Patent Publication Number: US-2005122723-A1

Title: Decorative light strings and repair device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application is a continuation in part of PCT application PCT/US/02/07609 filed Mar. 13, 2002, claiming priority to U.S. provisional applications 60/277,346 filed Mar. 19, 2001, 60/277,481 filed Mar. 20, 2001, 60/287,162 filed Apr. 27, 2001, 60/289,865 filed May 9, 2001, and U.S. applications Ser. No, 09/854,255 filed May 14, 2001, 10/041,032 filed Dec. 28, 2001 and Ser. No. 10/068,452 filed Feb. 2, 2002. 
    
    
     FIELD OF THE INVENTION  
      The present invention relates to decorative lights, including lights for Christmas trees, including pre-strung or “pre-lit” artificial trees.  
     SUMMARY OF THE INVENTION  
      In accordance with one embodiment of the present invention, one or more strings of decorative lights are supplied with power by converting a standard residential electrical voltage to a low-voltage, and supplying the low-voltage to at least one pair of parallel conductors having multiple decorative lights connected to the conductors along the lengths thereof, each of the lights, or groups of the lights, being connected in parallel across the conductors. A string of decorative lights embodying this invention comprises a power supply having an input adapted for connection to a standard residential electrical power outlet, the power supply including circuitry for converting the standard residential voltage to a low-voltage e.g. 12 volts to 30 volts output; a pair of conductors connected to the output of the power supply for supplying the low-voltage output to multiple decorative lights; and multiple lights connected to the conductors along the lengths thereof, each of the lights, or groups of the lights, being connected in parallel across the conductors. The lights preferably require voltages of about  6  volts or less, and are preferably connected in parallel groups of 2 to 5 lights per group with the lights within each group being connected in series with each other.  
      In one particular embodiment, a supply providing low-voltage DC is used in combination with a string having dual-bulb sockets and associated diode pairs to permit different decorative lighting effects to be achieved by simply reversing the direction of current flow in the string, by changing the orientation of the string plug relative to the power supply.  
      In another embodiment of the present invention, one or more strings of decorative lights are supplied with power by a power supply including either circuitry for converting the standard residential voltage to one or more DC voltages and circuitry for switching the polarity and/or amplitude of the DC voltage(s), or circuitry for allowing only a predetermined portion of every AC cycle of an AC voltage source to reach the multiple lights.  
      In another embodiment of the present invention, a string of decorative lights includes a plurality of elongated electrical conductors having multiple electrical lamps inserted into sockets. The multiple electrical lamps and sockets are connected at intervals along the lengths of the conductors. A small compartment is also included and includes a wall forming a first opening adapted to receive in frictional engagement a base of an electrical lamp. The compartment also includes a first member designed to engage a second member on the socket to assist in removing the electrical lamp from the socket. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The invention may best be understood by reference to the following description taken in conjunction with the accompanying drawings, in which:  
       FIG. 1  is a schematic diagram of a string of decorative lights embodying the present invention;  
       FIG. 2  is a more detailed diagram of the light string shown in  FIG. 1 ;  
       FIG. 3  is an enlarged and more detailed perspective view of a portion of the light string of  FIG. 2 ;  
       FIG. 4  is an exploded perspective view of a bulb and socket for use in the light string of  FIGS. 1-3 ;  
       FIG. 5  is a schematic circuit diagram of a suitable power supply for use in the light string of  FIGS. 1-3 ;  
       FIG. 6  is a front elevation of a power supply for supplying multiple light strings on a prelit artificial tree;  
       FIG. 7  is a side elevation of the power supply of  FIG. 6 ;  
       FIG. 8  is a top plan view of the power supply of  FIG. 6 ;  
       FIG. 9  is an exploded perspective view of bulbs and a modified socket for use in the light string of  FIGS. 1-3 ;  
       FIG. 9   a  is a schematic circuit diagram of a reversible DC power supply for use with the bulbs and modified socket shown in  FIG. 9 ;  
       FIG. 9   b  is an exploded perspective view of dual-filament bulbs and sockets;  
       FIG. 9   c  is a schematic circuit diagram of a power supply permitting simultaneous control of both filaments in the lights strings of  FIG. 9  or  FIG. 9   b.    
       FIG. 9   d  is a schematic circuit diagram of a power supply and filament combination illustrating the operation of the dual filament lamps shown in  FIG. 9   b.    
       FIG. 9   e  is a schematic circuit diagram of a dual-power supply and filament combination according to one embodiment of the present invention;  
       FIG. 9   f  is a schematic circuit diagram of a power supply, rectifier bridge, and filament combination according to another embodiment of the present invention;  
       FIG. 10  is an exploded perspective view of another modified bulb and socket for use in the light string of  FIGS. 1-3 ;  
       FIG. 11  is an exploded view of the bulb and socket shown in  FIG. 10 ;  
       FIG. 12  is a schematic circuit diagram of a modified power supply for use with the light string of  FIGS. 1-3 ;  
       FIG. 13  is a perspective view of a power supply housing mounted on a prelit artificial tree for supplying power to multiple light strings on the tree; and  
       FIG. 14  is a schematic circuit diagram of a modified power supply for use with the light string of  FIGS. 1-3 . 
    
    
     DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS  
      Although the invention will be described next in connection with certain preferred embodiments, it will be understood that the invention is not limited to those particular embodiments. On the contrary, the description of the invention is intended to cover all alternatives, modifications, and equivalent arrangements as may be included within the spirit and scope of the invention as defined by the appended claims.  
      Turning now to the drawings and referring first to  FIGS. 1-3 , a power supply  10  is connected to a standard residential power outlet that supplies electrical power at a known voltage and frequency. In the United States, the known voltage is 120 volts and the frequency is 60 Hz, whereas in Europe and some other countries the voltage is 220-250 volts and the frequency is 50 Hz. The power supply  10  converts the standard power signal to a 24-volt, 30-KHz AC waveform, which may be a pulse amplitude modulated waveform (PAM), which is supplied to a pair of parallel conductors  11  and  12  that supply power to multiple 6-volt incandescent lights L. A typical light “string” contains  52  lights L.  
      Multiple groups of the lights L are connected across the two conductors  11  and  12 , with the lights within each group being connected in series with each other, and with the light groups in parallel with each other. For example, lights L 1 -L 4  are connected in series to form a first light group G 1  connected across the parallel conductors  11  and  12 . Lights L 5 -L 8  are connected in series to form a second group G 2  connected across the conductors  11  and  12  in parallel with the first group G 1 , and so on to the last light group Gn.  
      If one of the bulbs fails, the group of four series-connected lights containing that bulb will be extinguished, but all the other 96 lights in the other groups will remain illuminated because their power-supply circuit is not interrupted by the failed bulb. Thus, the failed bulb can be easily and quickly located and replaced. Moreover, there is no need for shunts to bypass failed bulbs, which is a cost saving in the manufacture of the bulbs. If it is desired to avoid extinguishing all the lights in a series-connected group when one of those lights fails, then the lights may still be provided with shunts that are responsive to the low-voltage output of the power supply. That is, each shunt is inoperative unless and until it is subjected to substantially the full output voltage of the power supply, but when the filament associated with a shunt fails, that shunt is subjected to the full output voltage, which renders that shunt operative to bypass the failed filament. A variety of different shunt structures and materials are well known in the industry, such as those described in U.S. Pat. Nos. 4,340,841 and 4,808,885.  
      As shown in  FIG. 4 , each of the individual lights L uses a conventional incandescent bulb  20  attached to a plastic base  21  adapted to be inserted into a plastic socket  22  attached to the wires that supply power to the bulb. Each bulb contains a filament  23  that is held in place by a pair of filament leads  25  and  26  extending downwardly through a glass bead  24  and a central aperture in the base  21 . The lower ends of the leads  25 ,  26  are bent in opposite directions around the lower end of the base  21  and folded against opposite sides of the base to engage mating contacts  27  and  28  in the socket  22 . The interior of the socket  22  has a shape complementary to the exterior shape of the lower portion of the bulb base  21  so that the two components fit snugly together.  
      As shown most clearly in  FIG. 4 , the contacts  27  and  28  in each bulb base  22  are formed by tabs attached to stripped end portions of the multiple wire segments that connect the lights L in the desired configuration. If a lamp is at one end of a group, these wire segments may include multiple segments of either the conductor  11  or the conductor  12  from  FIGS. 1-3 . As can be seen in  FIG. 4 , the connector tabs  27 ,  28  in each socket  22  are fed up through a hole in the socket and seated in slots formed in the interior surface of the socket on opposite sides of the hole. Prongs  27 a and  28 a on the sides of the tabs engage the plastic walls of the slots to hold the tabs securely in place within the slots. When the bulb base  21  is inserted into its socket  22 , the bent filament leads  25 ,  26  on opposite sides of the bulb base  21  are pressed into firm contact with the mating tabs  27 ,  28 .  
      As can be most clearly seen at the lower right-hand corner of  FIG. 4 , the tab  27  at each end of each series-connected group G is generally connected to two wires, both of which are segments of one of either the conductor  11  or the conductor  12 . The other wire, which connects to tab  28 , leads to the next light in that particular series-connected group G.  
      After all the connections have been made, the wires are twisted or wrapped together as in conventional light sets in which all the lights are connected in series.  
      Turning next to the power supply  10  (shown in  FIG. 1 ), a switching power supply is preferred to minimize size and heat. Power supplies of this type generally use switching technology to make the device smaller. An alternative is a power supply that uses switching technology and pulse width modulation or frequency modulation for output regulation, although this type of power supply is generally more expensive than those using electronic transformers. One suitable electronic transformer is available from ELCO Lighting of Los Angeles, Calif., Cat. No. ETR150, which converts a 120-volt, 60-Hz input into a 12-volt, 30-KHz output.  
       FIG. 5  is a generalized schematic diagram of a power supply for converting a standard 120-volt, 60-Hz input at terminals  30  and  31  into a 24-volt AC output at terminals  32  and  33 . It will be understood that devices for supplying low-voltage, high-frequency signals are well known and vary to some degree depending on the output wattage range of the supply, and the particular design of the device is not part of the present invention.  FIG. 5  illustrates a standard self-oscillating half-bridge circuit in which two transistors Q 1  and Q 2  and parallel diodes D 10  and D 11  form the active side of the bridge, and two capacitors C 1  and C 2  and parallel resistors R 11  and R 12  form the passive side.  
      The AC input from terminals  30  and  31  is supplied through a fusing device (in this case fuse F 1 ) to a rectifier circuit, such as diode bridge  34 , consisting of diodes D 1 -D 4  to produce a full-wave rectified output across busses  35  and  36  leading to the capacitors C 1  and C 2 , transistor Q 1 , and transistor Q 2  (through R 13 ). The capacitors C 1 , C 2  form a voltage divider, and one end of the primary winding T 1   a  of an output transformer T 1  is connected to a point between the two capacitors. The secondary winding T 1   b  of the output transformer is connected through RT 1 , RT 2 , and S 1  to the output terminals  32  and  33 , which are typically part of a socket for receiving one or more plugs on the ends of light strings. The resistors R 11  and R 12  are connected in parallel with the capacitors C 1  and C 2  to equalize the voltages across the two capacitors, and also to provide a current bleed-off path for the capacitors in the event of a malfunction.  
      When power is supplied to the circuit, a capacitor C 3  begins charging to the input voltage through a resistor R 2 . A diac D 6  and a current-limiting resistor R 1  are connected in series from a point between the capacitor C 3  and the resistor R 2  to the base-drive circuitry of the transistor Q 2 . When the capacitor C 3  charges to the trigger voltage of the diac D 6 , the capacitor C 3  discharges, supplying current to the base of the transistor Q 2  and turning on that transistor. This action is required to start the switching process. During normal operation, diode D 7  prevents the capacitor C 3  from acquiring sufficient voltage to trigger diac D 6  by repeatedly discharging capacitor C 3  via transistor Q 2 . A resistor R 2  limits the current from the bus  35 . Resistors R 3  and R 4 , connected to the bases of the respective transistors Q 1  and Q 2  stabilize the biases, and diodes D 8  and D 9  in parallel with the respective resistors R 3  and R 4  provide for fast turn off.  
      Self-oscillation of the illustrative circuit is provided by an oscillator transformer T 2  having a saturable core. A ferrite core having a B/H curve as square as possible is preferred to provide a reliable saturation point. The number of turns in the primary and secondary windings T 2   b  and T 2   a  of the transformer T 2  are selected to force the operating gain of the transistors Q 1  and Q 2 , based on the following equation: 
 
N p *I P =N s *I s  
 
 where N p  is the number of turns in the primary winding T 2   b,  N s  is the number of turns in the secondary winding T 2   a,  I p  is the peak collector current, and I s  is the base current. Suitable values for N p  and N s  are 1 and 3, respectively, and assuming a one-volt supply across the primary winding N p , the forced gain is 3. The nominal collector current I c  is: 
 
 I   c =( P   out /η)*(2 NV   line ) 
 
 where I c  and V line  are RMS values, η is the efficiency of the output transformer T 1 , and P out  is the average output power. 
 
      The saturable transformer T 2  determines the oscillation frequency F according to the following equation: 
 
 F =( V   p *10 4 )/(4*B s   * A*N   p ) 
 
 where F is the chopper frequency, V p  is the voltage across the primary winding T 2   b  of the oscillator transformer T 2  in volts, B s  is the core saturation flux in Tesla, and A is the core cross section in cm 2 . 
 
      The output transformer T 1  has a non-saturable core with a ratio N p /N s  to meet the output requirements, such as 24 volts (RMS). It must also meet the power requirements so that it may operate efficiently and safely. The peak voltage V p (pri) across the primary winding T 1   a  is one half of the peak rectified voltage V peak  at bus  35 . 
 
 V   p (pri)= V   peak /2=(120*1.414)/2=85 volts 
 
      The desired 24-volt output translates to: 
 
 V   p (sec)=24*1.414=33.9 volts 
 
      Thus, the required ratio of turns in the primary and secondary windings of the transformer T 1  is 85/33.9 or 2.5/1.  
      A third winding T 1   c  with a turns ratio of 10/1 with respect to the primary winding provides a nominal 6-volt output for a bulb checker, described below.  
      The illustrative circuit also includes a light dimming feature. Thus, a switch Si permits the output from the secondary winding T 1   b  to be taken across all the turns of that winding or across only a portion of the turns, from a center tap  37 . A pair of thermistors RT 1  and RT 2  are provided in the two leads from the secondary winding T 1   b  to the terminals  32  and  33  to limit inrush current during startup.  
      To automatically shut down the circuit in the event of a short circuit across the output terminals  32  and  33 , a transistor Q 3  is connected to ground from a point between a diac D 6  and a diode D 9 . The transistor Q 3  is normally off, but is turned on in response to a current level through resistor R  3  that indicates a short circuit. The resistor R 13  is connected in series with the emitter-collector circuits of the two transistors Q 1  and Q 2 , and is connected to the base of the transistor Q 3  via resistors R 14  and R 15 , a diode D 12 , and capacitor C 4 . The current in the emitter-collector circuit of transistors Q 1  and Q 2  rises rapidly in the event of a short circuit across the output terminals  32 ,  33 . When this current flow through resistor R 13  rises to a level that causes the diode D 12  to conduct, the transistor Q 3  is turned on, thereby disabling the entire power supply circuit.  
      The light string is preferably designed so that the load on the power supply remains fixed so that there is no need to include voltage-control circuitry in the power supply to maintain a constant voltage with variable loads. For example, the light string preferably does not include a plug or receptacle to permit multiple strings to be connected together in series, end-to-end. Multiple strings may be supplied from a single power supply by simply connecting each string directly to the power supply output via parallel outlet sockets. Extra lengths of wire may be provided between the power supply and the first light group of each string to permit different strings to be located on different portions of a tree. Because ripple is insignificant in decorative lighting applications, circuitry to eliminate or control such fluctuations is not necessary, thereby reducing the size and cost of the power supply.  
      The low-voltage output of the power supply may have a voltage level other than 24 volts, but it is preferably no greater than the 42.4 peak voltage specified in the UL standard UL1950, SELV (Safe Extra-Low Voltage). With a 30-volt rms supply, for example, 10-volt lights may be used in groups of three, or  6 -volt lights may be used in groups of five. Other suitable supply voltages are 6 and 12 volts, although the number of lights should be reduced when these lower output voltages are used.  
      The power supply may produce either a DC output or low-voltage AC outputs. The frequency of a low-voltage AC output is preferably in the range from about 10 KHz to about 150 KHz within a 60 Hz envelope to permit the use of relatively small and low-cost transformers.  
      The voltage across each light must be kept low to minimize the complexity and cost of the light bulb and its socket. Six-volt bulbs are currently in mass production and can be purchased at a low cost per bulb, especially in large numbers. These bulbs are small and simple to install, and the low voltage permits the use of thin wire and inexpensive sockets, as well as minimizing the current in the main conductors. In the illustrative light string of  FIG. 1  with a 24-volt supply and four lights per group, the voltage available for each light is 6 volts. Consequently, the bulbs can be the simple and inexpensive bulbs that are mass produced for conventional Christmas light strings using series-connected lights. Similarly, the simple and inexpensive sockets used in such conventional Christmas light strings can also be used. Simple crimped electrical contacts may be provided at regular intervals along the lengths of the parallel conductors  11  and  12  for connection to the end sockets in each group of four lights. The maximum current level is only about 2 amperes in a 100-light string using four 6-volt lights per group and a 24-volt supply, and thus the two conductors  11  and  12  can also be light, thin, and inexpensive.  
      Light strings embodying the present invention are particularly useful when used to pre-string artificial trees, such as Christmas trees. Such trees can contain well over 1000 lights and can cost several hundred dollars (US) at the retail level. When a single light and its shunt fail in a series light string, the lights in an entire section of the tree can be extinguished, causing customer dissatisfaction and often return of the tree for repair or replacement pursuant to a warranty claim. When the artificial tree is made in sections that are assembled by the consumer, only the malfunctioning section need be returned, but the cost to the warrantor is nevertheless substantial. With the light string of the present invention, however, the only lights that are extinguished when a single light fails are the lights in the same series-connected group as the failed light. Since this group includes only a few lights, typically 2 to 5 lights, the failed bulb can be easily located and replaced.  
      When pre-stringing artificial trees, the use of a single low-voltage power supply for multiple strings is particularly advantageous because it permits several hundred lights to be powered by a single supply. This greatly reduces the cost of the power supply per string, or per light, and permits an entire tree to be illuminated with only a few power supplies, or even a single power supply, depending on the number of lights applied to the tree.  
       FIGS. 6-8  illustrate a single power supply  50  for supplying power to a multiplicity of light strings on a prelit artificial tree having a hollow artificial trunk  51 . The power supply is contained in a housing  52  having a concave recess  53  in its rear wall  54  to mate with the outer surface of the artificial trunk  51 . A pair of apertured mounting tabs  55  and  56  are provided at opposite ends of the rear wall  54  to permit the power supply to be fastened to the trunk  51  with a pair of screws. The power input to the supply  50  is provided by a conventional three-conductor cord  57  that enters the housing through the bottom wall  58 . The free end of the cord  57  terminates in a standard three-prong plug.  
      The power output of the supply  50  is accessible from a terminal strip  59  mounted in a vertically elongated slot in the front wall  60  of the housing  52 . This terminal strip  59  can receive a multiplicity of plugs  61  on the ends of a multiplicity of different light strings, as illustrated in  FIG. 7 . Thus, if each light string contains 100 lights and the terminal strip can receive ten plugs, the power supply can accommodate a total of 1000 lights for a given tree. Each plug  61  is designed to fit the terminal strip  59  but not standard electrical outlets, to avoid accidental attachment of the low-voltage light string to a 120-volt power source. A latch  62  extends along one elongated edge of the terminal strip  59  to engage each plug  61  as it is inserted into the strip, to hold the plugs in place. When it is desired to remove one of the plugs  61 , a release tab  63  is pressed to tilt the latch enough to release the plug.  
      The front wall of the power supply  50  also includes a bulb-testing socket  64  containing a pair of electrical contacts positioned to make contact with the exposed filament leads on a 6-volt bulb when it is inserted into the socket  64 . The contacts in the socket  64  are connected to a 6-volt power source derived from the power-supply circuit within the housing  52 , so that a good bulb will be illuminated when inserted into the socket  64 .  
      If desired, dimmer, flicker, long-life and other operating modes can be provided by the addition of minor circuitry to the power supply. In the illustrative power supply  50 , a selector switch  65  is provided on the front of the housing  52  to permit manual selection of such optional modes.  
      The front wall  60  of the housing  52  further includes an integrated storage compartment  66  for storage of spare parts such as bulbs, tools and/or fuses. This storage compartment  66  can be molded as a single unit that can be simply pressed into place between flanges extending inwardly from the edges of an aperture in the front wall  60  of the housing  52 . The flange on the top edge of the aperture engages a slightly flexible latch  67  formed as an integral part of the upper front corner of the storage compartment  66 . The lower front corner of the compartment and the adjacent flanges form detents  68  that function as pivot points to allow the storage compartment  66  to be pivoted in and out of the housing  52 , as illustrated in  FIG. 7 , exposing the open upper end of the storage compartment.  
      As can be seen in  FIGS. 7 and 8 , the bottom and rear walls  58  and  54  of the housing  52  are preferably provided with respective holes  69  and  70  that allow air to flow by convection through the housing to provide airflow desired of the circuit elements within the housing.  
       FIG. 9  illustrates a modified bulb-socket construction for use with a low-voltage DC power supply. A DC power supply may be the same device described above with the addition of a full-wave rectifier at the output to convert the low-voltage, high-frequency voltage to a low-voltage, DC voltage. The plug on the light string to be connected to the DC power supply is reversible so that the plug may be inserted into the socket of the power supply in either of two orientations, which will cause the DC current to flow through the light string in either of two directions. As will be described in more detail below, the direction of the current flow determines which of two bulbs in each of the multiple sockets along the length of the string are illuminated. This permits different decorative effects to be achieved with the same string by simply reversing the orientation of the string plug relative to the power-supply socket. For example, the bulbs illuminated by current flow in one direction may be clear bulbs, while the bulbs illuminated by current flow in the opposite direction may be colored and/or flashing bulbs.  
      As can be seen in  FIG. 9 , each socket  100  forms receptacles  101  and  102  for two different bulbs  103  and  104 , respectively. For example, bulb  103  may be clear and bulb  104  colored. Power is delivered to both receptacles  101  and  102  by the same pair of wires  105  and  106 , but the connector tabs  107  and  108  attached to the wires have increased widths to permit either an electrical connection to one of the exposed filament leads on the base of each bulb or to permit the diodes discussed below to be mounted. The rear connector tab  108  makes direct contact with one of the filament leads on the base of each bulb. The front connector tab  107  carries a pair of inexpensive, oppositely poled, surface-mount diodes  109  and  110  having metallized contact surfaces III and  112  at their upper ends. Each of the metallized contact surfaces  111  and  112  makes contact with a filament lead on only one of the bulb bases, so that each diode  109  and  110  is connected to only one bulb. Because a diode conducts current in only one direction, and the two diodes are poled in opposite directions, the DC current supplied to the socket  100  will flow through only one of the two bulbs  103  or  104 , depending upon the direction of the current flow, which in turn depends upon the orientation of the string plug relative to the power-supply socket.  
      As shown in  FIG. 9 , the two bulbs  103  and  104  preferably diverge from each other to reduce reflections from the non-illuminated bulb in each pair. If desired, a non-reflective barrier may be provided between the two bulbs.  
      A modified construction is to provide only a single pair of diodes for each of the parallel groups of lights. The diodes are provided at one end of each parallel group, with two separate wires connecting each diode to one of the two bulbs in each socket in that group. Another modified construction uses only a single bulb in each socket, with each bulb having two filaments and two diodes integrated into the base of the bulb for controlling which filament receives power.  FIG. 9   b  shows a typical example of such a construction. As shown in  FIG. 9   b , each bulb  203 ,  204  and socket  201 ,  202  include a key  213 ,  214  and a slot  215 ,  216  to insure bulb insertion in only one direction. This guarantees that the same filament in each bulb will glow in response to current in a particular direction, which is desirable for producing a uniform effect. The two filaments are spaced from each other along the axis of the bulb, and one end portion of the bulb is colored so that illumination of the filament within that portion of the bulb produces a colored light, while illumination of the other filament produces a clear light. Alternatively, the opposite end portions of the bulb can both be colored, but of two different colors.  
       FIG. 9   a  is a diagram of a circuit for reversing the polarity of a DC power supply. The standard AC power source is connected across a pair of input terminals  120  and  121  and full-wave rectified by a rectifier circuit, such as diode bridge  122 , as described above. The rectified output of the bridge  122  is supplied to the light string  123  connected to output terminals  124  and  125 . Between the bridge  122  and the terminals  124 ,  125 , a dual pole switch SW can change the direction of current flow so that the polarity of the terminals  124  and  125  is reversed.  
      In some cases, light strings using the bulb and socket configurations of  FIGS. 9 and 9   b  would make use of the power supply described in  FIG. 9   a . The dual pole switch SW causes one of the lamps or filaments to light, but not the other. In other words, one of the two lamps in the dual socket of  FIG. 9  (or one of the dual filaments of  FIG. 9   b ) might be lit at any given time, but not both.  
      Other known power supplies may be used such that power is supplied to both lamps (or filaments), causing both lamps or filaments to be lit simultaneously. These circuits all take advantage of the thermal time lag in the filaments of the lamps. One method drives the light string with an AC current. This causes both of the lamps or filaments to glow with equal intensity. A second DC current (or lower frequency AC current) is added to the original AC current. The combined AC and DC currents cause one lamp or filament to glow brighter, while the second becomes dimmer. By adjusting the amplitudes of the AC and DC currents, independent control can be obtained over each lamp in  FIG. 9  (or filament in  FIG. 9   b ). If the second source were a slowly varying AC source instead of DC, the lamps could be made to fade from one into another and back at the frequency of that source.  
      Another approach is to rectify an AC power source to generate one or more DC sources. The DC source (or sources) is then electronically switched at a fast rate, supplying positive current, negative current, and zero current to the light string. By controlling the length of time a switch is ‘on’ or ‘off,’ independent control can be obtained over the bulbs or filaments. This approach would also include circuits using SCRs, TRIACs, transistors, or similar devices, triggered asymmetrically on positive and negative half cycles of AC input current.  
       FIG. 9   c  is an example of the above approach. Electronic switches SW 1  and SW 2  can include SCRs, TRIACs, transistors and/or similar devices, as well as other appropriate control circuitry. If terminal T 100  is positive and terminal T 200  is negative, current flows from T 100  to SW 1 . From SW 1 , the current then flows through diode D 300  into filament L 100   a,  then to diode D 500 , filament L 200   a,  and back to switch SW 2 . Switch SW 2  is turned off at this time, so the current goes through diode D 200  and returns to terminal T 200 . The brightness of filaments L 100   a  and L 200   a  is controlled by the percentage of time that switch SW 1  remains ‘on’ during this half cycle. When terminal T 200  becomes positive and terminal T 100  is negative, the current flows from terminal T 200  to switch SW 2 , to filament L 200   b,  to diode D 600 , to filament L 100   b,  to diodes D 400  and D 100 , and then back to terminal T 100 . Switch SW 1  is off at this time, and switch SW 2  controls the brightness of filaments L 100   b  and L 200   b.  Switching occurs at such a high rate that the filaments L 100   a,  L 100   b,  L 200   a,  and L 200   b,  do not have time to cool. Thus, both lamps glow. Relative brightness between the lamps and overall brightness are thus controlled by the amount of time switches SW 1  and SW 2  are ‘on’ during their respective half cycles.  
      These methods are described for illustrative purposes only. There are numerous other well-known methods that can be used. These methods are beneficial effects. For example, if one lamp or filament were colored red and the other were white, it would be possible to cause the lamps to fade from white to red every 10 seconds or so. By fading from one bulb into the other at a faster rate, it is possible to achieve a shimmering effect wherein the lamps appear to be in motion. The lamps could also be made to change color or brightness in time with music or other special effects.  
      Turning now to  FIG. 9   d , a schematic of a light string in combination with a power source having terminals T 300  and T 400  is shown. In this embodiment, if the current is flowing from terminal T 300  to T 400 , the current flows through diode D 700 , the top filament F 100  to T 400 , thus only lighting the top filament F 100 . If the direction of the current is reversed, so that it travels from terminal T 400  to terminal T 300 , the current flows through the bottom filament F 200 , through the diode D 800  and to terminal T 300 . The advantage of this design, is that the diodes D 700  and D 800  are part of the base of the light string, and not included in the power supply. This allows the light string to operate with fewer wires on the outside, which is more aesthetically pleasing and cheaper to manufacture.  
       FIG. 9   e  illustrates an embodiment of the present invention where two power sources are used. In this embodiment, power is supplied by both a DC power supply  500  and an AC power supply  510 . Depending upon the direction of the current flow, the current passes through either diode  720  to the bulb or filament  520  or through diode  710  to the bulb or filament  540 . By varying the amplitude of each supply relative to the other, the individual brightness of each bulb ( 520  or  540 ) can be controlled at will. This is just one example of using multiple power supplies. Other known methods may also be utilized.  
       FIG. 9   f  illustrates another embodiment of the present invention for manipulating current flow. In this embodiment, an AC power supply  600  produces a low voltage AC output. A center tap  605  is attached to the power supply  600 . A full wave rectifier bridge  610  is connected to the AC power supply  600  and generates two DC sources. One is positive and the other negative. A single pole triple throw electronic switch  620  switches between the positive DC source, the negative DC source, or no source (position NC) at all. This then controls which of the two bulbs or filaments  630 ,  640 , if either, receive any current. By switching at a sufficiently fast rate, and controlling the amount of time switch  620  remains closed in each position, the individual brightness of each bulb ( 630  or  640 ) can be controlled at will.  
       FIGS. 10 and 11  illustrate a modified bulb base and socket construction that facilitates the replacement of a failed bulb. The bulb  130  in  FIGS. 10 and 11  has the same construction described above, including a filament  131  and a pair of filament leads  132  and  133  held in place by a glass bead  134 . The leads  132  and  133  extend downwardly through a molded plastic base  135  that fits into a complementary socket  136 . In this modified embodiment, the bulb base  135  includes a pair of diametrically opposed lugs  137  and  138  that support a bulb-removal ring  139  between the top surfaces of the lugs and the underside  140  of the flange  141  of the base  135 . The central opening  142  of the ring  139  is dimensioned to have a diameter just slightly smaller than that of the flange  141  so that the ring can be forced upwardly over the lugs  137 ,  138  until the ring  139  snaps over the top surfaces of the lugs, adjacent the underside of the flange  141 . The ring  139  is then captured on the base  135 , but can still rotate relative to the base.  
      To hold the bulb base  135  in the socket  136 , the ring  139  forms a hinged, apertured tab  143  that can be bent downwardly to fit over a latching element  144  formed on the outer surface of the socket  136 . When the bulb fails, the tab  143  is pulled downwardly and away from the socket  136  to release it from the socket  136 , and then the tab  143  is used to rotate the ring  139  to assist in removing the bulb and its base  135  from the socket  136 . As the ring  139  is rotated, a descending ramp  145  molded as an integral part of the ring engages a ramp  146  formed by a complementary notch  147  in the upper end of the socket  136 . When the bulb base  135  and the socket are initially assembled, the ramp  145  on the ring  139  nests in the complementary notch  147 . But when the ring  139  is rotated relative to the socket  136 , the engagement of the two ramps  145  and  146  forces the two parts away from each other, thereby lifting the bulb base  135  out of the socket  136 .  
       FIG. 12  is a generalized schematic diagram of a power supply for converting a standard 120-volt, 60-Hz input at terminals  161 ,  162  into a 24-volt AC output at terminals  163 ,  164  and  165 ,  166 . This circuit uses a switching power supply to deliver a low-voltage, high-frequency AC signal while also providing the following features for the light strings: 
          continuous dimming capability from very low light level to full light level,     multi-level dimming capability,     energy-saving and minimum-light-setting features,     soft-start feature to increase the lamp life,     soft start feature to reduce inrush current in the circuit, and     low cost with multi-feature lighting.        
      The AC input from the terminals  161 ,  162  is supplied through a fusing device, shown as fuse F 21 , to a diode bridge DB 21  consisting of four diodes to produce a full-wave rectified output across buses  167  and  168 , leading to a pair of capacitors C 23  and C 24  and a corresponding pair of transistors Q 21  and Q 22  forming a half bridge. The input to the diode bridge DB 21  includes inductor T 21 , a MOV (metal oxide varistor) or dual zener diode V Z21  and a pair of capacitors C 21  and C 22  which are part of the radio frequency interference and line noise filtering circuitry. Capacitors C 25  and C 26  are connected in parallel with capacitors C 23  and C 24 , respectively, to provide increased ripple current rating and high-frequency performance. The capacitors C 23  and C 24  may be electrolytic capacitors while capacitors C 25  and C 26  are film-type capacitors offering high-frequency characteristics to the parallel combination. A pair of resistors R 30  and R 31  are connected in parallel with the capacitors C 23  and C 24 , respectively, to equalize the voltages across the two capacitors, and also to provide a current bleed-off path for the capacitors in the event of a malfunction.  
      The capacitors C 23 , C 24  form a voltage divider, and one end of the primary winding T P  of an output transformer T 22  is connected to a point between the two capacitors. The secondary windings T S21  and T S22  of the transformer T 22  are connected to the output terminals  163 ,  164  and  165 ,  166 , which are typically part of a socket for receiving one or more plugs on the ends of light strings. A capacitor C 27  is connected in parallel with the primary winding T P  and acts as a snubber across the transformer T 22  to reduce voltage ringing.  
      An integrated circuit driver IC 21 , such as an IR2153 driver available from International Rectifier, drives the half bridge MOSFET transistors Q 21  and Q 22 . The power supply for the driver IC 21  is derived from the DC bus through a resistor R 25  and a parallel combination of capacitors C 28  and C 29 . The capacitor C 28  may be an electrolytic or an a film capacitor, and the capacitor C 29  is preferably a film-type capacitor offering a high-frequency de-coupling characteristic to the driver IC 21 . A zener diode V Z22  clamps the voltage at V CC  input pin  1  of IC 21  to ensure a safe operating limit. The zener diode V Z22  along with the resistor R 25  provide a regulated power supply for the driver IC 21 . A diode D 22  and a capacitor C 31  provide a boot-strap mechanism for power storage to turn on the MOSFET Q 21  of the half bridge.  
      The frequency of oscillation of the MOSFET driver is determined by the total resistance connected across pins  2  and  3  of the driver IC 21  together with the capacitance from pin  3  to ground. The two outputs of IC 21 , pins  7  and  5 , are connected to the gates of the MOSFETs Q 21  and Q 22 . A resistor R 21  limits the gate current of the MOSFET Q 21 , while R 24  limits the gate current of MOSFET Q 22 . A pair of resistors R 22  and R 23  are connected across the MOSFETs Q 21  and Q 22  to reduce noise sensitivity to avoid any spurious turn-on of the MOSFETs. Resistor/capacitor combinations R 27 /C 32  and R 28 /C 33  are tied across the two MOSFETs Q 21  and Q 22  as snubbers to quench transient voltage surges at the turn-off of these transistors.  
      When power is applied to the circuit, the voltage developed on the bus  167  causes voltage to be applied to the IC 21 &#39;s V CC  input. This causes the driver IC 21  to start oscillating and start driving the half-bridge transistors Q 21  and Q 22  alternately. This applies voltage across the primary winding T P  of the transformer T 22 , which in turn applies voltage across the secondary windings T S21  and T S22  of the transformer, which is applied to the load.  
      The rectified output of the DC bus  167  is applied is applied to the Vcc pin  1  of the driver IC 21  through a resistor R 25 . A zener diode V Z22  and capacitors C 28  and C 29 , connected between the Vcc pin  1  and ground, provide decoupling and voltage regulation for the driver IC 21 . The two outputs of IC 21  at pins  7  and  5 , provide drive to the gates of the MOSFETs Q 21  and Q 22 .  
      The RMS output voltage can be varied by controlling the on/off ratio of the pulse width applied to the primary of the transformer T 22 . A limited dimming control can be achieved by varying the frequency of the oscillation signal from the integrated circuit IC 21 . The output voltage is controlled by the potentiometer P 21  connected to the integrated circuit, which permits the user to adjust the light output to the desired level.  
      The dimming feature can be used to provide different fixed light levels, such as a low light output, an energy-saving output, or a full-light output. These three light levels can be achieved by use of three fixed resistors in place of the potentiometer P 21 . The three resistor settings can be selected by use of a three-position switch. A low-light output corresponds to a minimum output voltage, and a full-light output corresponds to maximum output voltage. An energy-saving output corresponds to an intermediate light level such as a 75% light output.  
      The bulb life can be extended by soft starting the driver IC 21 , so that the IC starts with minimum light output and slowly ramps up to the full or desired light level. At the time of start, the bulbs in the light string are normally cold, and the cold resistance of the bulbs is very low. The cold resistance of a bulb is typically ten times lower than the steady state, full-light operating resistance. If the full voltage were applied to a cold bulb at startup, the inrush bulb current could be ten times the rated current of the bulb, which could cause the bulb filament to weaken and ultimately break. By soft starting the control circuit, the voltage applied during starting of the bulb is significantly lower. As the bulb heats up and the bulb resistance increases, the voltage is increased. Thus the bulb current never exceeds its hot rating, which increases bulb life.  
      Soft starting of the circuit also helps reduce the inrush current from the circuit, thereby avoiding any interaction with other circuits or appliances. Soft starting in this circuit can be achieved by starting the driver IC 21  at a high frequency and then reducing it to the normal operating frequency after a short delay, e.g. one second. This is possible because it is characteristic of this supply that higher switching frequencies tend to reduce supply output, causing the lamps to dim. A typical method for achieving soft starting is shown in  FIG. 12 . When the power supply is first turned on, voltage is applied to pin  1  (Vcc) of IC 21 , enabling it to operate. Voltage is also applied to resistor R 93 . This causes capacitor C 96  to begin to charge up. During this time, transistor Q 90  is ‘off’. The switching frequency of the supply is determined by the resistance between pins  2  and  3  of IC 21  in combination with the capacitance from pin  3  to ground. Since the transistor Q 90  is ‘off’, that capacitance is capacitor C 30  in series with capacitor C 95 , which causes the supply to switch at a very high frequency and its output to be correspondingly low. The lights attached glow dimly. After about one second capacitor C 96  charges up, causing transistor Q 90  to turn on. Transistor Q 90  and diode D 95  now effectively short out capacitor C 95  so that only capacitor C 30  is left in the circuit. This causes the power supply to switch at a lower frequency, insuring normal lamp brightness. It should be understood that this is only one of many known methods of achieving the soft-start function.  
      If a wider range of dimming control is needed, the driver IC 21  can be replaced by another integrated circuit, such as an IR21571, to drive the FETs, it is capable of providing pulse width modulation. The output can be controlled from low light to full light.  
      The particular embodiment illustrated in  FIG. 12  is a half bridge circuit and a typical example, but it will be understood that the features of this circuit can be incorporated in other topologies such as flyback, forward, buck, full bridge or other power converters, including isolated as well as non-isolated power converter designs.  
       FIG. 13  illustrates a mounting arrangement for a housing  170  containing any of the power supplies described above, on a pre-lit artificial tree having a central “trunk” pole  171  and multiple branches such as branches  172 - 174  extending laterally from a support collar  175  on the pole  171 . Each branch carries a portion of one of multiple light strings attached to connectors on the housing  170 . In the illustrative embodiment, two such connectors  176  and  177  project upwardly from the top of the housing  170  for receiving mating connectors  178  and  179  attached to respective ends of two pairs of conductors  180  and  181 . When the connectors  178  and  179  are mated to the connectors  176  and  177 , the conductors are connected to the power supply contained within the housing  170 .  
      In an artificial tree having two or more vertical sections, the power supply housing  170  is preferably mounted on the uppermost collar  175  in the lowest of the three sections. Then one of the two connectors  176 ,  177  can supply power to the lowest section(s) of the tree, which generally is(are) the largest section(s), while the other connector supplies power to the smaller, upper sections of the tree. The electrical loads in the light strings in these two portions of the tree are typically about equal, and thus the output of the power supply can be split evenly between the two output connectors  176 ,  177 .  
      As can be seen in  FIG. 13 , the outer end panel  182  of the housing  170  is most accessible to the user. This end panel  182  carries a manually operated on-off switch  183  for turning the power supply on and off, and an indicator light  184  that is illuminated whenever the power supply is connected to a power source. A dimmer knob  185  connected to a potentiometer permits the user to control the light level by adjusting the position of the potentiometer. A bulb socket  186  permits the user to test a bulb by connecting the bulb to an appropriate power source within the housing. The panel  182  also contains a drawer  187  for storage of spare bulbs and fuses. Power for the circuitry within the housing  170  is supplied via cord  188 .  
      To mount the housing  170  on the collar  175 , a hook  189  extends upwardly from the housing. The weight of the housing  170  forces the lower end of the inside panel  190  against the pole  171 , and a yoke  191  projecting from the inside panel keeps the housing centered on the pole.  
      The two pairs of conductors  180  and  181  are connected to respective connector blocks  192  and  193  each of which includes multiple connectors for receiving mating connectors crimped onto the ends of the wires of multiple light strings. For example, the connector block  193  typically receives the connectors on a multiplicity of light strings mounted on the bottom section(s) of a pre-lit tree. The other connector block  192  typically receives a multiplicity of light strings for the middle section of the tree. The top section(s) of the tree typically includes two or more light strings, which are connected to a smaller third connector block  196  connected to the block  192  via mating connectors  194  and  195  on the ends of two pairs of conductors leading to the respective blocks  192  and  196 .  
       FIG. 14  is another schematic diagram of a power supply for converting a standard 120-volt, 60-Hz input at terminals  261 ,  262  into a  24 -volt AC output at terminals  263 ,  264  and  265 ,  266 . This circuit uses a switching power supply to deliver a low-voltage, high-frequency PAM signal while also providing the following features for the light strings: 
          continuous dimming capability from very low light level to full light level,     multi-level dimming capability,     energy-saving and minimum-light-setting features,     soft-start feature to increase the lamp life,     soft start feature to reduce inrush current in the circuit, and     low cost with multi-feature lighting.        
      The AC input from the terminals  261 ,  262  is supplied through a fuse FH 201  to a diode bridge DB 221  consisting of four diodes to produce a full-wave rectified output across buses  267  and  268 , leading to a pair of capacitors C 223  and C 224  and a corresponding pair of transistors Q 221  and Q 222  forming a half bridge. The input to the diode bridge DB 221  includes a passive component network consisting of C 203 , C 204 , C 206 , C 207 , L 201 , L 204  and RV 201  which are part of the radio frequency interference and line noise filtering circuitry. Capacitors C 225  and C 226  are connected in parallel with capacitors C 223  and C 224 , respectively, to provide increased ripple current rating and high-frequency performance. The capacitors C 223  and C 224  may be electrolytic capacitors while capacitors C 225  and C 226  are film-type capacitors offering high-frequency characteristics to the parallel combination.  
      The capacitors C 223 , C 224  form a virtual center tap. One end of the primary winding T P  of an output transformer T 222  is connected to a point between the two capacitors. The secondary winding T S  of the transformer T 222  is connected to the output terminals  263 ,  264  and  265 ,  266 , through series inductors L 202  and L 203  (along with C 214 , C 215 , C 216  and R 216 ) which act as filters to minimize electromagnetic interference. The output terminals receive one or more plugs on the ends of light strings.  
      An integrated circuit driver U 201 , such as a IR21571D controller available from International Rectifier, controls the switching frequency of oscillation and other features indicated above. The power supply V cc  for the driver U 201  is derived from the DC bus  267  through resistors R 201  and R 202  to an internal zener diode. The device includes protection elements which prohibit starting oscillation (operation) until the power supply voltages are in tolerance or if there is a fault which interferes with the proper sequencing of voltages V DC , V CC , and V SD . Diodes D 202 , D 203 , D 204  and capacitors C 209 , C 210  and C 211  provide a boot-strap mechanism for powering the IC. Capacitors C 212  and C 218  provide bulk storage to start the controller at power up.  
      The frequency of oscillation of the controller is determined by the total resistance connected between pin  12  (Corn) and pin  4  of the controller U 201  and a capacitor C 213  connected between pin  6  and pin  12  (Corn) of the controller U 201 . The two outputs of U 201  at pins  11  and  16  are connected to the gates of the MOSFETs Q 221  and Q 222 . A resistor R 208  limits the gate current of the MOSFET Q 221 . A second resistor R 215  limits the gate current of the MOSFET Q 222 .  
      When power is applied to the circuit, the voltage developed on the bus  267  causes voltage to be applied to U 201  V CC , V DC , and SD. This causes U 201  to start oscillating and start driving the half-bridge transistors Q 221  and Q 222  alternately. This applies voltage across the primary winding T P  of the transformer T 222 , which in turn applies voltage across the secondary winding T S  of the transformer, which is applied to the load.  
      The rectified output of the DC bus  267  is applied to the Vcc and V DC  pins of the controller U 201  through resistors R 201  and R 202 . An internal zener diode and capacitors C 218  and C 212  maintain the operating voltages for the controller. A voltage divider consisting of a thermistor TH 201  and R 205  sets the voltage at pin  9  (SD) of U 201 . The controller uses these three voltages to determine the state of the power bus  267  to prevent operation when the power bus has collapsed.  
      The preset output voltage is set by the turns ratio of the output transformer T 222 . A limited dimming control is achieved by adjusting the resistance that appears between pins  6  and  7  of controller U 201 . This resistance controls the amount of dead time for the output FETs, which reduces the RMS value of the output voltage of T 222  and thereby reduces the intensity of the light strings connected to terminals  263 ,  264  and  265 ,  266   
      The dimming feature can be used to provide different fixed light levels, such as a low light output, an energy-saving output, or a full-light output. These three light levels can be achieved by use of three fixed resistors in place of the potentiometer R 214 . The three resistor settings can be selected by use of a three-position switch. A low-light output corresponds to a maximum output dead time, and a full-light output corresponds to minimum dead time. An energy-saving output corresponds to an intermediate light level such as a 75% light output.  
      The controller has an additional control pin (SD) which can be used as a thermal shutdown control to protect the power supply from overheating. As the air temperature in the unit rises, the value of TH 201  will decline until the voltage appearing at pin  9  of U 201  rises above the shut down value of approximately 2.0 volts.  
      The particular embodiment illustrated in  FIG. 14  employs a half bridge circuit, but it will be understood that the features of this circuit can be incorporated in other topologies such as flyback, forward, buck, full bridge or other power converters, including isolated as well as non-isolated power converter designs.  
      While the present invention has been described with reference to one or more particular embodiments, those skilled in the art will recognize that many changes may be made thereto without departing from the spirit and scope of the present invention. Each of these embodiments and obvious variations thereof is contemplated as falling within the spirit and scope of the claimed invention, which is set forth in the following claims.