Abstract:
A multi-color white LED which can be driven by a single current, and more particularly, to the use of a specific ratio of numbers of different color LEDs to obtain a specific desired color while running all of them at the same current.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to multi-color white LEDs, which can be driven by a single current, and more particularly, to the use of a specific ratio of a plurality of LEDs having a different color spectrum to obtain a desired color and while running each of the LEDs at the same current. 
       BACKGROUND OF THE INVENTION 
       [0002]    Although most LEDs (light emitting diodes) today are devices which run at low voltage, the technology exists to run LEDs directly from a high voltage source, such as an AC (alternating current) line. Two common circuits for running LEDs directly from a high voltage source both place enough LEDs in a series string that their forward voltage at a desired current is approximately equal to that of the AC line voltage. In one circuit technique, two anti-parallel strings are placed directly across the AC line, with one string conducting during the first half line-cycle and the other during the second half. In a second circuit technique, a single string of LEDs is placed after a bridge rectifier, so that the single string of LEDs conducts during both halves of the line-cycle. 
         [0003]    In both of these circuit techniques up until now, white LEDs have been created by using a string of white LED dice. It can be appreciated that this works well, if the dice are available to produce the exact shade of white, which is desired. However, in many cases the particular desired white is not available, in which case there is no way to use an AC LED, and designers must go to DC (direct current) LEDs and use complicated and expensive AC/DC converters to run them. 
         [0004]    The other method of obtaining white light from LEDs is to use a combination of two or more different color LEDs. This is commonly done in DC LEDs. However, in circuits to date this has been impracticable in AC LEDs, because each LED color must be run at its own current. The reason for this is that to create white light requires a different amount of each LED color. Further, each LED color has different efficacy, the two factors together resulting in each LED color typically being run on its own converter. 
         [0005]    Accordingly, it would be desirable to have the ability to select the precise shade of white delivered by an AC LED, which would permit more inexpensive LED lights to be produced for more applications. In addition, it would be desirable to generate arbitrary colored light using AC LEDs rather than needing one or more AC/DC converters. 
       SUMMARY OF THE INVENTION 
       [0006]    In accordance with an exemplary embodiment, one of the objects of this invention is developing a circuit with white or arbitrary light color output, and which provides an AC LED whose light color may be selected by the designer arbitrarily, without requiring one or more AC/DC converters. In accordance with an exemplary embodiment, the circuit includes a specific ratio of a plurality of different color LEDs, the ratio being based on the desired output light color and the efficacy of each color LED. In accordance with an embodiment, the total number of LEDs in the circuit is determined by the forward voltage required to obtain the desired average LED current, factoring in the difference in forward voltage of the different color LEDs. In addition, it can be appreciated that the same current can be run through each of the LEDs, permitting operation as a single string. 
         [0007]    In accordance with one embodiment, three colors of LEDs are used, red, green and blue (RGB), and wherein the ratios of the numbers of LEDs of each of the three colors is selected to give the correct output light color, factoring in the efficacy of the individual color LEDs by considering their light output at the same current. In addition, the total number of LEDs is selected to give the appropriate forward voltage to be operated from the AC line, accounting for the differing forward voltage (or forward voltage drop) of each of the differing colors of LEDs. 
         [0008]    In accordance with an exemplary embodiment, an LED includes at least two colors of LEDs, a ratio of the at least two colors of LEDs being selected to obtain a desired color spectrum, and wherein all of the at least two colors of LEDs are run in series at the same current. In accordance with another exemplary embodiment, an LED includes a plurality of different color LEDs, which are selected based on a desired output light color and an efficacy of each of the plurality of different color LEDs, and wherein the plurality of different color LEDs are all run in series at the same current. 
         [0009]    In accordance with an exemplary embodiment, a method of producing a spectrum of white light from a plurality of color LEDs, includes the steps of selecting at least one LED from at least two colors of LEDs; establishing a ratio of the at least one LED from at least two colors of LEDs to obtain a desired color spectrum; and running the plurality of LEDs in series at a desired current. In accordance with another exemplary embodiment, a method of developing a circuit with white or arbitrary light color output, includes the steps of providing a plurality of color LEDs; establishing a ratio of the plurality of color LEDS based on the desired output light color and efficacy of the each of the selected color LEDs; and running the plurality of color LEDs in series at a desired current. In accordance with a further exemplary embodiment, a method of producing a spectrum of white light from a plurality of LEDs having a different color spectrum, includes the steps of selecting a plurality of LEDs having a different color spectrum; establishing a specific ratio of the plurality of LEDs having a different color spectrum to obtain a desired color; and running the plurality of LEDs having a different color spectrum in series at the same desired current. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings, 
           [0011]      FIG. 1  is a drawing of the spectrum of white light from an incandescent source. 
           [0012]      FIG. 2  is a schematic of a circuit that uses a set of RGB (red, green, and blue) LEDs to produce a white AC LED. 
           [0013]      FIG. 3  is a view of a device using RGB (red, green, and blue) LEDs to produce a white AC LED. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0014]    Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts. 
         [0015]    According to the design characteristics, a detailed description of the current practice and preferred embodiments is given below. 
         [0016]      FIG. 1  is a drawing of a spectrum  10  of white light from an incandescent source. In this drawing, with arbitrary scale, red light  20  at 650 nm is approximately 230 units, green light  30  at 525 nm is approximately 75 units, and blue light  40  at 450 nm is approximately 45 units. 
         [0017]      FIG. 2  is a schematic of an exemplary circuit  60  that uses a plurality of red LEDs  21 , a plurality of green LEDs  31  and a plurality of blue LEDs  41  to produce a white AC LED  100 . In accordance with an exemplary embodiment, a red LED  22  typically has a forward voltage (or forward voltage drop) of approximately 2.2V at a specified current, a green LED  32  typically has a forward voltage of approximately 3.4V at the same current, and a blue LED  42  typically has a forward voltage of approximately 3.4V at the same current. In accordance with this exemplary embodiment, a single red LED  22  has a luminous flux of approximately 40 in arbitrary units at this same current, a single green LED  32  has a luminous flux of 52 in the same units at this same current, and a single blue LED  42  has a luminous flux of 15 in the same units at this same current. 
         [0018]    In accordance with this exemplary embodiment, the desired ratio of red to green to blue (i.e., R:G:B) LEDs  21 ,  31 ,  41  to obtain the spectrum  10  of white light from an incandescent source as shown in  FIG. 1  is approximately equal to the following ratio: 230:75:45. In order to achieve this with luminous fluxes in the ratio R:G:B=40:52:15, the LED would have the number of LEDs in the ratio R:G:B equal to approximately (230/40):(75/52):(45/15)=5.75:1.44:3.00≈4:1:2. 
         [0019]    In this exemplary embodiment, using 4 (four) red LEDs  21  plus 1 (one) green LED  31  plus 2 (two) blue LEDs  41  in series would produce a forward voltage of 4*2.2V+1*3.4V+2*3.4V=19V. However, in order to achieve a forward voltage of 120V, suitable for the AC line, the number of such sets of LEDs should be approximately 120V/19V≈6. Thus, this gives a total of 24 (twenty-four) red LEDs  21 , 6 (six) green LEDs  31  and 12 (twelve) blue LEDs  41 . By selecting 7 sets, the total number of LEDs  100  may be made equal to 49, a square number. 49 (forty-nine) LEDs  100  may be made suitable for the AC line by suitable adjustment of the operating current for the circuit. It can be appreciated that circuits  60  with more or less than three colors of LEDs  21 ,  31 ,  41  and more or less sets of LEDs can be implemented based on the desired color spectrum. 
         [0020]    In addition, the exemplary embodiment as shown in  FIG. 2 , is only an exemplary embodiment, and that the at least two color LEDs are not limited to red, blue and green LEDs, such that the circuits and methods as described herein can be extended to any color LED or LED having a different color spectrum, and which is capable of producing a desired color spectrum in combination with at least one other color LED. In addition, the parameters for the color LEDs as they pertain to wavelength, forward voltage, and luminous fluxes are only examples, and should not be construed as limiting parameters for those color LEDs or other suitable color LEDs. 
         [0021]      FIG. 3  is a view of a device using RGB LEDs to produce a white AC LED  110 . In a preferred embodiment, 28 (twenty-eight) red LEDs  21 , 7 (seven) green LEDs  31  and 14 blue LEDs  41 , which are randomly arranged in a 7×7 die matrix  120 . The random arrangement is selected in order to aid mixing of the colors of light. It can be appreciated that the red LEDs  21 , green LEDs  31 , and the blue LEDs  41  can be randomly arranged in any suitable matrix and is not limited to a 7×7 die matrix  120 . For example, the die matrix  120  can be a 3×3, a 5×5, a 7×7, a 9×9, or larger, or can be non-square, for example 6×7. 
         [0022]    It will be apparent to those skilled in the art that various modifications and variation can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.