BALLAST WITH LAMP FILAMENT DETECTION

A ballast (10) for powering one or two gas discharge lamps (30,40) includes an inverter (100), an output circuit (200), and a control circuit (500). During a period prior to startup of inverter (100), control circuit (500) monitors a signal within output circuit (200) in order to determine the presence of lamps with intact filaments that are present at the ballast output connections (202,204, . . . ,210,212). Preferably, control circuit (500) is realized by a programmable microcontroller which implements a dual timing scheme in order to accurately determine the number of lamps with both filaments intact. The resulting determination may be used for various purposes, such providing appropriate levels of filament heating and/or for setting thresholds for accurately detecting and protecting against various lamp fault conditions.