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A dimming module for controlling the power supplying to a fluorescent lamp ballasted by a nondimmable electronic ballast is proposed. It is physically connected across the lamp tube to perform the dimming operation. The methodology is based on diverting the current supplying to the lamp tube to adjust the lamp power. The current diverted is in phase quadrature with the lamp voltage. The power stage of the module consists of a series-resonant inverter operated as a reactive-power controller. On dimming the lamp, it will increase the filament voltage for maintaining good thermionic emission of electrons from the filament. More importantly, it enables dimming function without detriment to the input-power quality of the ballast and lamp-current crest factor. The operating principle, control of the module, and the design procedure are described in this paper. An experimental prototype for dimming a 28 W T5 lamp has been built. The module is applied to one of the two lamps that are both ballasted by a commercial-electronic ballast for dual lamps. Experimental results show that the proposed module can adjust the power supplying to the lamp with the dimming module from 100% to 30% of the full power. A comprehensive study into the electrical characteristics of the ballast, lamp operational characteristics, and system efficiency at different dimmed conditions will be presented.