terminals  50   b  and  50   c  by using a thin solder. With this, a finished product of the holder block  60  is provided, which is illustrated by only the solid line in FIG.  1 . As shown in this drawing, the terminals  50   b  and  50   c  for the fuse  12  have upper portions longer than those of the remaining terminals  50   a ,  50   d  and  50   e.    
     For assembling the resistor unit according to the present invention, the resistor block  4  is mounted to the holder block  60  by using the rivets  10 . That is, the circular openings  17   a  and  17   b  of the connecting lugs  90   a  and  90   b  tightly receive the rivets  10  which extend between the heat radiation plates  3   a  and  3   b.    
     In the following, advantages possessed by the above-mentioned resistor unit of the invention will be described. 
     Since the connecting lugs  90   a  and  90   b  are constructed of metal, the drawbacks possessed by the conventional plastic connecting lugs  9   a  and  9   b  are eliminated. That is, due to usage of metal, durability of the connecting lugs  90   a  and  90   b  and thus that of the resistor unit is greatly increased. In addition, the dimensional stability of the lugs  90   a  and  90   b  is greatly improved, which induces a precise and tight assembly of the resistor unit. 
     Since the connecting lugs  90   a  and  90   b  and the terminals  50   a  to  50   e  are supplied by the same metal sheet  13 , the method of producing the resistor unit is quite simplified as compared with the production method of the conventional resistor unit of FIG.  8 . 
     In the following, two modifications of the present invention will be described with reference to FIGS. 3 to  7  of the accompanying drawings. 
     Referring to FIGS. 3,  4  and  5 , there is shown a first modification of the resistor unit of the present invention. 
     That is, as is well shown in FIG. 3, in this modification, a unique fuse holding structure  100  is provided by the terminals  50   b  and  50   c  of the terminal-mounted plastic holder block  60 . 
     As is seen from FIG. 4, the terminals  50   b  and  50   c  are formed at portions below the fuse  12  with respective rectangular recesses  50   b ′ and  50   c ′ which face each other. In the illustrated modification, the recesses  50   b ′ and  50   c ′ are symmetric with respect to an imaginary plane vertically extending between the two terminals  50   b  and  50   c . With the rectangular recesses  50   b ′ and  50   c ′, a so-called safety fuse holding space “S” is defined. The space “S” is sufficiently larger than the fuse  12 . More specifically, the size and shape of the space “S” are so determined as not to establish an electric connection or short-circuit between the two terminals  50   b  and  50   c  by the fuse  12  once the fuse  12  is operated or burnt out, as is understood from FIG.  5 . That is, the distance “A” between the portion of the terminal  50   b  or  50   c  to which the fuse  12  is connected and a lower wall of the safety fuse holding space “S” is greater than the length “L” of the fuse  12 . That is, “A&gt;L” is established. In the illustrated modification, the distance between opposed walls of the recesses  50   b ′ and  50   c ′ is substantially equal to the length “L” of the fuse  12 . Of course, the length “L” of the fuse  12  is greater than the distance “D” between opposed portions of the two terminals  50   b  and  50   c  where the fuse  12  is arranged. Due to the provision of the above-mentioned fuse holding structure  100 , it never occurs that the fuse  12  accidentally establishes a connection or short-circuit between the two terminals  50   b  and  50   c  when the fuse is operated or burnt out. That is, as is illustrated by a phantom line in FIG. 5, if the size of the fuse holding space “S” is not sufficiently large, the possibility of establishing such undesired connection or short-circuit increases. 
     Referring to FIGS. 6 and 7, there is shown a second modification of the resistor unit of the present invention. 
     That is, in this modification, a unique fuse holding structure  200  is installed in the resistor proper part  40 . 
     As is seen from FIG. 6, the flat resistor  1  is formed at separated resistor pattern parts  1   a  and  1   b  thereof with respective projections  1   a ′ and  1   b ′. A fuse  12  is welded to these two projections  1   a ′ and  1   b ′ by using a thin solder. Thus, the fuse  12  and the projections  1   a ′ and  1   b ′ constitute a raised structure ( 12 ,  1   a ′ and  1   b ′) provided on the flat resistor  1 . 
     Thus, in this modification, the terminals  50   b  and  50   c  of the terminal-mounted plastic holder block  60  has no fuse  12  welded thereto, as is understood from FIG.  6 . 
     The insulating plate  2   a  has, at a portion thereof facing the raised structure ( 12 ,  1   a ′ and  1   b ), a hinged tongue portion  2   a ′ pressed out therefrom. In this modification, the insulating plate  2   a  is constructed of a resilient member, such as a glass fiber cloth, flexible mica sheet, silicon sheet, polyimide resin sheet or the like. The heat radiation plate  3   a  is formed at a portion facing the tongue portion  2   a ′ with the fuse receiving portion  11 . The fuse receiving portion  11  is formed with ventilation openings  11   a  (only one is shown). 
     As is seen from FIG. 7, when the resistor proper part  40  is properly assembled, the raised structure ( 12 ,  1   a ′ and  1   b ′) of the flat resistor  1  is projected into the fuse receiving portion  11  while pressing the tongue portion  2   a ′ of the insulating plate  2   a  against an inner wall  11 ′ of the fuse receiving portion  11 . That is, the insulating tongue portion  2   a ′ is intimately sandwiched between the fuse  12  and the heat radiation plate  3   a . Due to the provision of having the insulating tongue portion  2   a ′, electric insulation between the fuse  12  and the heat radiation plate  3   a  of metal is assured. Since the raised structure ( 12 ,  1   a ′ and  1   b ′) of the flat resistor  1  is pressed against the inner wall  11 ′ of the fuse receiving portion  11 , the same can be tightly and stably held in the fuse receiving portion  11  without play. Due to the nature of the resilient member of which the insulating plate  2   a  is constructed, the opened tongue portion  2   a ′ is biased toward a closed position. This biasing force of the tongue portion  2   a ′ promotes proper operation of the fuse  12 . Furthermore, because of the same reason, when the fuse  12  is operated or burnt out, the tongue portion  2   a ′ is forced to take the closed position. This means that the resistor pattern parts  1   a  and  1   b  of the flat resistor  1  are protected from being contaminated by air pollutants. 
     In addition to the above , the following modifications are possible in the second modification. 
     If desired, the fuse  12  may be arranged to the terminals  50   b  and  50   c  in a manner as is shown in FIG.  1 . In this case, a raised structure like the above-mentioned raised structure ( 12 ,  1   a ′ and  1   b ′) should be provided by the fuse  12  and the terminals  50   b  and  50   c.    
     Furthermore, if desired, the other insulating plate  2   a  may be constructed with the above-mentioned resilient member. In this case, the tight and stable installation of the raised structure ( 12 ,  1   a ′ and  1   b ′) in the fuse receiving portion  11  is greatly assured. 
     In addition to the above, many modifications are available in the present invention without departing from the novel teachings and advantages of the invention. All such modifications are intended to be included within the scope of the present invention as defined in the following Claims.

Technology Category: 4