Patent Publication Number: US-9900945-B1

Title: Color temperature control

Description:
TECHNICAL FIELD 
     The present disclosure relates generally to lighting solutions, and more particularly to dimmable LED lighting. 
     BACKGROUND 
     Lighting devices generally adjust the color temperature of a light emitted by the LEDs of the lighting device in response to changes in the dim level of the light or the current amount from a power source such as an LED driver. For example, the lighting device may include a first string of LEDs and a second string of LEDs, where the two strings of LEDs have the same number of LEDs and are in parallel with each other. The first string of LEDs may emit a light that has a first correlated color temperature (CCT), and the second string of LEDs may emit a light that has a second CCT that is higher (cooler) than the first CCT. The CCT of the light emitted by the lighting device is generally the flux weighted combination of the CCTs of the two strings of LEDs. 
     When the current provided to the strings of LEDs is reduced to dim the combined light such that the combined light has a CCT that closely matches the first CCT (warmer), the second string of LEDs may remain powered on, which may prevent the CCT of the combined light from reaching the desired CCT. Thus, a solution that enables a light emitted by a lighting device to have a desired CCT at lower dim levels of the light is desirable. 
     SUMMARY 
     The present disclosure relates generally to lighting solutions, and more particularly to dimmable LED lighting. In an example embodiment, a lighting device includes a first group of light emitting diodes (LEDs) that are in series with each other and that emit a first light having a first correlated color temperature (CCT). The lighting device further includes a second group of LEDs that are in series with each other and that emit a second light having a second CCT. The lighting device also includes an active electrical component in series with the second group of LEDs. A voltage across both the second group of LEDs and the active electrical component that is needed for the second group of LEDs to start emitting the second light is higher than a voltage across the first group of LEDs that is needed for the first group of LEDs to start emitting the first light. The lighting device further includes a switch coupled in series with the second group of LEDs and the electrical component. The first group of LEDs is in a parallel configuration with the switch, the second group of LEDs, and the electrical component. 
     In another example embodiment, a lighting device includes a first group of light emitting diodes (LEDs) that are in series with each other and that emit a first light having a first correlated color temperature (CCT). The lighting device further includes a second group of LEDs that are in series with each other and that emit a second light having a second CCT. The lighting device also includes a third group of LEDs that are in series with each other and emit a third light having the second CCT. The lighting device  1000  further includes an electrical component in series with the second group of LEDs, where a voltage across both the second group of LEDs and the electrical component that is needed for the second group of LEDs to start emitting the second light is higher than a voltage across the first group of LEDs that is needed for the first group of LEDs to start emitting the first light. The lighting device also includes a switch coupled in series with the third group of LEDs, where the first group of LEDs is in a parallel configuration with the switch and the third group of LEDs and with the second group of LEDs and the electrical component. 
     In another example embodiment, a lighting device includes a first group of light emitting diodes (LEDs) that are in series with each other and that emit a first light having a first correlated color temperature (CCT). The lighting device further includes a second group of LEDs that are in series with each other and emit a second light having a second CCT. The lighting device also includes a third group of LEDs that are in series with each other and emit a third light having the first CCT. The lighting device further includes an electrical component in series with the second group of LEDs, where a voltage across both the second group of LEDs and the electrical component that is needed for the second group of LEDs to start emitting the second light is higher than a voltage across the first group of LEDs that is needed for the first group of LEDs to start emitting the first light. The lighting device also includes a switch coupled in series with the third group of LEDs, where the first group of LEDs is in a parallel configuration with the switch and the third group of LEDs and with the second group of LEDs and the electrical component. 
     These and other aspects, objects, features, and embodiments will be apparent from the following description and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       Reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein: 
         FIG. 1  is a plot illustrating voltage-current (V-I) characteristic of a typical light emitting diodes (LED) according to an example embodiment; 
         FIG. 2  is a plot illustrating voltage-current (V-I) characteristics of two groups of LEDs that are designed to emit lights having different color temperatures according to an example embodiment; 
         FIG. 3  is a plot illustrating a voltage-current (V-I) characteristic of a typical light emitting diode where the horizontal axis is the current and the vertical axis the voltage, and approximated by a third order polynomial according to an example embodiment; 
         FIG. 4  is a light source  400  with two groups of LEDs as described with respect to  FIG. 2  according to an example embodiment; 
         FIG. 5  illustrates a Flux-Current plot of LEDs of a first group of LEDs that emit light having a lower CCT (CCT1) of 1000° K to 2700° K and the LEDs of a second group of LEDs that emit light having a higher CCT (CCT2) of 3000° K to 5000° K; 
         FIG. 6  illustrates a lighting device including two groups of LEDs according to another example embodiment; 
         FIG. 7  illustrates a lighting device including three groups of LEDs according to an example embodiment; 
         FIG. 8  illustrates a lighting device including three groups of LEDs according to another example embodiment; 
         FIG. 9  illustrates a lighting device including three groups of LEDs according to another example embodiment; and 
         FIG. 10  is illustrates a system for adjusting the color temperature of a light according to an example embodiment 
     
    
    
     The drawings illustrate only example embodiments and are therefore not to be considered limiting in scope. The elements and features shown in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the example embodiments. Additionally, certain dimensions or placements may be exaggerated to help visually convey such principles. In the drawings, reference numerals designate like or corresponding, but not necessarily identical, elements. 
     DETAILED DESCRIPTION OF THE EXAMPLE EMBODIMENTS 
     In the following paragraphs, example embodiments will be described in further detail with reference to the figures. In the description, well known components, methods, and/or processing techniques are omitted or briefly described. Furthermore, reference to various feature(s) of the embodiments is not to suggest that all embodiments must include the referenced feature(s). 
     Turning now to the figures, particular embodiments are described.  FIG. 1  is a plot illustrating voltage-current (V-I) characteristic of a typical light emitting diodes (LED) according to an example embodiment. As illustrated in  FIG. 1 , as the forward voltage (V f ) across the LED reaches and exceeds approximately 2.6 V, the current (I f ) starts flowing through the LED and increases along with an increase in the forward voltage (V f ). The non-linear relationship between V f  and I f  shown in  FIG. 1  illustrates that a current source is better suited for providing power to the LED as a small change in V f  may result in a large current swing that may damage the LED. 
       FIG. 2  is a plot illustrating voltage-current (V-I) characteristics of two groups of LEDs that are designed to emit lights having different color temperatures according to an example embodiment. The curve LED 1  corresponds to the V-I characteristics of a first group of ten LEDs (in series) (LED 1 ) that are designed to emit a light having a lower CCT. The curve LED 2  corresponds to the V-I characteristics of a second group of eleven LEDs (in series) (LED 2 ) that are designed to emit a light having a higher CCT. The additional one LED that is included in the second group of LEDs (LED 2 ) results in the second group of LEDs requiring a higher voltage for current to flow through the second group of LEDs (LED 2 ). 
     To illustrate, if the first group of LEDs (LED 1 ) and the second group of LEDs (LED 2 ) are coupled to each other in parallel, as a drive current provided to the two groups of LEDs increases, the first group of LEDs (LED 1 ) starts to conduct while the second group of LEDs (LED 2 ) remains turned off until the forward voltage (V f ) across the first group of LEDs (LED 1 ) and the second group of LEDs (LED 2 ) reaches approximately 28.6 V. Similarly, as the drive current provided to the groups of LEDs decreases (e.g., due to dimming by a dimmer), the second group of LEDs (LED 2 ) stops conducting current at approximately at 28.6 V while first group of LEDs (LED 1 ) continues to conduct current until approximately 26 V. Thus, in some example embodiments, the light resulting from the lights emitted by the first and second groups of LEDs may have a CCT that is the same as or that closely matches the CCT of the light emitted by the first group of LEDs (LED 1 ) when the second group of LEDs (LED 2 ) is not conducting current because of the additional LED that the second group of LEDs (LED 2 ) includes. 
     Although the first group of LEDs (LED 1 ) is described above as including ten LEDs and the second group of LEDs (LED 2 ) is described as including eleven LEDs, in alternative embodiments, the two groups of LEDs may include more or fewer LEDs while having a different number of LEDs such that the second group of LEDs (LED 2 ) has more LEDs than the first group of LEDs (LED 1 ) to maintain a difference in the threshold forward voltages of the two groups of LEDs. 
       FIG. 3  is a plot illustrating a voltage-current (V-I) characteristic of a light emitting diode and approximated by a third order polynomial according to an example embodiment. In some example embodiments, the third order polynomial is:
 
 V= 108 i   3 −53 i   2 +11 i+ 2.59
 
       FIG. 4  is a light source/device  400  with two groups of LEDs as described with respect to  FIG. 2  according to an example embodiment. In some example embodiments, LED 1  emits a relatively warm light (e.g., 1800 CCT) and LED 2  emits a relatively cool light (e.g., 3000). 
     In some example embodiments, the light source  400  may be modeled using ideal diodes (D 1  or D 2 ) in series with a respective current dependent dynamic resistance as represented by the following equation:
 
 V=V   D   +iR ( i )
 
where the voltage V represents a voltage across each of the first group of LEDs (LED 1 ) and the second group of LEDs (nLED 2 ); V D  equals 2.59 V; and R(i)=108 i 2 −53 i+11.
 
     In some example embodiments, the voltage across the first group of LEDs (LED 1 ) and across the second group of LEDs (nLED 2 ) may be such that the first group of LEDs (LED 1 ) conduct a current while the second group of LEDs (nLED 2 ) does not. To illustrate, when the current (i) is decreased to an amount where the second group of LEDs (nLED 2 ) no longer conducts current to emit a light, the CCT of the light emitted by the light source  400  may transition from Cool White (reflecting the contribution of the second group of LEDs (nLED 2 )) to Warm White, specifically, from 3000° K to 1800° K or less. 
     The voltage across the first group of LEDs (LED 1 ) may be represented by the following equation:
 
 V=V   D   +i   1   R ( i )
 
     The voltage across the second group of LEDs (nLED 2 ), which is the same as the voltage across the first group of LEDs (LED 18 ), may be represented by the following equation:
 
 V=nV   D   +i   2   nR ( i )
 
     The current provided to the light source  400  may be represented by the following equation:
 
 i=i   1   +i   2 ,
 
where i 1  and i 2  are the currents in the first group of LEDs (LED 1 ) and the second group of LEDs (nLED 2 ), respectively. In the above equations, V D  is the ideal diode voltage (e.g., 2.59 V for a single LED, or 25.9V for 10 LEDs in series), and the dynamic resistance R(i)=108 i 2 −53 i+11. n is a multiplier that is greater than 1 (one) and that reflects the addition of one or more LEDs (in series) in the second group of LEDs (nLED 2 ) as compared to the number of LEDs (in series) in the first group of LEDs (LED 1 ).
 
     The above three equations can be solved for the three unknowns, V, i 1  and i 2 , in terms of V D , n and i. The solution equation is a cubic function of i 1  of the form:
 
 Ai   1   3   +Bi   1   2   +Ci   1   +D= 0,
 
where A=−108(n+1); B=324 n i+53(n−1); C=−324 n i 2 −106 n i−11(n+1); and D=2.59(n−1)+108 n i 3 +53 n i 2 +11 ni, and where only real and positive value solutions that belong to the current range are applicable. The solution equation with respect to i 2  is also a similar cubic function of i 2  and can be determined by the equation i 2 =i−i 1  
 
     When the current through each group of LEDs is determined for all values of the input current (i), the total flux (i.e., the flux of the light resulting from the combination lights emitted by the two groups of LEDs (LED 1 ) and (nLED 2 )), can be determined from the Flux-Current plot that is supplied by the LED manufacturer of the LEDs.  FIG. 5  illustrates a Flux-Current plot of the LEDs of the first group of LEDs (LED 1 ) that emit light having a lower CCT of 2700° K to 1000° K and the LEDs of the second group of LEDs (nLED 2 ) that emit light having a higher CCT of 3000° K to 5000° K. 
     The Flux-Current relationship for each group of LEDs (LED 1 ) and (nLED 2 ) in  FIG. 5 , can be approximated by following third order polynomials:
 
φ 2 =2460 i   2   3 −1230 i   2   2 +408 i   2  and φ 1 =1160 i   1   3 −579 i   1   2 +193 i   1  
 
The total flux is represented by φ Total =φ 2 +φ 1 , and the combined CCT (i.e., the CCT of the combined lights emitted by the first group of LEDs (LED 1 ) and the second group of LEDs (nLED 2 ) is approximated as CCT Combined 
 
     
       
         
           
             
               CCT 
               Combined 
             
             ≈ 
             
               
                 
                   
                     
                       CCT 
                       2 
                     
                     ⁢ 
                     
                       °K 
                       · 
                       
                         ϕ 
                         2 
                       
                     
                   
                   + 
                   
                     
                       CCT 
                       1 
                     
                     ⁢ 
                     
                       °K 
                       · 
                       
                         ϕ 
                         1 
                       
                     
                   
                 
                 
                   ϕ 
                   Total 
                 
               
               . 
             
           
         
       
     
     In some example embodiments, for a known value of n above (e.g., n=1.1 representing that the second group of LEDs (nLED 2 ) has eleven LEDs while the first group of LEDs (LED 1 ) has ten LEDs), a controller/processor may determine the total input current flowing in both groups of LEDs (LED 1 ) and (nLED 2 ) as described above and adjust the CCT of the light emitted by the light source  400  to a desired CCT value. In some alternative embodiments, a lookup table that has a predetermined current-CCT mapping may be used to adjust the CCT of the light emitted by the light source based on the current flowing through either groups of LEDs (LED 1 ) or (nLED 2 ) as determined above for a known value of n. 
     In some example embodiments, a similar analysis as above may be performed for groups of LEDs that emit lights having CCT values other than 1800° K and 3000° K. In some example embodiments, the second group of LEDs (nLED 30 ) may be replaced with another group of LEDs that has the same number of LEDs as the first group of LEDs (LED 18 ), where the other group of LEDs is in series with an electrical component, such as one or more diodes. 
       FIG. 6  illustrates a lighting device  600  including two groups of LEDs according to another example embodiment. The lighting device  600  includes a first group of LEDs (LED 1 ) that emit a first light having a first CCT. For example, the first group of LEDs (LED 1 ) may include a number of LEDs that are in series with each other. In some example embodiments, the first group of LEDs (LED 1 ) may include multiple subgroups of LEDs where the LEDs in each subgroup are in series with each other, and the different subgroups are parallel with each other. In some example embodiments, the first group of LEDs (LED 1 ) may correspond to the first group of LEDs (LED 1 ) described with respect to  FIG. 4 . 
     The light source/source  600  includes a second group of LEDs (LED 2 ) that emit a second light having a second CCT. The second group of LEDs (LED 2 ) may include number of LEDs that are in series with each other. In some example embodiments, the number of LEDs in the second group of LEDs (LED 2 ) is the same as the number of LEDs in the first group of LEDs (LED 1 ). In some example embodiments, the second group of LEDs (LED 2 ) may include LEDs that have the same configurations as described above with respect to the first group of LEDs (LED 1 ). 
     In some example embodiments, the lighting device  600  includes an active electrical component  604  that is in series with the second group of LEDs (LED 2 ). Because of a voltage drop across the active electrical component  604 , the voltage across both the second group of LEDs (LED 2 ) and the active electrical component  604  that is needed for the second group of LEDs (LED 2 ) to start emitting the second light is higher than the voltage across the first group of LEDs (LED 1 ) that is needed for the first group of LEDs (LED 1 ) to start emitting the first light. 
     In some example embodiments, the lighting device  600  includes a switch  602  coupled in series with the second group of LEDs (LED 2 ) and the electrical component  604 . As illustrated in  FIG. 6 , the first group of LEDs (LED 1 ) is in a parallel configuration with the switch  602 , the second group of LEDs (LED 2 ), and the electrical component  604 . In some example embodiments, the electrical component  604  is or includes one or more diodes that are in series with each other. Alternatively, the electrical component  604  is or includes one or more LEDs that are in series with each other and that emit a third light having the second CCT. For example, the first CCT may be 1800° K, and the second CCT may be 3000° K. When the electrical component  604  is one or more LEDs that emit a third light having the second CCT, the electrical component  604  and the second group of LEDs (LED 30 ) correspond to the second group of LEDs (nLED 2 ) shown in  FIG. 4 . 
     In some example embodiments, the switch  602  includes one or more transistors that operate as a switch to enable and disable current flow through the second group of LEDs (LED 2 ), which affects the CCT of the combined light emitted by the lighting device  600 . 
     In some example embodiments, the lighting device  600  includes a controller (such as the controller shown in  FIG. 10 ) that outputs a control signal to open and close the switch  602 . For example, the controller may be an integrated circuit device from Microchip Technology (e.g., part number PIC16F1827) or another suitable controller. For example, the controller may control durations of time that the switch  602  is open and closed by controlling a duty cycle of the control signal. To illustrate, increasing a duration of time that the switch  602  is closed may result in a combined light that is cooler (e.g., closer to 3000° K), where the combined light is a combination of at least the first light emitted by the first group of LEDs (LED 1 ) and the second light emitted by the second group of LEDs (LED 2 ). Decreasing the duration of time that the switch  602  is closed may result in the combined light having a warmer color temperature (e.g., closer to 1800° K). The controller may adjust the duty cycle of the control signal to adjust the contribution of the second light (e.g., the second light having a CCT of 3000° K) to the combined light emitted by the lighting device  600 , for example, based on a lookup table that current to CCT mapping. For example, the control signal may have a frequency of 1 KHz. 
     In some example embodiments, the number of parallel groups of LEDs that emit a light that has the first CCT may be less or more than the number of parallel groups of LEDs that emit a light that has the second CCT. For example, the lighting device  600  may include a third group of LEDs that are in series with each other and that emit a third light having the second CCT, where the third group of LEDs is in a parallel configuration with the second group LEDs. 
     In some example embodiments, the switch  602  may be kept closed (as compared to being toggled) such that current flows through the second group of LEDs (LED 2 ) and the electrical component  604  without disruption by the opening of the switch  604 . For example, with the first group of LEDs (LED 1 ) and the second group of LEDs (LED 2 ) having the same number of LEDs that are connected in series within each group, the first group of LEDs (LED 1 ) may start emitting a light, as the voltage V increases, before the second group of LEDs (LED 2 ) because of the additional voltage drop across the electrical component  604  (e.g., an LED that emits a light having the same or substantially the same CCT as the light emitted by the second group of LEDs (LED 2 )). Similarly, the first group of LEDs (LED 1 ) may continue emitting a light after the second group of LEDs (LED 2 ) seize emitting a light, for example, during dimming down (i.e., the voltage V decreasing) of the overall light emitted by the lighting device  600 . In some example embodiments, the first group of LEDs (LED 1 ) and the second group of LEDs (LED 2 ) may operate as desired such that the switch  604  can be kept closed. For example, variations in manufacturing, wear, etc. of the LEDs may be within acceptable ranges. In some example embodiments, the switch  602  may be replaced by a wire (creating a short). 
       FIG. 7  illustrates a lighting device  700  including three groups of LEDs according to an example embodiment. The lighting device  700  includes a first group of LEDs (LED 1 ) that emit a first light having a first CCT. For example, the first group of LEDs (LED 1 ) may include a number of LEDs that are in series with each other. In some example embodiments, the first group of LEDs (LED 1 ) may correspond to the first group of LEDs (LED 1 ) described with respect to  FIGS. 4 and 6 . 
     The lighting device  700  may include a second group of LEDs (LED 2 ) that are in series with each other and that emit a second light having a second CCT. For example, the first group of LEDs (LED 1 ) and the second group of LEDs (LED 2 ) may have the same number of LEDs. In some example embodiments, the lighting device  700  may include a third group of LEDs (LED 2A ) that are in series with each other and that emit a third light having the second CCT. 
     In some example embodiments, the lighting device  700  includes an active electrical component  704  that is in series with the second group of LEDs. Because of a voltage drop across the active electrical component  604 , a voltage across both the second group of LEDs (LED 2 ) and the electrical component  704  that is needed for the second group of LEDs (LED 2 ) to start emitting the second light is higher than a voltage across the first group of LEDs (LED 1 ) that is needed for the first group of LEDs (LED 1 ) to start emitting the first light. 
     In some example embodiments, the lighting device  700  includes a switch  702  coupled in series with the third group of LEDs (LED 2A ). As illustrated in  FIG. 7 , the first group of LEDs (LED 1 ) is in a parallel configuration with the switch  602  and the third group of LEDs (LED 2A ), and with the second group of LEDs (LED 2 ) and the electrical component  704 . 
     In some example embodiments, the electrical component  704  is or includes one or more diodes that are in series with each other. Alternatively, the electrical component  704  is or includes one or more LEDs that are in series with each other and that emit a fourth light having the second CCT. For example, the first CCT may be 1800° K, and the second CCT may be 3000° K. When the electrical component  704  is one or more LEDs that emit a fourth light having the second CCT, the electrical component  704  and the second group of LEDs (LED 2 ) correspond to the second group of LEDs (nLED 2 ) shown in  FIG. 4 . 
     In some example embodiments, the switch  702  includes one or more transistors that operate as a switch to alternatingly enable and disable current flow through the second group of LEDs (LED 2 ), which affects the CCT of the combined light emitted by the lighting device  700 . 
     In some example embodiments, the lighting device  700  includes a controller (such as the controller shown in  FIG. 10 ) that outputs a control signal to open and close the switch  602 . For example, the controller may control durations of time that the switch  602  is open and closed by controlling a duty cycle of the control signal. To illustrate, increasing a duration of time that the switch  702  is closed may result in the combined light emitted by the lighting device  700  having a cooler color temperature (e.g., closer to 3000° K). Decreasing the duration of time that the switch  702  is closed and reducing the current provided to the lighting device  700  may result in the combined light having a warmer color temperature (e.g., closer to 1800° K). The controller may adjust the duty cycle of the control signal to adjust the contribution of the third light (e.g., the third light having a CCT of 3000° K) to the combined light emitted by the lighting device  700 , for example, based on a lookup table that has a current-CCT mapping. For example, the control signal may have a frequency of 1 KHz. 
     In some example embodiments, the switch  602  may be kept open (as compared to being toggled) such that current does not flow through the third group of LEDs (LED 2A ). For example, with the first group of LEDs (LED 1 ) and the second group of LEDs (LED 2 ) having the same number of LEDs that are connected in series within each group, the first group of LEDs (LED 1 ) may start emitting a light, as the voltage V increases, before the second group of LEDs (LED 2 ) because of the additional voltage drop across the electrical component  704  (e.g., an LED that emits a light having the same or substantially the same CCT as the light emitted by the second group of LEDs (LED 2 )). Similarly, the first group of LEDs (LED 1 ) may continue emitting a light after the second group of LEDs (LED 2 ) seize emitting a light, for example, during dimming down (i.e., the voltage V decreasing) of the overall light emitted by the lighting device  700 . To illustrate, in some example embodiments, the first group of LEDs (LED 1 ) and the second group of LEDs (LED 2 ) may operate as desired such that the switch  704  can be kept open. For example, variations in manufacturing, wear, etc. of the LEDs may be within acceptable ranges. In some example embodiments, the switch  702  and the third group of LEDs (LED 2A ) may be omitted. 
       FIG. 8  illustrates a lighting device  800  including three groups of LEDs according to another example embodiment. The light device  800  includes a first group of LEDs (LED 1 ) that emit a first light having a first CCT. For example, the first group of LEDs (LED 1 ) may include a number of LEDs that are in series with each other. In some example embodiments, the first group of LEDs (LED 1 ) may correspond to the first group of LEDs (LED 1 ) described with respect to  FIGS. 4 and 6 . 
     The lighting device  800  may include a second group of LEDs (LED 2 ) that are in series with each other and that emit a second light having a second CCT. For example, the first group of LEDs (LED 1 ) and the second group of LEDs (LED 2 ) may have the same number of LEDs. In some example embodiments, the lighting device  800  may include a third group of LEDs (LED 1A ) that are in series with each other and that emit a third light having the first CCT. 
     In some example embodiments, the lighting device  800  includes a switch  802  coupled in series with the third group of LEDs (LED 1A ). As illustrated in  FIG. 8 , the first group of LEDs (LED 1 ) is in a parallel configuration with the switch  702  and the third group of LEDs (LED 1A ), and with the second group of LEDs (LED 2 ) and the electrical component  804 . 
     In some example embodiments, the electrical component  804  is or includes one or more diodes that are in series with each other. Alternatively, the electrical component  804  is or includes one or more LEDs that are in series with each other and that emit a fourth light having the second CCT. For example, the first CCT may be 1800° K, and the second CCT may be 3000° K. When the electrical component  804  is one or more LEDs that emit the fourth light having the second CCT, the electrical component  804  and the second group of LEDs (LED 2 ) correspond to the second group of LEDs (nLED 2 ) shown in  FIG. 4 . 
     In some example embodiments, the switch  802  includes one or more transistors that operate as a switch to alternatingly enable and disable current flow through the second group of LEDs (LED 2 ), which affects the CCT of the combined light emitted by the lighting device  800 . 
     In some example embodiments, the lighting device  800  includes a controller (such as the controller shown in  FIG. 10 ) that outputs a control signal to open and close the switch  802 . For example, the controller may control durations of time that the switch  802  is open and closed by controlling a duty cycle of the control signal. To illustrate, increasing a duration of time that the switch  802  is closed may result in the combined light emitted by the lighting device  800  having a warmer color temperature (e.g., closer to 1800° K). Decreasing the duration of time that the switch  802  is closed may result in the combined light having a cooler color temperature. The controller may adjust the duty cycle of the control signal to adjust the contribution of the third light (e.g., the third light having a CCT of 1800° K) to the combined light emitted by the lighting device  800 , for example, based on a lookup table that has a current-CCT mapping. For example, the control signal may have a frequency of 1 KHz. 
     In some example embodiments, the switch  802  and the third group of LEDs (LED 1A ) may be omitted. 
       FIG. 9  illustrates a lighting device  900  including three groups of LEDs according to another example embodiment. The light device  900  includes a first group of LEDs (LED 1 ) that emit a first light having a first CCT. For example, the first group of LEDs (LED 1 ) may include a number of LEDs that are in series with each other. In some example embodiments, the first group of LEDs (LED 1 ) may correspond to the first group of LEDs (LED 1 ) described with respect to  FIGS. 4 and 6 . 
     The lighting device  900  may include a second group of LEDs (LED 2 ) that are in series with each other and that emit a second light having a second CCT. For example, the first group of LEDs (LED 1 ) and the second group of LEDs (LED 2 ) may have the same number of LEDs. In some example embodiments, the lighting device  900  may include a third group of LEDs (LED 2A ) that are in series with each other and that emit a third light having the second CCT. 
     In some example embodiments, the lighting device  900  includes a switch  902  coupled in parallel with the third group of LEDs (LED 2A ). By toggling the switch  902 , the contribution of the third light emitted by the third group of LEDs (LED 2A ) to the combined light emitted by the lighting device  900  may be controlled. To illustrate, the switch  902  may include one or more transistors that operate as a switch to alternatingly enable and disable current flow through the third group of LEDs (LED 2A ), which affects the CCT of the combined light emitted by the lighting device  900 . 
     In some example embodiments, the lighting device  900  includes a controller (such as the controller shown in  FIG. 10 ) that outputs a control signal to open and close the switch  902 . For example, the controller may control durations of time that the switch  902  is open and closed by controlling a duty cycle of the control signal. The controller may adjust the duty cycle of the control signal to adjust the contribution of the third light (e.g., the third light having a CCT of 1800° K) to the combined light emitted by the lighting device  900 , for example, based on a lookup table that has a current-CCT mapping. For example, the control signal may have a frequency of 1 KHz. 
       FIG. 10  is illustrates a system  1000  for adjusting the color temperature of a light according to an example embodiment. The system  1000  includes a driver  1002  such as an LED driver that, for example, provides output current to power a light source such as one or more LEDs. The driver  1002  receives an AC (alternating current) input, for example, from a mains power supply and provides power to a controller  1004  via a regulator  1008  that adjusts the voltage level that is seen by the controller  1002 . The controller may be an integrated circuit device from Microchip Technology (e.g., part number PIC16F1827) or another suitable controller. 
     The driver  1002  also provides power to the lighting device  1006  that may include two or more groups of LEDs. For example, the lighting device  1006  may correspond to the lighting device  600  of  FIG. 6 . Alternatively, the lighting device  1006  may be replaced by one of the lighting devices of  FIGS. 6-9 . The controller  1004  may provide a control signal via connection  1110  to control (i.e., open and close) a switch, such as the switch  602  shown in  FIG. 6 , to adjust the color temperature of the light emitted by the lighting device  1006 . For example, the controller  1004  may adjust the duty cycle of the control signal to change the on or off durations of the switch, which may be one or more transistors. 
     In some example embodiments, the controller  1004  may sense the total current flowing through groups of LEDs of the lighting device  1006  via the connection  1112  to adjust the CCT of the light emitted by the lighting device  1006 , for example, by adjusting the duty cycle of the control signal based on a lookup table that has a predetermined current-CCT mapping. In some example embodiments, the controller  1004  may adjust the duty cycle of the control signal to produce a warmer light (e.g., 1800° K) at low dim levels. In some example embodiments, the controller  1004  may adjust the duty cycle of the control signal to produce a cooler light (e.g., 3000° K) at high dim levels. 
     In some example embodiments, the switch  902  may be omitted. Alternatively, the switch  902  may be permanently kept open. 
     Although particular embodiments have been described herein in detail, the descriptions are by way of example. The features of the example embodiments described herein are representative and, in alternative embodiments, certain features, elements, and/or steps may be added or omitted. Additionally, modifications to aspects of the example embodiments described herein may be made by those skilled in the art without departing from the spirit and scope of the following claims, the scope of which are to be accorded the broadest interpretation so as to encompass modifications and equivalent structures.