Abstract:
An apparatus for detecting the reconnection of a lamp filament to an electronic ballast driving a fluorescent lamp. A control circuit controls an inverter circuit to providing power to a filament control circuit. The filament control circuit preheats and powers the lamp filament of the one or more lamps. A pulse generator generates an input signal as a function of the number of lamp filament connected to the filament control circuit. A current sensor generates a first voltage indicative of whether a lamp filament has been reconnected to the circuit. A peak detector generates a peak voltage signal when the first voltage indicates a reconnection of a lamp filament has occurred. A sensing circuit generates a command signal to provide to the control circuit to supply power to the filament control circuit to preheat and power the lamp when the peak detector generates the peak voltage signal.

Description:
TECHNICAL FIELD 
     The present invention relates to ballasts for powering gas discharge lamps. In particular, the invention relates to an electronic ballast for powering multiple series-connected fluorescent lamps having filaments connected in parallel. The ballast includes relamping circuitry for detecting the reconnection of a lamp filament in order to energize the reconnected lamp. 
     BACKGROUND OF THE INVENTION 
     Electronic ballasts for gas discharge lamps are often classified into two groups according to how the lamps are ignited: (1) a preheat type ballast; and (2) an instant start type ballast. In preheat ballasts, the lamp filaments are preheated at a relatively high level (e.g., 7 volts peak) for a limited period of time (e.g., one second or less) before a moderately high voltage (e.g., 500 volts peak) is applied across the lamp in order to ignite the lamp. In instant start ballasts, the lamp filaments are not preheated, so a higher starting voltage (e.g., 1000 volts peak) is required in order to ignite the lamp. It is generally acknowledged that instant start operation offers certain advantages, such as the ability to ignite the lamp at a lower ambient temperatures and greater energy efficiency (i.e., light output per watt) due to no expenditure of power on filament heating during normal operation of the lamp. On the other hand, instant start operation usually results in considerably lower lamp life than preheat operation. 
     Because a significant amount of power can be unnecessarily expended heating the lamp filaments after the lamp is ignited, it is desirable to have preheat type ballasts in which filament power is minimized or eliminated once the lamp has ignited. One approach for preheating ballasts employs switching circuitry such as a filament control circuit that disconnects the source of filament power from each of the filaments after the lamp ignites. However, when such switching circuitry is used with ballasts driving multiple fluorescent lamps, there have been problems preheating and igniting lamps which have been disconnected from the ballast and then reconnected back to the ballast. One solution to ignite such reconnected lamps has been to cycle the power supplied to the ballast (i.e., turn the power off, and then back on). 
     In ballast circuits driving three (3) or more lamps, the outermost lamps are usually connected directly to the ballast circuit. Thus, disconnecting the outer lamps may cause an open circuit which can be detected. When an outer lamp is reconnected, it closes the circuit so that preheating and/or ignition can be initiated. However, the inner lamps, such as the middle lamp in a three lamp circuit, are connected with one or more of the outer lamps but are not directly connected to the ballast circuit. Hence, removing and reconnecting an inner lamp may not close an open circuit so that its reconnection is difficult to detect. Accordingly, re-igniting a disconnected and reconnected inner lamp has typically required cycling of the power. To avoid the need for cycling the ballast power when an inner lamp of a plurality of lamps connected to the ballast circuit is taken out and then reconnected to the circuit, there is a need for a ballast circuit that detects the reconnection of an inner lamp to preheat and/or ignite the reconnected lamp without requiring cycling of the power. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the invention, a ballast circuit is provided for detecting the reconnection of a lamp filament to a power bus in an electronic ballast driving a fluorescent lamp. The ballast circuit includes an inverter control circuit that controls an inverter circuit to provide power to the power bus. The ballast circuit also includes a filament control circuit that interconnects the power bus and the lamp filament to preheat and power the lamp filament and to inhibit the inverter circuit when sensing that the lamp filament has been disconnected from the filament control circuit. The ballast circuit also includes a pulse generating circuit coupled to the lamp filament that generates an input signal indicative of a reconnection of the lamp filament to the filament control circuit. The ballast circuit further includes a detection circuit coupled to the pulse generating circuit that detects the reconnection of the lamp filament and is operative to produce a command signal that is provided to the inverter control circuit to cause the inverter circuit to supply power to the filament control circuit to preheat the lamp filament and supply power to the lamp. 
     In accordance with another aspect of the invention, a detection circuit is provided for detecting the reconnection of a lamp filament in an electronic ballast that includes a filament control circuit for preheating and powering lamp filaments of a plurality of fluorescent lamps. The ballast includes an inverter control circuit that controls an inverter circuit to provide an AC voltage signal to power the filament control circuit to preheat and power each lamp filament of the plurality of lamps. The ballast also includes a pulse generating circuit coupled to the plurality of lamps to generate an input signal indicative of a reconnection of one of the lamp filaments to the filament control circuit. The ballast also includes a current sensor that is connected to the pulse generating circuit and responsive to the input signal for generating an input voltage signal that has a first magnitude when the filament is disconnected from the filament control circuit and has a second magnitude when the filament is reconnected to the filament control circuit. The ballast also includes a peak detection circuit connected to the current sensor that senses a magnitude of the input voltage signal, and generates a detected voltage signal as a function of the sensed magnitude of the input voltage signal. The detected voltage signal has a peak magnitude when the input voltage signal has the second magnitude. The ballast further includes a sensing circuit connected to the peak detection circuit that senses a magnitude of the detected voltage signal, and generates a command signal that is provided to the inverter control circuit to supplying power to the filament control circuit to preheat and power the lamp filament when the detected voltage signal has the peak magnitude. 
     In accordance with yet another aspect of the invention, a method is provided for detecting the reconnection of a lamp filament to a power bus in a ballast circuit driving a fluorescent lamp. The method includes supplying an alternating current (AC) signal to the lamp via an inverter circuit. The method also includes preheating and powering the lamp filament when the lamp filament is connected to the power bus. The method also includes generating an input signal that has a first magnitude when the lamp filament is disconnected from the power bus and has a second magnitude when the lamp filament is reconnected to the power bus. The method also includes generating a detection signal as a function of the magnitude of the generated input signal. The generated detection signal has a peak magnitude when the generated input signal has the second magnitude. The method further includes supplying the AC signal to preheat and power the lamp filament when the detection signal has the peak magnitude. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating a ballast circuit for powering a plurality of gas discharge lamps 
         FIG. 2  is a combination block and schematic diagram illustrating components of a ballast circuit according to one embodiment of the invention. 
         FIG. 3  illustrates components of a detection circuit for detecting the reconnection of a filament in the ballast according to one embodiment of the invention 
     
    
    
     Corresponding reference characters indicate corresponding parts throughout the drawings. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  illustrates a ballast circuit  100  for powering a plurality of gas discharge lamps  102 ,  104 ,  106 . The ballast circuit  100  includes a control circuit  107  connected to and controlling an inverter  108  to supply power to output terminals  110 ,  112 ,  114 ,  1 . 15 ,  116 ,  117  via an isolation transformer  122 , and via a filament control circuit  124 . 
     The inverter  108  receives a substantially direct current (DC) input voltage, V DC , from a DC bus  125  via input terminals  126 ,  128 , and is responsive to a control signal  129  from the control circuit  107  to provide an alternating current (AC) output voltage at output bus terminal  130  for powering the lamps  102 ,  104 ,  106 . The DC input voltage can be provided from a DC source (not shown) such as a rectified input AC source, a battery, or any other source of DC power. As known to those skilled in the art, the AC output voltage at inverter output bus terminal  130  has a high frequency (e.g., 20,000 hertz or greater) at or near to the natural resonant frequency of an inductor  119  and a capacitor  120  of a resonant tank circuit  121  connected to the inverter  108 , the isolation transformer  122 , and circuit ground  133 . 
     The isolation transformer  122  provides the increased voltage necessary for igniting the lamps  102 ,  104 ,  106  and minimizes power dissipation. The isolation transformer  122  includes a primary winding  132  connected between the inductor  119  and capacitor  120  and connected to circuit ground  133  via a DC blocking capacitor  123  such that the primary winding  132  and the DC blocking capacitor  123  form a series combination that is connected in parallel with the capacitor  120  of the resonant tank circuit  121 . A secondary winding  134  of the isolation transformer  122  outputs the increased voltage via terminals  110 ,  112 . More specifically, the isolation transformer  122  is responsive to the AC output voltage at bus terminal  130 , and the resulting voltage across the capacitor  120 , to provide an increased AC output voltage at the secondary winding  134  for preheating and/or igniting the lamps  102 ,  104 ,  106 . 
     The filament control circuit  124  coupled to bus terminal  130  supplies a preheat voltage to lamps  102 ,  104 ,  106  via output terminals  114 ,  115 ,  116 ,  117  to preheat the filaments  135 – 140  of lamps  102 ,  104 ,  106 . As explained above, in order minimize the amount of power expended on heating lamp filaments in a preheat ballast, it is desirable to preheat lamp filaments prior to ignition by supplying a preheat voltage during a preheat mode in which the voltage applied across each of the lamps is substantially less than an ignition voltage required to ignite the lamp. Output terminals  110 ,  112 ,  114 ,  115 ,  116 ,  117  and input terminal  126  are adapted for connection to the filaments  135 – 140  of the lamps  102 ,  104 ,  106 . More specifically, input terminal  126  is connected to a connection  141  of a first filament  135  of lamp  102  via a current limiting resistor  142 , and output terminal  110  is connected to a connection  143  of the first filament  135  of lamp  102 . Output terminals  116 ,  117  are connected to a second filament  138  of lamp  102  via connections  144 ,  146 , respectively, and to a second filament  139  of lamp  104  via connections  148 ,  150 . Output terminals  114 ,  115  are connected to a first filament  136  of lamp  104  via connections  152 ,  154 , and to a first filament  137  of lamp  106  via connections  156 ,  158 . A second filament  140  of lamp  106  is connected to output terminal  112  via connection  162 , and to circuit ground  133  via a current limiting resistor  159  and connection  160 . Thus, as can be seen, the first filament  135  of lamp  102  is connected in series with output terminal  110  and input terminal  126  via resistor  142 . The first filament  136  of lamp  104  is connected in parallel with the first filament  137  of lamp  106  via output terminals  114 ,  115 . The second filament  138  of lamp  102  is connected in parallel with the second filament  139  of lamp  104  via output terminals  116 ,  117 . The second filament  140  of lamp  106  is connected in series with the secondary winding  134  of the isolation transformer  122 . 
     In this particular circuit  100 , the filament control circuit  124  is configured to provide the preheat voltage to filaments  136 – 139 . For example, the preheat voltage produced across output terminals  114 ,  115  preheats filaments  136  and  137  of the second and third lamps  104 ,  106 , and the preheat voltage across output terminals  116 ,  117  preheats filaments  138  and  139  of the first and second lamps  102 ,  104 . After filament preheat is complete, the filament control circuit  124  shuts down, and only re-activates when power to ballast circuit  100  is cycled. 
     In operation, when either of the outer lamps  102 ,  106  are removed an open circuit occurs between terminals  126  and  141  or between terminal  160  of the filament  140  of the outer lamp  106  and ground  133 . This causes the voltage across the resistor  159  to fall to zero. When the outer lamps  102 ,  106  are reconnected, the circuit is closed. As such a voltage appears across the resistor  159  which can be used to re-trigger the control circuit  107  to start the ballast again. Notably, resistor  159  should be of sufficiently high value so that the isolation between the input and the output because of the presence of isolation transformer  122  remains substantially unaffected. However, as noted above, the filaments  136 , 139  of middle lamp  104  are connected in parallel with the filaments  137 ,  138  of lamps  106 ,  102 , respectively. Because of this parallel connection, the control circuit  107  cannot detect an open circuit when lamp  104  is removed or a closed circuit when lamp  104  is reconnected. As explained in more detail below in reference to  FIG. 2 , the filament control circuit  124  includes a shut down circuit  164  responsive to the removal of the any of the three lamps to generate a fault signal  166 . The control circuit  107  connected to the shut down circuit  164  is responsive to the fault signal  166  to shut down the inverter  108 . As a result, minimal, if any, voltage is present across the primary and secondary windings  132 ,  134  of the isolation transformer  122 , and, thus, the filament control circuit  124  is de-energized and shuts down. The middle lamp  104  becomes a floating system. That is, even when the middle lamp  104  is reconnected, the filament control circuit  124  remains deactivated until the ballast power is cycled. 
     Referring now to  FIG. 2 , a combination block and schematic diagram illustrates components of a ballast circuit  200  according to one embodiment of the invention. As described above, the inverter circuit  108  is responsive to an input DC voltage signal received via input terminal  126 ,  128  to generate an output AC voltage signal, as indicated by reference character  204 , for powering the lamp filaments  135 – 140  via a filament control circuit  206  (e.g., filament control circuit  124  in  FIG. 1 ). In this embodiment, the inverter circuit  108  includes switching transistors such as MOSFETs  208 ,  210 , connected between DC input terminals  126 ,  128 . MOSFETs  208 ,  210  are driven by first and second control signal  212 ,  214 , respectively, supplied from a control circuit  216  (e.g., control circuit  107  in  FIG. 1 ) to generate the output AC voltage signal  204 . The control circuit  216  can be a L6569 Half Bridge Driver manufactured by STMicroelectronics of Plan les Ouates, Geneva, Switzerland. 
     A drain  218  of the MOSFET  208  is coupled to input terminal  126 . A gate  220  of the MOSFET  208  connected to the control circuit  216  is responsive to the first control signal  212  generated by the control circuit  216  to turn the MOSFET  208  on and off. For example, when the magnitude of the first control signal  212  is equal to or greater than a threshold voltage (i.e., first control signal has at least a minimum magnitude), the MOSFET  208  turns on and positive current flows through the ballast circuit  200  via a power bus  222 . A drain  218  of the MOSFET  210  is coupled to a source  224  of MOSFET  208 . A gate  220  of the MOSFET  210  connected to the control circuit  216  is responsive to the second control signal  214  generated by the control circuit  216  to turn the MOSFET  210  on and off. For example, when the magnitude of the second control signal  214  is equal to or greater than a threshold voltage (i.e., second control signal has at least a minimum magnitude), the MOSFET  210  turns on and negative current flows through the circuit via power bus  222 . By selectively activating MOSFETs  208 ,  210  in an alternating fashion, the control circuit  216  causes the inverter circuit  108  to generate the output AC signal to preheat, ignite and operate lamps  102 ,  104 ,  106 . 
     As described above, the filament control circuit  206  provides a preheat voltage to the filaments  136 – 139  to preheat the lamps  102 ,  104 ,  106  prior to ignition. In this embodiment, the filament control circuit  206  includes a second transformer  225 , a capacitor  226 , a switching device  228  (e.g., a MOSFET), and a diode  230 . The second transformer  225  has a primary winding  232 , a first auxiliary winding  234 , and a second auxiliary winding  236 . The primary winding  232  is connected to the inverter circuit  108  and circuit ground  133 , via capacitor  226  and the switching device  228 , and is responsive to the output AC voltage signal  204  from inverter  108  to generate the preheat voltage across each of the first and second auxiliary windings  234  and  236 . The MOSFET  228  is connected between the capacitor  226  and circuit ground  133 . More specifically, a drain  238  of the MOSFET  228  is connected to capacitor  204  and a source  240  of the MOSFET  228  is connected to circuit ground  133 . A pulse generator  241  supplies a pulse signal  242  to a gate  244  of the MOSFET  228  to turn the MOSFET  228  on and off. For example, the pulse generator  241  is configured to generate the pulse signal  242  when the DC input voltage between input terminals  126 ,  128  reaches a threshold value. When the pulse signal  242  is supplied to the gate  244  of the MOSFET  228 , the MOSFET  228  turns on and current flows thru the primary winding  232  of the second transformer  224 . As a result, current flows through each of the first and second auxiliary windings  234 ,  236  producing the preheat voltage across each of the first and second auxiliary windings  234 ,  236 . 
     The filaments  138  and  139  of the first and second lamps  102 ,  104 , respectively, are connected in parallel with each other, via connections  144 ,  146  and connections  148 ,  150 , respectively, and with the first auxiliary winding  234 . The filaments  136  and  137  of the second and third lamps  104 ,  106 , respectively, are connected in parallel with each other, via connections  152 ,  154  and connections  156 ,  158 , respectively, and with the second auxiliary winding  236 . When the pulse signal  242  being applied to the gate  224  of MOSFET  228  is removed, the MOSFET  228  turns off and current stops flowing to the primary winding  232  of the second transformer  225 , and, thus, no voltage is generated across the first and secondary auxiliary windings  234 ,  236 . Notably, as illustrated in phantom lines, the filament control circuit  206  may also include third and fourth auxiliary windings  245 ,  246  for preheating the remaining filaments  135  and  140  of outer lamps  102 ,  106 , respectively. However, for purposes of illustration the filament control circuit  108  is described herein as supplying a preheat voltage to filament  138  of outer lamp  102 , to filament  137  of outer lamp  106 , and to filaments  136 ,  139  of middle lamp  104 . 
     The shut down circuit  164  includes a current sensing resistor  247 , and generates a fault signal  248  representative of the voltage drop across the resistor  247 . The control circuit  216  connected to the shutdown circuit  164  is responsive to the fault signal  248  (e.g., fault signal  166  in  FIG. 1 ) having a magnitude greater than a specified value (e.g., 1V) to shut down the ballast  200 . For example, as known to those skilled in the art, when any one of the lamps  102 ,  104 ,  106  is removed from the circuit  200 , the MOSFETS  208 ,  210  go into hard switching. As a result, the current through the inverter  108  increases resulting in current spikes within the ballast circuit  200 . These current spikes cause the voltage drop across resistor  247  to increase beyond the specified value. The control circuit  216  is responsive to the increased voltage to inhibit operation of the inverter circuit  108  by preventing control signals  212  and  214  (i.e., gate-drive signals for MOSFETs  208 ,  210 ) from being supplied to the inverter circuit  108 . This terminates AC power from being supplied to the lamps  102 ,  104 ,  106 . 
     According to the present invention, a detection circuit  252  connected to the filament control circuit  206  and the control circuit  216  is responsive to an input signal indicative of the reconnection of one or more lamps  102 ,  104 ,  106  to generate a command signal  254  provided to the control circuit  216  to override the fault signal  248  to operate the inverter  108  without cycling of the power to the ballast. 
     Referring now to  FIG. 3 , a schematic diagram illustrates components of a detection circuit  252  of the ballast circuit  200  for detecting the disconnection and reconnection of any of lamps  102 ,  104 ,  106  according to one embodiment of the invention. In this particular embodiment, the detection circuit  252  senses a magnitude of an input voltage signal generated within the ballast circuit  200  and generates the command signal  254  provided to the control circuit  216  as a function of the magnitude of the sensed voltage. 
     A pulse generating circuit  300  connected to the filament control circuit  206  and the lamps  102 ,  104 ,  106  generates an input signal, as indicated by reference character  301 , indicative of a disconnection or reconnection of a lamp filament from the filament control circuit  206 . The pulse generating circuit  300  includes a pulse transformer  302 , having a primary winding  304  and first and second auxiliary windings  306 ,  308 . The primary winding  304  is connected to a second pulse generator  310  supplying a pulse signal  312  of high frequency. The pulse transformer  302  is responsive to the pulse signal  312  supplied to the primary winding  304  to generate an output voltage across each of the first and second auxiliary windings  306 ,  308 . The pulse generator  310  is, for example, an astable multivibrator 555 timer capable of providing a high frequency voltage signal. The first and second auxiliary windings  306 ,  308  of the pulse transformer  302  are connected in series with the first and second auxiliary windings  234 ,  236 , respectively, of the filament control circuit  206  (see  FIG. 2 ). As a result of the output voltage signal generated across the first and second auxiliary windings  306 ,  308 , a current is continuously supplied to filaments  138  and  139  of the first and second lamps  102 ,  104  and to filaments  136  and  137  of the second and third lamps  104 ,  106 . As known to those skilled in the art, when a circuit includes resistive elements (e.g., filaments) connected in parallel, and one of the resistive elements is removed, the effective resistance of the circuit increases. From  FIG. 3  it can be seen that the filament  138  of lamp  102  is connected in parallel with the filament  139  of lamp  104 . Accordingly, if filament  139  of lamp  104  is disconnected, the current through first auxiliary winding  306  of pulse transformer  302  is reduced because the corresponding effective resistance on the secondary side increases. When the second filament  139  of lamp  104  is reconnected, the corresponding effective resistance on the secondary side decreases, and, thus, current through the first auxiliary winding  306  increases. 
     As a result of the current supplied to the first and/or second auxiliary windings  306 ,  308 , there is a reflected current (e.g., input signal  301 ) in the primary winding  304 . The primary winding  304  of the pulse transformer  302  is connected to output terminal  316  of the pulse generator  310  and circuit ground  133  via a filtering capacitor  318  and a current sensing resistor  320  (e.g., current sensing resistor  247  of  FIG. 2 ). Thus, the magnitude of the current flowing through the resistor  320  corresponds to the number of filaments connected to the filament control circuit  206 . For example, if the filament  139  of lamp  104  is disconnected, the current through resistor  320  is reduced, and, thus, the voltage drop across resistor  320  decreases. When the filament  139  of lamp  104  is reconnected, the reflected current onto the primary winding  304  increases resulting in an increased voltage drop across the current sensing resistor  320 . 
     A peak detector circuit  322  connected to the current sensing resistor  320  detects when the voltage drop across the current sensing resistor  320  increases. In this embodiment, the peak detector  322  includes a first operational amplifier (opamp)  324  having a first input terminal (non-inverting terminal)  326 , a second input terminal  328  (inverting terminal), and an output terminal  330 . The non-inverting terminal  326  is connected to the filtering capacitor  318  and the current sensing resistor  320  via an input resistor  332 . The inverting terminal  328  is tied to the output terminal  330  so that the first opamp  324  acts as a voltage follower. Thus, the first opamp  324  receives an input voltage at the non-inverting terminal  326  determined as a function of the magnitude of the voltage drop across the current sensing resistor  320 , and is responsive to the input voltage at the non-inverting input terminal  326  to generate an output voltage signal V out , as indicated by reference character  334 . In other words, the output voltage signal  334  follows the voltage across the current sensing resistor  320 . A diode  336  connected to the output terminal  330  is forward biased by the output voltage signal  334  and charges a capacitor  338 . The capacitor  338  continues to charge until the inverting and non-inverting terminals are at same voltage. In other words, when the voltage at the non-inverting input terminal  326  exceeds the voltage at the inverting input terminal  328 , the capacitor  338  continues to charge until the voltage across the capacitor  338  is equal to the input voltage at the non-inverting input terminal  326 . Because the output voltage signal  334  follows the voltage across the current sensing resistor  320 , the voltage across capacitor  338  decreases when a filament is removed and increases (i.e., peaks) when a filament is connected. 
     A sensing circuit  340  connected to the peak detection circuit  322  is responsive to the output voltage signal  334  to generate a command signal  341  (e.g., command signal  254  of  FIG. 2 ) provided to the control circuit  216  to control operation of the inverter circuit  108 . The sensing circuit  340  includes a second operational amplifier (opamp)  344  having a first input terminal (non-inverting terminal)  346 , a second input terminal  348  (inverting terminal), and an output terminal  350 . In this particular embodiment, the first and second opamps  324 ,  344  include positive voltage input terminals  351 ,  352 , respectively, that are tied together and connected to a DC voltage source  349  (e.g., 15 volt DC source), and negative voltage input terminals  353 ,  354  that are both connected to ground  133 . The non-inverting terminal  346  is connected to the peak detector  322  via a resistor network  355 . The resistor network  355  comprises resistors  356 ,  357  connected in series with each other and connected in parallel with resistors  358 ,  360 . The values of the resistors  356 ,  357 ,  358 ,  360  in the resistor network  348  determine the input voltages supplied to the non-inverting terminal  346  and the inverting terminal  348 . The inverting terminal  344  is connected to the peak detector  322  via the resistor network  355 , and a delay capacitor  362  connected in parallel with resistor  360 . The non-inverting terminal  346  and the inverting terminal  348  are connected to the resistor network  355  such that the effective resistance ultimately causes input voltage at the inverting terminal  348  to be greater than the input voltage at the non-inverting terminal  346 . However, the inverting terminal  348  is also connected to the capacitor  362 , which operates to delay this condition. That is, the delay capacitor  362  slowly charges so that the input voltage at the non-inverting terminal  346  is initially greater than the input voltage at the inverting terminal  348 , and the opamp  316  is responsive to the voltage difference to generate an output voltage signal, as indicated by  341 , having a peak magnitude (e.g., 5 volts), which is indicative of the reconnection of a filament. After a delay, the capacitor  362  charges so that the input voltage at the inverting terminal  348  becomes greater than the input voltage at the non-inverting terminal  346 , at which time the output voltage signal  341  (i.e., command signal) goes low (e.g., 0 volts). Thus, in operation the command signal  341  generated by the sensing circuit  340  can have two different states. For example, when the detection signal has a peak magnitude (i.e., filament connected), the command signal  341  generated by the sensing circuit  340  has a first state (e.g., peak magnitude). In contrast, when the detection signal has a minimum magnitude (i.e., filament disconnected), the command signal  341  generated by the sensing circuit  340  has a second state (e.g., low magnitude). The control circuit  112  is responsive to the command signal  341  having a peak magnitude to activate the MOSFETs  208 ,  210  (See  FIG. 2 ) to supply power to the lamps  102 ,  104 ,  106 . Notably, it can be seen that after implementation of this circuit  200 , resistors  142  and  159  (see  FIG. 1 ), which can be used to detect the removal of the outer lamps  102 ,  106  can be eliminated from the circuit. That is, because the outer lamp filaments  137 ,  138  are connected in parallel with the middle lamp filaments  136 ,  139 , respectively, the re-lamping of the outer lamps  102 ,  106  will also get detected in the same way as re-lamping of the middle lamp. 
     When introducing elements of the present invention or the embodiment(s) thereof, the articles “a,” “an,” “the,” and “said” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. 
     In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained. 
     As various changes could be made in the above constructions and methods without departing from the scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.