Abstract:
A lampholder for miniature light sets, which may be of two-piece snap-together construction or a molded, one-piece unit, has a socket at one end to receive a push-in lamp unit and a wireway at the opposite end. The socket receives two identical push-in contact plates which are laterally reversible so that in one position they project by a respective insulation-severing element into one portion of the wireway to engage a first wire, and in a second position the severing element projects into a second portion of the wireway to engage a second wire. The wires are preferably contained in an insulated cord having cutouts at regular intervals which are located in the lampholders to be bridged via the contact plates and lamp units for a series connection. The lampholders at the ends of each series may have one of their contact plates reversed to engage non-interrupted wires in the cord to provide a serial-parallel arrangement. The cutouts may be formed at the location where the lampholders are applied to the cord or previously.

Description:
RELATED APPLICATIONS 
     This application is a continuation-in-part of my copending application Ser. No. 131,027, filed Dec. 10, 1987, now U.S. Pat. No. 4,807,098, which is a continuation-in-part of Ser. No. 945,602, filed 12-22-86, now U.S. Pat. No. 4,779,177, which is in turn a continuation-in-part of Ser. No. 664,153, filed 664,153, now U.S. Pat. No. 4,631,650. 
    
    
     TECHNICAL FIELD 
     The present invention relates to lampholders for series-parallel strings of lights, and particularly to those having miniature pushing type bulbs operating at relatively low voltage. Aspects of the invention are applicable, however, to larger bulbs and other types of light sets. 
     BACKGROUND ART 
     Decorative light strings in which all of the bulbs are in a single series have the disadvantage that if one bulb fails to light, the entire string goes out and it may be difficult to determine which light failed. Also, in a series string, the voltage available for each light is the line voltage divided by the number of bulbs. If, on the other hand, all the bulbs are in a parallel arrangement, failure of one of the bulbs does not affect the others, but each bulb is subjected to the full line voltage unless a transformer is used. 
     A suitable compromise between a series lighting circuit and a parallel lighting circuit is one in which sets of series-arranged bulbs are wired in parallel relation to make up a string. This is called a &#34;series parallel&#34; string. In such a string, the voltage for each light is the line voltage divided by the number of bulbs in each set. If a bulb fails in a series-parallel string, only the bulbs in the series set containing that bulb will fail to light. Hence, there are fewer bulbs to check to find the faulty bulb than in a string where all the bulbs are in a single series string. 
     Normally, in a series-parallel string of miniature push-in type bulbs, the lampholders in each series set are interconnected by using multiple short lengths of insulated lead wire connected to contact plates in a manner similar to that shown, for example, in U.S. Pat. No. 3,104,924. The lead wires to and from the first and last lampholders in each series set are connected, respectively, to parallel wires from the wall plug. Alternatively, the connection to the parallel wires is made by interrupting the parallel wires at the first and last bulbs of each series set and connecting both interrupted ends to the appropriate contact plate of the first and last lampholders. Hence, assembly of a series-parallel string of miniature lights has involved the handling and end-stripping of many pieces of wire, and normally there has been a need to wind the series wires and the parallel wires together between the bulbs for ease of handling when applying the string to a Christmas tree. 
     DISCLOSURE OF THE INVENTION 
     The present invention provides an improved lamp holder preferably used in conjunction with an insulated three-strand cord, making it unnecessary to use multiple short lengths of wire, and making it possible to have all of each lampholders identical and to make both contact elements in the lampholder identical, thus making it more economical and faster to assemble a string. 
     In carrying out the invention, two-piece lampholder housings are provided with a snap-on cover at their base end which forms a wireway for the cord with the rest of the housing, one-piece lampholder housings are provided which are injection-molded onto the cord. Two identical contact plates in each lampholder have insulation-severing contact elements projecting into the wireway to pierce the insulation of the cord and make the proper electrical connection to the wire. The cord has three side-by-side wires separated by insulation, the outer two wires being the parallel wires of the circuit and the center wire providing the series connection wires. The center wire is preformed with cutouts for the lampholder locations. When the two-piece lampholders are used, the lampholder covers have cover-fastening means passing through the respective cutout to separate the ends of the center wire exposed at the cutout as well as securing the cover in place. When the one-piece lampholders are used, the cutouts in the center wire can be filled with plastic when the lampholders are molded or can be left unfilled. 
     Each contact plate has an insulation-piercing element to engage the center wire and make a series connection via the bulb across the cutout in the center wire. As part of the present invention, each of the contact plates is laterally reversible so as to alternatively position its piercing element in engagement with the center wire or one of the other two wires in the cord. Thus, the lead-in contact plate can be reversed in the first lampholder in each series set and the lead-out contact plate of the last lampholder in each series set may be reversed to engage the appropriate one of the outer parallel wires so as to make the parallel connection for the series set. 
     If precut, the cutouts in the center wire are useful for properly positioning the cord when the two-piece lampholders are assembled on the wire, or when the one-piece lampholders are positioned in the mold. The snap-on cover of the two-piece lampholders has a positioning element onto which the cutouts are placed. The mold for the one-piece lampholders has a positioning pin onto which the cutouts are placed. As an alternative, this positioning pin can be used as a punch to form the cutouts. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic drawing of a series-parallel light string which is achieved using the present invention. 
     FIG. 2 is an isometric view showing a power cord with cutouts used with the present invention. 
     FIG. 3 is an exploded isometric view of a lamp unit of the present invention with a two-piece lampholder for use in the light string of FIG. 1. 
     FIG. 4 is a longitudinal sectional view of the lampholder with the cover in place and taken as indicated by line 44 in FIG. 3. 
     FIG. 5 is a longitudinal sectional view of the lampholder taken as indicated by line 5-5 of FIG. 4, but with the cover shown in phantom and with the alternative position of the illustrated contact plate shown in phantom. 
     FIG. 6 is a top plan view of the two-piece lampholder with the cover in operative position. 
     FIG. 7 is a bottom plan view of the two-piece lampholder housing before insertion of the conductor plate elements. 
     FIG. 8 is a longitudinal sectional view of the one-piece lampholder taken in the manner of FIG. 4. 
     FIG. 9 is a longitudinal sectional view of the one-piece lampholder taken as indicated by line 9-9 in FIG. 8. 
     FIG. 10 is a longitudinal sectional view of a mold for making the one-piece lampholder and taken in correspondence with FIG. 9. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown in FIG. 2, there is provided an insulated cord 10 having three wires 11, 12, and 13 arranged in generally coplanar relation as a ribbon and separated by insulation 14. The cord 1 has a series of cutouts 15 severing and passing through the center wire 12 and spaced apart along the length of the cord according to the desired spacing of the lampholders. At its ends, the cord 10 is provided with a wall plug 16 and an add-on socket 18. The outer two wires 11, 13 are electrically connected to the two contacts of the plug 16 and socket 18, and the center wire 12 deadends within the plug and socket. 
     Each lamp unit of the invention has a miniature push-in type lamp assembly comprising a bulb 20 and a lamp base 22 in which the lamp 20 is mounted, and has a lampholder unit 24 receiving the lamp base. The lampholder unit 24 has a husk or housing 26, two identical push-in brass contact plates 28-28&#39;, and a cover 30. 
     As is common in the decorative lighting string art, each lamp 20 seats in a circular socket 22a in the lamp base 22. The latter is necked beneath the socket 22a and has an extension 22b with a generally rectangular cross section which tapers at the outer end to assist in entry into the lampholder housing 26. The lamps 20 have a pair of wire leads 20a extending therefrom. These leads 20a extend from the circular socket portion 22a of the lamp base 22 through respective longitudinal passages in the extension 22b and double back over opposite faces 22c of the extension. 
     The lampholder housing 26 has a cylindrical socket portion 26a to receive the socket portion 22a of the lamp base 22, and has a generally rectangular bore extension 26b. This bore extension has the wider two of its opposite sides formed with a laterally stepped configuration to provide recessed faces 26c, 26c&#39;, each extending about two-thirds of the wider width of the bore extension 26b. Significantly, the recessed faces 26c, 26c&#39; extend laterally in opposite directions so that only about half of the width of each of the recessed faces is positioned directly opposite the other. 
     The narrower faces 26d, 26d&#39; of the bore extension 26b extend longitudinally from the socket portion 26a to a pair of opposed, cantilevered base flanges 26e, 26e&#39; of generally rectangular configuration in plan view which are separated by a base entry 26f. At one of their ends, the base flanges 26e, 26e&#39; are separated from the rest of the body of the housing 26 by slots 26g, 26g&#39; which terminate shortly above the base flanges, as seen in FIG. 5. At their other ends, the base flanges 26e, 26e&#39; are separated from the rest of the body of the housing 26 by slots 26h, 26h&#39; which continue laterally from the recesses providing the bore extension faces 26c, 26c&#39;. Preferably, the recesses providing the bore extension faces 26c, 26c&#39; are extended laterally outward as longitudinal keeper grooves 26k, 26k&#39;, best seen in FIG. 6. 
     The base entry 26f connects the bore extension 26b with a wireway 26m that is also intersected by the slots 26h, 26h&#39; and the keeper grooves 26k, 26k&#39;. The longitudinal sides of the wireway 26m are defined by opposed housing base extensions 26o, 26o&#39; which have a height adequate to laterally confine the cord 10 and the cover 30. 
     The cover 30 has a rectangular base 30a of a size to fit between the housing base extensions 26o, 26o&#39; and cover the underside of the wireway. At its upper side, the cover 30 has a central projecting divider leg 30b formed with a tapered keeper head 30c. This head is formed with a pair of keeper faces 30d at opposite sides for engaging the upper faces of the base flanges 26e, 26e&#39;, as shown in phantom in FIG. 5. The housing 26 and cover 30 are injection-molded plastic selected to have sufficient resiliency to permit the keeper head 30c to snap-fit into position responsive to pushing through the entry 26f between the flanges 26e, 26e&#39;. As shown in FIG. 5, the mouth of the entry 26f is preferably tapered along the opposed longitudinal edges of the flanges 26e, 26e&#39;. 
     Referring to FIGS. 3 and 5, the contact plates 28, 28&#39; each have a pair of tapered fingers 29 which have opposed insulation-severing edges 29a and function as contact elements. These fingers 29 are adapted to pierce the insulation around a wire until the wire nests at the narrow end of the tapered slot between the fingers so that contact is made between the fingers and the wire, as indicated in FIG. 5 with respect to the center wire 12 shown in phantom. The contact plates 28, 28&#39; are inserted through the cylindrical socket portion 26a of the housing in alignment with the slots 26h, 26h&#39; so that the outer faces of the contact members will rest against the recessed faces 26c, 26c&#39; and the outer longitudinal edge portions of the contact members will fit within the keeper grooves 26k, 26k&#39;. When the upper edges of the contact members are flush with the upper ends of the recessed faces 26c, 26c&#39;, the tapered contact fingers 29 project into the wireway. The contact members have tapered barbs 32 to resist movement thereof out of the bore extension 26b by biting into the adjoining housing material. 
     When assembling the light string, the covers 30 may be positioned with the divider legs 30b passing through the cutouts 15 in the cord 10. Then the covers and cord can be positioned in the wireways of the housings 26 between the housing base extensions 26o, 26o&#39;, whereupon the covers and housings can be pressed together to give them a snap-fit. This pressure also forces the contact fingers 29 through the cord insulation 14 into wire contact so that a circuit will be completed to the leads 20a of the lamps 20. 
     As indicated in FIG. 1, multiple sets of the lampholders 24 are placed in series with respect to center wire 12 between the parallel wires 11, 13. All of the lampholders 24 in each series set, except lampholders 24a, 24b at the two ends of the series set, have the contact plates 28, 28&#39; arranged with their contact fingers 29 directly opposite one another so as to make contact with the center wire 12 on opposite sides of the cutouts 15. The end lampholders 24a, 24b have only one of the contact plates 28, 28&#39; positioned so that its contact elements will make contact with the center wire 12. Each end lampholder 24a has one of the contact plates laterally reversed from its normal position so that its contact fingers 29 are arranged to make contact with wire 11, and each end lamp holder 24b has one of its contact plates laterally reversed in the opposite direction so that its contact fingers 29 will make contact with wire 13. It will be apparent that the end lampholders 24a, 24b in each series section can be identical to the other lampholders, the only difference being the lateral reversal of one of the contact plates. 
     To assist in case of entry of the contact plates 28, 28&#39; into the lampholder housing 26, it is preferred to round the entry end at the lead-in corner that is spaced from the tapered contact fingers 29. The resulting rounded edge 28a is complemented by the taper of the outer edge 29b of the outermost finger in guiding the contact plates into position in the housing 26 during assembly. In this regard, the contact plates 28 are adapted to be nested like staples loaded in a magazine and spring fed for dispensing into the lampholders by a pressure element, thus making assembly easier and more economical. 
     Although it is preferred to use a pair of tapered fingers as the contact element for the contact plates 28, it will be appreciated that a single tapered contact element can be used which is centered over the wire to be contacted so as to pierce the insulation and contact the wire by its point. 
     Referring to FIGS. 8 and 9, it is seen that the lampholder housing can be injection-molded as one piece, designated 24&#39;, and that the socket end portion can have the same interior configuration for receiving the contact plates 28, 28&#39;. A two-part mold 40 is used, with one part 40a forming the socket end portion and the other part 40b forming the wireway end portion of the lampholder housing. At their meeting ends, the mold parts 40a, 40b present sets of opposed, semi-circular grooves 42, 42&#39; for passage of the cord 10 through the mold when the two parts of the mold are placed together. The mold parts fit snugly over the cord at the grooves 42, 42&#39; to adequately seal the mold with the cord in place. If the cutouts 15 in the center wire of the cord are preformed, the cord is preferably positioned in the mold by fitting the respective cutout 15 on a positioning pin 44 projecting into the wireway end forming part 40b of the mold. This positioning pin may be left in position during the molding operation or partially withdrawn to expose the cutout 15 so that it will fill with plastic. The positioning pin 44 can also be provided as part of the socket forming part 40a of the mold. 
     As an alternative, the positioning pin 44 can function as a punch to form the cutout 15 in the center wire, either by the act of closing the mold, or by moving the pin 44 as a punch relative to the mold parts. 
     After the lampholder housings 24, have been molded onto the cord 10, the contact plates 28,28&#39; are pushed into position in the housings so that their insulation-severing fingers 29 pierce the cord insulation and contact the appropriate wire. As with the two-piece lampholder housings 24, the one-piece units 24, can all be identical and the contact plates 28,28&#39; can be identical, and yet a continuous string of lights can be formed in a series-parallel arrangement. 
     When the lampholders are molded directly onto the cord, several cords may be advanced in parallel-spaced relation through an injection-molding machine containing multicavity molds. The cords can have the cutouts 15 precut before reaching the molding machine and used to assist in positioning the cord in the mold, or the cords can be advanced to the molding machine in preset increments by a suitable feeder and the cutouts can then be made by a punching operation within the mold as previously described, or in any other suitable manner. If the cutouts are punched within the mold, the punch may be retracted before the molding operation so that the cutouts are filled with plastic when the lampholder housings are formed. This procedure assists in locking the lampholders in position on the cord. However, this is not essential, because the plastic forming the lampholder housings firmly grips the cord on completion of the molding operation. Locking of the lampholders on the cord is also assisted by the interfit of the insulation-piercing contact fingers 29 with the cord. 
     Although the lampholders 24,24&#39; of the present invention are illustrated as being used for a series-parallel string, it will be understood that the lampholders could also be used in a series set having a two-wire cord with only wires 11 and 12. In that case, the wires 11,12 would both be connected to the contacts in the plug 16 and the end lampholder remote from the plug 16 would have its contact plates 28, 28&#39; arranged in the same manner as lampholder 24a and namely with the one of its contact plates closest to the nearest lampholder 24 and the other contact element engaging wire 11. In that case, the end of wire 11 in lampholder 24a could be covered by an extension on the housing 26 or cover 30, or covered by an independent element. The wire 12 would terminate within lampholder 24a at its cutout 15 therein. 
     From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not to be limited except as by the appended claims.