Duty factor probing of a triac-based dimmer

摘要

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.

主权项

1. A method of determining a line frequency of an AC power source supplying power to a thyristor-switched dimmer circuit, the method comprising:
during non-adjacent half cycles of a voltage of the AC power source that supplies power to the thyristor-switched dimmer circuit, predicting zero-crossing times of the AC power source for the non-adjacent half cycles by measuring a voltage at an output of the thyristor-switched dimmer circuit at times other than the zero-crossing times and generating an indication of the zero-crossing times by extrapolating the zero-crossing times from the voltage measured at the times other than the zero-crossing times; and determining the line frequency from a time interval extending between the predicted zero crossing times of the AC power source.