A light source may be driven by an appropriate driver, such as a light-emitting diode (LED) driver or ballast, in order to control (e.g., illuminate) the light source. For example, a LED light source may be driven by a LED driver circuit to turn the LED light source on or off. A fluorescent lamp may be driven by a ballast to turn the fluorescent lamp on or off. A driver may be used to control the intensity of a light source, for example, to dim the light source. For instance, the driver may use zero-to-ten-volt (0-10V) control to vary the intensity of the light source. Zero-to-ten-volt control may sometimes be referred to as 1-10V control. A 0-10V driver receives power from an AC power source. An external mechanical switch may be coupled between the AC power source and the 0-10V driver, for example to provide a switched-hot voltage to the driver.
A 0-10V control device may provide a 0-10V control signal to the 0-10V driver, such that the 0-10V driver may control the intensity of the light source accordingly. The 0-10V control device may be an external device. Often, the 0-10V control device is mounted in an electrical wall box and comprises an intensity adjustment actuator (e.g., a slider control). The 0-10V control device regulates the direct-current (DC) voltage level of the 0-10V control signal provided to the driver. For example, the 0-10V control device may set the DC voltage level between a substantially low voltage (e.g., 0-1 V) to a maximum voltage (e.g., approximately 10 V). The 0-10V control device may select the DC voltage level in response to an actuation of an intensity adjustment actuator by a user who may adjust the intensity adjustment actuator to indicate a desired intensity for the light source.
A driver may act as a capacitive load, for example, when power is applied to the driver. Due to the capacitive nature of the driver, there may be a large in-rush of current into the driver when a mechanical switch is closed to turn on a light source. The in-rush current may quickly subside as the input capacitance of the driver charges up to line voltage. However, a temporary current surge can be problematic as the number of drivers controlled by a mechanical switch increases. For example, in the case of a full 16-amp (e.g., steady-state) circuit of drivers, the in-rush current can approach 560 amps. Although the in-rush current may be short-lived (e.g., only a few line cycles or shorter), such high current surges can wreak havoc on the contacts of even a relatively large relay with a high current rating (e.g., 50 amps).
A large in-rush current may be problematic because it may deteriorate the contacts of a mechanical switch in a relay. The contacts of a mechanical switch have a tendency to bounce apart when the switch is closed and when the contacts bounce apart during a large current surge, the intervening medium (e.g., gas or air) between the contacts may ionize. Although the intervening medium may be nonconductive, a high current surge may cause the nonconductive medium to become conductive. When the medium becomes conductive, the in-rush current may flow from one contact to another even though the contacts may not be touching each other. This phenomenon may be referred to as arcing. The contacts may be coated with a conductive material, for example, to improve the conductivity of the relay and arcing between the contacts may inadvertently cause some of the coating to be removed, for example, by blasting away the conductive coatings. A significant removal of the conductive coating from the relay contacts may eventually cause the relay to fail. For example, the relay may fail due to erosion of the contact material, or due to welding of the contacts in the closed position.
Some prior art lighting control systems including 0-10V drivers have required heavy-duty mechanical switches that can be physically large and costly. Also, physically large mechanical switches may be too large to fit in a single electrical wall box. Hence, physically large mechanical switches may need to be mounted in an enclosure separate from the 0-10V control device. An example of a prior art 0-10V control device that requires an externally-mounted relay is the Nova T-Star® 0-10V Control, model number NTFTV, manufactured by Lutron Electronics Co., Inc.
Other prior art switching circuits for drivers have required advanced components and structures, for example microcontrollers and multiple relays per driver circuit. Also, some prior art switching circuits for drivers have required complex wiring topologies, for example requiring a neutral connection. An example of such a switching circuit is described in greater detail in commonly-assigned U.S. Pat. No. 5,309,068, issued May 3, 1994, entitled TWO RELAY SWITCHING CIRCUIT FOR FLUORESCENT LIGHTING CONTROLLER, and U.S. Pat. No. 5,633,540, issued May 27, 1999, entitled SURGE-RESISTANT RELAY SWITCHING CIRCUIT. The entire disclosures of both patents are hereby incorporated by reference.
Some prior art 0-10V control devices are able to fit in a single electrical wallbox and provide both the switched hot voltage and the 0-10V control signal to a 0-10V ballast, as described in commonly-assigned U.S. Pat. No. 8,274,240, issued Sep. 25, 2012, and U.S. Pat. No. 8,278,839, issued Oct. 2, 2012, both entitled SWITCHING CIRCUIT HAVING DELAY FOR INRUSH CURRENT PROTECTION, the entire disclosures of which are hereby incorporated by reference. However, these prior art 0-10 V control devices can only be used in a two-wire lighting system, not a three-wire lighting system.
The prior art two-wire control devices cannot be used in a three-wire lighting system because they cannot be connected to an external three-way switch. An external three-way switch has three terminals. One of the three terminals of the external three-way switch may be connected to either an alternating current (AC) power source or an electrical load. But the remaining two terminals of the external three-way switch must be connected to a switching circuit of a load control device. A prior art two-wire control device has two terminals. One of the two terminals of the prior art two-wire control device may be connected to either the AC power source or the electrical load and the remaining one terminal may be connected to an external switch. Due to the mismatch in the number of available terminals, the prior art two-wire control device cannot be connected to the external three-way switch. Hence, the prior art two-wire control device cannot be used in a three-wire lighting system.
Therefore, there is a need for a switching circuit that can be used in a three-way lighting system, can handle a large inrush current, and does not require a neutral connection or a heavy-duty mechanical switch or relay.