Patent Publication Number: US-9414472-B2

Title: Filament miswire protection in an electronic dimming ballast

Description:
TECHNICAL FIELD 
     The present disclosure relates to electronic ballasts and, more particularly, to electronic dimming ballasts for gas discharge lamps, such as fluorescent lamps. 
     BACKGROUND 
     A typical fluorescent lamp includes a sealed glass tube containing a rare earth gas, and an electrode at each end for striking and maintaining an electric arc through the gas. The electrodes are typically constructed as filaments to which a filament voltage is applied to heat the electrodes, thereby improving their capability to emit electrons. This results in improved electric arc stability and longer lamp life. 
     Typical prior art ballasts apply the filament voltages to the filaments prior to striking the arc and maintain the filament voltages throughout the entire dimming range of the lamp. At low end, when light levels are lowest and, consequently, the electric arc is at its lowest level, the filament voltages help maintain a stable arc current. At high end, when light levels are highest, and the electric arc current is at its highest level, the electric arc current contributes to heating the filaments. 
       FIG. 1  is a perspective view of an example gas discharge lamp fixture  100 . The fixture  100  includes a ballast  102 , lamp sockets  104 , and a housing  106 . The ballast  102  and the sockets  104  may be fixed to the housing  106 . The lamp sockets  104  may be sized and situated within the housing  106  to hold lamps  108 . The ballast  102  may have wires  110  to connect the ballast  102  to the sockets  104  for driving the lamps  108  and for providing heating current, discussed above. In practice, the ballast  102  may be wired by a fixture supplier, as is common in new construction, or it may be wired by an on-site installer, as is common in retrofit projects. 
     Some ballasts are manufactured with the expectation that certain of the filaments are to be wired to the ballast in parallel with one another. Sometimes, such a ballast may be installed such that the filaments are inadvertently “miswired” in series with one another. Other ballasts are manufactured with the expectation that certain of the filaments are to be wired in series with one another. Sometimes, such a ballast may be installed such that the filaments are inadvertently “miswired” in parallel with one another. Certain problems may arise when the filaments are miswired. Not all of these problems are immediately apparent, and symptoms of these problems, such as shortened lamp life, may show up much later. 
     SUMMARY OF THE INVENTION 
     An electronic dimming ballast that accommodates miswiring of lamp filaments (e.g., miswiring the corresponding lamp sockets) is disclosed. The electronic dimming ballast may drive a plurality of gas discharge lamps. Each gas discharge lamp may have a respective filament. The electronic dimming ballast may include a filament winding and a filament miswire protection element. The filament winding may be magnetically coupled to an inductor. The filament winding may be operable to supply an AC filament voltage to each of the filaments. The filament miswire protection element may be coupled to the filament winding. The filament miswire protection element may be connectable to the filaments. 
     The electronic dimming ballast, via the filament miswire protection element, may establish the same voltage across a first of the filaments regardless of whether the filaments are wired in series or in parallel. For example, the electronic dimming ballast may establish a first voltage across each of the filaments when the filaments are wired in series and a second voltage across each of the filaments when the filaments are wired in parallel. The first and second voltages may be approximately equal. 
     The filament miswire protection element may have an impedance, at an operating frequency, that is approximately equal to an impedance of at least one of the filaments. For example, the filament miswire protection element may include one or more capacitors, inductors, and/or resistors. In an embodiment, the filament miswire protection element may include only a capacitor. In an embodiment, the filament miswire protection element may include only an inductor. 
     Other features and advantages of the disclosed ballast will become apparent from the following description that refers to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an example gas discharge lamp fixture. 
         FIG. 2  is a simplified block diagram of a prior art dimming ballast for driving multiple lamps. 
         FIG. 3  is a simplified schematic diagram of the back end of the prior art dimming ballast of  FIG. 2 . 
         FIGS. 4A and 4B  are diagrams of a ballast and corresponding gas discharge lamps having filaments wired in parallel and in series, respectively. 
         FIGS. 5A and 5B  are isometric views of example gas discharge lamp sockets wired in parallel and in series, respectively. 
         FIGS. 6A and 6B  are schematic diagrams illustrating filaments wired in parallel and in series, respectively. 
         FIG. 7  is a plot of the magnitude of filament voltage versus the dimming level of the ballast illustrating a lamp safe operating area (SOA). 
         FIGS. 8A and 8B  are simplified schematic diagrams of example ballast back ends each having a filament miswire protection element. 
         FIGS. 9A and 9B  are schematic diagrams illustrating an example filament miswire protection element and filaments wired in parallel and in series, respectively. 
         FIGS. 10A-E  are schematic diagrams illustrating example filament miswire protection elements. 
         FIG. 11  is a flow chart illustrating an example method of manufacturing a ballast with a filament miswire protection element. 
     
    
    
     DETAILED DESCRIPTION 
     An example of an electronic dimming ballast  200  for driving three fluorescent lamps L 1 , L 2 , L 3  in parallel is shown in  FIG. 2 . The electronic dimming ballast  200  may drive any number of lamps. Electronic ballasts typically can be analyzed as comprising a front end  210  and a back end  220 . The front end  210  typically includes a rectifier  230  for generating a rectified voltage from an alternating-current (AC) line voltage, and a filter circuit, for example, a valley-fill circuit  240 , for filtering the rectified voltage to produce a direct-current (DC) bus voltage. The valley-fill circuit  240  may be coupled to the rectifier  230  through a diode  242  and may include one or more energy storage devices that selectively charge and discharge so as to fill the valleys between successive rectified voltage peaks to produce a substantially DC bus voltage. The DC bus voltage may be the greater of either the rectified voltage or the voltage across the energy storage devices in the valley-fill circuit  240 . 
     The back end  220  typically includes an inverter  250  for converting the DC bus voltage to a high-frequency AC voltage and an output circuit  260  comprising a resonant tank circuit for coupling the high-frequency AC voltage to the lamp electrodes. A balancing circuit  270  may be provided in series with the three lamps L 1 , L 2 , L 3  to balance the currents through the lamps and to prevent any lamp from shining brighter or dimmer than the other lamps. A control circuit  280  may generate drive signals to control the operation of the inverter  250  so as to provide a desired load current I LOAD  to the lamps L 1 , L 2 , L 3 . A power supply  282  may be connected across the outputs of the rectifier  230  to provide a DC supply voltage, V CC , for powering the control circuit  280 . 
       FIG. 3  shows a simplified schematic diagram of the back end  220  of the electronic dimming ballast  200  for driving the lamps L 1 , L 2 , L 3 . As previously mentioned, the back end  220  may include an inverter  250  and an output circuit  260 . The inverter input terminals A, B are connected to the output of the valley-fill circuit  240 . The inverter  250  may generate a high-frequency AC voltage for driving the lamps L 1 , L 2 , L 3  and may include series-connected first and second switching devices  352 ,  354 , for example, two field-effect transistors (FETs). The control circuit  280 , shown in  FIG. 2 , may drive the FETs  352 ,  354  of the inverter  250  using a complementary duty cycle switching mode of operation, e.g., a D(1-D) switching technique. This means that one, and only one, of the FETs  352 ,  354  is conducting at a given time. When the FET  352  is conducting, then the output of the inverter  250  is pulled upwardly toward the DC bus voltage. When the FET  354  is conducting, then the output of the inverter  250  is pulled downwardly toward circuit common. 
     The output of the inverter  250  is connected to the output circuit  260  comprising a resonant inductor  362  and a resonant capacitor  364 . The output circuit  260  filters the output of the inverter  250  to supply a substantially sinusoidal voltage to the parallel-connected lamps L 1 , L 2 , L 3 . A DC blocking capacitor  366  prevents DC current from flowing through the lamps L 1 , L 2 , L 3 . Filament windings W 1 , W 2 , W 3 , W 4  are magnetically coupled to the resonant inductor  362  of the output circuit  260  and are coupled to the filaments of the lamps L 1 , L 2 , L 3 . 
     The windings W 1 , W 2 , W 3  may be referred to as independent filament windings because each is coupled to a respective filament of each of several different lamps (e.g., winding W 1  is coupled to a filament of lamp L 1 ; winding W 2  is coupled to a filament of lamp L 2 ; and winding W 3  is coupled to a filament of lamp L 3 ). The winding W 4  may be referred to as a common filament winding because it is coupled to the filaments of all three lamps L 1 , L 2 , L 3 . The common filament winding may be electrically connected to the filaments such that the filaments are in series with one another or in parallel with one another.  FIG. 3  illustrates the common filament winding as being electrically connected to the filaments such that the filaments are in parallel to one another. 
     The filament windings provide AC filament voltages within a range appropriate for the specific lamp type being driven. A lamp type, such as the T8 lamp type for example, may be provided with an AC filament voltage of approximately 3 to 5 V RMS . Another lamp type, such as the T5HE lamp type for example, may be provided with an AC filament voltage of approximately 5 to 8 V RMS . The filaments especially need to be heated when the ballast is dimming the lamps to low end and during preheating of the filaments before striking the lamp. 
     As mentioned above, the example ballast of  FIG. 2  and  FIG. 3  illustrates the common filament winding W 4  wired such that the filaments are in parallel to one another. Another example ballast may have the common filament winding wired to the filaments, such that the filaments are in series with one another.  FIGS. 4A and 4B  are example wiring diagrams showing how a ballast  402  may be wired to lamps  404 ,  406  (i.e., wired to the sockets holding the lamps  404 ,  406 ). Two lamps  404 ,  406  are shown here and below for ease of illustration. The principles described may be applied to any number of lamps. In both  FIGS. 4A and 4B , the ballast  402  has six output wires. Two sets of wires are from independent filament windings, such as two red wires  408  and two blue wires  416 , in this example. One set of wires is from the common filament winding, such as two yellow wires  424 , in this example. The red wires  408  electrically connect to the terminal ends of a filament  410  at a first end  412  of the first lamp  406 . Similarly, the blue wires  416  electrically connect to the terminal ends of a filament  418  at a first end  420  of a second lamp  404 . The yellow wires  424  are electrically connected to the filaments  426 ,  428 , at the second ends  430 ,  432  of the first and second lamps  404 ,  406 .  FIG. 4A  shows the yellow wires connected to the filaments  426 ,  428  in parallel.  FIG. 4B  shows the yellow wires connected to the filaments  426 ,  428  in series. 
     Certain ballasts are manufactured with the expectation that the common filament winding (i.e., connected to the yellow wires) is to be wired in the parallel configuration. When such a ballast has the yellow wires wired in series, the resultant fixture is miswired. Similarly, other ballasts are manufactured with the expectation that the common filament winding (i.e., connected to the yellow wires) is to be wired in the series configuration. When such a ballast has the yellow wires wired in parallel, the resultant fixture is miswired. 
     Because both wiring configurations are used in the industry, it is not uncommon for technicians, such as fixture manufacturers and/or installers, to wire the yellow wires of a ballast in series when they should be wired in parallel or to wire the yellow wires of the ballast in parallel when they should be wired in series. To illustrate the wiring from the technician&#39;s point-of-view,  FIG. 5A  illustrates two rapid start lamp sockets  502 ,  504  wired in parallel, and  FIG. 5B  illustrates the two rapid start lamp sockets  502 ,  504  wired in series. 
     Proper wiring of the yellow wires for a ballast is relevant to the proper operation of the ballast. Typically, the ballast is designed to impart a particular filament voltage to the filaments. This filament voltage generates a corresponding current that properly heats the filaments. When the yellow wires are miswired (e.g., wired in series when they are expected to be in parallel or wired in parallel when they are expected to be in series), the actual voltage across each of the filaments, and thus the corresponding current, may not be what was intended when the ballast was designed. 
     To illustrate,  FIGS. 6A and 6B  show two filaments, R 1 , R 2 , (shown as resistors) wired in parallel and in series. Because the lamp types in a given fixture would typically be the same, we can assume that the resistance values R 1  and R 2  are equal. When the filaments are wired in parallel and a common winding voltage V w  generated by the common winding W 4  is coupled across the filaments as shown in  FIG. 6A , the voltage across each filament is the common winding voltage V w . However, when the filaments are wired in series, as shown in  FIG. 6B , the filaments divide the common winding voltage V w  in half, as shown by the following equation: 
     
       
         
           
             
               V 
               
                 R 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 1 
               
             
             = 
             
               
                 
                   
                     V 
                     w 
                   
                   · 
                   
                     R 
                     1 
                   
                 
                 
                   
                     R 
                     1 
                   
                   + 
                   
                     R 
                     2 
                   
                 
               
               = 
               
                 
                   V 
                   w 
                 
                 2 
               
             
           
         
       
     
     For sophisticated ballasts, this difference in voltage across each filament is particularly problematic when the ballast attempts to provide a relatively fine control of the heating current through the filaments. Typically, the manufacturers of gas discharge lamps establish a safe operating area (SOA) for a particular lamp-type. The SOA defines an acceptable filament voltage and/or current at various dimming levels to maximize the life of the lamp.  FIG. 7  illustrates an example safe operating area (SOA). One can appreciate that a ballast designed to impart a filament voltage within a particular SOA may fail to provide the appropriate filament voltage when the yellow wires are miswired. In the example of a two-lamp ballast, the difference in filament voltage was a factor of two. Such a ballast, when miswired, would likely be outside the SOA, particularly at low dimming levels. Accordingly, it would be desirable for a ballast to accommodate miswirings, keeping the magnitudes of the filament voltages within a given SOA regardless of whether the filaments are wired in parallel or series. Moreover, it would be desirable for a ballast to achieve this result with a minimum of additional parts and cost and with little to no detriment to ballast performance. 
     The inclusion of a miswire protection element, for example the miswire protection element described below, may accommodate miswirings, by keeping the magnitudes of the filament voltages within a given SOA regardless of whether the filaments are wired in parallel or in series. Moreover, the inclusion of a miswire protection element may provide this miswire accommodation with a minimum of additional parts and with little to no detriment to ballast performance. 
       FIG. 8A  is a simplified schematic diagram of an example ballast back end  820  having a filament miswire protection element  822 . Similar to the back end  220  described in  FIGS. 2 and 3 , the back end  820  includes the inverter  250  and an output circuit  260 . The inverter input terminals A, B are connected to the output of the valley-fill circuit  240 . The inverter  250  generates a high-frequency AC voltage for driving the lamps L 1 , L 2 , L 3  and includes series-connected first and second switching devices  352 ,  354 , for example, two field-effect transistors (FETs). The control circuit  280  drives the FETs  352 ,  354  of the inverter using a complementary duty cycle switching mode of operation. This means that one, and only one, of the FETs  352 ,  354  is conducting at a given time. When the FET  352  is conducting, then the output of the inverter  250  is pulled upwardly toward the DC bus voltage. When the FET  354  is conducting, then the output of the inverter  250  is pulled downwardly toward circuit common. 
     The output of the inverter  250  is connected to the output circuit  260  comprising a resonant inductor  362  and a resonant capacitor  364 . The output circuit  260  filters the output of the inverter  250  to supply a substantially sinusoidal voltage to the parallel-connected lamps L 1 , L 2 , L 3 . A DC blocking capacitor  366  prevents DC current from flowing through the lamps L 1 , L 2 , L 3 . 
     Filament windings W 1 , W 2 , W 3 , W 4  are magnetically coupled to the resonant inductor  362  of the output circuit  260 . The filament windings provide AC filament voltages to the filaments to keep the filaments warm through the entire dimming range. The filaments especially need to be heated when the ballast is dimming the lamps to low end and during preheating of the filaments before striking the lamp. 
     The windings W 1 , W 2 , and W 3  are independent filament windings. The independent filament windings W 1 , W 2 , W 3  are coupled to respective filaments of lamps L 1 , L 2 , L 3 . The winding W 4  is a common filament winding. The common filament winding W 4  is connected to each of the filaments of lamps L 1 , L 2 , L 3  via a filament miswire protection element  822 . The filament miswire protection element may be a two-node element. A first node  824  of the filament miswire protection element  822  may be connected a branch (either branch, for example) of the common filament winding W 4 . A second node  826  of the filament miswire protection element  822  may be connected to a filament or filaments of the lamps. As illustrated, the filaments connected to the common filament winding W 4  are wired in parallel. However, as will be discussed further below, the filaments connected to the common filament winding W 4  could be wired in series with the filament miswire protection element  822  accommodating for the difference in the wiring. 
     The filament miswire protection element  822  may be an electrical component, system, or sub-system that accommodates for miswiring of the common filament winding W 4 . For example, the filament miswire protection element  822  may be an electrical component, system, or sub-system that has an impedance that is approximately equal to an impedance of at least one of the filaments of lamps L 1 , L 2 , L 3 . Because the ballast with back end  820  may operate within a given range of frequencies, the filament miswire protection element  822  may have an impedance that, within the relevant operating frequency/frequencies, is approximately equal to an impedance of at least one of the filaments of lamps L 1 , L 2 , L 3 . 
     The filament miswire protection element  822  may be coupled to the filament winding, such as for example the common filament winding W 4 . The filament miswire protection element  822  may be connectable to the filaments. For example, the electronic dimming ballast may have a pair of terminals T 1 , T 2 . The filament miswire protection element  822  may be connected to one of the pair of terminals T 1 , T 2 . The pair of terminals T 1 , T 2 , may be connectable to the filaments of lamps L 1 , L 2 , L 3 . For example, the pair of terminals T 1 , T 2 , may be a pair of wires. For example, the pair of terminals T 1 , T 2  may be in a terminal block. As a result, the electronic dimming ballast with back end  820  may establish, via the filament miswire protection element  822 , the same voltage across a first of the filaments regardless of whether the filaments are wired in series or in parallel. The electronic dimming ballast with back end  820  may establish, via the filament miswire protection element  822 , for example, a first voltage across each of the filaments when the filaments are wired in series and a second voltage across each of the filaments when the filaments are wired in parallel. Here, the first and second voltages may be approximately equal. In other words, the electronic dimming ballast with back end  820  may establish, via the filament miswire protection element  822 , a voltage across each of the filaments when the filaments are wired in series that is approximately equal to a voltage that the electronic dimming ballast establishes across each of the filaments when the filaments are wired in parallel. 
       FIG. 8B  is a simplified schematic diagram of example ballast back end  830  having a filament miswire protection element  832 . The ballast back end  830  includes a first inverter  834  and a second inverter  836 . The second inverter  836  may be different from the first inverter  834 . Similar to the back end  220  described in  FIGS. 2 and 3 , the back end  830  includes an inverter (e.g., the first inverter  834  operates similar to the inverter  250 ) and an output circuit  838 . The first inverter  834  may drive the lamps L 1 , L 2 , L 3  via the resonant inductor  840 , resonant capacitor  842 , and DC blocking capacitor  844 . The second inverter  836  may operate to provide an AC filament voltage via a second inductor  846 . 
     The second inverter  836  may enable independent control of the AC filament voltage. For example, the second inverter  836  may be controlled by the control circuit  280 , i.e., the same control circuit  280  that controls the first inverter  834 . Alternatively, the second inverter  836  may be controlled by a control circuit (not shown) that is different from the control circuit  280  that controls the first inverter  834 . The frequency of the second inverter  836  may be driven independently of the frequency of the first inverter  834 . The frequency of the second inverter  836  may be driven somewhat independently of the frequency of the first inverter  834 , such as operating at one-half of the frequency of the first inverter  834 , for example. 
     The second inverter  836  may include series-connected first and second switching devices  848 ,  850 , for example, two field-effect transistors (FETs). The FETs  848 ,  850  of the second inverter  836  may be driven using a complementary duty cycle switching mode of operation. This means that one, and only one, of the FETs  848 ,  850  is conducting at a given time. When the FET  848  is conducting, then the output of the second inverter  836  is pulled upwardly toward the DC bus voltage. When the FET  850  is conducting, then the output of the second inverter  836  is pulled downwardly toward circuit common. 
     Filament windings W 1 , W 2 , W 3 , W 4  are magnetically coupled to the second inductor  846 . The filament windings provide AC filament voltages to the filaments to keep the filaments warm through the entire dimming range. The filaments especially need to be heated when the ballast is dimming the lamps to low end and during preheating of the filaments before striking the lamp. 
     The windings W 1 , W 2 , and W 3  are independent filament windings and are coupled to respective filaments of lamps L 1 , L 2 , L 3 . The winding W 4  is a common filament winding and is connected to each of the filaments of lamps L 1 , L 2 , L 3  via a filament miswire protection element  832 . The filament miswire protection element  832  may be a two-node element. A first node  852  of the filament miswire protection element  832  may be connected to a branch (either branch, for example) of the common filament winding W 4 . A second node  854  of the filament miswire protection element  832  may be connected to a filament or filaments of the lamps. As illustrated, the filaments connected to the common filament winding are wired in parallel. However, as will be discussed further below, the filaments connected to the common filament winding could be wired in series with the filament miswire protection element accommodating for the difference in the wiring. 
     The filament miswire protection element  832  may be an electrical component, system, or sub-system that accommodates for miswiring of the common filament winding W 4 . For example, the filament miswire protection element  832  may be an electrical component, system, or sub-system that has an impedance that is approximately equal to an impedance of at least one of the filaments of lamps L 1 , L 2 , L 3 . Because the ballast with back end  830  may operate within a given range of frequencies, the filament miswire protection element  832  may have an impedance that, within the relevant operating frequency/frequencies, is approximately equal to an impedance of at least one of the filaments of lamps L 1 , L 2 , L 3 . 
     The filament miswire protection element  832  may be coupled to the filament winding, such as for example the common filament winding W 4 . The filament miswire protection element  832  may be connectable to the filaments. For example, the electronic dimming ballast may have a pair of terminals T 1 , T 2 . The filament miswire protection element  832  may be connected to one of the pair of terminals T 1 , T 2 . The pair of terminals T 1 , T 2 , may be connectable to the filaments of lamps L 1 , L 2 , L 3 . For example, the pair of terminals T 1 , T 2 , may be a pair of wires. For example, the pair of terminals T 1 , T 2  may be in a terminal block. As a result, the electronic dimming ballast with back end  830  may establish, via the filament miswire protection element  832 , the same voltage across the filaments regardless of whether the filaments are wired in series or in parallel. The electronic dimming ballast with back end  830  may establish, via the filament miswire protection element  832 , for example, a first voltage across each of the filaments when the filaments are wired in series and a second voltage across each of the filaments when the filaments are wired in parallel. Here, the first and second voltages may be approximately equal. In other words, the electronic dimming ballast with back end  830  may establish, via the filament miswire protection element  832 , a voltage across each of the filaments when the filaments are wired in series that is approximately equal to a voltage that the electronic dimming ballast establishes across each of the filaments when the filaments are wired in parallel. 
     To illustrate how the miswire protection element accommodates for filament miswiring,  FIGS. 9A and 9B  show an example filament miswire protection element  900  with filaments R 1 , R 2 , (shown as resistors) wired in parallel and in series, respectively. The filament miswire protection element  900  may be wired in series with the network of the two filaments R 1 , R 2 . For example, in  FIG. 9A , the filament miswire protection element  900  is shown in series with the two filaments R 1 , R 2 , being in parallel with each other. In  FIG. 9B , the filament miswire protection element  900  is shown in series with two filaments R 1 , R 2 , with the two filaments being in series with each other. Accordingly, in  FIG. 9B , the three components, the filament miswire protection element  900  and the two filaments R 1 , R 2 , are in series with one another. 
     Because lamp types in a given fixture would typically be the same, we can assume that the resistance values R 1  and R 2  are equal, having a value R. The filament miswire protection element  900  may have an impedance, Z. The impedance, Z, may be approximately equal to the resistance R 1 , R 2  of one of the filaments. For example, the impedance, Z, may have the value R, the same as each of the filaments. To the extent that the impedance, Z, is a function of frequency, the absolute value of Z may have the value R at the relevant frequency of the common winding voltage V w . 
     When the filaments are wired in parallel and the common winding voltage V w  is coupled across the filaments as shown in  FIG. 9A , the voltage across each filament is equal to the voltage divided between the network of parallel filaments R 1 , R 2 , and the filament miswire protection element  900 . With the impedance Z being equal to R and with equivalent resistance of the network of parallel filaments, as shown below, the filament voltage is one-third of the common winding voltage V w , as shown by the following equation: 
     
       
         
           
             
               V 
               R 
             
             = 
             
               
                 
                   
                     V 
                     w 
                   
                   · 
                   
                     R 
                     
                       Network 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       of 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       parallel 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       filaments 
                     
                   
                 
                 
                   Z 
                   + 
                   
                     R 
                     
                       Network 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       of 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       parallel 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       filaments 
                     
                   
                 
               
               = 
               
                 
                   
                     
                       V 
                       w 
                     
                     · 
                     
                       
                         
                           R 
                           1 
                         
                         · 
                         
                           R 
                           2 
                         
                       
                       
                         
                           R 
                           1 
                         
                         + 
                         
                           R 
                           2 
                         
                       
                     
                   
                   
                     R 
                     + 
                     
                       
                         
                           R 
                           1 
                         
                         · 
                         
                           R 
                           2 
                         
                       
                       
                         
                           R 
                           1 
                         
                         + 
                         
                           R 
                           2 
                         
                       
                     
                   
                 
                 = 
                 
                   
                     
                       
                         V 
                         w 
                       
                       · 
                       
                         R 
                         2 
                       
                     
                     
                       R 
                       + 
                       
                         R 
                         2 
                       
                     
                   
                   = 
                   
                     
                       V 
                       w 
                     
                     3 
                   
                 
               
             
           
         
       
     
     When the filaments R 1 , R 2 , are wired in series, as shown in  FIG. 9B , the voltage across each filament R 1 , R 2 , is also one-third of the common winding voltage V w . Here, the voltage across each filament R 1 , R 2 , is equal to the voltage divided between a given filament R 1 , for example, and the collection of remaining filaments, R 2  for example, and the filament miswire protection element  900 . Again, with the impedance Z being equal to R, the filament voltage is one-third of the common winding voltage V w , as shown by the following equation: 
     
       
         
           
             
               V 
               
                 R 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 1 
               
             
             = 
             
               
                 
                   
                     V 
                     w 
                   
                   · 
                   
                     R 
                     1 
                   
                 
                 
                   
                     ( 
                     
                       Z 
                       + 
                       
                         R 
                         2 
                       
                     
                     ) 
                   
                   + 
                   
                     R 
                     1 
                   
                 
               
               = 
               
                 
                   
                     
                       V 
                       w 
                     
                     · 
                     R 
                   
                   
                     3 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     R 
                   
                 
                 = 
                 
                   
                     V 
                     w 
                   
                   3 
                 
               
             
           
         
       
     
     With the proper selection of the impedance of the filament miswire protection element  900 , the filament miswire protection element  900  accommodates for miswiring of the filaments. For example, the filament miswire protection element  900  may be connectable to the filaments R 1 , R 2 , such that the same AC filament voltage is established across a first of the filaments R 1 , R 2 , regardless of whether the filaments are wired in series or in parallel. 
     For example, the filament miswire protection element  900  may be connectable to the filaments R 1 , R 2 , such that a first AC filament voltage is established across each of the filaments R 1 , R 2 , when the filaments R 1 , R 2 , are wired in series, e.g., V R1  in  FIG. 9B , and a second AC filament voltage is established across each of the filaments R 1 , R 2 , when the filaments R 1 , R 2 , are wired in parallel, e.g., V R  in  FIG. 9A . The first and second AC filament voltages may be approximately equal, e.g., one-third of the common winding voltage V w  in the above example. 
     For example, the filament miswire protection element  900  may be connectable to the filaments R 1 , R 2 , such that the AC filament voltage across each of the filaments R 1 , R 2 , when the filaments R 1 , R 2 , are wired in series is approximately equal to the AC filament voltage across each of the filaments R 1 , R 2 , when the filaments R 1 , R 2 , are wired in parallel, e.g., one-third of the common winding voltage V w  in the above example regardless of whether the filaments R 1 , R 2 , are wired in series or in parallel to each other. 
       FIGS. 10A-E  are schematic diagrams illustrating example filament miswire protection elements  1002 ,  1006 ,  1010 ,  1014 ,  1020 . As shown in  FIG. 10A , the filament miswire protection element  1002  may include a capacitor  1004 . In an embodiment, the filament miswire protection element  1002  may be only a capacitor  1004 . The impedance of the capacitor  1004  may be selected to be approximately equal to the filament impedance Z of the lamp type being served by the ballast. Because the impedance of the capacitor  1004  varies as a function of frequency, the capacitance value of the capacitor may be selected such that the absolute value of the impedance of the capacitor  1004  is approximately equal to the absolute value of the filament impedance Z at the relevant operating frequency, e.g., the frequency of the filament voltage. 
     As shown in  FIG. 10B , the filament miswire protection element  1006  may include an inductor  1008 . In an embodiment, the filament miswire protection element  1006  may be only an inductor  1008 . The impedance of the inductor  1008  may be selected to be approximately equal to the filament impedance Z of the lamp type being served by the ballast. Because the impedance of the inductor  1008  varies as a function of frequency, the inductance value of the inductor  1008  may be selected such that the absolute value of the impedance of the inductor  1008  is approximately equal to the absolute value of the filament impedance Z at the relevant operating frequency, e.g., the frequency of the filament voltage. 
     As shown in  FIG. 10C , the filament miswire protection element  1010  may include a resistor  1012 . In an embodiment, the filament miswire protection element  1010  may be only a resistor  1012 . The resistance of the resistor  1012  may be selected to be approximately equal to the absolute value of the filament impedance Z of the lamp type being served by the ballast. 
     Table 1 contains example capacitance and inductance values corresponding to common lamp types at a relevant operating frequency, i.e., 50 kHz. These values are examples, and acceptable values may range within, for example, ±10% of the values shown. Acceptable values may be within a range greater than or less than the ±10% range based on the ballast design and application requirements. Such a range would result in similarly acceptable impedances being approximately equal to the corresponding filament resistances. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                   
                 Values of filament miswire protection 
               
               
                   
                 Filament 
                 element (at 50 kHz) 
               
            
           
           
               
               
               
               
            
               
                 Lamp type 
                 resistance (ohms) 
                 Capacitor (nF) 
                 Inductor (uH) 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                 T8 family 
                 12 
                 265 
                 38 
               
               
                 T5HE family 
                 40 
                 79 
                 127 
               
               
                 T5HO 80 W 
                 7 
                 454 
                 22 
               
               
                 T5HO 54 W 
                 8 
                 397 
                 25 
               
               
                 T5HO 39 W 
                 12 
                 265 
                 38 
               
               
                 T5HO 24 W 
                 12 
                 265 
                 38 
               
               
                   
               
            
           
         
       
     
       FIG. 10D  illustrates a filament miswire protection element  1014  with a selectable impedance. The filament miswire protection element  1014  may include a plurality of capacitors  1016 A,  1016 B,  1016 C that are selectable based on the shorting and/or opening of one or more jumpers  1018 . The selectable impedance may be selectable by a user. For example, the selectable impedance may be selectable by a user during the manufacturing process or in the field. Though capacitors are shown in  FIG. 10D , it should be understood that inductors or resistors could be used instead of or in addition to the capacitors. 
       FIG. 10E  illustrates a filament miswire protection element  1020  with a selectable impedance. The filament miswire protection element  1020  may include a plurality of capacitors  1022 A,  1022 B,  1022 C that are selectable based on a controllable switch  1024 . A controller (not shown) may control the controllable switch  1024  to select the appropriate capacitance value. The filament miswire protection element  1020  may include a filament current sensor  1026  that may be used by the controller to facilitate the correct selection of the appropriate capacitance value. The controller may be a microprocessor. The controller may be a control circuit of the ballast, for example control circuit  280 , as illustrated in  FIG. 2 . Again, though capacitors are shown in  FIG. 10E , it should be understood that inductors or resistors could be used instead of or in addition to the capacitors. 
       FIG. 11  illustrates a method of manufacturing a ballast with a miswire protection element. At  1100 , the method may start. When manufacturing and/or designing a ballast, one may, at  1102 , identify a ballast for a lamp type (e.g., the T8 family, T5HE family, T5HO 80W, T5HO 54W, T5HO 39W, T5HO 24W, and the like). Various lamp types may have characteristics provided by the lamp manufacturer, including for example filament resistance and/or impedance and a safe operating area. 
     At  1104 , a miswire protection element may be selected based on the lamp type. For example, an impedance, at an operating frequency, that is approximately equal to a filament resistance of the lamp type may be selected when selecting a miswire protection element. 
     At  1106 , a ballast with the miswire protection element may be provided. For example, instructions indicating that two terminals are to be connected to a plurality of filaments may be provided. The instructions may be written to not require that the plurality of filaments be connected in series. Similarly, the instructions may be written also to not require that the plurality of filaments be connected in parallel. Alternatively, the instructions may indicate that the plurality of filaments may be connected either in series or in parallel. At  1108 , the method ends. 
     Although the disclosed ballast and methods have been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims.