Patent ID: 8247997

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; and a control circuit coupled to the output circuit and to the inverter, wherein the control circuit is operable, during a detection period prior to startup of the inverter: (i) in an arrangement wherein the lamp load includes only a single lamp, to detect whether or not the single lamp has both filaments intact; and (ii) in an arrangement wherein the lamp load includes multiple lamps, to detect whether or not all of the lamps have both filaments intact; and wherein the control circuit includes: (i) a filament detection input coupled to the output circuit; and (ii) at least a first control output coupled to the inverter; and the control circuit is further operable: (i) during the detection period prior to startup of the inverter, to receive at the filament detection input a voltage signal from the output circuit that is indicative of whether or not intact lamp filaments are connected to the output connections; and (ii) to provide a control signal at the first control output in dependence upon the voltage signal; and the control circuit provides a first timing function and a second timing function; and the control circuit is further operable: (a) in response to the voltage signal at the filament detection input exceeding a first predetermined threshold, to start a first timer, and then to periodically increment the first timer until such time as: (i) the voltage signal exceeds a second predetermined threshold; or (ii) the first timer reaches a predetermined overflow limit; and (b) in response to the voltage signal at the filament detection input exceeding the second predetermined threshold, to: (i) stop the first timer, if the first timer had previously started; (ii) start a second timer; and (iii) periodically increment the second timer until such time as the second timer reaches the predetermined overflow limit.