Abstract:
A series-wired LED light string formed of a plurality of LEDs connected in series with a current limiting resistor. A resistive component is electrically connected across each LED and serves as a shunt to electrically bypass a failed LED to keep the remaining LEDs in the light string fully illuminated. The LED light string has only two wires and is powered by a full wave rectified voltage, generated external to the light string.

Description:
This application claims the benefit of Provisional Application No. 62/473,323, filed on Mar. 18, 2017. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a series connected light string and, more particularly, to an LED light string for pre-lit Christmas trees. 
     BACKGROUND OF THE INVENTION 
     Light Emitting Diode (LED) light strings have become quite popular recently for Christmas holiday decorating. They are much more energy efficient than incandescent lighting that has been around for many years. Since both the LED and the more conventional incandescent mini-light operate at very low voltage—usually between 2.0 to 3.5 volts—they are wired in electrical series connection with approximately 35 to 50 lights in a light string. As with incandescent lighting, when an LED bulb burns out or otherwise fails, the entire series light string goes out. To prevent this, bypass shunts can be wired across each LED to continue current through the light string in the event of such a failure. Such shunts are often resistive shunts, such as disclosed in U.S. Pat. Application No. 2014/0042903, incorporated by reference herein. 
     One of the big drawbacks in the assembly of pre-lit Christmas trees is the wiring. Many manufacturers today power pre-lit LED Christmas trees with half wave rectification to save costs. These light strings run on half the A.C. voltage rather than full wave rectified A.C. (alternating current). Such light strings flicker and are offensive to some people. Low flicker, full wave rectified LED light strings exist, but such light strings require three, four, or sometimes even five wires in the string. 
     Accordingly, it would be desirable to provide a low cost, low flicker, LED light string for pre-lit LED Christmas trees. 
     SUMMARY OF THE INVENTION 
     The present invention provides a new way to construct and power light strings for pre-lit Christmas trees. Advantageously, the present invention requires only two wires in each light string for full wave low flicker operation. 
     Each light string of the present invention consists of a plurality of LEDs connected in series with a current limiting resistor. A resistive component is electrically connected across each LED and serves as a shunt to electrically bypass a failed LED to keep the remaining LEDs in the light string fully illuminated. The LED light string is powered by a full wave rectified voltage, generated external to the light string. The light string of the present invention simply plugs into a DC socket in the Christmas tree. 
     Other advantages, variations and other features of the invention will become apparent from the drawings, the further description of examples and the claims to follow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows the basic LED circuit of the present invention. 
         FIG. 2  show the power box containing a full wave rectifier. 
         FIG. 3  shows a bridge rectifier with a DPDT switch to reverse polarity of the DC voltage. 
         FIG. 4  shows a light string with clear and multi-color LEDs connected in an anti-parallel configuration. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The basic LED circuit  2  is shown in  FIG. 1 . The basic circuit consists of a plurality of LED housings  4  (preferably, at least 15) disposed in sockets and connected in series with a current limiting resistor  6 . The current limiting resistance can be between zero and 1000 Ohms. As shown in  FIG. 1  and as described more fully below, each LED  4  in the string is shunted, preferably by a resistor  8 , to allow current to flow (and the string to remain lit) in the event of a failure of one or more of the LEDs. 
     Multiple basic LED light string circuits  2  may be strung on a pre-lit Christmas tree and plugged into the center trunk of the tree. These strings are all connected in parallel and connected to a power box at the bottom of the tree or in the main plug. In the power box or in the main plug is a bridge rectifier  10  converting the 120 volt house power into D.C. (direct current) to power the light strings with full wave rectification. This is shown in  FIG. 2 . 
     In one embodiment of the invention, the operating power—the output of the bridge rectifier  10 —is sent up the center trunk of the tree terminating in female receptacles  18  every three to eight inches. The light strings  2  of the present invention would be connected to these receptacles via male plugs  20 . The light strings as shown in  FIG. 1  are “naked” regarding power conversion. They have no operating power conversion circuitry to convert the 120 volts AC house current to DC voltage required to operate the LEDs  4 . This is a huge wire savings in that only two wires are needed for operation. 
     A shunt  8  is connected across each LED housing  4  to keep the light string operating in case of an LED failure. Shunt  8  is optional. Shunts can be resistors (approximately 680 Ohms); or NTC thermistors; TVS devices; metal oxide varistors or other semiconductor devices. The resistor R is a current limiting resistor of approximately 750 ohms. A diode (not shown) can be inserted in each light string to prevent reverse voltage from damaging the LED&#39;s. 
       FIG. 3  shows a bridge rectifier  10  with a DPDT switch  12  to reverse polarity. Such a switch can be used in color changing light strings such as described in U.S. Pat. No. 9,510,423, incorporated herein in its entirety. 
     Accordingly, the present invention provides the following important features over the prior art: 
     The present invention is a light string consisting of “naked” series connected LEDs, void of any operating power conversion circuitry. Operation of the light string is full wave low flicker. 
     Presently, light strings used in pre-lit Christmas trees have conversion circuitry to convert the 120 volt AC source to DC—which is necessary to operate LEDs. Some strings contain four or even five wires. The light string of the present invention requires only two wires because the AC to DC conversion is done externally to the string. 
     The “naked” LED light string of the present invention may have a series current limiting resistor R in the line with a resistance between zero and 1000 Ohms. The series wired string of LEDs may contain a diode  14  (see  FIG. 4 ) for reverse voltage protection. 
     Strings of clear and multicolor LEDs may overlap each other physically but polarity will determine which one illuminates. The circuit shown in  FIG. 4  is a better way to go from clear to multicolor, using LEDs  16  connected in an anti-parallel configuration, one clear, the other multicolor, as disclosed and claimed in co-pending U.S. patent application Ser. No. 15/341,747, filed Nov. 2, 2016, incorporated by reference herein. In either case, operating polarity will determine which color illuminates. 
     The output DC from the bridge rectifier can be sent up the trunk of the tree with female receptacles every 3 to 8 inches or can be at the bottom of the tree awaiting strings to be powered. The “naked” LED strings will have male receptacle plugs. The receptacles may have polarity identification marks. 
     Although the invention has been described in detail in connection with the exemplary embodiments, it should be understood that the invention is not limited to the above disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alternations, substitutions, or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Accordingly, the invention is not limited by the foregoing description or drawings, but is only limited by the scope of the appended claims.