Patent Abstract:
A low THD load adapter system is disclosed. The load adapter system includes a first lighting module and a second lighting module connected parallel to the first lighting module. During each AC cycle the first lighting module conducts current for a first portion of the cycle and the second lighting module conducts current for a second portion of the cycle. When combined, the total current drawn from the power source substantially tracks the shape of the applied AC voltage. Accordingly, there is minimal distortion, and low total harmonic distortion level is achieved.

Full Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This patent application claims the benefit of priority under 35 USC sections 119 and 120 of a regular patent application filed May 29, 2009 having application Ser. No. 12/455,127, and a provisional patent application filed Sep. 7, 2008 having Application Ser. No. 61/191,307 and a provisional patent application filed Nov. 16, 2008 having Application Ser. No. 61/199,493. The entirety of the application Ser. Nos. 12/455,127, 61/191,307, and 61/199,493 are all incorporated herein by reference. The applicant claims benefit to Sep. 7, 2008 as the earliest priority date. 
     
    
     BACKGROUND 
       [0002]    The present invention relates generally to lighting systems having low total harmonic distortion characteristics, and more particularly to a lighting system including an inventive configuration of light emitting devices such as, for example, LEDs, to achieve low total harmonic distortion characteristics. 
         [0003]    In lighting systems and technology, there has been and continues to be an ever increasing desire to achieve a number of competing and often conflicting goals. For example, these goals include, inter alia, reliability, minimal cost, and minimization of electrical interferences. This is not a complete list. In particular, the goal of minimizing electrical interferences has proven difficult to achieve without increasing costs and decreasing reliability. 
         [0004]    Lighting systems typically connect to alternating current (AC) electrical power source and generate light by drawing current from the AC power source. In the U.S., the AC power provides a cyclical voltage of approximately 120 volts RMS (root mean square) with a peak voltage value ranging from approximately positive 170 volts (V) to approximately negative 170 volts. In Europe and other countries, the available AC power is approximately 240 volts RMS. Other countries may use a different frequency, for example, 50 Hz. Other platforms (for example, aircraft avionics) may use another frequency such as 400 Hz. The same principles apply to the following discussion regardless of applied oscillatory voltage or frequency. 
         [0005]    The AC power is cyclical with an oscillation frequency of approximately 60 Hertz (Hz) for the example application. Each complete voltage oscillation is considered a complete power cycle and includes 360 degrees. A sample AC power cycle is often illustrated as a sinusoidal graph as illustrated in  FIG. 1 , which illustrates a number of oscillations of the AC power voltage as represented by a solid line graph  120   v . In  FIG. 1 , the horizontal axis represents time flowing from left to right, and the vertical axis for solid graph  120   v  represents voltage amplitude in volts. As illustrated, a single power cycle, in this example, lasts approximately 16.7 milliseconds (ms) which is one second divided by 60 cycles. 
         [0006]    Electrical interferences are often measured in total harmonic distortion (THD) compared to the input AC power. In the present context, THD is a measure of extent or magnitude to which the wave shape of the electrical current drawn from the AC power is distorted compared to the sinusoidal shape of the AC voltage  120   v . In numerical terms, THD is expressed as a percentage calculated as the ratio of the sum of the powers of all harmonic frequencies above the fundamental frequency to the power of the fundamental frequency. In the present example, the fundamental frequency of the AC power is 60 Hz. It is desirable to minimize electrical interferences generated by a lighting system by minimizing lighting system THD. 
         [0007]    Many current lighting systems use fluorescent bulbs, especially for industrial and commercial applications. Fluorescent bulbs are more efficient compared to incandescent bulbs. However, fluorescent bulbs are notoriously noisy. That is, fluorescent bulbs draw current from the AC power source such that undesirably high levels of total harmonic distortions (THD) are generated. This is illustrated using  FIGS. 1A and 2A . 
         [0008]      FIG. 2A  illustrates a lighting system including a fluorescent bulb  10  connected to an electrical plug  12 . The plug  12  is adapted to engage in a socket that provides the electrical power  120 , the alternating current (AC) described above. In  FIG. 2A , the load on the provided AC power  120  is the fluorescent bulb  20 . Often, an inductor  15  is serially connected with the bulb  10  to limit the current flowing through the bulb  10 . A representative dashed graph  10   i  is an approximation of the shape of the current through the bulb  10 . The actual conduction duration, the maximum and minimum currents +I MAX  and −I MIN , and the exact shape of the representative dashed graph  10   i  depend on a number of factors. The factors may include, for example only, wattage rating of the bulb  10 , ambient temperature, exact waveshape and characteristics of the power voltage  120   v , characteristics of the inductor  15 , many others not listed here, or a combination of any one or more of these factors. For the purpose of discussing the background, the exact numerical value and the exact shape of these curves are not important; however, the maximum positive and negative currents, +I MAX  and −I MAX  typically range between plus and minus 670 mA (peak of the AC waveform). The shape of the illustrated curve  10   i  is one possible sample shape only and may not indicate the exact shape of the current flow graph which may vary widely as already noted above. 
         [0009]      FIG. 1A  is a graph illustrating electrical characteristics of the lighting system of  FIG. 2A . Referring to  FIGS. 1A and 2A , the AC power voltage  120   v  is a sinusoidal shaped graph  120   v  having 60 Hz oscillation. Current through the fluorescent bulb  10  is represented by representative dashed graph  10   i . The applied AC power  120  drives current flow (as illustrated by the representative dashed graph  10   i ) through the fluorescent bulb  10 . As illustrated in  FIG. 1A , the shape of the current  10   i  through the fluorescent bulb  10  is highly dissimilar to the sinusoidal shape of the AC voltage  120   v . In fact, the shape of the current  10   i  is exceedingly distorted compared to the shape of the AC voltage  120   v . This is because the fluorescent bulb  20  presents a highly non-linear load to the applied AC voltage  120   v . This is caused by a number of factors including, for example only, the operating characteristics of fluorescent bulbs. The high degree of distortion of the current  10   i  means that the total harmonic distortion is correspondingly high. 
         [0010]    In some implementations, the THD value of fluorescent bulbs exceeds 100 percent. That is, more current is drawn at non-fundamental frequencies compared to the current drawn at the fundamental frequency. Such high THD value leads to a number of undesired affects such as, for example, stresses to wires, circuits, and all other systems connected to the same AC source  120 . Further, the high THD value results in undesired levels of electrical noise to all surrounding and commonly connected circuits and electrical systems. In some jurisdictions, there are efforts to limit and regulate the THD values of various circuits allowed to be operated within the jurisdiction. 
         [0011]    In most fluorescent bulb based lighting systems, the fluorescent bulb is isolated from the AC power  120  by a ballast circuit that operates to reduce the THD.  FIG. 2B  illustrates the lighting system of  2 A with a ballast  17  connected to the fluorescent bulb  10  on one side and the electrical plug  12  on the other side. The ballast  17  regulates the current flowing through the fluorescent bulb  10  to decrease distortion of the shape of the current, thereby reducing the THD. However, the ballast  17  introduces additional electrical components. These additional electrical components increase the costs and reduce the reliability of the fluorescent bulb based lighting system. 
         [0012]    New and increasing popular lighting technology involves the use of light emitting diodes (LEDs). LEDs are cost effective and have higher luminous efficacy compared to incandescent bulbs and fluorescent bulbs.  FIG. 3  illustrates a lighting system including a first light emitting diode (LED)  21  connected to the plug  12  in a first direction and a second light emitting diode (LED)  22  connected to the plug  12  in the opposite direction and also connected to the LED  21  in parallel. Collectively, the LEDs  21  and  22  are referred to herein as the LED pair  20 . As with the lighting system  FIG. 2A , the plug  12  is adapted to engage in a socket that provides the electrical power  120  as described above. In  FIG. 3 , the load to the electrical power  120  is the LED pair  20 . LEDs are diodes that conduct electricity in one direction. To take advantage of the alternating current power source  120 , two LEDs are configured as shown to produce light. Often, a resistor  25  is serially connected with the LED pair  20  to limit the current flowing through the LED pair  20 . 
         [0013]      FIG. 1B  is a graph illustrating electrical characteristics of the lighting system of  FIG. 3 . Referring to  FIGS. 1B and 3 , during the positive portion  121  (also, the “positive swing”) of each power cycle, node  122  is at positive voltage compared to node  124 . During the positive swing  121 , the first LED  31  is forward biased and the second LED  32  is reverse biased, thus, no current flows through the second LED  32 . However, after a threshold voltage (+V TH ) is reached, current flows through the first LED  31 , generating light. 
         [0014]    During the negative portion  123  (also, the “negative swing”) of each of the power cycles, tab point  124  is at positive voltage compared to tab point  122 . During the negative swing  123 , the first LED  31  is reverse-biased and the second LED  32  is forward biased, thus, no current flows through the first LED  31 . However, after a threshold voltage (−V TH ) is reached, current flows through the second LED  33 , generating light. 
         [0015]    The lighting system of  FIG. 3  has electrical characteristics similar to that of the lighting system of  FIG. 2A , though possibly with a different current waveform. The representative dashed graph  16   i  of  FIG. 1B  approximates the shape of the current through the LED pair  20 . The actual conduction duration, the maximum and minimum currents +I MAX  and −I MIN , and the exact shape of the representative dashed graph  16   i  depend on a number of factors. The factors may include, for example only, wattage rating of the LED pair  20 , ambient temperature, exact shape and characteristics of the power voltage  120   v , characteristics of the resistor  25 , many others not listed here, or a combination of any one or more of these factors. For the purpose of discussing the background, the exact numerical value and the exact shape of these curves are not important. In one example, the maximum positive and negative currents, +I MAX  and −I MAX  typically range between plus and minus 80 mA in either direction. 
         [0016]    The value of the threshold voltage (positive and negative) depends on the value of the resistor  25  and characteristics of the LED pair  20 . The amount of current depends on a number of factors including the wattage rating of the LEDs  20  and the value of the resistor  25 . Again, for our purposes here, the exact numerical values of these are not important. 
         [0017]    As illustrated in  FIG. 1B , the shape of the current (represented by dashed line graph  16   i ) through the LED pair  20  is not similar to the sinusoidal shape of the AC voltage  120   v  and is, in fact, very distorted compared to the shape of the AC voltage  120   v . This is because the LED pair  20  presents a highly non-linear load to the applied AC voltage  120   v . This is caused by a number of factors including, for example only, the way LEDs operate to generate light. The high degree of distortion of the current  16   i  means that the total harmonic distortion is correspondingly high. In fact the THD for the LED pair is often over 100 percent. 
         [0018]    To realize even lower THD values for LED based lighting systems, some suggested use of complex LED driver circuits between the LEDs and the power source. For example, U.S. Pat. No. 6,304,464 to Jacobs teaches the use of a complex “flyback converter” for, inter alia, THD reduction. In another example, U.S. patent application Ser. No. 11/086,955 having a filing date of Mar. 22, 2005 and publication date of Sep. 28, 2006 teaches the use of a complex “digital power converter for driving LEDS.” The use of these LED driver circuits introduces additional electrical components. These additional electrical components increase the complexity and the costs, and reduce the reliability of these LED systems. 
         [0019]    Accordingly, the need remains for LED based lighting systems having even lower levels of THD values while eliminating or minimizing the need for additional circuits and components. 
       SUMMARY OF THE INVENTION 
       [0020]    The need is met by the present invention. In a first embodiment of the present invention, a lighting system includes a first lighting module, a second lighting module, a first capacitor, and a second capacitor. The first lighting module includes at least one light emitting element. The second lighting module includes at least one light emitting element. The second lighting module is connected in parallel to the first lighting module. The first capacitor is connected in series with the first lighting module. The first capacitor is connected in parallel to the second lighting module. The second capacitor is connected in series with both the first lighting module and the second lighting module. When electrical power is applied to the lighting system, the first lighting module conducts electrical current during a first conduction period within each power cycle and the second lighting module conducts electrical current during a second conduction period within each power cycle. 
         [0021]    In the lighting system, a portion of the first conduction period overlaps a portion of the second conduction period. The first lighting module, when connected to the electrical power source, also conducts during a third conduction period within each power cycle, and the second lighting module, when connected to the electrical power source, also conducts during a fourth conduction period within each power cycle. A portion of the third conduction period overlaps a portion of the fourth conduction period. 
         [0022]    The lighting system&#39;s first and second lighting modules may each include a plurality of LED pairs wherein each LED pair includes a first LED connected in forward direction and a second LED connected in reverse direction. 
         [0023]    Alternatively, the lighting system&#39;s first and second lighting modules may each include two parallel sets of LEDs wherein a first set of plural LEDs is serially connected in forward direction and a second set of plural LEDs is serially connected in reverse direction. 
         [0024]    The first lighting module includes a first predetermined number of LEDs and the second lighting module includes a second predetermined number of LEDs wherein the first predetermined number is less than the second predetermined number. 
         [0025]    In a second embodiment of the present invention, a lighting system is adapted to connect to an electrical power source providing alternating current (AC) electrical power, the electrical power having power cycles. The lighting system includes a first lighting module, a first rectifier, a second lighting module, and a second rectifier. The first lighting module includes at least one light emitting element. The first rectifier is connected to the first lighting module to provide a first rectified signal to the first lighting module. The second lighting module includes at least one light emitting element. The second rectifier is connected to the second lighting module to provide a second rectified signal to the second lighting module. The first rectifier and the first lighting module are connected in parallel to the second rectifier and the second lighting module. With electrical power applied to the lighting system, the first lighting module conducts electrical current during a first conduction period within each power cycle and the second lighting module conducts electrical current during a second conduction period within each power cycle. 
         [0026]    The lighting system may also include a first capacitor connected in series with the first lighting module. The lighting system may also include a second capacitor. The second capacitor is connected in series with both the first lighting module and the second lighting module. The lighting system may also include a third capacitor connected parallel to the first lighting module and a fourth capacitor connected parallel to the second lighting module. 
         [0027]    In the lighting system, a portion of the first conduction period overlaps a portion of the second conduction period. In the lighting system the first lighting module, when connected to the electrical power source, conducts during a third conduction period within each power cycle, and the second lighting module, when connected to the electrical power source, conducts during a fourth conduction period within each power cycle. A portion of the third conduction period overlaps a portion of the fourth conduction period. The first lighting module includes a first predetermined number of LEDs and the second lighting module includes a second predetermined number of LEDs wherein the first predetermined number is less than the second predetermined number 
         [0028]    In a third embodiment of the present invention, a lighting system is adapted to connect to an electrical power source providing alternating current (AC) electrical power, the electrical power having power cycles. The lighting system includes a first current path and a second current path. The first current path includes at least one lighting emitting element. The second current path includes at least one light emitting element and is connected in parallel to the first current path. The first current path is adapted to conduct electrical current during a first conduction period within each power cycle and the second current path is adapted to conduct electrical current during a second conduction period within each power cycle. 
         [0029]    In a fourth embodiment of the present invention, a method of generating light from an alternating current (AC) electrical power source having power cycles, the method includes the following steps: First, an alternating current power source is provided, the alternating current having a substantially sinusoidal flow characteristics and including continuous power cycles; light is generated during a first conduction period during each power cycle using a first set of light emitting devices (LEDs) by conducting current during the first conduction period; light is generated during a second conduction period during each power cycle using a second set of light emitting devices (LEDs) by conducting current during the second conduction period; and the current conducted during the first conduction period and the second conduction period aggregate to a total conduction current flow that has substantially sinusoidal flow characteristics. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]      FIG. 1A  is a graph illustrating electrical characteristics of the lighting system of  FIG. 2A ; 
           [0031]      FIG. 1B  is a graph illustrating electrical characteristics of the lighting system of  FIG. 3 ; 
           [0032]      FIG. 2A  is a schematic diagram of a prior art lighting system including a fluorescent lamp; 
           [0033]      FIG. 2B  is schematic diagram of a prior art lighting system illustrated in  FIG. 2A  with an additional component; 
           [0034]      FIG. 3  is a schematic diagram of a prior art lighting system including light emitting diodes; 
           [0035]      FIG. 4  is a schematic diagram of a lighting system in accordance with one embodiment of the present invention; 
           [0036]      FIGS. 5 ,  6 ,  7 ,  8 , and  9  illustrate graphs representing various electrical characteristics of the lighting systems of  FIGS. 4 ,  10  and  11 ; 
           [0037]      FIG. 10  is a schematic diagram of a lighting system in accordance with another embodiment of the present invention; 
           [0038]      FIG. 11  is a schematic diagram of a lighting system in accordance with yet another embodiment of the present invention; 
           [0039]      FIGS. 12   a  through  12   e , inclusive, illustrate graphs representing various electrical characteristics of the lighting systems of  FIG. 11 ; and 
           [0040]      FIG. 13  is a schematic diagram of a lighting system in accordance with yet another embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0041]    The lighting system of the present invention includes lighting elements such as, but not limited to, light emitting diodes (LED) in a configuration to minimize total harmonic distortion while not requiring separate and complex driver circuitry. Here, the challenge, as discussed above, is to generate light from an alternating current (AC) electrical power (the electrical power having power cycles) while generating lower distortion levels (THD, the total harmonic distortion) than previously possible. In the present invention, this is accomplished by having at least two lighting modules in parallel, each module conducting (drawing current thereby generating light) during different periods of each power period. These currents combine such that the shape of the total current drawn by the lighting system is more similar to the sinusoidal shape of the AC power. That is, the lighting system current graph of the present invention has less distortion compared to the AC power sinusoidal shape, than the current graph distortions of prior art lighting systems. 
         [0042]      FIG. 4  illustrates one embodiment of the lighting system  100  of the present invention. The lighting system  100  of the present invention includes a first lighting module  30  and a second lighting module  40 . The first lighting module is adapted to connect to an electrical power source  120  via an electrical plug  12 . The electrical power source  120  provides alternating current (AC) electrical power, the electrical power having power cycles. In the U.S., the AC power provides a cyclical voltage of approximately 120 volts RMS (root mean square) with a peak voltage value ranging from approximately positive 170 volts to approximately negative 170 volts. In Europe and other countries, the available AC power is approximately 220 volts RMS. The first lighting module  30  defines a first current path and the second lighting module  40  defines a second current path. 
         [0043]    The AC power  120  is cyclical in that the AC power has an oscillation frequency of approximately 60 Hertz (Hz).  FIG. 5  illustrates a number of oscillations of the AC power voltage as represented by a solid line graph  120   v . Each complete oscillation of voltages is considered a complete power cycle and includes 360 degrees. In  FIG. 5 , the horizontal axis represents time flowing from left to right, and the vertical axis for graph  120   v  represents voltage amplitude in volts. As illustrated, a single power cycle, in this example, lasts approximately 16.7 milliseconds (ms) which is one second divided by 60 cycles. For convenience of discussion herein, a single power cycle period  125  is used to discuss the operations of the lighting system  100  of  FIG. 4 . As for the beginning and the ending of the power cycle period  125 , it is arbitrary where the power cycle is deemed to begin and to end as long as the power cycle period  125  includes a complete oscillation, the entire 360 degrees. 
         [0044]    Continuing to refer to  FIG. 4 , the first lighting module  30  includes at least one light emitting element. In the illustrated sample embodiment, the first lighting module  30  includes 12 LED pairs (for a total of 24 individual LEDs), each LED pair having one forward biased LED and one reversed biased LED. In the illustrated sample embodiment, each of the LEDs of the first lighting module  30  has a 2.5 volt turn-on (threshold) voltage with operating voltage of 3.3 volts as observed. Such LEDs are available in the marketplace as, for example, LW540 from Seoul Semiconductor Company, Ltd. Accordingly, for each direction of electrical flow, the first lighting module  30  presents a turn-on threshold voltage of 30.0 volts, V THRESHOLD . This number is 2.5 volts multiplied by 12 LEDs in a particular direction. Again, the present invention is not limited in scope to this illustrated embodiment. Selection of direction as “forward” or “reverse” is arbitrary for the purposes of the present invention; however, for the purposes of discussion herein, directions beginning at node  126 , through the modules  30  and  40 , and ending at node  150  are considered “forward.” The number of LEDs may range from one to many depending on the characteristics of the LEDs, the desired current graph, etc. 
         [0045]    In the illustrated embodiment, the lighting elements are light emitting diodes (LEDs); however, the present invention is not limited to LEDs as the light emitting element but may include other light emitting devices such as, for example only, Organic Light Emitting Diode (OLED), Light Emitting Polymer (LEP), and Organic Electro Luminescence (OEL), or other lighting means. 
         [0046]    The second lighting module  40  is also adapted to connect to the electrical power source  120  via the electrical plug  12 . The second lighting module  40  includes at least one light emitting element. In the illustrated sample embodiment, the second lighting module  40  includes 21 LED pairs (for a total of 42 individual LEDs), each LED pair having one forward biased LED and one reverse biased LED. The second lighting module  40  is connected in parallel to the first lighting module  30 . 
         [0047]    In the illustrated sample embodiment, each of the LEDs of the first lighting module  40  has a 2.5 volt turn-on (threshold) voltage. Accordingly, for each direction of electrical flow, the first lighting module  40  presents a turn-on threshold voltage of 52.5 volts, V THRESHOLD . This number is 2.5 volts multiplied by 21 LEDs in a particular direction. The number of LEDs may range from one to many depending on the characteristics of the LEDs, the desired current graph, etc. The second lighting module  40  includes a greater number of lighting elements compared to the number of lighting elements of the first lighting module  30 . 
         [0048]    A first capacitor  50  is connected in series with the first lighting module  30 . The first capacitor is connected in parallel to the second lighting module  40 . In the illustrated embodiment, the first capacitor  50  has value of approximately 2.7 microfarad (μf). 
         [0049]    A second capacitor  52  is connected in series with both the first lighting module  30  and the second lighting module  40  as illustrated. Further, the second capacitor  52  is connected in series with the first capacitor. In fact, the second capacitor  52  connects to the power source  120  on the one side, and on its other side, the second capacitor  52  connects to the first capacitor  50  and to the second lighting module  40 . In the illustrated embodiment, the second capacitor  52  has a value of approximately 3.3 μF. 
         [0050]    Operations of the lighting system  100  of  FIG. 4  are described below with reference to  FIGS. 5 through 9 .  FIG. 5  is a graph illustrating AC voltages over time of the AC power supply  120  as AC supply voltage  120   v , AC voltage at node  130  as AC voltage  130   v , and AC voltage at node  140  as AC voltage  140   v . Nodes  130  and  140  of  FIG. 4  and other “nodes” of the Figures of the present invention merely indicate a location or a point (of the circuit or apparatus) indicated by the reference number and its callout line. Accordingly, the term “node” does not indicate any structure or special protuberance. 
         [0051]    Referring to  FIGS. 4 and 5 , the AC power voltage oscillates between approximately positive 170 V and approximately negative 170 V. Again, this is in the U.S. where the typical AC power outlets supply 120 volts RMS of AC power. In Europe and other countries, the available AC power is approximately 220 volts RMS. A single cycle of the AC power voltage  120   v  is illustrated as power cycle period  125  which begins at time T 1  and ends at time T 2 . As for the beginning and the ending of the power cycle period  125 , it is arbitrary where the power cycle is deemed to begin and to end as long as the power cycle period  125  includes a complete oscillation, the entire 360 degrees. In  FIG. 5 , for convenience of discussion, the power cycle period  125  is illustrated as beginning at T 1  when the voltage is at zero, extending through its positive swing period  121  (180 degrees), passing through zero volts, and through its negative swing period  123  (180 degrees) back to the zero voltage at T 2 , thereby completing its 360 degrees. In the present example, the power cycle period  125  is approximately 16.7 milliseconds (ms). Time references (on the Figures and also used herein) are labeled such as, in general, T N  where the subscripts N used herein indicate various points on the time line and therefore do not indicate that these references occur in the sequence according to the numerical value of N. 
         [0052]    The power voltage  120   v  is available from the power supply  120  through connected plug  12 , and is operated on by the second capacitor  52 . The second capacitor  52  presents capacitance and capacitive reactance to the incoming power voltage such that, at node  140 , the power cycle  120   v  is delayed by almost approximately 15.1 ms. The delayed AC voltage  140   v  at node  140  is illustrated in  FIG. 5 . A single AC voltage cycle  140   v  is illustrated as cycle period  145 , which begins at time T 5  and ends at time T 6 . As for the beginning and the ending of the cycle period  145 , it is arbitrary where the cycle period is deemed to begin and to end as long as the cycle period  145  includes a complete oscillation, the entire 360 degrees. 
         [0053]    In  FIG. 5 , for convenience of discussion, the cycle period  145  is illustrated as beginning at T 5  when the voltage is at zero, extending through its positive swing period, passing through zero volts, and through its negative swing period back to the zero voltage at T 6 , thereby completing its 360 degrees. In the present example, the cycle period  145  is also approximately 16.7 ms. The cycle period  145  lags the power cycle period  125  by about 15.0 ms which is about 335 degrees in the sinusoidal curve. This conditional is operationally equivalent to the cycle period  145  leading the power cycle period  125  by about 1.6 ms or about 25 degrees (360 less 335 degrees). Such lagging conditions (where the lag is over 180 degrees) are conventionally referred to as the cycle period  145  leading the power cycle period  125 . This convention is used in this document. The lead of the voltage  140   v  compared to the power voltage  120   v  is illustrated as gap  149 . 
         [0054]    The voltage  140   v  at node  140  is operated on by the first capacitor  50 . The first capacitor  50  presents capacitance and capacitive reactance to the voltage  140   v  such that, at node  130 , the voltage  130   v  leads the voltage  140   v  by about 1.9 ms and leads the power voltage  120   v  by approximately 3.2 ms. The delayed AC voltage  130   v  at node  130  is illustrated in  FIG. 5 . A single cycle of the AC voltage  130   v  is illustrated as cycle period  135 , which begins at time T 3  and ends at time T 4 . As for the beginning and the ending of the cycle period  135 , it is arbitrary where the cycle period is deemed to begin and to end as long as the cycle period  135  includes a complete oscillation, the entire 360 degrees. The actual peak (both positive and negative) values of the AC voltage  140   v , V PEAK-140 , may vary depending on implementation and the peaks of the power voltage  120   v . In the illustrated sample implementation, positive and negative peak voltages V PEAK-140  are approximately plus and minus 92 volts. The lead of the voltage  130   v  compared to the power voltage  120   v  is illustrated as gap  139 . 
         [0055]    In  FIG. 5 , for convenience of discussion, the cycle period  135  is illustrated as beginning at T 3  when the voltage is at zero, extending through its positive swing period, passing through zero volts, and through its negative swing period back to the zero voltage at T 4 , thereby completing its 360 degrees. In the present example, the cycle period  135  is also approximately 16.7 ms. The cycle period  135  leads the power cycle period  125  by about 3.1 ms or about 86 degrees. The AC voltage  130   v  is experienced by the first lighting module  30 . The actual peak (both positive and negative) values of the AC voltage  130   v , V PEAK-130 , may vary depending on implementation and the peaks of the power voltage  120   v , V PEAK-140 , or both. In the illustrated sample implementation, V PEAK-130  is approximately plus and minus 52 volts. 
         [0056]      FIG. 6  is a graph illustrating AC voltages at node  130  as AC voltage  130   v  and current conducting through the first lighting module  30  as graph  130   i  having a dash line. The operations of portions of the lighting system  100  are described here with reference to  FIGS. 4 and 6  beginning at time T 3 . During the positive swing  131  of the AC voltage  130   v , the voltage  130   v  increases from zero to some threshold turn-on voltage (in the forward direction) at time T 3A . Beginning at T 3A , forward biased LEDs  32  of the first lighting module  30  begin to conduct electrical current thereby generating light. During the positive swing  131 , reverse biased LEDs  34  do not conduct electricity. The forward biased LEDs  32  continue to conduct current until time T 3B  when the AC voltage  130   v  decreases below the threshold voltage. The temporal period between T 3A  and T 3B  is referred to as the first conduction period  136 . The actual value of the threshold voltage, V THESHOLD , is implementation dependent. In the illustrated embodiment, +V THESHOLD  is approximately 34 volts. The actual peak (both positive and negative) values of the current  130   i , I PEAK-130 , may vary depending on implementation. In the illustrated sample implementation, positive and negative peak currents I PEAK-130  are approximately plus and minus 80 mA. 
         [0057]    During the negative swing  133  of the AC voltage  130   v , the voltage  130   v  decreases from zero to some threshold turn-on voltage (in the reverse direction) at time T 3C . Beginning at T 3C , the reverse biased LEDs  34  of the first lighting module  30  begin to conduct electrical current thereby generating light. During the negative swing  133 , forward biased LEDs  34  do not conduct electricity. The reverse biased LEDs  34  continue to conduct current until time T 3D  when the AC voltage  130   v  increases above the threshold voltage (in the reverse direction). The temporal period between T 3C  and T 3D  is referred to herein as the third conduction period  138 . 
         [0058]      FIG. 7  is a graph illustrating AC voltages at node  140  as AC voltage  140   v  and current conducting through the second lighting module  40  as graph  140   i  having a dash-dot line. The operations of portions of the lighting system  100  are described here with reference to  FIGS. 4 and 7  beginning at time T 5 . During the positive swing  141  of the AC voltage  134   v , the voltage  140   v  increases from zero to some threshold turn-on voltage (in the forward direction) at time T 5A . Beginning at T 5A , forward biased LEDs  42  of the second lighting module  40  begin to conduct electrical current thereby generating light. During the positive swing  141 , reverse biased LEDs  44  do not conduct electricity. The forward biased LEDs  42  continue to conduct current until time T 5B  when the AC voltage  140   v  decreases below the threshold voltage. The temporal period between T 5A  and T 5B  is referred to as the second conduction period  146 . The actual value of the threshold voltage, V THESHOLD , is implementation dependent. In the illustrated embodiment, +V THESHOLD  is approximately 55 volts. The actual peak (both positive and negative) values of the current  140   i , I PEAK-140 , may vary depending on implementation. In the illustrated sample implementation, positive and negative peak currents I PEAK-140  are approximately plus and minus 80 mA. 
         [0059]    During the negative swing  143  of the AC voltage  140   v , the voltage  140   v  decreases from zero to some threshold turn-on voltage (in the reverse direction) at time T 5C . Beginning at T 5C , the reverse biased LEDs  44  of the second lighting module  40  begin to conduct electrical current thereby generating light. During the negative swing  143 , forward biased LEDs  44  do not conduct electricity. The reverse biased LEDs  44  continue to conduct current until time T 5D  when the AC voltage  140   v  increases above the threshold voltage (in the reverse direction). The temporal period between T 5C  and T 5D  is referred to herein as the fourth conduction period  148 . 
         [0060]      FIG. 8  illustrates a graph including portions of  FIGS. 5 through 7 .  FIG. 8  overlays the AC power voltage as represented by a solid line graph  120   v  with the first module current  130   i  (dash line, same as  130   i  of  FIG. 6 ) and the second module current  140   i  (dash-dot line, same as  140   i  of  FIG. 7 ). Referring to  FIG. 8 , an AC power cycle  155  is illustrated, the power cycle period  155  spanning a complete oscillation, the entire 360 degrees from time T 7  and time T 8 . The power cycle period  155  is same as the power cycle period  125  of previous Figures but for the fact that it begins at a different time T 7  compared to the beginning time of T 1  of the power cycle  125 . However, this is irrelevant. Again, it is arbitrary where the power cycle is deemed to begin and to end as long as the power cycle period includes a complete oscillation, the entire 360 degrees. In  FIG. 8 , for convenience of discussion, the power cycle period  155  is illustrated as beginning at T 7  which is before the beginning T 3A  of the first conduction period  136  and is after the end T 5D  of the fourth conduction period  138 . 
         [0061]    Referring now to  FIGS. 4 and 8 , during the application of the power cycle  155  to the lighting system  100 , the first lighting module  30  conducts electrical current (in the forward direction) during the first conduction period  136  and during the third conduction period  138 . This is illustrated by the first module current  130   i . Additionally, during the application of the power cycle  155  to the lighting system  100 , the second lighting module  40  conducts electrical current (in the reverse direction) during the second conduction period  146  and during the fourth conduction period  148 . This is illustrated by the second module current  140   i . As illustrated, the lighting modules  30  and  40  are connected in parallel to each other. Accordingly, these currents are added to determine the total current for the lighting system  100 . The total current drawn by the lighting system  100  is the sum of currents  130   i  (drawn by the first lighting module  30 ) and  140   i  (drawn by the second lighting module  40 ) and is referred herein as the light system current. 
         [0062]      FIG. 9  illustrates the total current (light system current) as dash line graph  126   i  as measured at the node  126  and the power cycle  155  from T 7  to T 8 . As is apparent from  FIG. 9 , the shape of the light system current  126   i  is similar to the shape of the power supply voltage  120   v . That is, the shape of the light system current  126   i  is only slightly distorted compared to the shape of the power supply voltage  120   v . Accordingly, the total harmonic distortion (THD) generated by the lighting system  100  of  FIG. 4  when connected to the AC power  120  is low. In fact, in some tests, the THD generated by the lighting system  100  of the present invention was in the range of less than ten percent. 
         [0063]      FIG. 10  illustrates another embodiment of the present invention. Referring to  FIGS. 4 and 10 , a lighting system  200  includes the lighting system  100  of  FIG. 4  and supporting circuit  190 . The supporting circuit  190  includes one or more components to protect the lighting system  100 , to support the operations of the lighting system  100 , or both. For example, the supporting circuit  190  is used to limit in-rush current at turn-on. If the in-rush current is not limited, the in-rush current may charge the capacitors  50  and  52  too rapidly, potentially damaging power switches used to activate the lighting system. 
         [0064]    In the illustrated embodiment, thermistor  198  specifically provides in-rush current limiting when first powering the circuit. In case the mains voltage is at the peak of its waveform when first applied to the circuit, there would be a relatively fast voltage surge across capacitive elements, leading to a large in-rush or surge current that could harm the LEDs or other components. When cold, the thermistor  198  acts as a resistor to minimize surge current. When heated (due to the operation of the system  200 ) the thermistor  198  offers decreased resistance so as minimize the resistive effects against the flow of current through the system  200 . Additionally, a fuse  194  may briefly experience a large current that could cause it to fail open, were it not for the thermistor  198 . 
         [0065]    The supporting fuse  194  is connected in series with the lighting system  100 . The fuse  194  protects the lighting system  100  by opening the circuit (thereby disconnecting the lighting system  100  from the power source  120 ) in case of excessive current flows. Rating of the fuse  194  varies depending on the implementation. In the illustrated embodiment, as an example only, the fuse  194  may have a rating in the order of one or two amperes. 
         [0066]    Another protective device is a spark gap  196  that protects the lighting system  100  from excessive input voltage. When excessive voltage is applied to the lighting system  100 , the current jumps the spark gap  196  rather than being directed to the lighting system  100  thereby protecting the lighting system  100  from the excessive voltage. Rating of the spark gap  196  varies depending on the implementation. In the illustrated embodiment, as an example only, the spark gap  196  may have a rating on the order of one kilo-volts. 
         [0067]    In the illustrated embodiment, the supporting circuit  190  includes a transient voltage suppressor  192  such as, for example, a metal oxide variable (MOV) resistor  192  to prevent a voltage spike on lighting system  100  when transient voltage surges appear on the power source  120 . The MOV resistor  192  can be, for example, MOV resistor known as part VE13M00151K in the marketplace. The MOV resistor  192  is connected in parallel with the lighting system  100 , through the fuse  194 . 
         [0068]    The supporting circuit  190  need not include all the components illustrated in  FIG. 10 . For example, the supporting circuit  190  can be as simple as including only the MOV resistor  192  and still be within the scope of the present invention. The supporting circuit  190  may include any combination of the components illustrated. Furthermore, the supporting circuit  190  may include additional components not illustrated therein and still be within the scope of the present invention. 
         [0069]      FIG. 11  illustrates yet another embodiment of the present invention. Referring to  FIG. 11 , a lighting system  300  includes a first lighting module  330  including at least one light emitting element. In the illustrated embodiment, the first lighting module  330  includes a plurality light emitting diodes of serially connected in a forward direction. Again, the designation of forward or reverse is arbitrary. A first rectifier  332  is connected to the first lighting module  330 . A first capacitor  50  is connected to the first rectifier  332 . For the first lighting module  330 , each light emitting element can be a light emitting diode (LED) such as, for example LED model LW540A which operate generally between three to four forward volts. LW540A and similar LEDs are available in the marketplace. In the illustrated embodiment, the first lighting module  330  includes 12 serially connected LEDs. The first rectifier  332  can have any known rectifier configuration. In the illustrated embodiment, the first rectifier  332  is a diode-bridge type rectifier having the illustrated configuration, each diode being, for example, a 1N4004 rectifier diode available in the marketplace. The first capacitor  50  can be, for example, a 1.47 μF 100V Polyester type capacitor. The actual model, value, and type of these diode and capacitor components and the number of LEDs in the first lighting module  330  may vary depending on application. 
         [0070]    In the illustrated embodiment, the second lighting module  340  includes a plurality of light emitting diodes of connected in a forward direction. Again, the designation of forward or reverse is arbitrary. A second rectifier  342  is connected to the second lighting module  340 . For the second lighting module  340 , each light emitting element can be a light emitting diode (LED) such as, for example type LW540A discussed above. In the illustrated embodiment, the second lighting module  340  includes 23 serially connected LEDs. The second rectifier  342  can have any known rectifier configuration. In the illustrated embodiment, the second rectifier  342  is a diode-bridge type rectifier having the same configuration and components as the first rectifier  332 . The actual model, value, and type of these diode and capacitor components and the number of LEDs in the second lighting module  340  may vary depending on application. The second lighting module  340  and the second rectifier  342  are connected to the first lighting module  330  and the first rectifier  332  in parallel. Continuing to refer to  FIG. 11 , a second capacitor  52  is connected in series with both the first rectifier  332  and the second rectifier  342 . The second capacitor can be, for example, a 3.75 μF 250V Polyester type capacitor. The lighting system  300  may but not necessarily include the supporting circuit  190  illustrated in more detail in  FIG. 10  and discussed above. 
         [0071]    The operations of the lighting system  300  are mostly similar to the operations of the lighting system  100  of  FIG. 4  and discussed above using  FIGS. 4 through 9 , inclusive, with minor differences. The AC power source  120  provides AC voltage  120   v  illustrated in  FIGS. 5 ,  8 , and  9  as it may appear at node  126 . The AC voltage is operated by the second capacitor  52  as illustrated in  FIG. 6  and discussed above such that voltage at node  140  appears as graph  140   v  illustrated in  FIGS. 5 and 7  and discussed above. The voltage  140   v  at node  140  is operated on by the first capacitor  50 , resulting as the voltage  130   v  at node  130  illustrated in  FIGS. 5 and 6  and discussed above. 
         [0072]    Referring now to  FIGS. 5 through 9  and  11 , in the lighting system  300 , the voltage  130   v  at node  130  is rectified by the first rectifier  332  such that, at node  331 , a pulsed-DC (direct current) voltage is present. The pulsed-DC voltage at node  331  causes the current to flow through the LEDs of the first lighting module  330 . The pulsed-DC voltage at node  331  is illustrated by graph  331   v  of  FIG. 12   a . Referring to Figures to  FIGS. 5 through 9 ,  11 , and  12   a , the illustrated pulsed-DC voltage graph  331   v  is a measured waveform between nodes  331   a  and  331   b .  FIG. 12   a  also illustrates the approximate sine wave  126   v  as the voltage measured between nodes  126  and  127 . 
         [0073]    As the graph  331   v  indicates, the first rectifier  332  rectifies the input voltage into a pulsed-DC voltage waveform. The pulsed-DC voltage at  331   v  may be conditioned, or smoothed, by a third capacitor  54  placed in parallel to the first lighting module  330 . The third capacitor  54 , for example only, can be a 1.0 μF 200V electrolytic type capacitor. The third capacitor  54  reduces ripples of the pulsed-DC voltage at  331 . Such ripple reduction may be useful for some types of light emitting elements. 
         [0074]    Continuing to refer to  FIGS. 5 through 9 , and  11 , and also referring to  FIG. 12   b , in the lighting system  300 , the voltage  140   v  at node  140  is rectified by the second rectifier  342  such that, at node  341 , a pulsed-DC (direct current) voltage is present. The pulsed-DC voltage at node  341  causes the current to flow through the LEDs of the second lighting module  340 . The pulsed-DC voltage at node  341  is illustrated by graph  341   v  of  FIG. 12   b . In  FIG. 12   b , the illustrated pulsed-DC voltage graph  341   v  is a measured waveform between nodes  341   a  and  341   b .  FIG. 12   a  also illustrates the approximate sine wave  126   v  as the voltage measured between nodes  126  and  127 . 
         [0075]    As the graph  341   v  indicates, the second rectifier  342  rectifies the input voltage into a pulsed-DC waveform. The pulsed-DC voltage at  341  may be conditioned, or smoothed, by a fourth capacitor  56  placed in parallel to the second lighting module  340 . The fourth capacitor  56 , for example only, can be a 1.0 μF 200V electrolytic type capacitor. The fourth capacitor  56  reduces ripples of the pulsed-DC voltage at  341 . Such ripple reduction may be useful for some types of light emitting elements. 
         [0076]    The lighting system  300  of  FIG. 11  is different from the lighting system  100  of  FIG. 4  in that the internal AC voltages at nodes  130  and  140  are rectified before being applied to lighting modules to generate light. However, the current flow characteristics of the lighting system  300  of  FIG. 11  are substantially similar to that of the lighting system  100  of  FIG. 4 . 
         [0077]    The current drawn by the first lighting module  330  is illustrated in  FIG. 12   c  as graph  330   i . The current graph  330   i  was measured by placing the oscilloscope probes across a ten-ohm resistor in series at node  331   a .  FIG. 12   c  also illustrates the measured input current at node  126  as current graph  126   i . The current graph  126   i  was measured with a floating probe across a ten-ohm resistor. Note that the use of the floating probe introduced noise on that signal trace such that the measured current graph  126   i  is not smooth but appears serrated. The current drawn by the second lighting module  340  is illustrated in  FIG. 12   d  as graph  340   i . The current graph  340   i  was measured by placing the oscilloscope probes across a ten-ohm resistor in series at node  341   a .  FIG. 12   d  also illustrates the measured input current at node  126  as current graph  126   i.    
         [0078]    When the currents at nodes  331   a  and  341   a  combine, they sum to the current graph  126   i . The current graph  126   i  measured between nodes  126  and  127  is illustrated in  FIG. 12   e  as current graph  126   i . The current graph  126   i  of  FIGS. 12   c  and  12   d ; however, the probe used is not floating and no noise is introduced to the measurement. 
         [0079]    Note that the overall system current as represented by the current graph  126   i  of  FIG. 12   e  is similar to the  126   i  of  FIG. 9 . Comparing  FIG. 9 , with respect to the system  100  of  FIG. 4 , it is apparent that the shape of the light system current  126   i  (of  FIG. 9 ) is similar to the shape of the power supply voltage  120   v . That is, the shape of the light system current  126   i  (of  FIG. 9 ) is only slightly distorted compared to the shape of the power supply voltage  120   v . Accordingly, the total harmonic distortion (THD) generated by the lighting system  100  of  FIG. 4  when connected to the AC power  120  is low. Likewise, comparing  FIG. 9  with respect to the system  300  of  FIG. 11 , it is apparent that the shape of the light system current  126   i  (of  FIG. 12   e ) is similar to the shape of the power supply voltage  126   v  (of  FIGS. 12   a  and  12   b ). That is, the shape of the light system current  126   i  (of FIG.  12   e ) is only slightly distorted compared to the shape of the power supply voltage  126   v  (of  FIGS. 12   a  and  12   b ). Accordingly, the total harmonic distortion (THD) generated by the lighting system  300  of  FIG. 11  when connected to the AC power  120  is low. 
         [0080]      FIG. 13  illustrates an alternative embodiment of the lighting system  100   a  of the present invention. The lighting system  100   a  of  FIG. 13  is substantially similar to the lighting system  100  of  FIG. 4 . However, in the lighting system  100   a  of  FIG. 13 , the first lighting module includes two sets of LEDs  32   a  and  34   a . The first set of LEDs  32   a  includes a plurality of LEDs serially connected in forward direction and a second set of LEDs  34   a  includes a plurality of LEDs serially connected in reverse direction. Likewise, the second lighting module includes two sets of LEDs  42   a  and  44   a . The first set of LEDs  42   a  includes a plurality of LEDs serially connected in forward direction and a second set of LEDs  44   a  includes a plurality of LEDs serially connected in reverse direction. 
         [0081]    Note that although the invention has been described in terms of LEDs, the invention and embodiments described herein are not limited to LEDs but may be used with other light emitting devices such as, for example only, Organic Light Emitting Diode (OLED), Light Emitting Polymer (LEP), and Organic Electro Luminescence (OEL), or any other lighting element that generates or causes total harmonic distortion at a level that is higher than desired. The present invention is applicable to and includes regions where the supplied AC power is at 240 volts such as in Europe or other parts of the world. The present invention is applicable to and includes regions where the supplied AC power is at 50 Hz such as in Europe or 400 Hz such as on board an aircraft. The present invention is applicable to and includes use of rectifiers other than the illustrated example rectifiers which are used only for the purposes of disclosing the invention. The lighting system of the present invention can be, for example, a light bulb, a lighting surface, a light wall, a projection system, and the like that includes a plurality of light emitting elements such as LEDs.

Technology Classification (CPC): 8