A line-frequency determining circuit for coupling to the output of a thyristor-switched dimmer that determines a line-frequency of an AC power source that supplies an input to the thyristor-controlled dimmer permits accurate control of periodic probing of the dimmer output. The probing is performed to predict zero-cross times of the AC power source that, in turn, are used to determine a dimming control value of the thyristor-switched dimmer. A minimum conductance is applied across the output of the dimmer during the probing intervals that begin at the turn-on time of the dimmer and last until enough information has been gathered to correctly predict a next zero crossing of the AC line voltage that supplies the input of the dimmer. The probing can be performed at intervals of an odd number of half-cycles of the AC line frequency so that internal dimmer timer operation is not affected by DC offset.