Abstract: A dimmer applies a phase-controlled AC voltage to a lighting assembly which includes a fluorescent lamp operated with a magnetic ballast. A microprocessor performs a calibration routine in which the phase angle used in phase control is increment from 0 toward 180 degrees. The microprocessor monitors current conducted by the lighting assembly in response to each incrementing, effectively determining an operative range of phase angles free of flickering or drop-out. During steady-state operation, the microprocessor converts nominal power settings into corresponding phase angle settings within the operative range, preventing operation in undesirable states.