Patent ID: 8203273
Filing Date: 2012-06-19
Classification: H05B

Abstract:
1. A ballast circuit for pre-heating lamp filaments in a plurality of gas-discharge lamps, comprising: an inverter operable to convert a DC voltage into an AC voltage for powering the gas-discharge lamps; a primary pre-heating winding coupled to the inverter to receive a pre-heat voltage related to the AC voltage; secondary pre-heating windings that are each connectable to one of the lamp filaments, the secondary pre-heating windings being magnetically coupled to the primary pre-heating winding to receive the pre-heat voltage; a filament cut-back circuit coupled to the primary pre-heating winding, the filament cut-back circuit being operable to reduce the pre-heat voltage after the lamp filaments have been pre-heated; the inverter including a resonant inductor, the primary pre-heating winding being coupled to the resonant inductor; a pre-heat capacitor coupled between the resonant inductor and the primary pre-heating winding; and the filament cut-back circuit having a filament cut-back capacitor, a capacitance of the filament cut-back capacitor being greater than a capacitance of the pre-heat capacitor so that the pre-heat voltage is reduced when the inverter is generating the AC voltage at or above a lamp ignition power level.