Two-wire dimmer with improved zero-cross detection
A lighting device, such as a two-wire lighting control device, may include a controllably conductive device and a control circuit. The controllably conductive device may supply an AC line voltage to a load in response to a dive signal such that the controllable conductive device is non-conductive for a first duration of time and conductive for a second duration of time within a half-cycle of the AC line voltage. The control circuit may receive a signal from the controllably conductive device that represents a voltage developed across the controllable conductive device during the first duration of time. The control circuit may generate a sine-wave-shaped signal that complements the voltage developed across the controllably conductive device during the second duration of time. The control circuit may also filter the signal from the controllably conductive device during the first duration of time and the sine-wave-shaped signal during the second duration of time.
1. A lighting control device comprising:
a controllably conductive device for supplying an AC line voltage to a load in response to a drive signal, wherein the controllably conductive device is non-conductive for a first duration of time and is conductive for a second duration of time, wherein the first duration of time and the second duration of time are within the same half-cycle of the AC line voltage;
a control circuit configured to:
receive a signal from the controllably conductive device that represents a voltage developed across the controllably conductive device during the first duration of time;
generate a sine-wave-shaped signal that complements the voltage developed across the controllably conductive device during the second duration of time; and
filter the signal from the controllably conductive device during the first duration of time and the sine-wave-shaped signal during the second duration of time.
2. The lighting control device of claim 1 , wherein the control circuit comprises:
a controller configured to generate the sine-wave-shaped signal;
a combiner circuit for combining a signal from the controllably conductive device during the first duration of time and the sine-wave-shaped signal during the second duration of time; and
a filter circuit configured to filter the combined signal from the combiner circuit.
3. The lighting control device of claim 2 , wherein the sine-wave-shaped signal is a step-sine wave.
4. The lighting control device of claim 3 , wherein the step-sine wave comprises steps based on at least one pulse-width modulated signal.
5. The lighting control device of claim 3 , wherein the step-sine wave comprises steps based on at least one phase correction corresponding to a zero-crossing detected by a zero crossing detector.
6. The lighting control device of claim 2 , wherein the combiner circuit comprises a switch configured to switch between the signal from the controllably conductive device and the sine-wave-shaped signal based on the drive signal, the switch configured to select the signal from the controllably conductive device during the first duration of time and the sine-wave-shaped signal during the second duration of time.
7. The lighting control device of claim 6 , wherein the switch comprises a multiplexer that switches in concert with the drive signal.
8. The lighting control device of claim 2 , wherein the filter circuit is a low-pass filter.
9. The lighting control device of claim 8 , wherein the low-pass filter provides a substantially linear phase delay of less than one-half of a period of the fundamental frequency and attenuates frequency components of a filter-input signal that are substantially equal to third order harmonics and greater of the fundamental frequency.
10. The lighting control device of claim 1 , wherein the control circuit comprises a controller configured to sample the signal from the controllably conductive device during the first duration of time.
11. The lighting control device of claim 10 , wherein the controller is configured to filter the signal from the controllably conductive device and the sine-wave-shaped signal using a digital Bessel filter.
12. The lighting control device of claim 1 , wherein the control circuit is configured to:
generate a filtered signal by filtering the signal from the controllably conductive device during the first duration of time and filtering the sine-wave-shaped signal during the second duration of time; and
determine a zero-crossing of the AC line voltage from the filtered signal.
13. The lighting control device of claim 12 , wherein the control circuit is configured to generate the drive signal to control the controllably conductive device based on the determined zero-crossing of the AC line voltage.
14. The lighting control device of claim 1 , wherein the sine-wave-shaped signal is shaped such that delay associated with filtering the signal from the controllably conductive device during the first duration of time and the sine-wave-shaped signal during the second duration of time is consistent with delay associated with filtering the AC line voltage.
15. The lighting control device of claim 1 , wherein the sine-wave-shaped signal is shaped to maintain stability of the two-wire lighting control device.