Patent Publication Number: US-2007103047-A1

Title: Fuse light for a light string

Description:
CROSS REFERENCE TO RELATED PATENTS  
      Not applicable  
     BACKGROUND OF THE INVENTION  
      The present invention relates to strings of lights and to safety features in strings of lights in particular.  
      Lights strings such as the type used to decorate Christmas trees includes sets of miniature lights arranged electrically in series. In order to avoid the problem of having the whole string go out when any one light burns out, many manufacturers include shunts in the bulbs or in the sockets. However, if a bulb burns out, the current flowing though the remaining bulbs can increase, sometimes to levels unacceptable from a safety standpoint. Typically, manufacturers will install fuses in a housing in line with the lights of the string or in the plug of the light set to prevent an over-current condition from occurring. Other manufacturers will simply not provide a shunt for one of the bulbs but rather allow that bulb to burn out and thus open the circuit when the current exceeds a pre-determined level as a form of fuse. However, this type of bulb, a so-called fuse bulb but in reality just a bulb without a shunt, will burn out eventually regardless of whether any other lights have failed in the interim, leaving the whole set to fail.  
      There remains a need for a better way to provide a light string that will operate following the failure of any individual bulb but still protect against over-current conditions.  
     SUMMARY OF THE INVENTION  
      According to its major aspects and briefly recited, the present invention is a fuse light for use in in a string of lights in which the lights of the string are connected electrically in series. The present fuse light acts as a fuse in the event the current in the string of lights exceeds a pre-determined level, a so-called “over-current” condition. When inserted into a light socket of the string of lights and the light string is plugged into a source of electrical power, the present fuse light lights up in the same manner as a conventional light. If the filament of the present light burns out, a shunt inside the light switches the electrical current from the filament to a fuse that acts to limit current that would otherwise exceeds the pre-determined level. The filament has failed so the light is dark, but the remaining lights of the light string continue to glow.  
      An important feature of the present invention is the use of a shunt as an electrical bypass circuit for maintaining the current in the other lights of a light string flowing when the filament of the present light burns out. When the filament of the present fuse light burns out, the potential for an over-current condition to occur is greater than before. Yet, as long as the current is below a pre-selected level, say, for example, when the fuse light filament is the only filament in the light string that has burned out, the balance of the light string can continue to function.  
      Another feature of the present invention is that the filament of the present light does not limit electrical current, unlike filaments of prior art fuse lights. Rather a fuse limits current in a light that is otherwise a regular light. Therefore the present light string is unlikely to be subjected to an over-current condition.  
      These and other features and their advantages will be apparent to those skilled in the art of light string design from a careful reading of the Detailed Description of Preferred Embodiments accompanied by the following drawings. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
      In the drawings,  
       FIG. 1  is a cross sectional side view of a light according to a preferred embodiment of the present invention; and  
       FIG. 2  is a cross sectional side view of the light of  FIG. 1  with the filament burned out.  
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The present invention is a fuse light for use in a light string.  
      Referring now to the figures, there is shown a cross sectional view of a light according to a preferred embodiment of the present invention. Generally referred to using the reference number  10 , light  10  has a bulb  12 , typically made of glass or plastic, with a first terminal  14  and a second terminal  16  extending into bulb  12  from outside. First and second terminals  14 ,  16 , which are both made of an electrically conductive material such as metal, are connected to a light base (not shown) that holds the terminals in spaced relation and in relatively fixed positions so that they may be placed in contact with electrical conductors. The present light is made to be used with a light base and a socket that connect terminals  14 ,  16  to electrical current carrying wires and are both typical of prior art miniature holiday light bases and sockets used in decorative light strings. These standard lights and light strings meet industry standards such as Underwriter&#39;s Laboratory standard 588, which is incorporated herein by reference.  
      First and second terminals  14 ,  16 , pass through an insulating glass bead  22 , which is also typically made of glass or plastic but which may be made of any electrically insulating material. Bead  22  spaces first and second terminals  14 ,  16 , apart from each other and from the interior surface of bulb  12 . A filament  26  is connected to the distal ends  28 ,  30 , of first and second terminals  14 ,  16 , respectively, through which electrical current can pass from first terminal  14  to second terminal  16 , thus completing the electrical circuit. Filament  26  incandesces when electrical current passes through it, thus producing light.  
      There is a third terminal  38  provided in bulb  12  of light  10 . Third terminal  38  does not extend outside bulb  12  but is anchored in bead  22 . A shunt  40  is applied between first and third terminals  14 ,  38 , near bead  22 , either just above it or just below it. Shunt  40  is typically a coil of fine wire wrapped around first and third terminals  14 ,  38 . Ordinarily, shunt  40  does not pass current, but, when filament  26  burns out, as shown in  FIG. 2 , first and third terminals  14 ,  38 , heat up quickly, burning off the thin oxide layer on shunt  40  where shunt  40  contacts first and third terminals  14 ,  38 , and quickly welding shunt to terminals  14 ,  38 , thus allowing current to flow from one to the other. At that point, shunt  40  will conduct electricity from first terminal  14  to third terminal  38  and thence through a fuse  44  to second terminal  16 , thus completing a new circuit that by passes filament  26 . In the event of an over-current condition, fuse  44  will burn out, leaving light  10  to fail but preventing current from flowing to lights in series with light  10 .  
      Filament  26  of light  10  may be designed to last longer than ordinary filaments. Nonetheless, filament  26  may fail from the electrical current in the light string exceeding the pre-selected value or merely from long use. In the case where filament  26  fails from excessive current, shunt  40  will pass the excessive current to fuse  44 , which will also burn out immediately, thus protecting the light string from the excessive current. If however, filament  26  merely fails from long use, fuse  44  will allow the balance of the lights in the light string to operate although light  10  itself will be dark.  
      It is intended that the scope of the present invention include all modifications that incorporate its principal design features, and that the scope and limitations of the present invention are to be determined by the scope of the appended claims and there equivalents. It also should be understood, therefore, that the inventive concepts herein described are interchangeable and/or they can be used together in still other permutations of the present invention, and that other modifications and substitutions will be to those skilled in the art from the foregoing description of the preferred embodiments without departing from the spirit or scope of the present invention.