Patent ID: 7986111

Claim:
A ballast for operating a first lamp set and for selectively operating a second lamp set in combination therewith, said ballast comprising: a rectifier configured to receive alternating current (AC) power from a power supply and to provide a direct current (DC) voltage to a DC voltage bus; a first lamp driver circuit and a second lamp driver circuit, each configured to receive power from the DC voltage bus and to provide AC power to operate its lamp set when said lamp driver circuit is enabled; a controller configured to control the second lamp driver circuit, to monitor a first value corresponding to a current through a filament of a lamp in the second lamp set, to monitor a second value corresponding to a reference current, to determine a calculated ratio of the first value to the second value, and to enable the second lamp driver circuit based on the calculated ratio such that: the controller disables the second lamp driver circuit when the calculated ratio of the first value to the second value is less than a predetermined ratio, and the controller enables the second lamp driver circuit when the calculated ratio is more than the predetermined ratio; and a control circuit comprising a first input terminal and a second input terminal, the first input terminal and the second input terminal configured to selectively connect to the power supply, the control circuit configured to reduce the current through the filament of the lamp in the second lamp set when a positive potential exists between the first input terminal and the second input terminal, such that the calculated ratio determined by the controller is less than the predetermined ratio.