Patent Publication Number: US-RE49537-E

Title: Electronic switch having an in-line power supply

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a reissue of U.S. Pat. No. 9,418,809, issued on Aug. 16, 2016 from U.S. patent application Ser. No. 14/529,267, filed Oct. 31, 2014, entitled ELECTRONIC SWITCH HAVING AN IN-LINE POWER SUPPLY, which is a divisional application of commonly-assigned U.S. patent application Ser. No. 12/751,324, filed Mar. 31, 2010, entitled SMART ELECTRONIC SWITCH FOR LOW-POWER LOADS, which is a non-provisional application of commonly-assigned U.S. Provisional Application Ser. No. 61/172,511, filed Apr. 24, 2009, entitled SMART LOAD CONTROL DEVICE HAVING A ZERO-CURRENT OFF STATE, and U.S. Provisional Application Ser. No. 61/226,990, filed Jul. 20, 2009, entitled SMART ELECTRONIC SWITCH FOR LOW-POWER LOADS, the entire disclosures of which are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to load control devices for control of the power delivered from an alternating-current (AC) power source to an electrical load, and more particularly, to a “smart” two-wire electronic switch having a controller, a latching relay, and a power supply that provides substantially all of the line voltage of the AC power source to the load and draws current through the load in a manner that does not result in inappropriate operation of the load. 
     2. Description of the Related Art 
     Typical load control devices are operable to control the amount of power delivered to an electrical load, such as a lighting load or a motor load, from an alternating-current (AC) power source. Wall-mounted load control devices are adapted to be mounted to standard electrical wallboxes. A dimmer switch comprises a controllably conductive device (e.g., a bidirectional semiconductor switch, such as, a triac), which is coupled in series between the power source and the load. The controllably conductive device is controlled to be conductive and non-conductive for portions of a half cycle of the AC power source to thus control the amount of power delivered to the load (e.g., using a phase-control dimming technique). A “smart” dimmer switch (i.e., a digital dimmer switch) comprises a microprocessor (or similar controller) for controlling the semiconductor switch and a power supply for powering the microprocessor. In addition, the smart dimmer switch may comprise a memory, a communication circuit, and a plurality of light-emitting diodes (LEDs) that are all powered by the power supply. 
     An electronic switch (i.e., a digital switch) comprises a controllably conductive device (such as a relay or a bidirectional semiconductor switch), a microprocessor, and a power supply. In contrast to a smart dimmer switch, the controllably conductive device of an electronic switch is not controlled using the phase-controlled dimming technique, but is controlled to be either conductive or non-conductive during each half cycle of the AC power source to thus toggle the electrical load on and off. Often, wall-mounted electronic switches do not require a connection to the neutral side of the AC power source (i.e., the electronic switch is a “two-wire” device). This is particularly useful when the electronic switch is installed in a retro-fit installation (i.e., to replace an existing switch or load control device in an electrical wallbox in which there is no neutral connection). 
     In order to charge, the power supply of a two-wire electronic switch must develop an amount of voltage across the power supply. As a result, not all of the AC line voltage of the AC power source is available to power the electrical load and the electrical load may not operate properly. For example, if the electrical load is a lighting load, the lighting load may not be illuminated to the maximum possible intensity. In addition, the power supply must draw current through the controlled electrical load in order to charge, which may cause problems for some types of electrical loads. For example, when the electrical load is a lighting load, the magnitude of the power supply current must not be great enough to cause the lighting load to illuminate or to flicker. Further, some electrical loads, such as compact fluorescent lamps, do not conduct sinusoidal currents, and as a result, current cannot be conducted through these electrical loads during certain portions of the line cycle of the AC power source. 
     Therefore, there exists a need for an electronic switch that has a controller for turning the load on and off and a single power supply that operates in a manner that does not result in inappropriate operation of the load. 
     SUMMARY OF THE INVENTION 
     According to an embodiment of the present invention, a two-wire electronic switch adapted to be coupled between an AC power source and an electrical load for turning the electrical load on and off comprises a controllably conductive device adapted to be coupled in series electrical connection between the source and the load, a controller operatively coupled the controllably conductive device for controlling the controllably conductive, an output capacitor operable to develop to the DC supply voltage for powering the controller, and an in-line power supply that controls when the output capacitor charges asynchronously with respect to the frequency of the AC power source, such that the in-line power supply is operable to start and stop charging at any time during each half cycle. The controllably conductive device is adapted to conduct a load current through the load when the controllably conductive device is conductive. The controller renders the controllably conductive device conductive and non-conductive to turn the load on and off, respectively. The in-line power supply is coupled in series with the controllably conductive device, and further coupled to the output capacitor for controlling when the output capacitor charges in order to generate the DC supply voltage across the output capacitor when the controllably conductive device is conductive. A voltage developed across the in-line power supply when the output capacitor is charging has a substantially small magnitude as compared to a peak voltage of an AC line voltage of the AC power source. The output capacitor is adapted to conduct the load current for at least a portion of a line cycle of the AC power source when the controllably conductive device is conductive. The power supply starts and stops charging the output capacitor at least once during each half cycle of the AC power source. 
     In addition, a power supply for an electronic switch that comprises a controllably conductive device adapted to be coupled between an AC power source and an electrical load for turning the electrical load on and off is also described herein. The electronic switch comprises an output capacitor operable to develop to a DC supply voltage, a bidirectional semiconductor switch adapted to be coupled in series with the controllably conductive device and in parallel with the output capacitor, and a control circuit coupled to the bidirectional semiconductor switch for rendering the bidirectional semiconductor switch conductive and non-conductive. The output capacitor is operable to charge when the bidirectional semiconductor switch is non-conductive. The control circuit is responsive to the magnitude of the DC supply voltage to render the bidirectional semiconductor switch conductive when the magnitude of the DC supply voltage reaches a maximum DC supply voltage threshold and to render the bidirectional semiconductor switch non-conductive when the magnitude of the DC supply voltage drops to a minimum DC supply voltage threshold. A voltage developed across the power supply when the output capacitor is charging has a substantially small magnitude as compared to a peak voltage of an AC line voltage of the AC power source when the output capacitor is charging. The power supply controls when the output capacitor charges asynchronously with respect to the frequency of the AC power source, such that the in-line power supply is to start and stop charging at any time during each half cycle. The power supply starts and stops charging the output capacitor at least once during each half cycle of the AC power source. 
     As described herein, a two-wire electronic switch for controlling the power delivered from an AC power source to an electrical load may comprise a latching relay adapted to be coupled in series electrical connection between the source and the load, a controller, an output capacitor operable to develop a DC supply voltage for powering the controller, and an in-line power supply coupled in series with the relay and further coupled to the output capacitor for generating the DC supply voltage across the output capacitor when the relay is conductive. The latching relay conducts a load current through the load when the relay is conductive. The controller is operatively coupled to the relay for controlling the relay to be conductive and non-conductive to turn the load on and off, respectively. The output capacitor is adapted to conduct the load current for at least a portion of a line cycle of the AC power source when the relay is conductive. The relay is rendered non-conductive in response to an over-temperature condition in the electronic switch (e.g., in the power supply). 
     In addition, a two-wire electronic switch for controlling the power delivered from an AC power source to an electrical load may comprise a latching relay adapted to be coupled in series electrical connection between the source and the load for turning the load on and off, a first bidirectional semiconductor switch coupled in parallel electrical connection with the relay, and a controller operatively coupled to the relay and a control input of the first bidirectional semiconductor switch. The controller turns on the load by first rendering the first bidirectional semiconductor switch conductive and then rendering the relay conductive, and turns off the load by first rendering the relay non-conductive and then rendering the first bidirectional semiconductor switch non-conductive. The electronic switch further comprises an output capacitor operable to develop a DC supply voltage for powering the controller, and an in-line power supply coupled in series electrical connection with the relay, such that the first bidirectional semiconductor switch is coupled in parallel with the series combination of the relay and the power supply. The in-line power supply is further coupled to the output capacitor for generating the DC supply voltage across the output capacitor when the relay is conductive. The output capacitor is adapted to conduct the load current for at least a portion of a line cycle of the AC power source when the relay is conductive. The first bidirectional semiconductor switch is rendered conductive in response to an over-current condition in the output capacitor of the power supply. In addition, the power supply may further comprise a second bidirectional semiconductor switch coupled in series with the relay and in parallel with the output capacitor, such that the output capacitor is operable to charge when the relay is conductive and the second bidirectional semiconductor switch is non-conductive. 
     Further, a two-wire electronic switch for controlling the power delivered from an AC power source to an electrical load may comprise: (1) a latching relay adapted to be coupled in series electrical connection between the source and the load for turning the load on and off; (2) a first bidirectional semiconductor switch coupled in parallel electrical connection with the relay, the first bidirectional semiconductor switch comprising a control input; (3) a controller operatively coupled to the relay and the control input of the first bidirectional semiconductor switch, the controller operable to turn on the load by first rendering the first bidirectional semiconductor switch conductive and then rendering the relay conductive, the controller operable to turn off the load by first rendering the relay non-conductive and then rendering the first bidirectional semiconductor switch non-conductive; (4) an output capacitor operable to develop a DC supply voltage for powering the controller; and (5) an in-line power supply coupled in series electrical connection with the relay, such that the first bidirectional semiconductor switch is coupled in parallel with the series combination of the relay and the power supply. The in-line power supply is further coupled to the output capacitor for generating the DC supply voltage across the output capacitor when the relay is conductive. The power supply comprises a second bidirectional semiconductor switch coupled in series with the relay and in parallel with the output capacitor, such that the output capacitor is operable to charge when the relay is conductive and the second bidirectional semiconductor switch is non conductive. The first bidirectional semiconductor switch is rendered conductive in response to an over-current condition in the output capacitor of the power supply, and the relay is rendered non-conductive in response to an over-temperature condition in the power supply. 
     According to another aspect of the present invention, a two-wire electronic switch for controlling the power delivered from an AC power source to an electrical load comprises a latching relay adapted to be coupled in series electrical connection between the source and the load for turning the load on and off, an output capacitor operable to develop a DC supply voltage, an in-line power supply, and a controller operable to measure a charging time required to charge the output capacitor, and to determine if an overload condition is occurring if the length of the charging time is less than a predetermined charging time threshold. The in-line power supply is coupled in series electrical connection with the relay, and further coupled to the output capacitor for generating the DC supply voltage across the output capacitor when the relay is conductive. The power supply comprises a bidirectional semiconductor switch coupled in series with the relay and in parallel with the output capacitor, such that the output capacitor is operable to charge when the relay is conductive and the bidirectional semiconductor switch is non conductive. The bidirectional semiconductor switch is rendered conductive when the magnitude of the DC supply voltage reaches a maximum DC supply voltage threshold and rendered non-conductive when the magnitude of the DC supply voltage drops to a minimum DC supply voltage threshold. The output capacitor is adapted to conduct the load current for at least a portion of a line cycle of the AC power source when the relay is conductive. 
     Other features and advantages of the present invention will become apparent from the following description of the invention that refers to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described in greater detail in the following detailed description with reference to the drawings in which: 
         FIG.  1    is a simplified diagram of a radio-frequency (RF) lighting control system comprising a two-wire electronic switch and two remote vacancy sensors according to a first embodiment of the present invention; 
         FIG.  2    is a simplified block diagram of the two-wire electronic switch of  FIG.  1   ; 
         FIG.  3    is a simplified schematic diagram of an in-line on-state power supply of the two-wire electronic switch of  FIG.  2   ; 
         FIG.  4 A  is a simplified diagram of waveforms illustrating the operation of the power supply of  FIG.  3    showing an asynchronous charging current conducted through an output capacitor of the power supply; 
         FIG.  4 B  is a simplified diagram of waveforms illustrating the operation of the power supply of  FIG.  3    showing a synchronous charging current conducted through the output capacitor of the power supply; 
         FIG.  5    is a simplified schematic diagram of a latching relay, a bidirectional semiconductor switch, a drive circuit, and the in-line on-state power supply of the two-wire electronic switch of  FIG.  2   ; 
         FIG.  6    is a simplified flowchart of a button procedure executed by a controller of the electronic switch of  FIG.  2   ; 
         FIG.  7    is a simplified flowchart of a received message procedure executed by the controller of the electronic switch of  FIG.  2   ; 
         FIG.  8    is a simplified flowchart of a relay timer procedure executed by the controller of the electronic switch of  FIG.  2   ; 
         FIG.  9    is a simplified flowchart of a bidirectional semiconductor switch (BSS) timer procedure executed by the controller of the electronic switch of  FIG.  2   ; and 
         FIG.  10    is a simplified flowchart of an overload detection procedure executed by the controller of the electronic switch of  FIG.  2   . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The foregoing summary, as well as the following detailed description of the preferred embodiments, is better understood when read in conjunction with the appended drawings. For the purposes of illustrating the invention, there is shown in the drawings an embodiment that is presently preferred, in which like numerals represent similar parts throughout the several views of the drawings, it being understood, however, that the invention is not limited to the specific methods and instrumentalities disclosed. 
       FIG.  1    is a simple diagram of a radio-frequency (RF) lighting control system  100  comprising a two-wire electronic switch  110 , a keypad  120 , and two remote occupancy sensors  130  according to a first embodiment of the present invention. The electronic switch  110  and the keypad  120  are adapted to be wall-mounted in standard electrical wallboxes. Alternatively, the electronic switch  110  and the keypad  120  could be implemented as table-top control devices. In addition, the electronic switch  110  may comprise a controllable plug-in module adapted to be plugged into an electrical receptacle or a controllable screw-in module adapted to be screwed into the electrical socket (e.g., an Edison socket) of a lamp. 
     The electronic switch  110  comprises a hot terminal H and a switched hot terminal SH and is adapted to be coupled in series electrical connection between an AC power source  102  (e.g., 120 V AC @60 Hz or 240 V AC @50 Hz) and a lighting load  104  for controlling the power delivered to the lighting load. The electronic switch  110  generates a switched hot voltage V SH  at the switched hot terminal SH. The electronic switch  110  comprises a control actuator  112  (i.e., a control button) for toggling (i.e., turning off and on) the lighting load  104 , and a visual indicator  114  for providing feedback of whether the lighting load is on or off. The electronic switch  110  is also operable to turn the lighting load  104  off in response to digital messages received from the keypad  120  and the occupancy sensors  130  via RF signals  106 . 
     The keypad  120  is coupled to the hot and neutral connections of the AC power source  102  via a hot terminal H′ and a neutral terminal N, respectively. The keypad  120  comprises an on button  122  and an off button  124  for turning the lighting load  104  on and off, respectively. The keypad  120  is operable to transmit a digital message including an “on” command to the electronic switch  110  in response to an actuation of the on button  122 , and to transmit a digital message including an “off” command to the electronic switch in response to an actuation of the off button  124 . The keypad  120  further comprises visual indicators  126  provided on the button  122 ,  124  for providing feedback of whether the lighting load  104  is on or off. 
     The occupancy sensors  130  are removably mountable to a ceiling or a wall, for example, in the vicinity of (i.e., a space around) the lighting load  104  controlled by the electronic switch  110 . The occupancy sensors  130  are operable to detect the presence of an occupant in the space (i.e., an occupancy condition) and the absence of the occupancy (i.e., a vacancy condition) in the vicinity of the lighting load  104 . The occupancy sensors  130  may be spaced apart to detect occupancy conditions in different areas of the vicinity of the lighting load  104 . The occupancy sensors  130  and the electronic switch  110  operate to turn on the lighting load when one of the occupancy sensors detects that an occupant has entered the space (i.e., at least one sensor detects an occupancy condition) and then to turn off the lighting load when both occupancy sensors detect that the user has left the space (i.e., both sensors detect vacancy conditions). 
     Alternatively, the occupancy sensors  130  could be implemented as vacancy sensors. A vacancy sensor only operates to turn off the lighting load  104  when the vacancy sensor detects a vacancy in the space. Therefore, when using vacancy sensors, the lighting load  104  must be turned on manually (e.g., in response to a manual actuation of the control actuator  112 ). Examples of wireless battery-powered occupancy sensors are described in greater detail in U.S. patent application Ser. No. 12/203,500, filed Sep. 3, 2008, entitled BATTERY-POWERED OCCUPANCY SENSOR, the entire disclosure of which is hereby incorporated by reference. 
     The occupancy sensors  130  each include an internal detector (not shown), e.g., a pyroelectric infrared (PIR) detector. The internal detector is housed in an enclosure  132 , which has a lens  134  for directing infrared energy from an occupant in the space to the internal detector for sensing the occupancy condition in the space. The occupancy sensors  130  are operable to process the output of the internal detector to determine whether an occupancy condition or a vacancy condition is presently occurring in the space, for example, by comparing the output of the PIR detector to a predetermined occupancy voltage threshold. Alternatively, the internal detector could comprise an ultrasonic detector, a microwave detector, or any combination of PIR detectors, ultrasonic detectors, and microwave detectors. The occupancy sensors  130  each operate in an “occupied” state or a “vacant” state in response to the detections of occupancy or vacancy conditions, respectively, in the space. If one of the occupancy sensors  130  is in the vacant state and the occupancy sensor determines that the space is occupied, the occupancy sensor changes to the occupied state. Similarly, the occupancy sensor  130  changes to the vacant state, if the occupancy sensor is in the occupied state and the occupancy sensor determines that the space is unoccupied. 
     During a setup procedure of the RF lighting control system  100 , the electronic switch  110  and the keypad  120  may be assigned to (i.e., associated with) the occupancy sensors  130 . The setup and configuration of a lighting control system including occupancy sensors is described in greater detail in U.S. patent application Ser. No. 12/371,027, filed Feb. 13, 2009, entitled METHOD AND APPARATUS FOR CONFIGURING A WIRELESS SENSOR, the entire disclosure of which is hereby incorporated by reference. 
     The occupancy sensors  130  transmit digital messages wirelessly via the RF signals  106  in response to the present state of the occupancy sensors (i.e., whether an occupancy condition or a vacancy condition has been detected). The electronic switch  110  turns the lighting load  104  on and off in response to the digital messages received via the RF signals  106 . A digital message transmitted by the remote occupancy sensors  130  may include a command and identifying information, for example, a serial number associated with the transmitting occupancy sensor. The electronic switch  110  is responsive to messages containing the serial numbers of the remote occupancy sensors  130  to which the electronic switch is assigned. The operation of the RF lighting control system  100  is described in greater detail in U.S. patent application Ser. No. 12/203,518, filed Sep. 3, 2008, entitled RADIO-FREQUENCY LIGHTING CONTROL SYSTEM WITH OCCUPANCY SENSING, the entire disclosure of which is hereby incorporated by reference. 
     The commands included in the digital messages transmitted by the occupancy sensors  130  may comprise an occupied command (e.g., an occupied-take-action command or an occupied-no-action command) or a vacant command. When the lighting load  104  is off, the electronic switch  110  is operable to turn on the lighting load in response to receiving a first occupied-take-action command from any one of the occupancy sensors  130 . The electronic switch  110  is operable to turn off the lighting load  104  in response to the last vacant command received from those occupancy sensors  130  from which the occupancy sensor received either occupied-take-action or occupied-no-action commands. For example, if the occupancy sensors  130  both transmit occupied-take-action commands to the electronic switch  110 , the electronic switch will not turn off the lighting load  104  until subsequent vacant commands are received from both of the occupancy sensors. 
     Each occupancy sensor  130  also comprises an internal ambient light detector (not shown), e.g., a photocell, for detecting the level of ambient light around the occupancy sensor. The occupancy sensor  130  measures the ambient light level when an occupancy condition is first detected and compares the ambient light level to a predetermined ambient light level threshold. lf the measured ambient light level is less than the predetermined level when an occupancy condition is first detected by one of the occupancy sensors  130 , the occupancy sensor transmits the occupied-take-action command to the electronic switch  110 . On the other hand, if the measured ambient light level is greater than the predetermined level when an occupancy condition is first detected, the occupancy sensor  130  transmits the occupied-no-action command to the electronic switch  110 . Accordingly, the electronic switch  110  does not turn on the lighting load  104  if the ambient light level in the space is sufficient. 
     The occupancy sensors  130  are each characterized by a predetermined occupancy sensor timeout period T TIMEOUT , which provides some delay in the adjustment of the state of the occupancy sensor, specifically, in the transition from the occupied state to the vacant state. The predetermined timeout period T TIMEOUT  denotes the time between the last detected occupancy condition and the transition of the occupancy sensor  130  from the occupied state to the vacant state. The predetermined occupancy sensor timeout period T TIMEOUT  may be user-selectable, for example, ranging from approximately five to thirty minutes. Each occupancy sensor  130  will not transmit a vacant command until the occupancy sensor timeout period T TIMEOUT  has expired. Each occupancy sensor  130  maintains an occupancy timer to keep track of the time that has expired since the last detected occupancy condition. The occupancy sensors  130  periodically restart the occupancy timers in response to detecting a continued occupancy condition. Accordingly, the occupancy sensors  130  do not change to the vacant state, and the lighting load  104  is not turned off, in response to brief periods of a lack of movement of the occupant in the space. If the occupancy sensor  130  fails to continue detecting the occupancy conditions, the occupancy sensor uses the occupancy timer to wait for the length of the occupancy sensor timeout period T TIMEOUT , after which the occupancy sensor changes to the vacant state and transmits a vacant command to the electronic switch  110 . 
       FIG.  2    is a simplified block diagram of the electronic switch  110 . The electronic switch  110  comprises a controllably conductive device (e.g., a latching relay  210 ) connected in series electrical connection between the hot terminal H and the switched hot terminal SH. The relay  210  conducts a load current I L  from the AC power source  102  to the lighting load  104  when the relay is closed (i.e., conductive). The load current I L  may have, for example, a magnitude of approximately five amps depending upon the type of lighting load  104 . The electronic switch  110  further comprises a bidirectional semiconductor switch  212  coupled in parallel electrical connection with the relay  210  for minimizing the inrush current conducted through the relay  210  (and thus limiting any arcing that may occur at the contacts of the relay) when the lighting load  104  is first turned on. Specifically, the bidirectional semiconductor switch  212  is controlled to be conductive before the relay  210  is rendered conductive when the electronic switch  110  is turning on the lighting load  104 , and is controlled to be non-conductive after the relay is rendered non-conductive when the electronic switch is turning of the lighting load. The bidirectional semiconductor switch  212  may comprise, for example, a triac, a field-effect transistor (FET) in a rectifier bridge, two FETs in anti-series connection, one or more silicon-controlled rectifiers (SCRs), one or more insulated-gate bipolar junction transistors (IGBTs), or any other suitable type of bidirectional semiconductor switch. 
     The relay  210  and the bidirectional semiconductor switch  212  are independently controlled by a controller  214 . For example, the controller  214  may be a microcontroller, but may alternatively be any suitable processing device, such as a programmable logic device (PLD), a microprocessor, an application specific integrated circuit (ASIC), or a field-programmable gate array (FPGA). The controller  214  is coupled to SET and RESET terminals (e.g., SET and RESET coils) of the relay  210  for causing the relay to become conductive and non-conductive, respectively. Specifically, the controller  214  generates a relay-set control signal V RLY-SET  for driving the SET coil and a relay-reset control signal V RLY-RESET  for driving the RESET coil. The controller  214  also provides a BSS-drive control signal V BSS-DRIVE  to the a control input of the bidirectional semiconductor switch  212  via a gate drive circuit  216  for rendering the bidirectional semiconductor switch conductive. 
     The electronic switch  110  comprises two power supplies: an on-state (in-line) power supply  220  and an off-state power supply  222 . Both power supplies  220 ,  222  operate to generate a DC supply voltage V CC  (e.g., having an average magnitude of approximately five volts) across an output capacitor C OUT  (e.g., having a capacitance of approximately 680 μF). The controller  214  and other low-voltage circuitry of the electronic switch  110  are powered from the DC supply voltage V CC . The bidirectional semiconductor switch  212  is coupled in series electrical connection with the parallel combination of the relay  210  and the on-state power supply  220 . The on-state power supply  220  operates to generate the DC supply voltage V CC  when the relay  210  is closed and the lighting load  104  is on as will be described in greater detail below. The off-state power supply  222  is coupled in parallel electrical connection with the relay  210  and the bidirectional semiconductor switch  212  and operates to generate the DC supply voltage V CC  when the relay  210  is open and the lighting load  104  is off. Since the output capacitor C OUT  is referenced to the circuit common of the on-state power supply  220 , the off-state power supply  222  may comprise an isolated power supply. 
     The controller  214  receives inputs from a momentary tac- tile (i.e., mechanical) switch S 224 , which temporarily closes in response to actuations of the control actuator  112  of the electronic switch  110 . The series combination of the switch S 224  and a resistor S 226  (e.g., having a resistance of approximately 15 kΩ) is coupled between the DC supply voltage V CC  and the circuit common. When the control actuator  112  is actuated and the switch S 224  is temporarily closed, the input port of the controller  214  is pulled down towards circuit common, thus signaling to the controller  214  that the switch S 224  has been actuated. Accordingly, the controller  214  is operable to control the relay  210  and the bidirectional semiconductor switch  212  to toggle the lighting load  104  on and off in response to actuations of the switch  5224 . The controller  214  is further operable to control the visual indicator  114  to be illuminated when the lighting load  104  is on and not illuminated when the lighting load is off. 
     The controller  214  is also coupled to a memory  228  for storage of the serial number of the keypad  120  and the occupancy sensors  130  to which the electronic switch  110  is assigned. The memory  228  may be implemented as an external integrated circuit (IC) or as an internal circuit of the controller  214 . The electronic switch  110  further comprises an RF transceiver  230  and an antenna  232  for transmitting and receiving the RF signals  106  with the keypad  120  and the occupancy sensors  130 . The controller  214  is operable to control the relay  210  and the bidirectional semiconductor switch  212  in response to the digital messages received via the RF signals  106 . Examples of the antenna  232  for wall-mounted load control devices, such as the electronic switch  110 , are described in greater detail in U.S. Pat. No. 5,982,103, issued Nov. 9, 1999, and U.S. patent application Ser. No. 10/873,033, filed Jun. 21, 2006, both entitled COMPACT RADIO FREQUENCY TRANSMITTING AND RECEIVING ANTENNA AND CONTROL DEVICE EMPLOYING SAME, the entire disclosures of which are hereby incorporated by reference. 
     Alternatively, the electronic switch  110  could simply comprise an RF receiver for only receiving digital messages from the keypad  120  and the occupancy sensors  130  via the RF signals  106 . In addition, the electronic switch  110  could alternatively comprise an infrared (IR) receiver for receipt of IR signals, a wired communication circuit for connection to a wired communication link, a power-line carrier (PLC) communication circuit, or another type of communication circuit. Examples of lighting control system including other types of communication circuits are described in greater detail in U.S. Pat. No. 6,545,434, issued Apr. 8, 2003, entitled MULTI-SCENE PRESET LIGHTING CONTROLLER; U.S. Pat. No. 7,423,413, issued Sep. 8, 2009, entitled POWER SUPPLY FOR A LOAD CONTROL DEVICE; and U.S. patent application Ser. No. 11/447,431, filed Jun. 6, 2006, entitled SYSTEM FOR CONTROL OF LIGHTS AND MOTORS; the entire disclosures of which are hereby incorporated by reference. 
     The on-state power supply  220  generates the DC supply voltage V CC  while allowing the electronic switch  110  to provide substantially all of the AC line voltage to the lighting load  104  when the lighting load is on. When the output capacitor C OUT  is charging through the on-state power supply  220  (while the relay  210  is conductive), the voltage developed across the on-state power supply has a substantially small magnitude (e.g., approximately the DC supply voltage V CC , i.e., approximately five volts) as compared to the peak voltage of the AC line voltage of the AC power source  102 . In other words, the on-state power supply  220  imposes a substantially low voltage drop as compared to the peak voltage of the AC line voltage of the AC power source  102 , such that the voltage provided to the lighting load  104  (i.e., switched hot voltage V SH ) is only slightly smaller when the output capacitor C OUT  is charging. For example, the peak voltage of the AC line voltage is approximately 340 volts when the RMS voltage of the AC power source  102  is 240 V AC , while the voltage developed across the on-state power supply  220  is equal to approximately the DC supply voltage V CC  (i.e., approximately five volts) for only a portion of each half cycle of the AC power source  102 . 
     The on-state power supply  220  conducts a charging current I CHRG  ( FIG.  3   ) through the output capacitor C OUT  for charging the output capacitor. The output capacitor C OUT  is adapted to conduct the load current I L  for at least a portion of a line cycle of the AC power source  102  when the relay is conductive. Accordingly, the charging current I CHRG  is equal to the load current I L  for at least a portion of a line cycle of the AC power source  102  when the relay is conductive. The on-state power supply  220  is able to operate properly when the lighting load  104  is a low-power load, e.g., having a power rating down to approximately 25 W (and a voltage rating of 240 V AC ). In other words, the on-state power supply  220  is operable to appropriately charge the output capacitor C OUT  to keep the controller  214  powered when the load current I L  has a magnitude as low as approximately 100 mA. 
     Since the lighting load  104  may cause the load current I L  of the on-state power supply  220  to be a non-sinusoidal current (e.g., if the lighting is a compact fluorescent lamp), the output capacitor C OUT  may not be able to conduct the charging current I CHRG  through the lighting load during certain portions of the line cycle of the AC power source  102 . Accordingly, the on-state power supply  220  controls when the output capacitor C OUT  is able to charge in a manner that is asynchronous with respect to the frequency of the AC line voltage of the AC power source  102 , such that the power supply is operable to start and stop charging at any time during each half cycle (i.e., at any time between the beginning and the end of the half cycle). Specifically, the on-state power supply  220  is operable to begin charging the output capacitor C OUT  when the magnitude of the DC supply voltage V CC  drops to a minimum supply voltage V CC-MIN  (e.g., approximately five volts). However, the output capacitor C OUT  may not begin charging until the output capacitor C OUT  is able to conduct the load current I L  through the lighting load  104  (i.e., if the load current I L  is non-sinusoidal). The on-state power supply  220  always stops charging when the magnitude of the DC supply voltage rises to a maximum supply voltage V CC-MAX  (e.g., approximately six volts). When the lighting load  104  is a resistive load, such as an incandescent lamp (i.e., the load current I L  is sinusoidal), the charging current I CHRG  of the on-state power supply  220  may be asynchronous with respect to the frequency of the AC line voltage (as shown  FIG.  4 A ). Alternatively, if the lighting load  104  conducts a non-sinusoidal load current I L , the charging current I CHRG  may be synchronous with respect to the line voltage frequency (as shown in  FIG.  4 B ). 
     In order to minimize visible flickering in the lighting load  104 , the on-state power supply  220  draws current from the AC power source  102  at least once every half cycle of the AC power source  102 . Accordingly, the time period between any two consecutive pulses of the charging current I CHRG  is less than the period T HC  of a half cycle (e.g., approximately ten milliseconds for a 50-Hz power source), and thus the frequency of the pulses of the charging current I CHRG  is greater than the twice the line voltage frequency (e.g., approximately 100 Hz), so as avoid visible flickering in the lighting load  104 . The time period between any two consecutive pulses of the charging current I CHRG  may be approximately equal to the period T HC  of a half cycle if the charging current I CHRG  is synchronous with respect to the line voltage frequency (as shown in  FIG.  4 B ). 
     The controller  214  is operable to monitor the operation of the on-state power supply  220  in order to determine the appropriate times to perform actions that require larger amounts of current to be drawn from the output capacitor C OUT , such as energizing the coils of the relay  210 . The on-state power supply  220  provides to the controller  214  a feedback control signal V FB , which is representative of whether the output capacitor C OUT  is charging or not as will be described in greater detail below. The controller  214  may be operable to energize the SET and RESET coils of the relay  210  immediately after the output capacitor C OUT  stops charging, i.e., when the magnitude of the DC supply voltage V CC  is equal to the maximum supply voltage V CC-MAX  and the maximum amount of voltage is available to energize the coils. 
       FIG.  3    is a simplified schematic diagram of the in-line power supply  220  according to the first embodiment of the present invention. The on-state power supply  220  includes a bidirectional semiconductor switch  310  comprising, for example, two FETs Q 312 , Q 314  coupled in anti-series connection. The on-state power supply  220  also comprises a full-wave rectifier bridge that includes the body diodes of the two FETs Q 312 , Q 314  in addition to two diodes D 316 , D 318 , which are all coupled to the output capacitor C OUT , for allowing the output capacitor to charge from the AC power source  102  through the lighting load  104 . The rectifier bridge has AC terminals coupled in series between the switched hot terminal SH and the relay  210 , and DC terminals for providing a rectified voltage V RECT . The output capacitor C OUT  is coupled in series between the DC terminals of the rectifier bridge, such that the output capacitor is able to charge from the AC power source  102  through the rectifier bridge and the lighting load  104 . The anti-series-connected FETs Q 312 , Q 314  are coupled in parallel electrical connection with the AC terminals of the rectifier bridge, such that the FETs are operable to conduct the load current I L  from the AC power source  102  to the lighting load  104  when the FETs are conductive, and the output capacitor C OUT  is operable to conduct the load current I L  when the FETs are non-conductive. 
     The output capacitor C OUT  is also coupled in series with an over-current detect resistor R 320  (e.g., having a resistance of approximately 0.1Ω) and a positive-temperature-coefficient (PTC) thermistor R 322 , which allow for the detection of fault conditions (e.g., an over-current or an over-temperature condition in the electronic switch  110 ), as will be described in greater detail below with reference to  FIG.  5   . For example, the PTC thermistor R 322  may comprise part number B59807A0090A062, manufactured by EPCOS, Inc., which has a maximum nominal resistance of approximately 400Ω. A fault voltage V FAULT  is generated across the series combination of the PTC thermistor R 322  and the output capacitor C OUT  and has a magnitude approximately equal to the magnitude of the DC supply voltage V CC  during normal operating conditions (i.e., in absence of a fault condition). 
     The on-state power supply  220  comprises a control circuit  330 , which operates, during normal operation, to render the FETs Q 312 , Q 314  non-conductive to temporarily and briefly block the load current I L . This allows the output capacitor C OUT  to conduct the load current I L  and to thus charge for at least a portion of a line cycle of the AC power source  102  when the relay  210  in conductive. Accordingly, the magnitude of the DC supply voltage V CC  increases when the bidirectional semiconductor switch  310  is non-conductive and decreases when the bidirectional semiconductor switch is conductive. Specifically, the control circuit  330  renders the FETs Q 312 , Q 314  non-conductive when the magnitude of the DC supply voltage V CC  drops to the minimum supply voltage V CC-MIN  (i.e., approximately five volts) and renders the FETs conductive when the magnitude of the DC supply voltage V CC  rises to the maximum supply voltage V CC-MAX  (i.e., approximately six volts). 
     The control circuit  330  of the on-state power supply  260  comprises, for example, an analog circuit having a comparator U 332  for controlling when the FETs Q 312 , Q 314  are conductive in response to the magnitude of the DC supply voltage V CC . A resistor divider comprising two resistors R 334 , R 336  is coupled between the DC supply voltage V CC  and circuit common and provides a scaled voltage that is representative of the magnitude of the DC supply voltage V CC  to the positive terminal of the comparator U 332 . The resistors R 334 , R 336  may have, for example, resistances of approximately 40.2 kΩ and 11 kΩ, respectively. 
     The control circuit  330  comprises a shunt regulator D 338  (e.g., part number TLV431 manufactured by Texas Instruments) having a cathode connected to the DC supply voltage V CC  through a resistor R 340  (e.g., having a resistance of approximately 11 kΩ). The cathode of the shunt regulator D 338  is coupled to the reference terminal of the shunt regulator and to the negative terminal of the comparator U 332 , such that a fixed reference voltage (e.g., approximately 1.24 V) is provided at the negative terminal. A resistor R 342  (e.g., having a resistance of approximately 47 kΩ) is coupled between the positive terminal and the output terminal of the comparator U 332  for providing some hysteresis in the operation of the on-state power supply  220 . The output of the comparator U 332  is pulled up to the DC supply voltage V CC  through a resistor R 344  (e.g., having a resistance of approximately 11 kΩ). When the scaled voltage at the positive terminal of the comparator U 332  is less than the fixed reference voltage (i.e., 1.24 V) at the negative terminal of the comparator, the output terminal of the comparator U 332  is driven low, so as to render the FETs Q 312 , Q 314  non-conductive as will be described below. Alternatively, the control circuit  330  of the on-state power supply  220  could comprise a digital circuit that includes, for example, a microprocessor, a PLD, an ASIC, an FPGA, or other suitable type of integrated circuit. The comparator U 332  may comprise part number LM2903 manufactured by National Semiconductor Corporation. 
     The output of the comparator U 332  is coupled to the base of an NPN bipolar junction transistor Q 345  via a resistor R 346  (e.g., having a resistance of approximately 22 kΩ). The collector of the transistor Q 345  is coupled to the DC supply voltage V CC  via two resistors Q 348 , Q 350  (e.g., having resistances of 100 kΩ and 22 kΩ, respectively). The base of a PNP bipolar junction transistor Q 352  is coupled to the junction of the two resistors Q 348 , Q 350 . The collector of the transistor Q 352  is coupled to the gates of the FETs Q 312 , Q 314  via two respective gate resistors R 354 , R 356  (e.g., both having a resistance of approximately 8.2 kΩ). When the output terminal of the comparator U 332  is pulled high towards the DC supply voltage V CC , the transistors Q 345 , Q 352  are both rendered conductive. Accordingly, the DC supply voltage V CC  is coupled to the gates of the FETs Q 312 , Q 314  via the respective gate resistors R 354 , R 356 , thus rendering the FETs conductive. When the output terminal of the comparator U 332  is driven low (i.e., approximately at circuit common) and the transistors Q 345 , Q 352  are rendered non-conductive, the gate capacitances of the gates of the FETs discharge through a resistor R 358  (e.g., having a resistance of approximately 8.2 kΩ) and the FETs are rendered non-conductive. 
       FIG.  4 A  is a simplified diagram of example waveforms illustrating the operation of the on-state power supply  220  when the lighting load  104  is a resistive load, such as an incandescent lamp, and the charging current I CHRG  is asynchronous with respect to the frequency of the AC power source  102 . While the FETs Q 312 , Q 314  are non-conductive, the DC supply voltage V CC  increases in magnitude (from the minimum supply voltage V CC-MIN  to the maximum supply voltage V CC-MAX ) during a charging time T CHRG . During the charging time T CHRG , the scaled voltage at the positive terminal of the comparator U 332  (which is representative of the magnitude of the DC supply voltage V CC ) is less than the reference voltage of the shunt regulator D 338  at the negative terminal. When the magnitude of the DC supply voltage V CC  exceeds the maximum supply voltage V CC-MAX , the output of the comparator U 332  is driven high towards the DC supply voltage V CC  and the FETs Q 312 , Q 314  are rendered conductive (as shown by the gate voltages V G  in  FIG.  4 A ). At this time, the voltage at the positive terminal of the comparator U 332  is pulled high towards the DC supply voltage V CC . Since the FETs Q 312 , Q 314  are conductive, the magnitude of the DC supply voltage V CC  and the magnitude of the scaled voltage at the negative terminal of the comparator U 332  begin to decrease as the controller  214  and other low-voltage circuits of the electronic switch  110  draw current from the output capacitor C OUT . 
     When the magnitude of the DC supply voltage V CC  drops below the minimum supply voltage V CC-MIN , the scaled voltage at the positive terminal of the comparator U 332  becomes less than the reference voltage of the shunt regulator D 338  at the negative terminal. The output of the comparator U 332  is driven low towards circuit common, and the FETs Q 312 , Q 314  are rendered non-conductive, thus allowing the output capacitor C OUT  to charge and the DC supply voltage V CC  to increase in magnitude during the charging time T CHRG . As a result of the operation of the power supply  220 , only a low-voltage drop (i.e., approximately five volts) is developed across the power supply and the switched hot voltage V SH  has only small “notches” (i.e., small changes in magnitude) when the output capacitor C OUT  is charging as shown in  FIG.  4 A . Note that the worst case charging time T CHRG  may be equal to approximately the period T HC  of a half cycle of the AC power source  102  if the output capacitor C OUT  charges and discharges such that the magnitude of the DC supply voltage V CC  does not exceed the maximum supply voltage V CC-MAX . 
       FIG.  4 B  is a simplified diagram of example waveforms illustrating the operation of the on-state power supply  220  when the load current I L  is non-sinusoidal (e.g., the lighting load  104  is a compact fluorescent lamp), and the charging current I CHRG  is synchronous with respect to the frequency of the AC power source  102 . As shown in  FIG.  4 B , the charging current I CHRG  does not immediately begin flowing when the magnitude of the DC supply voltage drops below the minimum supply voltage V CC-MIN  even though the gate voltages V G  are driven low and the FETs Q 312 , Q 314  are rendered non-conductive. The charging current I CHRG  begins flowing when the lighting load  104  begins conducting the load current I L , which occurs at approximately the same time each half cycle, such that the charging current I CHRG  is symmetric with respect to the frequency of the AC power source  102 . Once again, only a low-voltage drop is developed across the power supply  220  and the switched hot voltage V SH  has only small notches when the output capacitor C OUT  is charging as shown in  FIG.  4 B . 
     Referring back to  FIG.  3   , the feedback control signal V FB , which is provided to the controller  214 , is generated at the collector of the transistor Q 345 . Thus, the feedback control signal V FB  is the inverse of the gate voltage V G  shown in  FIGS.  4 A and  4 B . When the transistor Q 345  is conductive (i.e., the FETs Q 312 , Q 314  are conductive and the output capacitor C OUT  is discharging), the feedback control signal V FB  is driven low towards circuit common (i.e., a logic low level). When the transistor Q 345  is non-conductive (i.e., the FETs Q 312 , Q 314  are non-conductive and the output capacitor C OUT  is charging), the feedback control signal V FB  is pulled up towards the DC supply voltage V CC  (i.e., a logic high level). When the controller  214  is ready to render the relay  210  conductive or non-conductive, the controller may wait until the feedback control signal V FB  transitions from high to low (i.e., the magnitude of the DC supply voltage V CC  is at the maximum supply voltage C CC-MAX ) before energizing either the SET coil or the RESET coil of the relay. 
     The controller  214  is operable to determine if the electronic switch  110  is overloaded (i.e., if an overload condition is occurring) in response to the charging time T CHRG  required to charge the output capacitor C OUT . For example, the electronic switch  110  may be overloaded if the lighting load  104  causes the load current I L  conducted through the relay  210  to have a magnitude of approximately eight amps. Specifically, the controller  214  is operable measure the length of the time period between the low-to-high and high-to-low transitions of the feedback control signal V FB  (i.e., the length of the charging time T CHRG  when the output capacitor C OUT  is charging). As the magnitude of the load current I L  increases, the charging time T CHRG  required to charge the output capacitor C OUT  decreases. Therefore, the controller  214  is operable to compare the time period between the low-to-high and high-to-low transitions of the feedback control signal V FB  to a predetermined charging time threshold T CHRG-TH  (e.g., approximately 85 μsec) to determine if an overload condition may be occurring. Specifically, the controller  214  determines that the overload condition is occurring in response to detecting that a percentage (e.g., 10%) of the charging times T CHRG  are less than the predetermined charging time threshold, for example, if ten of the last one hundred time periods between the low-to-high and high-to-low transitions of the feedback control signal V FB  are less than approximately 85 μsec. The controller  214  opens the relay  210  when the overload condition is detected. In addition, the controller  214  may blink the visual indicator  114  in response to detecting the overload condition. 
       FIG.  5    is a simplified schematic diagram showing how the in-line on-state power supply  220  is coupled to the latching relay  210  and the drive circuit  216  for the bidirectional semiconductor switch  212  to provide for fault detection and protection of the electronic switch  110 . The SET coil of the relay  210  is coupled between the relay-set control signal V RLY-SET  and the DC supply voltage V CC . When the controller  214  drives the relay-set control signal V RLY-SET  low to approximately circuit common, the mechanical switch of the relay  210  is rendered conductive. The RESET coil of the relay  210  is coupled between the relay-reset control signal V RLY-RESET  and the fault voltage V FAULT , which has a magnitude approximately equal to the magnitude of the DC supply voltage V CC  during normal operating conditions (i.e., in absence of an over-temperature condition). The relay-reset control signal V RLY-RESET  is also coupled to the DC supply voltage V CC  through a diode D 305 . When the controller  214  drives the relay-reset control signal V RLY-RESET  low to approximately circuit common during normal operating conditions, the mechanical switch of the relay  210  is rendered non-conductive. 
     If the output capacitor C OUT  were to fail shorted when the latching relay  210  is conductive, the temperatures of the FETs Q 312 , Q 314  of the on-state power supply  220  may increase to undesirable levels. According to an aspect of the present invention, when an over-temperature condition is detected in the FETs Q 312 , Q 314  of the on-state power supply  220 , the electronic switch  110  controls the latching relay  210  (e.g., to open the relay) in order to remove the over-temperature condition. Specifically, the PTC thermistor R 322  is thermally coupled to the FETs Q 312 , Q 314 , such that the resistance of the PTC thermistor increases as the combined temperature of the FETs increases during the over-temperature condition, thus causing the fault voltage V FAULT  to increase in magnitude. Since the series combination of the diode D 305  and the RESET coil of the relay  210  is coupled between the fault voltage V FAULT  and the DC supply voltage V CC  (i.e., in parallel with the output PTC thermistor R 322 ), current begins to flow through the RESET coil as the resistance of the PTC thermistor increases and the magnitude of the fault voltage V FAULT  increases. The relay  210  is rendered non-conductive when the combined temperature of the FETs Q 312 , Q 314  increases above a predetermined temperature threshold T FAULT  (e.g., approximately 90° F.). In other words, the relay  210  is rendered non-conductive when the fault voltage V FAULT  increases such that the voltage across the RESET coil renders the relay  210  non-conductive. Accordingly, the current through the FETs Q 312 , Q 314  is controlled to zero amps and the fault condition is removed (i.e., the temperatures of the FETs will decrease below the undesirable levels). The relay  210  is rendered conductive in response to the over-temperature condition independent of the magnitude of the relay-reset control signal V RLY-RESET . In addition, the relay  210  could be rendered conductive in response to an over-temperature condition in other circuits of the electronic switch  110 . 
     As shown in  FIG.  5   , the bidirectional semiconductor switch  212  is implemented as a triac. The drive circuit  216  comprises an optocoupler U 380  having an output phototriac coupled in series with the gate of the bidirectional semiconductor switch  212 . When the output phototriac of the optocoupler U 380  is conductive, the output phototriac conducts a gate current through two resistors  8382 ,  8384  each half cycle of the AC power source  102 , thus rendering the bidirectional semiconductor switch  216  conductive each half cycle. The resistors R 382 , R 384  may both have, for example, resistances of approximately 100 Ω. 
     The optocoupler U 380  also has an input photodiode having an anode coupled to the rectified voltage V RECT  of the on-state power supply  220 . An NPN bipolar junction transistor Q 385  is coupled in series with the input photodiode of the optocoupler U 380 . The controller  214  is coupled to the base of the transistor Q 385  via a resistor R 386  (e.g., having a resistance of approximately 1 kΩ). When the transistor Q 385  is rendered conductive, the transistor conducts a drive current through the input photodiode of the optocoupler U 380  and a resistor R 388  (e.g., having a resistance of approximately 330Ω), thus rendering the output optotriac and the bidirectional semiconductor switch  212  conductive. 
     According to another aspect of the present invention, when an over-current condition is detected in the in-line on-state power supply  220 , the electronic switch  110  uses the bidirectional semiconductor switch  212  to remove the over-current condition. The over-current condition may be caused by an inrush current conducted through the relay  210 , for example, when the lighting load  104  is a capacitive load, such as a screw-in compact fluorescent lamp or an electronic low-voltage (ELV) lighting load. For example, the inrush current may have a magnitude greater than approximately three hundred amps and last for approximately two milliseconds as defined by the NEMA 410 Standard published by the National Electrical Manufacturers Association (NEMA). To protect the on-state power supply  220  from the over-current condition, the bidirectional semiconductor switch  212  is rendered conductive when the current through the over-current detect resistor R 320  of the on-state power supply  220  exceeds a predetermined current threshold I FAULT  (e.g., approximately forty amps). At this time, the voltage across the on-state power supply  220  is reduced to approximately the on-state voltage of the bidirectional semiconductor switch  212  (e.g., approximately one volt), which causes the power supply to stop charging the output capacitor C OUT , and eliminates the over-current condition. 
     Referring back to  FIG.  5   , the over-current detect resistor R 320  of the on-state power supply  220  is coupled in parallel with the series combination of the input photodiode of the optocoupler U 380 , a diode D 390 , and a resistor R 392  (e.g., having a resistance of approximately 47Ω). When the current through the over-current detect resistor R 320  exceeds the predetermined current threshold I FAULT , the voltage generated across the series combination of the input photodiode of the optocoupler U 380 , the diode D 390 , and the resistor R 392  causes the output phototriac of the optocoupler to be rendered conductive. Accordingly, the bidirectional semiconductor switch  212  is rendered conductive and the over-current condition is eliminated. Since the bidirectional semiconductor switch  212  is a triac, the bidirectional semiconductor switch becomes non-conductive at the end of the half cycle when the current through the bidirectional semiconductor switch drops to approximately zero amps. The bidirectional semiconductor switch  212  will be rendered conductive once again during the next half cycle if the over-current condition remains. 
       FIG.  6    is a simplified flowchart of a button procedure  400  executed by the controller  214  of the electronic switch  110  is response to an actuation of the switch S 224  at step  410 . The controller  214  uses two timers, e.g., a relay timer and a bidirectional semiconductor switch (BSS) timer, to control when the relay  210  and the bidirectional semiconductor switch  212  become conductive and non-conductive. When the relay timer expires, the controller  214  executes a relay timer procedure  600  to render the relay  210  conductive if the lighting load  104  is off and to render the relay non-conductive if the lighting load is on (as will be described in greater detail below with reference to  FIG.  8   ). When the BSS timer expires, the controller  214  executes a BSS timer procedure  700  to control the bidirectional semiconductor switch  212  to become conductive if the lighting load  104  is off and to become non-conductive if the lighting load is on (as will be described in greater detail below with reference to  FIG.  9   ). The controller  214  executes a received keypad message procedure (not shown), which is similar to the button procedure  400 , in response to receiving an on command (when the on button  122  is actuated) and an off command (when the off button  124 ). 
     Referring to  FIG.  6   , if the lighting load  104  is off at step  412 , the controller  214  initializes the BSS timer to a BSS-turn-on time t BSS-ON , and starts the BSS timer decreasing in value with respect to time at step  414 . The controller  214  then initializes the relay timer to a relay-turn-on time t RLY-ON , and starts the relay timer decreasing in value with respect to time at step  416 , before the button procedure  400  exits. For example, the BSS-turn-on time t BSS-ON  may be approximately zero milliseconds and the relay-turn-on time t RLY-ON  may be approximately thirty milliseconds, such that the bidirectional semiconductor switch  212  will be rendered conductive before the relay  210  is rendered conductive. If the lighting load  104  is on at step  412 , the controller  214  immediately renders the bidirectional semiconductor switch  212  conductive at step  418 . The controller  214  then initializes the relay timer to a relay-turn-off time t RLY-OFF , and starts the relay timer decreasing in value with respect to time at step  420 . Finally, the controller  214  initializes the BSS timer to a BSS-turn-off time t BSS-OFF , and starts the BSS timer decreasing in value with respect to time at step  422 , before the button procedure  400  exits. For example, the relay-turn-off time t RLY-OFF  may be approximately thirty milliseconds and the BSS-turn-off time t BSS-OFF  may be approximately sixty milliseconds, such that the relay  210  will be rendered non-conductive before the bidirectional semiconductor switch  212  becomes non-conductive. 
       FIG.  7    is a simplified flowchart of a received occupancy message procedure  500  executed by the controller  214  of the electronic switch  110  in response to receiving a digital message from one of the occupancy sensors  130  via the RF signals  106  at step  510 . The controller  214  keeps track of the states of the occupancy sensor  130  to which the electronic switch  110  is assigned in response to the digital messages received from the occupancy sensors. Specifically, if the controller  214  receives an occupied-take-action command or an occupied-no-action command from an occupancy sensor  130 , the controller marks the serial number of the occupancy sensor as “occupied” in the memory  228 . If the controller  214  receives a vacant command from the occupancy sensor  130 , the controller marks the serial number of the occupancy sensor as “vacant” in the memory  228 . The controller waits for a vacant command from all of the occupancy sensors to which the electronic switch  110  is assigned before turning off the lighting load  104 . 
     Referring to  FIG.  7   , after receiving the digital message at step  510 , the controller  214  first determines whether the serial number provided in the received digital message is stored in the memory  228  at step  512 . If not, the controller  214  does not process the received digital message and the received occupancy message procedure  500  exits. If the serial number of the received digital message is stored in the memory  228  at step  512  and the received digital message is an occupied-take-action command at step  514 , the controller  214  determines if any of the serial numbers stored in the memory  228  are marked as occupied at step  516  to determine if the space is occupied or vacant. If there are no serial numbers marked as occupied at step  516  (i.e., the space has just become occupied), the controller  214  turns on the lighting load  104  by initializing and starting the BSS timer (using the BSS-turn-on time t BSS-ON ) at step  518 , and initializing and starting the relay timer (using the relay-turn-on time t RLY-ON ) at step  520 . The controller  214  then marks the serial number of the received digital message as occupied at step  522  and the received message procedure  510  exits. If there are serial numbers marked as occupied at step  516  (i.e., the space is occupied), the controller  214  marks the serial number of the received digital message as occupied at step  522 , before the received occupancy message procedure  500  exits. 
     If the received digital message is an occupied-no-action command at step  524 , the controller  214  does not adjust the amount of power delivered to the lighting load  104 . The controller  214  simply marks the serial number as occupied at step  522  and the received occupancy message procedure  500  exits. If the received digital message is a vacant command at step  526 , the controller  214  marks the serial number as vacant at step  528 . If any of the serial numbers are still marked as occupied at step  530  (i.e., the space is still occupied), the received occupancy message procedure  500  simply exits. However, if all of the serial numbers are marked as vacant at step  530  (i.e., the space is now vacant), the controller  214  controls the lighting load  104  off by immediately rendering the bidirectional semiconductor switch  212  conductive at step  532 , initializing and starting the relay timer (using the relay-turn-off time t RLY-OFF ) at step  534 , and initializing and starting the BSS timer (using the BSS-turn-off time t BSS-OFF ) at step  536 , before the received occupancy message procedure  500  exits. 
       FIG.  8    is a simplified flowchart of a relay timer procedure  600  executed by the controller  214  when the relay timer expires at step  610 . First, the controller  214  waits until the feedback control signal V FB  transitions from high to low at step  612  indicating that the magnitude of the DC supply voltage V CC  is equal to the maximum supply voltage V CC-MAX . When the controller  214  detects that the feedback control signal V FB  has transitioned from high to low at step  612 , the controller immediately renders the relay  210  conductive or non-conductive depending upon the present state of the lighting load  104 . If the lighting load  104  is off at step  614 , the controller  214  renders the relay  210  conductive at step  616  by conducting current through the SET coil of the relay and the relay timer procedure  600  exits. If the lighting load  104  is off at step  614 , the controller  214  renders the relay  210  non-conductive at step  618  by conducting current through the RESET coil and the relay timer procedure  600  exits. 
       FIG.  9    is a simplified flowchart of a BSS timer procedure  700  executed by the controller  214  when the BSS timer expires at step  710 . If the lighting load  104  is off at step  712 , the controller  214  controls the drive circuit  216  to render the bidirectional semiconductor switch  212  conductive at step  714  and illuminates the visual indicator  214  at step  716 , before the BSS timer procedure  700  exits. If the lighting load  104  is off at step  712 , the controller  214  controls the drive circuit  216  such that the bidirectional semiconductor switch  212  becomes non-conductive at step  718 . The controller  214  then controls the visual indicator  214  to be off at step  720  and the BSS timer procedure  700  exits. 
       FIG.  10    is a simplified flowchart of an overload detection procedure  800  executed by the controller  214  when the feedback control signal V FB  transitions from high to low or low to high at step  810 . If the detected transition of the feedback control signal V FB  is a low-to-high transition at step  812 , the controller  214  initializes a timer (e.g., to zero μsec) and starts the timer increasing in value with respect to time at step  814 , before the overload detection procedure  800  exits. If the detected transition of the feedback control signal V FB  is a high-to-low transition at step  812 , the controller  214  stores the present value of the timer at step  816 . If the timer value is greater than the predetermined charging time threshold T CHRG-TH  (i.e., approximately 85 μsec) at step  818 , the overload detection procedure  800  simply exits. However, if the timer value is less than or equal to approximately the predetermined charging time threshold T CHRG-TH  at step  818 , the controller  214  determines if an overload condition is occurring at step  820 . Specifically, the controller  214  determines at step  820  if a percentage (e.g., 10%) of the most recently stored timer values (from step  816 ) are less than the predetermined charging time threshold, for example, if ten of the last one hundred stored timer values are less than approximately 85 μsec. If the controller  214  does not detect the overload condition at step  820 , the overload detection procedure  800  simply exits. Otherwise, if the controller  214  detects the overload condition at step  820 , the controller  214  renders the relay  210  non-conductive at step  822  and blinks the visual indicator  114  at step  824 , before the overload detection procedure  800  exits. 
     While the present invention has been described with reference to the electronic switch  110  controlling the power delivered to a connected lighting load, the concepts of the present invention could be used in any type of control device of a load control system, such as, for example, a dimmer switch for adjusting the intensity of a lighting load (such as an incandescent lamp, a magnetic low-voltage lighting load, an electronic low-voltage lighting load, and a screw-in compact fluorescent lamp), a remote control, a keypad device, a visual display device, a controllable plug-in module adapted to be plugged into an electrical receptacle, a controllable screw-in module adapted to be screwed into the electrical socket (e.g., an Edison socket) of a lamp, an electronic dimming ballast for a fluorescent load, and a driver for a light-emitting diode (LED) light source, a motor speed control device, a motorized window treatment, a temperature control device, an audio/visual control device, or a dimmer circuit for other types of lighting loads, such as, magnetic low-voltage lighting loads, electronic low-voltage lighting loads, and screw-in compact fluorescent lamps. 
     Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims.