Duty factor probing of a triac-based dimmer

An integrated circuit for operating a circuit that powers one or more lighting devices from an output of a thyristor-switched dimmer circuit, comprising:

• a power converter control circuit for controlling a power converter for supplying the one or more lighting devices from the output;
• a zero-crossing predicting circuit for predicting a next zero-cross time of a voltage of an AC power source that supplies power to the thyristor-switched dimmer circuit during non-adjacent half cycles of the AC power source;
• a line-frequency determining circuit that determines a frequency of the voltage of the AC power source from a time interval between the predicted zero crossing times;
• and
• a probing control circuit that intermittently or periodically applies a minimum conductance to the output of the thyristor-switched dimmer circuit during durations of probing cycles, wherein a rate of initiating the probing cycles is less than a half-cycle rate of the AC power source, whereby some half-cycles of the AC power source are not probed, wherein the durations of the probing cycles extend from a turn-on time of the thyristor-switched dimmer circuit at least until the zero-crossing predicting circuit has gathered sufficient information to predict the next zero-cross time of the voltage of the AC power source.