A passive control network for connection between a pair of load output level setting terminals, as on a ballast for a dimmable fluorescent lamp in a lighting system, to provide a required variable impedance, where the magnitude of the impedance establishes the load output level. The passive control network includes an isolation transformer for coupling a periodic waveform at the load input terminals to the variable impedance component, the magnitude of which impedance is reflected through the transformer to provide the load level-setting impedance.