A dimming module for controlling power supply to a fluorescent lamp ballasted by a nondimmable electronic ballast

A dimming module for controlling the active power supply to a fluorescent lamp ballasted by non-dimmable electronic ballast is presented. The module is physically connected across the lamp tube that requires dimming. The operating concept is based on diverting the current being supplied to the lamp tube away from the lamp tube thereby dimming the lamp. The power stage of the module is a series resonant inverter operating as a reactive power controller. It is controlled to draw a current in phase quadrature with the lamp voltage. On dimming the lamp, it will increase the filament voltage so as to maintain thermal emission of electrons from the filament, without detriment to the input power quality of the ballast and lamp current crest factor. An experimental prototype for a 28W T5 lamp has been built and evaluated. It is applied to one of the two lamps that are both ballasted by commercial electronic ballast. Experimental results show that the power supply to the lamp with the dimming module connected can be reduced by 70% while the one without the dimming module remains unchanged. A comprehensive study into the electrical performance of the ballast, lamp operational characteristics and system efficiency at different dimmed conditions will be presented.