Patent ID: 8212497

Claim:
A ballast for powering a lamp load comprising at least one gas discharge lamp having a pair of lamp filaments, the ballast comprising: an inverter; an output circuit coupled to the inverter, the output circuit comprising a plurality of output connections adapted for coupling to the at least one gas discharge lamp; a filament heating control circuit coupled to the inverter and to the output circuit, and operable to provide, in conjunction with the output circuit, heating of the filaments of the at least one lamp, wherein the filament heating control circuit includes first and second inputs; a control circuit coupled to the output circuit, to the inverter, and to the filament heating control circuit, wherein the control circuit is operable: (a) during a lamp filament detection period prior to startup of the inverter, to determine the number of lamps with both filaments intact that are coupled to the output circuit; and (b) during a lamp type detection period following startup of the inverter, to determine the lamp type corresponding to the lamps with both filaments intact, wherein the control circuit includes: a filament detection input coupled to the output circuit; a current-sensing input coupled to the filament heating control circuit; and at least one control output coupled to the inverter; and wherein the control circuit is further operable: (i) during the lamp filament detection period prior to startup of the inverter, to receive at the filament detection input a first voltage signal from the output circuit that is indicative of whether or not intact lamp filaments are coupled to the output connections; (ii) during the lamp type detection period following startup of the inverter, to receive at the current-sensing input, a second voltage signal from the filament heating control circuit that is indicative of the lamp type of the lamps with intact filaments that are coupled to the output connections; (iii) after completion of the lamp filament detection period and the lamp type detection period, to provide a control signal at the at least one control output in dependence upon the first and second voltage signals; and wherein the ballast is operable, following the lamp type detection period, to provide heating of the lamp filaments in dependence upon: (i) the determined number of lamps with both filaments intact; and (ii) the determined lamp type of the lamps with both filaments intact, and wherein the inverter includes an inverter driver circuit, comprising: at least one input coupled to the at least one control output of the control circuit; and an output coupled to the second input of the filament heating control circuit; and wherein the inverter driver circuit is operable to provide a filament heating control signal at the output in dependence upon the at least one control signal provided by the control circuit.