Patent ID: 7187132

Claim:
A ballast for powering at least one gas discharge lamp having first and second lamp filaments, the ballast comprising: an inverter having first and second input terminals and first and second output terminals, wherein the second input terminal and the second output terminal are coupled to a circuit ground, the inverter including an inverter driver circuit having a preheat control output; an output circuit coupled to the inverter output terminals, the output circuit having first, second, third, and fourth output connections adapted for connection to the at least one gas discharge lamp, wherein the first and second output connections are adapted for connection to the first lamp filament, and the third and fourth output connections are adapted for connection to the second lamp filament, the output circuit including a first filament heating winding coupled between the first and second output connections, and a second filament heating winding coupled between the third and fourth output connections; a dimming control circuit having a pair of input connections adapted to receive a dimming control input; and a filament heating control circuit coupled to the dimming control circuit, the inverter, and the output circuit, the filament heating control circuit being operable such that: (i) during a preheat mode that occurs prior to ignition of the lamp, the lamp filaments are heated at a first level; (ii) during a full-light mode that occurs after ignition of the lamp, the lamp filaments are not heated, wherein the full-light mode includes operating the lamp at a current level that is substantially equal to a rated normal operating current of the lamp; and (iii) during a dimming mode that occurs after ignition of the lamp, the lamp filaments are heated at a second level, wherein the dimming mode includes operating the lamp at a current level that is substantially less than the rated normal operating current of the lamp; and wherein the filament heating control circuit includes: (i) a first electronic switch operably coupled to the first output terminal of the inverter, the preheat control output of the inverter driver circuit, and circuit ground, and operable, in response to a low level voltage at the preheat control output during the preheat mode, to turn on and control heating of the lamp filaments during the preheat mode; and (ii) a second electronic switch that operably coupled to the first output terminal of the inverter, the dimming control circuit, and circuit ground, and operable, in response to a low level voltage from the dimming control circuit during the dimming mode, to turn on and provide heating of the lamp filaments during the dimming mode.