Patent ID: 8896213

Claim:
An electronic ballast comprising: an inverter functional to receive power from a power source and provide an alternating current (AC) signal at an inverter frequency; first and second output ballast output terminals; a resonant tank coupled to the inverter and configured to receive the AC signal from the inverter and provide an output signal across the first and second ballast output terminals; an impedance sensing circuit comprising a direct current (DC) power supply operable to provide a DC voltage referenced to a ballast ground, a first resistor connected between the DC power supply and the first ballast output terminal, a second resistor having a first terminal connected to the first ballast output terminal and a second terminal, a third resistor connected between the ballast ground and the second terminal of the second resistor; and a controller connected to the inverter and the second terminal of the second resistor, wherein the controller is effective to sense a lamp impedance of a lamp coupled to the ballast output terminals by detecting a DC voltage impedance signal provided at the second terminal of the second resistor, calculate a change in the lamp impedance over time, determine whether the lamp impedance is unstable by determining whether the change in the lamp impedance over time exceeds a predetermined threshold, and increase a current provided to the lamp by the ballast for a predetermined period of time when the lamp impedance is unstable by adjusting the inverter frequency.