Patent ID: 7183714

Claim:
A circuit for detecting the reconnection of a lamp filament to a power bus in an electronic ballast driving a fluorescent lamp, said circuit comprising: an inverter control circuit for controlling an inverter circuit providing power to the power bus; a filament control circuit interconnecting the power bus and the lamp filament for preheating and powering the lamp filament and for inhibiting the inverter circuit when sensing that the lamp filament has been disconnected from the filament control circuit; a pulse generating circuit coupled to the lamp filament for generating an input signal indicative of a reconnection of the lamp filament to the filament control circuit; a detection circuit coupled to the pulse generating circuit for detecting the reconnection of the lamp filament and operative to produce a command signal 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; and wherein the pulse generating circuit comprises: a pulse generator supplying a control signal; and a transformer having a primary winding receiving the control signal and a secondary winding connected to the lamp filament, wherein the control signal at the primary winding corresponds to an output pulse signal at the secondary winding, said output pulse signal having an output current having a first magnitude when the lamp filament is disconnected from the filament control circuit, and having a second magnitude when the lamp filament is reconnected to the filament control circuit, and wherein the output current signal generated at the secondary winding corresponds to an input current signal at the primary winding, said input current signal having a peak magnitude when the output current signal has the second magnitude.