Load control device configured to operate in two-wire and three-wire modes

1. A load control device configured to control an electrical load, the load control device configured to be coupled between an alternating-current (AC) power source and the electrical load, the load control device comprising:

a hot terminal adapted to be electrically coupled to a hot side of the AC power source;

a dimmed-hot terminal adapted to be electrically coupled to the electrical load;

a neutral terminal;

a power supply capable of conducting a charging current through the electrical load;

a switching circuit configured to be rendered conductive and non-conductive to control when the charging current is conducted through the electrical load; and

a control circuit configured to determine whether the load control device should operate in a two-wire mode or a three-wire mode, the two-wire mode corresponding to the neutral terminal not being connected to the neutral side of the AC power source, the three-wire mode corresponding to the neutral terminal being connected to the neutral side of the AC power source;

wherein the control circuit is further configured to:

based on a determination that the load control device should operate in the two-wire mode, render the switching circuit conductive to allow the charging current to be conducted through the electrical load during positive half-cycles of an AC mains line voltage generated by the AC power source; and

based on a determination that the load control device should operate in the three-wire mode, render the switching circuit non-conductive to prevent the charging current from being conducted through the electrical load during the positive half-cycles of the AC mains line voltage.

2. The load control device of claim 1 , further comprising:

a neutral wire detect circuit configured to generate a neutral wire detect signal indicating whether the neutral terminal is connected to a neutral side of the AC power source, wherein the control circuit is configured to determine whether the load control device should operate the two-wire mode or the three-wire mode based on the neutral wire detect signal.

3. The load control device of claim 2 , wherein, in order to determine whether the load control device should operate in the two-wire mode or the three-wire mode based on the neutral wire detect signal, the control circuit is configured to:

count a number of times within a predetermined number of line cycles of the AC mains line voltage that the magnitude of the neutral wire detect signal transitions from a first value to a second value;

determine whether the number of transitions is above a threshold;

based on a determination that the number of transitions is equal to or above the threshold, control the load control device to operate in the three-wire mode; and

based on a determination that the number of transitions is below the threshold, control the load control device to operate in the two-wire mode.

4. The load control device of claim 3 , wherein the first magnitude corresponds to a low magnitude of the neutral wire detect signal and the second magnitude corresponds to a high magnitude of the neutral wire detect signal, the high magnitude being greater than the low magnitude.

5. The load control device of claim 4 , wherein the low magnitude is approximately equal to the magnitude of a circuit common and the high magnitude is approximately equal to the magnitude of a supply voltage of the control circuit.

6. The load control device of claim 3 , wherein the first magnitude corresponds to a high magnitude of the neutral wire detect signal and the second magnitude corresponds to a low magnitude of the neutral wire detect signal, the high magnitude being greater than the low magnitude.

7. The load control device of claim 6 , wherein the low magnitude is approximately equal to the magnitude of a circuit common and the high magnitude is approximately equal to the magnitude of a supply voltage of the control circuit.

8. The load control device of claim 2 , wherein the control circuit is configured to determine whether the load control device should operate in the two-wire mode or the three-wire mode in a start-up routine of the load control device.

9. The load control device of claim 8 , wherein, during the start-up routine, the control circuit is further configured to:

determine that the load control device is operating in the two-wire mode;

detect a number of transitions of the magnitude of the neutral wire detect signal from a first magnitude to a second magnitude within a predetermined number of line cycles of the AC mains line voltage, the number of transitions being equal to or above a threshold; and

switch the load control device to the three-wire mode.

10. The load control device of claim 9 , wherein, during the start-up routine, the control circuit is further configured to:

determine that the load control device is operating in the three-wire mode;

detect a number of transitions of the magnitude of the neutral wire detect signal from a first magnitude to a second magnitude within a predetermined number of line cycles of the AC mains line voltage, the number of transitions being below a threshold; and

switch the load control device to the two-wire mode.

11. The load control device of claim 1 , further comprising:

a controllably conductive device adapted to be coupled in series with the electrical load;

wherein the control circuit is configured to render the controllably conductive device conductive and non-conductive to control an amount of power delivered to the electrical load.

12. The load control device of claim 11 , further comprising:

a power supply configured to receive a rectified voltage and to generate a supply voltage for powering at least the control circuit by conducting a charging current through the electrical load when the controllably conductive device is non-conductive, the power supply comprising a bus capacitor configured to charge from the rectified voltage through a diode to generate a bus voltage.

13. The load control device of claim 12 , wherein the control circuit configured to adjust the amount of power delivered to the electrical load by adjusting a present phase angle of the controllably conductive device between a low-end phase angle and a high-end phase angle, the control circuit configured to decrease the high-end phase angle when a magnitude of the rectified voltage is less than a first threshold, and decrease the present phase angle when a magnitude of the bus voltage is less than a second threshold.

14. The load control device of claim 13 , wherein the control circuit is configured to turn off the electrical load when the magnitude of the bus voltage falls below a third threshold, where the third threshold is less than the second threshold.

15. The load control device of claim 14 , wherein the control circuit is configured to:

turn on the electrical load when the magnitude of the bus voltage rises above a fourth threshold that is greater than the third threshold;

repetitively turn off and on the electrical load in response to the magnitude of the bus voltage;

count the number of times that the electrical load is turned off in response to the magnitude of the bus voltage; and

change from a reverse phase-control dimming technique to a forward phase-control dimming technique when the number of times that the electrical load is turned off exceeds a fifth threshold.

16. The load control device of claim 11 , further comprising:

a first zero-cross detect circuit electrically coupled to between the hot terminal and the dimmed-hot terminal, and configured to detect zero-crossing points of an AC mains line voltage generated by the AC power source; and

a second zero-cross detect circuit electrically coupled between the hot terminal and the neutral terminal, and configured to detect the zero-crossing points of the AC mains line voltage, the second zero-cross circuit comprising an active filter;

wherein the control circuit is configured to determine the zero-crossing points of the AC mains line voltage in response to the first zero-cross detect circuit in the two-wire mode and in response to the second zero-cross detect circuit in the three-wire mode.

17. The load control device of claim 12 , further comprising:

a neutral wire detect circuit configured to generate, based on current conducted through the second zero-cross detect circuit, a neutral-wire detect signal indicating whether the neutral terminal is connected to a neutral side of the AC power source;

wherein the control circuit is configured to determine whether the load control device should operate in a two-wire mode or a three-wire mode based on the neutral wire detect signal.

18. The load control device of claim 12 , wherein the active filter comprises a half-wave filter circuit configured to conduct current through the controllably conductive device during negative half-cycles of the AC mains line voltage and not conduct current through the controllably conductive device during positive half-cycles of the AC mains line voltage.

19. The load control device of claim 12 , wherein the active filter comprises a full-wave filter circuit electrically coupled between the hot terminal and the neutral terminal and configured to conduct current through the controllably conductive device during positive and negative half-cycles of the AC mains line voltage.

20. The load control device of claim 11 , wherein the controllably conductive device comprises two semiconductor switches coupled together at a junction, and wherein the power supply is configured to conduct the charging current through the neutral terminal, a body diode of one of the semiconductor switches, and the hot terminal when the switching circuit is rendered conductive during the negative half-cycles.

21. The load control device of claim 1 , wherein the control circuit is configured to determine whether the load control device should operate in a two-wire mode or a three-wire mode based on a user input.

22. The load control device of claim 1 , further comprising:

a power supply capable of conducting a charging current through the electrical load; and

a switching circuit configured to be rendered conductive and non-conductive to control when the charging current is conducted through the electrical load;

wherein the control circuit is configured to render the switching circuit conductive during the two-wire mode and non-conductive during the three-wire mode so that, during the two-wire mode, the charging current is conducted through the electrical load during positive half-cycles of the AC mains line voltage, and, during the three-wire mode, the charging current is not conducted through the electrical load during the positive half-cycles of the AC mains line voltage.

23. The load control device of claim 1 , further comprising:

a user interface comprising one or visual indicators;

wherein the control circuit is configured to control the user interface to provide a visual indication of when the load control device is operating in the three-wire mode.