Patent ID: 8288956
Filing Date: 2012-10-16
Classification: H05B

Abstract:
1. A circuit for preheating filaments in lamps powered by an electronic ballast, the ballast having an inverter providing an inverter output voltage at an inverter output, the circuit comprising: an inverter tank circuit comprising a preheat tank circuit comprising a preheat capacitor connected in parallel with a primary winding of a filament transformer, the preheat tank circuit having a first circuit junction and a second circuit junction, the first circuit junction coupled to the first lamp terminal; a cut-back capacitor connected between the second circuit junction of the preheat tank circuit and the second lamp terminal; the filament transformer having a first secondary winding coupled to the first lamp terminal and a second secondary winding coupled to the second lamp terminal to provide a lamp filament current at each terminal; the inverter is functional to operate at a preheat frequency during a lamp preheat mode and at a steady-state frequency during a lamp steady-state mode, the steady-state frequency being lower than the preheat frequency; and the preheat tank circuit has a natural resonant frequency that is approximately the same as the preheat frequency so that when the ballast is operating in the steady state mode, a filament voltage will be substantially lower than when the ballast is operating in the preheat mode.