Abstract:
An apparatus and method for reducing the perception of flicker of solid state light sources. An alternating current (AC) signal is passed through an AC conditioning circuit and a passive resonating circuit before being supplied to a string of solid state light sources, such as light emitting diodes.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims benefit under 35 U.S.C. 119(e) of U.S. Provisional Patent Application Ser. No. 61/527,029, filed Aug. 24, 2011, which is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    This disclosure is generally related to reduction of flicker perception in solid state lighting systems. 
         [0004]    2. Description of the Related Art 
         [0005]    Solid state lighting has many advantages over traditional lighting, such as incandescent and fluorescent bulbs. Solid state lighting devices exhibit lower energy consumption, longer lifetime, and improved robustness over traditional lighting. However, the existing approaches to electrically drive solid state lighting are known to generate undesirable side effects. 
         [0006]    In particular, existing approaches may generate electromagnetic interference (EMI) and switching noise. Existing approaches generally employ a configuration of a switch-mode power supply to convert alternating current (AC) power to direct current (DC) power to operate the solid state light sources. Switch-mode power supplies excite a transformer by rapidly operating a switch, such as a metal-oxide semiconductor field effect transistor (MOSFET). The excitation of the transformer generates EMI that may interfere with signals intended for consumer electronics, such as cell phones, televisions, and radios. Additionally or alternatively, the rapid operation of the switch generates switching noise that may couple back to the main power line. Thus, using a switch-mode power supply to drive solid state lighting may generate the undesirable side effects of EMI and switching noise. 
         [0007]    As an alternative to employing a switch-mode power supply, a solid state light source may be connected to directly to an alternating current (AC) line voltage or to a rectified AC line voltage. However, such operation of a solid state light source may result in visually, physiologically, and/or cognitively perceptible flicker. Flicker is a lighting performance factor similar to color rendering, color temperature, and glare and may affect user satisfaction and technology adoption. Lighting industry standards groups are still developing a unified definition of flicker, so a unified industrial standard definition does not currently exist. However, flicker may be defined as a periodic change in the instantaneous light output of a light source. More particularly, flicker may be defined as a periodic change in the instantaneous light output of a light source that may be visually, physiologically, and/or cognitively perceived. Because the brightness of the output of a solid state light source is a function current, the drive current may be used to characterize some aspects of flicker. For example, frequency of the flicker may be defined by the occurrences of cycles of the periodic change in light output or drive current per unit time, e.g., seconds. Accordingly, solid state light sources may have a flicker frequency that matches the frequency of an AC line current (e.g., mains or grid) or that matches the frequency of a rectified AC line current. 
         [0008]    The intensity or amount of the flicker may be defined by or represented as a percent flicker or as a flicker index. The percent flicker is 100 multiplied by the maximum light output (A) in a period minus the minimum light output (B) in a period divided by the sum of the maximum and minimum, i.e., 100*(A−B)/(A+B). The flicker index depends upon an area under a graphical curve of a period of light output that is above an average light output (area 1) and the area under the curve that is below the average light output (area 2). The flicker index is defined by dividing area 1 by the sum of area 1 and area 2, i.e., area 1/(area 1+area 2). 
         [0009]    Some government agencies have restricted the use of solid state lighting based on the frequency of flicker and not necessarily on the intensity of the flicker. In particular, the government agencies restrict the use of solid state lighting that exhibits flicker frequencies occurring at 120 hertz or slower. Light flicker at low frequencies (e.g., less than 120 hertz) has been attributed to inducing seizures, causing migraine headaches, dizziness, nausea, impaired visual performance, and general malaise. Thus, light flicker occurring at less than about 120 hertz may be visually, physiologically, and/or cognitively perceived through physical health side effects. Until recently, it was believed that light flicker at frequencies up to about 160 hertz was also unhealthy. 
         [0010]    Perceptible flicker may become a problem for the LED industry because flicker may contribute to a negative perception of solid state lighting technology as a whole. If most LEDs exhibit flicker, the public may decide that all LEDs flicker. Such may discourage the adoption of LED lighting and/or may spur regulatory restrictions on the use of solid state lighting. Thus, EMI, switching noise, and visual, physiological, and/or cognitive side effects may also hinder the growth of solid state lighting, especially solid state lighting that is excited directly by AC power sources. 
       BRIEF SUMMARY 
       [0011]    Herein disclosed is a solid state lighting system that employs a light source ballast having passive signal conditioning circuitry to increase the frequency of solid state lighting flicker, making it less physiologically and visually perceivable to a human. The passive signal conditioning circuitry may also reduce EMI and switching noise emissions, as compared to active solid state lighting ballasts. 
         [0012]    A flicker control circuit for use with solid state lighting may be summarized as including a passive alternating current, or rectified alternating current, conditioner circuit that converts an alternating current input signal having a first frequency to an output signal having a second frequency that is greater than the first frequency of the alternating current, or rectified alternating current, input signal; and a passive resonating circuit coupled to the passive alternating current, or rectified alternating current, conditioner, to multiply the second frequency of the output signal to a multiplied frequency that is sufficiently high that a flicker from a plurality of solid state light sources powered by the output signal is undetectable by an unaided human eye. 
         [0013]    The passive resonating circuit may include a number of passive electrical elements operable to increase the frequency of the first output signal. The number of passive electrical elements may be operable to multiply the second frequency of the output signal by four to six times greater than the first frequency of the alternating current input signal. The number of passive electrical elements may include at least one inductor and at least one capacitor. The at least one inductor and the at least one capacitor may be coupled together electrically in series and one of the at least one inductor and the at least one capacitor may be connected to a first node between the passive alternating current conditioner circuit and the passive resonating circuit. The at least one inductor may be connected to the first node and the at least one capacitor may be connected to a voltage reference. The at least one inductor may have an inductance of approximately 88 millihenries and the at least one capacitor may have a capacitance of approximately 20 microfarads. The at least one inductor may include a first inductor and a second inductor and the at least one capacitor may include a first capacitor and a second capacitor, the first inductor being coupled between the first capacitor and a first node that is between the passive alternating current conditioner circuit and the passive resonating circuit, the first capacitor being coupled between the first inductor and a second node, the second inductor being coupled between the first node and the second node, the second capacitor being coupled to the second inductor and the first capacitor at the second node. The second capacitor may be coupled between the second node and a voltage reference. The second frequency of the output signal may be at least six times greater than the first frequency of the alternating current input signal. The first inductor may have an inductance that is approximately the same value as an inductance of the second inductor. A current supplied to the plurality of solid state light sources from the first node may be positive while the alternating current input signal may be received by the flicker control circuit. The first capacitor may have a capacitance of approximately 20 microfarads. The second capacitor may have a capacitance that is greater than 8 times the capacitance of the first capacitor. The circuit may further include a first diode electrically coupled in series to a second diode, the first diode and the second diode being coupled between a first node and a second node, wherein the at least one capacitor includes a first capacitor and a second capacitor, the first capacitor being coupled between the first node and the second node, and the at least one inductor being coupled between the first node and the second node. A cathode terminal of the first diode may be connected to the first node, an anode terminal of the first diode may be connected to an anode terminal of the second diode and a cathode terminal of the second diode may be connected to the second node, wherein the second capacitor may be coupled between the second node and a voltage reference. The circuit may further include the plurality of solid state light sources may be at least partially electrically coupled in series to one another and may be configured to receive the output signal from the passive alternating current conditioning circuit. 
         [0014]    A flicker control circuit for an alternating current lighting system employing solid state light sources may be summarized as including a diode rectifier circuit operable to receive an alternating current signal and provide a rectified current signal to a first node to provide power to a plurality of solid state light sources; and a passive signal conditioning resonant circuit electrically coupled to the first node and operable to increase a frequency of the rectified current signal at the first node to control a detectible flicker from the plurality of solid state light sources while providing a power factor in excess of 0.8. 
         [0015]    The passive signal conditioning resonant circuit may be operable to at least double the frequency of the rectified current signal at the first node. The plurality of solid state light sources may be electrically coupled in series to each other and at least one of the plurality of solid state light sources may be coupled to the first node. The plurality of solid state light sources may be light emitting diodes. The passive signal conditioning resonant circuit may consist of a number of passive electrical components comprising at least one inductor and at least one capacitor configured to resonate at a desired frequency to increase the frequency of the rectified current signal. The desired frequency may be 120 hertz. The desired frequency may be 100 hertz. The at least one inductor may be coupled in electrical series with the at least one capacitor and the at least one inductor and the at least one capacitor may be coupled between the first node and a voltage reference. The at least one inductor may include a first inductor and a second inductor and the at least one capacitor may include a first capacitor and a second capacitor, the first inductor being coupled between the first node and the first capacitor, the first capacitor being coupled between the first inductor and a second node, the second inductor being coupled between the first node and the second node, the second capacitor being coupled between the second node and a ground reference. The first capacitor may have a capacitance that is less than ⅛ of a capacitance of the second capacitor. The passive conditioning resonant circuit may further include a first diode connected in electrical series to a second diode, the at least one capacitor including a first capacitor and a second capacitor, the first and second diodes being connected electrically in parallel to the first capacitor and to the at least one inductor between a first node and a second node. The second capacitor may be coupled between the second node and a ground reference and the first and second diodes may be zener diodes. 
         [0016]    A method of operating a string of a plurality of solid state light sources without an active switch may be summarized as including receiving an alternating current signal at an input terminal; rectifying the alternating current signal with a rectifier bridge and providing a rectified current signal to a first node, the rectified current signal having a frequency that is approximately double a frequency of the alternating current signal; increasing the frequency of the rectified current signal with a resonant network including passive electrical elements to reduce the detectability of amplitude variations in the rectified current signal by an unaided human eye as emitted by the plurality of solid state light sources; and providing the rectified current signal to the string of the plurality of solid state light sources to forward bias the plurality of solid state light sources. 
         [0017]    Receiving the alternating current signal may include receiving the alternating current signal at a frequency of 120 hertz. Receiving may include receiving the alternating current signal at a frequency of 100 hertz. Increasing the frequency of the rectified current signal may include increasing the frequency to 200-260 hertz. Increasing the frequency of the rectified current signal may include increasing the frequency to 320-380 hertz. 
         [0018]    The structures and methods taught herein may be advantageously employed in a variety of solid state lighting systems and structures. For example, the structures and/or methods taught herein may be advantageously employed as the input to the structures taught in the U.S. patent application Publication No. 2010-0123403. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0019]    In the drawings, identical reference numbers identify similar elements or acts. The sizes and relative positions of elements in the drawings are not necessarily drawn to scale. For example, the shapes of various elements and angles are not drawn to scale, and some of these elements are arbitrarily enlarged and positioned to improve drawing legibility. Further, the particular shapes of the elements as drawn, are not intended to convey any information regarding the actual shape of the particular elements, and have been solely selected for ease of recognition in the drawings. 
           [0020]      FIG. 1  is a schematic diagram of a lighting system, according to one illustrated embodiment. 
           [0021]      FIG. 2  is a graph of voltage and current signals in the lighting system of  FIG. 1 , according to one illustrated embodiment. 
           [0022]      FIG. 3  is a schematic diagram of a lighting system, according to one illustrated embodiment. 
           [0023]      FIG. 4  is a graph of voltage and current signals in the lighting system of  FIG. 3 , according to one illustrated embodiment. 
           [0024]      FIG. 5  is a schematic diagram of a lighting system, according to one illustrated embodiment. 
           [0025]      FIG. 6  is a graph of a current signal in the lighting system of  FIG. 5 , according to one illustrated embodiment. 
           [0026]      FIG. 7  is a diagram showing an implementation of the lighting system of  FIG. 1 ,  3 , or  5 , according to one illustrated embodiment. 
           [0027]      FIG. 8  is a diagram showing an implementation of a lighting system of  FIG. 1 ,  3 , or  5 , according to one illustrated embodiment. 
           [0028]      FIG. 9  is a schematic diagram of a lighting system, according to one illustrated embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    In the following description, certain specific details are set forth in order to provide a thorough understanding of various disclosed embodiments. However, one skilled in the relevant art will recognize that embodiments may be practiced without one or more of these specific details, or with other methods, components, materials, etc. In other instances, well-known structures associated with lighting systems, for example power converters, thermal management structures and subsystems, and/or solid state lights have not been shown or described in detail to avoid unnecessarily obscuring descriptions of the embodiments. 
         [0030]    Unless the context requires otherwise, throughout the specification and claims which follow, the word “comprise” and variations thereof, such as, “comprises” and “comprising” are to be construed in an open, inclusive sense, that is as “including, but not limited to.” 
         [0031]    Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. 
         [0032]    As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the content clearly dictates otherwise. It should also be noted that the term “or” is generally employed in its sense including “and/or” unless the content clearly dictates otherwise. 
         [0033]    As used in the specification and the appended claims, references are made to a “node” or “nodes.” It is understood that a node may be a pad, a pin, a junction, a connector, a wire, or any other point recognizable by one of ordinary skill in the art as being suitable for making an electrical connection within an integrated circuit, on a circuit board, in a chassis, or the like. 
         [0034]    The headings and Abstract of the Disclosure provided herein are for convenience only and do not interpret the scope or meaning of the embodiments. 
         [0035]      FIG. 1  shows a lighting or illumination system  100 , according to one embodiment. The lighting system  100  includes a solid state light source  102 , an alternating current (AC) power supply  104 , an AC signal conditioner  106 , and a passive resonating circuit  108 . 
         [0036]    The solid state light source  102  may include a string of light emitting diodes (LEDs) D 5 -D 44 . Each of the LEDs D 5 -D 44  may have a forward voltage drop in the range of 2.5-4.5 volts. A voltage applied to an LED that is below this range may be insufficient to cause the LED to emit light, and a voltage applied to an LED that is above this range may render the LED inoperable or may significantly reduce the lifespan of the LED. Thus, a continuous string of 40 LEDs, e.g., LEDs D 5 -D 44 , that are connected electrically in series may operate by receiving a voltage in the range of 100-180 volts (i.e., 2.5 volts×40 to 4.5 volts×40). According to another embodiment, the solid state light source  102  may include multiple strings of LEDs with each of the strings of LEDs electrically coupled in parallel to each of the other strings of LEDs. 
         [0037]    As with many types of diodes, LEDs conduct current while forward-biased and leak minor amounts of current while reverse biased. As compared to monitoring current flow through an LED, it is more difficult to determine whether an LED is emitting light based solely on the voltage applied across the LED. Thus, current flow through the solid state light source  102 , rather than the voltage applied across the light source  102 , will be referenced during the discussion of the graphs of  FIGS. 2 ,  4 , and  6 . 
         [0038]    The power supply  104  supplies an AC power signal consisting of a voltage and a current to a node  110 . The frequency and amplitude of the AC power signal supplied by the power supply  104  may span a range of values. The frequency of the AC power signal may be any frequency greater than 30 hertz. The lower limit of this frequency will be discussed in more detail below. The power supply  104  may be a light receptacle which supplies AC voltage and current at, for example, 50 hertz in some parts of the world or 60 hertz in North America. Alternatively, the power supply  104  may supply the AC power signal as a rectified alternating current. 
         [0039]    The amplitude of the AC voltage supplied by the power supply  104  may span a wide range of values. The amplitude of the AC voltage may range from 3 volts to several-hundred volts, depending upon the characteristics of the light source  102 . For example, an AC voltage having an amplitude of 3 volts may be desirable when the light source  102  is a single LED or includes multiple LEDs coupled electrically in parallel. In North America, the amplitude of the AC voltage supplied to light fixtures and receptacles is approximately 170 volts zero-to-peak, or approximately 110-120 volts root-mean-squared (rms) zero-to-peak. Accordingly, the voltage supplied to a typical North American light receptacle (˜120 Vrms) is suitable to meet the operational range (141-187 volts zero to peak) of the string of 50 LEDs D 5 -D 44  as determined by a forward-voltage range of 2.5-4.5 volts per each LED, typically 3.25V at rated LED current. 
         [0040]    The AC conditioning circuit  106  may be coupled to the AC power supply  104  at the node  110  to receive the AC power signal. From the AC power signal, the AC conditioning circuit  106  may generate a rectified output signal having a frequency that is twice the frequency of the received AC power signal. Alternatively, the AC conditioning circuit  106  may be configured to generate a rectified output signal having a frequency that is greater than the frequency of the received AC power signal that is already a rectified alternating current signal. The frequency is determined by inverting a period of the rectified output signal. A period of the rectified output signal is the time elapsed between one amplitude maximum (voltage or current) to a next amplitude maximum. 
         [0041]    The AC conditioning circuit  106  may be a passive circuit, such as a full-wave diode rectifier bridge, as illustrated. Diodes D 1 -D 4  may be configured, as known in the art, to receive the AC power signal and generate the rectified output signal. Although rectified power signals are commonly considered to be direct current (DC) signals, the rectified output signal will be referred to as having a frequency that is the reciprocal of the period, discussed above. 
         [0042]    The AC conditioning circuit  106  may generate the rectified output signal at a first frequency at a node  112 . 
         [0043]    The passive resonating circuit  108  may be electrically coupled to the AC conditioning circuit  106  to receive the rectified output signal at the node  112 . The passive resonating circuit  108  may include an inductor L 1  electrically coupled or directly connected to the node  112 . The passive resonating circuit  108  may also include a capacitor C 1  electrically coupled or directly connected between a voltage reference or potential and the inductor L 1  at a node  114 . The voltage reference or potential may be ground or may be a voltage bias or potential held at a particular value to improve the operational characteristics of the passive resonating circuit  108 . 
         [0044]    The values of the passive components of the passive resonating circuit  108  may be selected or set to cause the passive resonating circuit  108  to resonate at the first frequency of the rectified output signal. In other words, the values of the inductor L 1  and the capacitor C 1  may be selected or set to have a resonant frequency that is approximately twice the frequency of the AC power signal from the AC power source  104 . Typically, an LC circuit resonates at a frequency in which the impedance of the inductor (Z L =jωL) summed with the impedance of the capacitor (Z C =1/(jωC)) is equal to zero. 
         [0045]    The resonant frequency f of an LC circuit may be determined mathematically as follows: 
         [0000]    
       
         
           
             f 
             = 
             
               
                 1 
                 
                   ( 
                   
                     2 
                      
                     π 
                      
                     
                       
                         L 
                          
                         
                             
                         
                          
                         1 
                         * 
                         C 
                          
                         
                             
                         
                          
                         1 
                       
                     
                   
                   ) 
                 
               
               . 
             
           
         
       
     
         [0000]    According to one embodiment, the inductor L 1  has an inductance of 88 millihenries and the capacitor C 1  has an inductance of 20 microfarads, so that the resonant frequency f is 120 hertz. The inductor L 1  may be a ferrite core or laminated iron core inductor with a single winding. The inductor L 1  may also be constructed with two windings on a single shared core. The capacitor C 1  may be an electrolytic, ceramic, tantalum, polypropylene film, or polyester metal film capacitor. Other values may be chosen for the inductor L 1  and the capacitor C 1  to achieve the desired frequency. Furthermore, the values of inductor L 1  and capacitor C 1  may be selected or set to produce resonant frequencies that are higher than, or lower than, twice the frequency of the AC power signal from the AC power source  104 . For example, in Europe where the AC power source  104  generates the AC power signal at 50 hertz, and the AC conditioning signal generates a rectified output signal at 100 hertz, a capacitor C 1  with a 27 microfarad capacitance may be used to cause the passive resonating circuit  108  to have a resonating frequency f of approximately 103 hertz. 
         [0046]    The passive resonating circuit  108  may multiply the frequency of the rectified output signal at the node  112  from the first frequency to a second frequency. In the absence of the passive resonating circuit  108 , the rectified output signal may have a first frequency of 120 hertz at the node  112 . The addition of the passive resonating circuit  108  that is excited by the 120 hertz rectified output signal may multiply the frequency of the rectified output signal at the node  112  from a first frequency of 120 hertz to a second frequency of 240 hertz. In countries supplying a 50 hertz AC power signal, the passive resonating circuit  108  may multiply the frequency of the rectified output signal at the node  112  from a first frequency of 100 hertz to, for example, a second frequency of 200 hertz. Thus, the passive resonating circuit  108  multiplies the frequency of the rectified output signal from a first frequency that is, for example, twice as high as the frequency of the AC power signal to a second frequency that is, for example, at least four-times as high as the frequency of the AC power signal. Accordingly, the lighting system  100  may receive an AC power signal having a frequency that is greater than 30 hertz and may multiply the frequency of that signal to exceed the 120 hertz threshold at which flicker has been determined to no longer be physiologically or visually perceived by humans. 
         [0047]      FIG. 2  shows a graph  200  which includes plots of signals measured at various nodes in the lighting system  100 . The graph  200  includes a plot of the voltage  202  of the AC power signal as measured at the node  110 , a plot of a current  204  measured through the inductor L 1 , and a plot of a current  206  flowing from the node  112  and through the solid state light source  102 . 
         [0048]    The AC power supply  104  generates the voltage  202  of the AC power signal at the node  110 . The voltage  202  has a zero-to-peak amplitude of approximately 180 volts (˜120 Vrms). The voltage  202  has a frequency of 60 hertz. 
         [0049]    The current  204  flows through the inductor L 1  oscillates at approximately the same frequency (e.g., 120 hertz) as the first frequency of the rectified output signal from the AC conditioning circuit  106 . The rectified output signal excites the passive resonating circuit  108  at the first frequency. The passive resonating circuit  108  doubles the frequency of the current of the rectified output signal. The passive resonating circuit  108  may double or multiply the frequency of the current at the node  112  by rectifying the current  204  that flows through the inductor L 1 . 
         [0050]    The current  206  flows from the node  112  through the solid state light source  102 . The current  206  has a frequency that is, for example, two-times greater than the current  204  through the inductor L 1 . According to the embodiment illustrated in graph  200 , the current  206  has a frequency of 240 hertz and is non-zero. According to another embodiment, the current  206  has a frequency of 360 hertz, which is six times greater than the frequency of the voltage  202  of the AC power signal. 
         [0051]    The passive resonating circuit  108  has several advantages over conventional techniques practiced, such as simply connecting a capacitor across the solid state light source  102 . Connecting a large capacitor to the output of a diode rectifier bridge is a common technique used to decrease the ripple from the diode rectifier bridge in order to provide a smoothed DC signal to LEDs. However, simply connecting a capacitor across the solid state light source  102  is not useful in practice because it creates unacceptable phase shifts relative to the AC power signal and results in a low power factor. By contrast, the embodiment of  FIG. 1  may have a power factor greater than 0.85. 
         [0052]    Another disadvantage of using a capacitor connected across the solid state light source  102  is that a large capacitor is needed to perform smoothing. A large capacitor, such as one having a capacitance of several hundred microfarads or more, causes high currents to flow relative to the peak of the AC power signal, resulting in high crest factors and high total harmonic distortion. By contrast, the embodiment of  FIG. 1  may use a smaller and therefore less costly capacitor, such as a 20 microfarad electrolytic capacitor, than used for DC smoothing. 
         [0053]    The embodiment of  FIG. 1  has advantages over LED drivers employing switch mode power supplies. As discussed previously, switch mode power supplies use active switches, such as MOSFETs, to regulate power supplied to LED load. Switch mode power supplies generate electromagnetic interference (EMI) by rapidly inducing magnetic fields in an inductor or transformer. The generated EMI can affect cell phone communication quality as well as television and radio reception. The rapid switching action of a switch mode power supply also generates switching noise which may couple to the main power line or AC power source  104 . The lighting system  100  advantageously employs passive components, such as the AC conditioning circuit  106  and the passive resonating circuit  108 , to ballast or drive the LEDs D 5 -D 44  while generating much less EMI with simpler, i.e., less costly, electrical components than a switch mode power supply. Notably, fewer and less costly components results in lower manufacturing costs. 
         [0054]    The lighting system  100  is advantageously robust to passive component tolerances. For example, the passive resonating circuit  108  may employ passive components having tolerances up to 20% and still operate desirably. Because passive components having tolerances up to 20% are less-costly to manufacture and purchase, the lighting system  100  may be manufactured more cost-effectively than lighting systems requiring components of more exacting tolerances. 
         [0055]      FIG. 3  illustrates a lighting system  300 . The lighting system  300  includes the solid state light source  102 , the (AC) power supply  104 , the AC signal conditioner  106 , and a passive resonating circuit  302 . For the sake of brevity, a description will not repeated for features having the same reference numbers as features illustrated in previous Figures. 
         [0056]    The passive resonating circuit  302  may be electrically coupled to the AC conditioning circuit  104  to receive the rectified output signal at a node  304 . The passive resonating circuit  302  may include an inductor L 2  electrically coupled or directly connected to the node  304  and a capacitor C 2  electrically coupled or directly connected between a node  306  and the inductor L 2 . An inductor L 3  may also be electrically coupled or directly connected between the node  304  and the node  306 . A capacitor C 3  may be electrically coupled or connected to the capacitor C 2  and the inductor L 3  at the node  306 . The capacitor C 3  may also be connected to a voltage reference or potential, such as ground or a voltage bias. 
         [0057]    The passive resonating circuit  302  may be configured to resonate at the first frequency of the rectified output signal. In other words, the values of the inductors L 2  and L 3  and the capacitors C 2  and C 3  may be selected or set so as to produce a resonant frequency that is, for example, approximately twice the frequency of the AC power signal from the AC power source  104 . 
         [0058]    According to one embodiment, the inductance of the inductor L 2  is 10 millihenries, the inductance of the inductor L 3  is 10 millihenries, the capacitance of the capacitor C 2  is 20 microfarads, and the capacitance of the capacitor C 3  is 100 microfarads, so that the passive resonating circuit  302  at least partially resonates at a frequency of approximately 120 hertz. Each inductor L 2 , L 3  may be implemented with a single-winding inductor or with a multiple-winding inductor. The capacitor C 2  may be a polyester film capacitor. The capacitor C 3  may be an electrolytic capacitor. Inductors L 2 , L 3  and/or capacitors C 2 , C 3  with other values may be selected or chosen to excite the passive resonating circuit  302  at a desired frequency. Furthermore, the values of the inductors L 2 , L 3  and the capacitors C 2 , C 3  may be selected or set to cause the passive resonating circuit  302  to resonate at frequencies that are higher, or lower, than the first frequency of the signal output, so as to decrease the magnitude of the response of the passive resonating circuit  302  to the first frequency of the rectified output signal. 
         [0059]    The passive resonating circuit  302  may multiply the frequency of the rectified output signal at the node  304  from the first frequency to the second frequency. Exciting the passive resonating circuit  302  with a 100-120 hertz rectified output signal may result in multiplication of the frequency of the rectified output signal at the node  304  from a first frequency of 100-120 hertz to a second frequency of 200-240 hertz. Accordingly, the passive resonating circuit  302  multiplies the frequency of the rectified output signal from a first frequency that is, for example, twice as high as the frequency of the AC power signal (50-60 hertz) to a second frequency that is, for example, at least four-times as high as the frequency of the AC power signal. 
         [0060]      FIG. 4  shows a graph  400  which includes plots of signals measured at various nodes in the lighting system  300 . The graph  400  includes a plot of the voltage  202  of the AC power signal as measured at the node  110  and a plot of the current  402  flowing from the node  304  through the solid state light source  102 . 
         [0061]    The current signal  402  flows from the node  304  through the solid state light source  102 . The current signal  402  has a frequency that is two-times greater than the frequency of the rectified output signal generated by the AC conditioning circuit  106 . According to an embodiment illustrated in graph  400 , the period  404  of the current signal  402  is approximately 4 milliseconds, the frequency of the current signal  402  is approximately 244 hertz, and the current signal  402  is non-zero. The frequency of the current signal  402  may be, for example, as low as 200 hertz or as high as 260 hertz. According to another embodiment, the current signal  402  has a frequency of approximately 360 hertz which is six times greater than the frequency of the voltage signal  202  of the AC power signal. The frequency of the current signal  402  may also be, for example, as low as 300 hertz or as high as 380 hertz. 
         [0062]    The passive resonating circuit  302  may be implemented with smaller and less-expensive components than the passive resonating circuit  108 . The passive resonating circuit  302  may use lower-inductance inductors L 2 , L 3  in place of a single higher-inductance inductor L 1  to multiply the frequency of the rectified output signal. The lower-inductance inductors L 2 , L 3  may have a lower resistance than a single larger inductor, the employment thereof may therefore result in lower losses due to power dissipation in the passive resonating circuit  302 . The inductors L 2 , L 3  of the passive resonating circuit  302  may be both dimensionally smaller and lighter weight than the inductor L 1  of the passive resonating circuit  108 . Because the passive resonating circuit  108  or  302  is part of an LED ballast and may be fitted into a ballast housing, both weight and size of the components of the passive resonating circuit  108  or  302  influence the marketability of the lighting system  300 . Additionally, two lower-inductance inductors L 2 , L 3  may be less costly than that of a single higher-inductance inductor L 1 . The lighting system  300  may be implement with passive components having tolerances of up to 20% and still operate as desired. 
         [0063]      FIG. 5  illustrates a lighting system  500 . The lighting system  500  includes a solid state light source  502 , the AC power supply  104 , the AC signal conditioner  106 , and a passive resonating circuit  504 . 
         [0064]    The solid state light source  502  may include more LEDs than the previously illustrated solid state light sources  102 . For example, the solid state light source  502  may include 48 LEDs D 5 -D 52 . Using a string of 48 LEDs may advantageously cause the solid state light source  502  to consume less current by decreasing the voltage drop across each of the 48 LEDs D 5 -D 52 . 
         [0065]    The passive resonating circuit  504  may be electrically coupled to the AC conditioning circuit  106  to receive the rectified output signal at a node  506 . The passive resonating circuit  504  may include an inductor L 4  electrically coupled or directly connected between the node  506  and a node  508 . The passive resonating circuit  504  may also include a capacitor C 4  electrically coupled or directly connected between the node  506  and the node  508 . A pair of zener diodes ZD 1 , ZD 2  may be electrically coupled in parallel to the capacitor C 4 , such that a cathode terminal of the first zener diode ZD 1  is connected to the node  506 , a cathode terminal of the second zener diode ZD 2  is connected to the node  508 , and anode terminals of the pair of zener diodes ZD 1 , ZD 2  are connected together at a node  510 . A capacitor C 5  may also be electrically coupled or connected to the capacitor C 4 , the inductor L 4 , and the cathode terminal of the second zener diode ZD 2  at the node  508 . The capacitor C 5  may also be connected to a voltage reference, such as ground or a voltage bias. 
         [0066]    The passive resonating circuit  504  may be configured to resonate at the first frequency of the rectified output signal. In other words, the inductor L 4  and the capacitors C 4  and C 5  may be selected with values that cause the passive resonating circuit  504  to have a resonant frequency that is, for example, approximately twice the frequency of the AC power signal (50-60 hertz) from the AC power source  104 . 
         [0067]    For instance, the inductance of the inductor L 4  is 10 millihenries, the capacitance of the capacitor C 4  is 40 microfarads, the capacitance of the capacitor C 5  is 200 microfarads, and the breakdown or reverse voltage of the zener diodes ZD 1 , ZD 2  is 33 volts so that the passive resonating circuit  504  resonates at a frequency of approximately 120 hertz. The inductor L 4  may be toroidal and may have multiple windings. The capacitor C 4  may be a polyester film capacitor. The capacitor C 5  may be an electrolytic capacitor. The inductor L 4  and the capacitors C 4 , C 5  may be selected with other inductance and capacitance values to excite the passive resonating circuit  504  at other frequencies. 
         [0068]    The passive resonating circuit  504  may multiply the frequency of the rectified output signal at the node  506  from the first frequency to the second frequency. The passive resonating circuit  504  that is excited by a 100-120 hertz rectified output signal may multiply the frequency of the rectified output signal at the node  506  from a first frequency of, for example, 100-120 hertz to a second frequency of, for example, 200-240 hertz, or higher. Accordingly, the passive resonating circuit  504  multiplies the frequency of the rectified output signal from a first frequency that is, for example, twice as high as the frequency of the AC power signal (50-60 hertz) to a second frequency that is, for example, at least four-times as high as the frequency of the AC power signal. 
         [0069]      FIG. 6  shows a graph  600  which includes a plot of a current signal  602  measured through the solid state lighting device  102 . 
         [0070]    The current signal  602  flows from the node  506  of the passive resonating circuit  504  through the solid state light source  502 . The current signal  602  has a frequency that is more than two-times greater than the rectified output signal generated by the AC conditioning circuit  106 . According to the illustrated graph  600 , the period  604  of the current signal  602  is approximately 3.8 milliseconds, the frequency of the current signal  602  is approximately 257 hertz, and the current signal  602  is non-zero, so the solid state light source  502  remains turned ON during operation of the lighting system  500 . Notably, the current signal  602  has an approximate average of 350 milliamperes, which is less than half of the average current of the previously disclosed embodiments. Thus, the quantity of power consumed by the lighting system  500  may be less than one-fourth the quantity of power consumed by the lighting system  100  or  300 . Alternatively, the current signal  602  may have a frequency of approximately 360 hertz, which is six times greater than the frequency of the voltage signal  202  of the AC power signal. 
         [0071]    Advantageously, the passive resonating circuit  504  may be implemented with smaller and less-expensive components than the previously illustrated passive resonating circuits  108  and  302 . The passive resonating circuit  504  may use a single inductor L 4  that is significantly smaller than the inductor L 1  of the passive resonating circuit  108  and that is one less component than the two inductors L 2 , L 3  of the passive resonating circuit  302 . Thus, the use of the single inductor L 1  results in a lower resistance and lower power dissipation in the passive resonator circuit  504  than may be exhibited by passive resonator circuits  108  and  302 . Additionally, two zener diodes ZD 1 , ZD 2  may be significantly lighter and less expensive than a single larger inductance (e.g., 10 millihenry) inductor, so the passive resonating circuit  504  may be lighter and less costly to manufacture than the passive resonating circuit  302 . 
         [0072]      FIG. 7  illustrates a retrofit AC solid state lighting system  700 . The system  700  may include a conventional light fixture  702 , a solid state light source  704 , and a solid state lighting ballast  706  configured to couple the solid state light source  704  to the light fixture  702 . 
         [0073]    The conventional light fixture  702  includes a receptacle  710 , such as a threaded socket, and may be, for example, mounted to a ceiling  708 . The light fixture  702  provides AC power to the receptacle  710 . 
         [0074]    The solid state light source  704  may include a string of LEDs such as those illustrated in  FIG. 1  or  FIG. 5 . The string of LEDs may be electrically coupled to the base  712  of the light source  704  to receive power from the light fixture  702 , through the solid state lighting ballast  706 . 
         [0075]    The solid state lighting ballast  706  may be used with a variety of light sources and may be used to retrofit solid state light sources  704  to conventional light fixtures  702 . The solid state lighting ballast  706  may include a base  714 , a ballast housing  716 , and a receptacle  718 . The base  714  is threaded to be adaptable with the receptacle  710  of the light fixture  702 . The base  714  is electrically coupled to ballast circuitry (not shown) that is within the ballast housing  716 . The ballast circuitry may include the AC conditioning circuit  106  and any one of the passive conditioning circuits  108 ,  302 , and  504  to provide a power signal to the receptacle  718  that results in a solid state light source having a frequency of flicker that is faster than may be visually, physiologically, or cognitively perceived by an unaided human, e.g., greater than 120 hertz. The receptacle  718  may be a threaded socket that is adaptable to receive a base  712  of the solid state light source  704 . 
         [0076]      FIG. 8  illustrates a solid state lighting system  800 . The lighting system  800  includes the light fixture  702  having the receptacle  710  and a lamp  802 . 
         [0077]    The lamp  802  includes a base  804 , an integrated ballast housing  806 , and a solid state light source  808  integrally coupled to the ballast housing  806 . The base  804  is electrically and mechanically coupleable to the receptacle  710  to secure the lamp  802  to the light fixture  702  and to transfer power from the light fixture  702  to the lamp  802 . The integrated ballast housing  806  encloses ballast circuitry (not shown) such as the type embodied in the AC conditioning circuit  106  and the passive conditioning circuits  108 ,  302 , and  502 . The ballast circuitry conditions the AC power delivered by the light fixture  702  so as to be suitable for use by the solid state light source  808 . The solid state light source  808  may include a plurality of LEDs configured to receive power from the ballast circuitry and, in response to receiving power, transmit visible light from the lamp  802 . 
         [0078]      FIG. 9  illustrates a lighting system  900 , according to one illustrated embodiment. The lighting system  900  includes a light source  902 , a passive resonating circuit  904  configured to supply power to the light source  902 , and a control circuit  906  to regulate an input power applied to a finite number of light emitters D 5 -D 56  within the light source  902 . 
         [0079]    The light source  902  may include a plurality of LEDs D 5 -D 56 . The LEDs D 5 -D 56  may form a string of LEDs connected to each other in electrical series. Alternatively, one or more of the LEDs D 5 -D 56  may be coupled in electrical parallel to one or more other LEDs D 5 -D 56 . The light source  902  may receive power from a passive resonating circuit  904 . 
         [0080]    The passive resonating circuit  904  may be any one of the passive resonating circuits  108 ,  302 , and  504  discussed above. The passive resonating circuit  904  receives a rectified output signal from the passive signal conditioning circuit  106  and increases the frequency of the rectified output signal from a first frequency to a second frequency higher than the first frequency to supply power to the light source  902 . 
         [0081]    The control circuit  906  regulates how many of the LEDs D 5 -D 56  receive power from the passive resonating circuit  904 . The control circuit  906  may receive power from the AC conditioning circuit  106  through optional connection  908 . The control circuit  906  may be coupled to, or electrically connected to, a plurality of switches Q 13 -Q 16  to switchably bypass portions of the string of LEDs D 5 -D 56 . Each of the switches Q 13 -Q 16  may be configured to switchably connect an anode terminal of one of the LEDs D 5 -D 56  to ground. The control circuit  906  may control the switches Q 13 -Q 16  to switchably bypass a portion of the string of LEDs D 5 -D 56  in order to increase or decrease the current and voltage being applied to each of the LEDs D 5 -D 56 . 
         [0082]    For example, one mode of operation of the light source  902  may include having the switch Q 15  turned ON in the closed position so that current flows from LED D 54  to ground. In such a mode of operation, the LEDs D 55 -D 56  are bypassed and do not contribute to the load on the current and voltage applied to the light source  902 . To decrease the voltage being applied to each of the LEDs D 5 -D 53 , the control circuit  906  may turn switch Q 15  OFF in the open position and turn switch Q 16  ON in the closed position so that current flows from the LED D 55  to ground, bypassing only the LED D 56 . Accordingly, the same voltage that was previously applied across the LEDs D 5 -D 54  is applied across the LEDs D 5 -D 55 , thereby decreasing the voltage drop across each of the LEDs D 5 -D 54 . 
         [0083]    Alternatively, the control circuit  906  may increase the voltage being applied to each of the LEDs D 5 -D 53  by turning switch Q 15  OFF and by turning switch Q 14  on so that current flows from the LED D 53  to ground, bypassing the LEDs D 54 -D 56 . This change in configuration decreases the overall number of LEDs across which the input voltage must drop and thereby increases the voltage being applied to each of the LEDs D 5 -D 53 . 
         [0084]    While only four switches Q 13 -Q 16  are illustrated, the control circuit  906  may be configured to control as many switches as there are LEDs, with each switch being connected between an anode terminal of each respective LED and ground. 
         [0085]    The control circuit  906  may regulate the current and voltage applied to the LEDs D 5 -D 56  based upon sensing an operational parameter of an input power. The operational parameter sensed by the control circuit  906  may be the current value of the input power in one embodiment or the voltage value of the input power in another embodiment. Alternatively, the control circuit  906  may sense both the current value and voltage value of the input power. The control circuit  906  may sense the input power from the output of the AC conditioning circuit  106  via connection  908 . Alternatively, the control circuit  906  may sense the input power directly from the power source  104  or from an input terminal of the light source  902 . 
         [0086]    In one embodiment, the control circuit  906  may employ an analog to digital converter (ADC) to convert a sensed voltage to an output for controlling the switches Q 13 -Q 16 . Alternatively, the control circuit may employ one or more analog controllers such as comparator circuits to control the switches Q 13 -Q 16 . 
         [0087]    In one embodiment, the control circuit  906  may comprise a semiconductor integrated circuit (e.g., a processor). In another embodiment, the control circuit  906  and the switches Q 13 -Q 16  may be integral parts of a processor. In yet another embodiment, the control circuit  906 , the switches Q 13 -Q 16 , and the LEDs D 5 -D 56  may be integral parts of a processor. 
         [0088]    Each of the plurality of switches Q 13 -Q 16  may be electrically coupled across a respective LED D 5 -D 56  to ground. There may be tens or dozens of LEDs and only four switches, with the switches being interleaved between groups of LEDs. Alternatively, there may be one switch for each of the plurality of LEDs D 5 -D 56 . Only four switches Q 13 -Q 16  are shown in  FIG. 9  for simplicity and to avoid unnecessarily obscuring the figure. 
         [0089]    Each of the switches Q 13 -Q 16  may be a solid-state switch. For example, each of the switches Q 13 -Q 16  may be a transistor, such as, for instance, an insulated-gate bipolar transistor (IGBT), a metal-oxide-semiconductor field-effect transistor (MOSFET) or a bipolar junction transistor (BJT). Typically, when a transistor is activated, the transistor conducts current; when the transistor is deactivated, no current flows through the transistor. Thus, when a switch Q 13 -Q 16  is activated, the switch Q 13 -Q 16  conducts current by creating a path of relatively minimal resistance and thereby electrically shorting the respective LEDs that are downstream of the connection to ground. 
         [0090]    The specific values, such as voltages, used herein are purely illustrative, and are not meant to be in anyway limiting on the scope unless expressly recited in the claim(s). Likewise, the arrangements and topologies are merely illustrative and other arrangements and topologies may be employed where consistent with the teachings herein. While specific circuit structures are disclosed, other arrangements that achieve similar functionality may be employed. 
         [0091]    The methods illustrated and described herein may include additional acts and/or may omit some acts. The methods illustrated and described herein may perform the acts in a different order. Some of the acts may be performed sequentially, while some acts may be performed concurrently with other acts. Some acts may be merged into a single act through the use of appropriate circuitry. 
         [0092]    The various embodiments described above can be combined to provide further embodiments. 
         [0093]    To the extent that they are not inconsistent with the teachings herein, the teachings of: U.S. provisional patent application Ser. No. 61/052,924 filed May 13, 2008; U.S. patent application Ser. No. 12/437,467 filed May 7, 2009 and published as U.S. patent application Publication No. 2009-0284155; U.S. provisional patent application Ser. No. 61/051,619 filed May 8, 2008; U.S. patent application Ser. No. 12/437,472 filed May 7, 2009 and published as U.S. patent application Publication No. 2009-0278474; U.S. provisional patent application Ser. No. 61/088,651 filed Aug. 13, 2008; U.S. patent application Ser. No. 12/540,250 filed Aug. 12, 2009 and published as U.S. patent application Publication No. 2010-0090577; U.S. provisional patent application Ser. No. 61/115,438 filed Nov. 17, 2008; U.S. provisional patent application Ser. No. 61/154,619 filed Feb. 23, 2009; U.S. patent application Ser. No. 12/619,535 filed Nov. 16, 2008 and published as U.S. patent application Publication No. 2010-0123403; U.S. provisional patent application Ser. No. 61/174,913 filed May 1, 2009(0.406P1); U.S. patent application Ser. No. 12/769,956 filed Apr. 29, 2010 and published as U.S. patent application Publication No. 2010-0277082; U.S. provisional patent application Ser. No. 61/180,017 filed May 20, 2009, U.S. patent application Ser. No. 12/784,080 filed May 2010 and published as U.S. patent application Publication No. 2010-0295946; U.S. provisional patent application Ser. No. 61/229,435 filed Jul. 29, 2009; U.S. patent application Ser. No. 12/846,516 filed Jul. 29, 2010 and published as U.S. patent application Publication No. 2011-0026264; U.S. provisional patent application Ser. No. 61/295,519 filed Jan. 15, 2010; U.S. provisional patent application Ser. No. 61/406,490 filed Oct. 25, 2010; U.S. patent application Ser. No. 13/007,080 filed Jan. 14, 2011; U.S. provisional patent application Ser. No. 61/333,983 filed May 12, 2010; U.S. patent application Ser. No. 12/784,091 filed May 20, 2010 and published as U.S. patent application Publication No. 2011-0295454; U.S. provisional patent application Ser. No. 61/346,263 filed May 19, 2010; U.S. patent application Ser. No. 12/784,093 filed May 20, 2010; U.S. patent application Ser. No. 13/166,626 filed Jun. 22, 2011; U.S. patent application Ser. No. 13/085,301 filed Apr. 12, 2011; and U.S. patent application Ser. No. 13/212,074 filed Aug. 17, 2011 are each incorporated herein by reference in their entirety. 
         [0094]    Aspects of the embodiments can be modified, if necessary to employ concepts of the various patents, applications and publications to provide yet further embodiments. For example, the structures and/or methods taught herein may be advantageously employed as an input to the structures taught in the U.S. patent application Publication No. 2010-0123403. 
         [0095]    These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.