Patent ID: 8008873

Claim:
A light source comprising: a ballast, wherein the ballast comprises: a controller; a dv/dt circuit: a rectifier configured to receive alternating current power from a power supply and to provide a direct current (DC) voltage to a DC voltage bus; an inverter configured to receive power from the DC voltage bus and to provide AC power during execution of a switching operation; and a resonant circuit configured to receive the AC power provided by the inverter, said resonant circuit comprising a first output pair, a second output pair, and a third output pair; a first lamp having a first filament and a second filament, wherein the first filament is connected across the first output pair; a second lamp having a first filament and a second filament, wherein the first filament of the second lamp is connected in series with the second filament of the first lamp across the second output pair, and the second filament of the second lamp is connected across the third output pair; wherein the controller is configured to monitor a first current corresponding to a DC current through at least one of the filaments of the lamps and to monitor a second current corresponding to a reference current, said controller further configured to determine a ratio of the first current to the second current, and to control the switching operation of the inverter as a function of the determined ratio, wherein: the controller inhibits the switching operation of the inverter when the determined ratio of the first current to the second current is less than a predetermined ratio, the controller drives the switching operation of the inverter when the determined ratio is more than the predetermined ratio, and the controller restarts the ballast in response to the ratio transitioning from below the predetermined ratio to above the predetermined ratio; and wherein the dv/dt circuit is configured to reduce the first current for a transient time period in response to a rapid voltage change at the second output pair; wherein the connection of at least one lamp filament to the resonant circuit causes the dv/dt circuit to reduce the first current for the transient time period, resulting in the determined ratio falling below the predetermined ratio for the transient time period.