Patent ID: 8482213

Claim:
An electronic ballast comprising: an inverter circuit having an output circuit coupled to a pair of lamp terminals; a protection circuit coupled to one of the lamp terminals; wherein the protection circuit is functional to detect lamp voltage pulses that occur at the lamp terminal when a lamp coupled to the lamp terminals reaches an end of life condition, and to accumulate the lamp voltage pulses into a ballast shut down signal that is usable by the ballast to initiate shut down of the ballast when the accumulated ballast shut down signal reaches a predetermined shutdown level; a DC blocking capacitor connected between the lamp terminal and circuit ground; and the protection circuit comprises a differential voltage sensing circuit coupled to the DC blocking capacitor and functional to sense the lamp voltage pulses as sudden changes in voltage across the DC blocking capacitor and, in response, to provide a positive AC voltage pulse, and a pulse accumulation circuit coupled to the differential voltage sensing circuit, the pulse accumulation circuit being responsive to the positive AC voltage pulses from the differential voltage sensing circuit to accumulate the positive AC voltage pulses into the ballast shutdown signal.