a ballast circuit includes an equalizer transformer for equalizing current between two or more lamps operated in parallel , combined on one core with a choke . in a 2 - lamp embodiment , four windings are provided on one main core . the first and second are closely coupled and have n turns each , each connected separately to one of the lamps . a third and fourth winding are loosely coupled with respect to each other , and are arranged symmetrically about the first and second windings on the main core . these latter windings may be connected in series or in parallel to be an inductive impedance in the ballast circuit .