Duty factor probing of a triac-based dimmer
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.
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.
2. The method of claim 1 , wherein the time interval is an odd number of the half cycles of the voltage of the AC power source.
3. The method of claim 1 , wherein the predicting the zero-crossing times comprises probing the output of the thyristor-switched dimmer by periodically or intermittently applying a minimum conductance across the output of the thyristor-switched dimmer from a turn-on time of the thyristor-switched dimmer circuit at least until the predicting has gathered sufficient information to predict a next zero-cross time of the voltage of the AC power source.
4. The method of claim 3 , wherein the probing applies the minimum conductance until the next zero-cross time.
5. The method of claim 3 , wherein the probing is performed at intervals corresponding to an odd number of half-cycles of the voltage of the AC power source.
6. The method of claim 3 , wherein the probing applies the minimum conductance to the output of the thyristor-switched dimmer circuit by controlling a power converter that transfers power from the AC power source to one or more lighting devices such that the power converter control circuit transfers energy continuously during the durations of the probing cycles.
7. The method of claim 3 , wherein the probing applies the minimum conductance to the output of the thyristor-switched dimmer circuit by controlling a controllable load impedance applied to the output of the thyristor-switched dimmer circuit.
8. The method of claim 3 , wherein a rate of initiating the probing cycles is less than a half-cycle rate of an AC power source that supplies the thyristor-switched dimmer circuit, whereby some half-cycles of the AC power source are not probed.
9. The method of claim 3 , further comprising:
detecting a leading edge of a waveform of an output voltage of the thyristor-switched dimmer circuit subsequent to the next zero-cross time; and
determining a dimming control value of the thyristor-switched dimmer from a time interval extending from the next zero-cross time of the voltage of the AC power source to the detected leading edge.
10. 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 zero-crossing predicting circuit for predicting zero-cross times 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 by measuring a voltage at an output of the thyristor-switched dimmer circuit at times other than the zero-crossing times, 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 a frequency of the voltage of the AC power source from a time interval extending between the predicted zero crossing times.
11. The integrated circuit of claim 10 , wherein the time interval is an odd number of the half cycles of the AC power source.
12. The integrated circuit of claim 10 , further comprising a probing control circuit that intermittently or periodically applies a minimum level of conductance to the output of the thyristor-switched dimmer circuit 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 a next zero-cross time of the voltage of the AC power source.
13. The integrated circuit of claim 12 , wherein the probing control circuit applies the minimum level of conductance until the next zero-cross time.
14. The integrated circuit of claim 12 , wherein the probing control circuit applies the minimum level of conductance at intervals corresponding to an odd number of half-cycles of the voltage of the AC power source.
15. The integrated circuit of claim 12 , wherein the probing control circuit applies the minimum level of conductance level to the output of the thyristor-switched dimmer circuit by controlling a power converter control circuit that powers the one or more lighting devices such that the power converter control circuit transfers energy continuously during the durations of the probing cycles.
16. The integrated circuit of claim 12 , wherein the probing control circuit applies the minimum level of conductance to the output of the thyristor-switched dimmer circuit by controlling a controllable load impedance applied to the output of the thyristor-switched dimmer circuit.
17. The integrated circuit of claim 12 , wherein a rate of initiating the probing cycles is less than a half-cycle rate of an AC power source that supplies the thyristor-switched dimmer circuit.
18. The integrated circuit of claim 12 , further comprising:
a detector for detecting a leading edge of a waveform of an output voltage of the thyristor-switched dimmer circuit subsequent to the next zero-cross time; and
a control circuit that determines the dimming control value of the thyristor-switched dimmer from a time interval extending from the next zero-cross time of the voltage of the AC power source to the detected leading edge.
19. 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 zero-cross times 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 by measuring a voltage at an output of the thyristor-switched dimmer circuit at times other than the zero-crossing times, 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 a frequency of the voltage of the AC power source from a time interval extending 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.
20. The integrated circuit of claim 19 , wherein the time interval is an odd number of the half cycles of the AC power source.