Patent Publication Number: US-11664679-B2

Title: Communication circuit for 2-wire protocols between HVAC systems and smart-home devices

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 16/010,271, filed Jun. 15, 2018, which is incorporated here by reference. U.S. application Ser. No. 16/010,271 is related to U.S. Pat. No. 9,568,201, entitled “Environmental Control System Retrofittable with Multiple Types of Boiler-Based Heating Systems,” which is hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     This patent specification relates generally to communication between an actuator device for a boiler-based HVAC system and the boiler itself. More specifically, this disclosure describes circuits and methods for enabling devices powering, power stealing, and communication between the boiler and the actuator device to maintain compatibility with predetermined thresholds. 
     BACKGROUND 
     In certain situations, it may be beneficial to have a thermostat located a distance away from a location at which connection to heating, ventilation, and air conditioning (HVAC) control wires are accessible. For instance, HVAC control wires may be run from an HVAC system, behind a wall, and exposed in an inconvenient location, such as within a utility closet. Such a location may not be convenient to be accessed by a user or such a location may not be ideal for accurately sensing the temperature of a region of the structure in which occupants are typically present. Furthermore, when an actuator for controlling a boiler system and/or a thermostat are located away from the HVAC system, power for these devices may not be readily available. 
     BRIEF SUMMARY 
     In some embodiments, a circuit for stealing power from an external system without interfering with a communication protocol may include a plurality of wiring connectors configured to receive a plurality of wires. The plurality of wiring connectors may receive a plurality of current levels set by the external system according to the communication protocol. The circuit may also include a first voltage regulator to regulate a voltage on the plurality of wiring connectors at a plurality of voltage levels according to the communication protocol. The circuit may additionally include a current monitor to measure the plurality of current levels received through the plurality of wiring connectors, and a second voltage regulator that provides a current-limiting output. The circuit may further include a power converter that optimizes an amount of power stolen from the plurality of wiring connectors based on the current-limiting output. 
     In some embodiments, a method for stealing power from an external system without interfering with a communication protocol may include receiving a plurality of current levels set by the external system according to the communication protocol, where the plurality of current levels may be received through a plurality of wiring connectors configured to receive a plurality of wires. The method may also include regulating a voltage on the plurality of wiring connectors at a plurality of voltage levels according to the communication protocol using a first voltage regulator. The method may additionally include measuring the plurality of current levels received through the plurality of wiring connectors using a current monitor, and providing a current-limiting output from a second voltage regulator. The method may further include optimizing an amount of power stolen from the plurality of wiring connectors by a power converter based on the current-limiting output. 
     In any embodiments, one or more of the following features may be included in any combination and without limitation. The circuit may also include a bridge rectifier coupled between the plurality of wiring connectors and the first voltage regulator. The bridge rectifier may include a diode bridge rectifier. The bridge rectifier may include a FET bridge rectifier with Zener-resistor clamps at FET gates. The first voltage regulator may include a plurality of the Zener diode clamps, where regulated voltages of the plurality of Zener diode clamps may correspond to the plurality of voltage levels. The current monitor may include a resistor and a current-sense amplifier that measures a voltage differential across the resistor. The circuit may also include a resistive network coupled to the first voltage regulator and a switch controlled by a transmit signal from a processor, where the switch may change a resistance of the resistive network to control a regulated voltage output by the first voltage regulator. The circuit may also include a resistive network coupled to the second voltage regulator and a switch controlled by a transmit signal from a processor, where the switch may change a resistance of the resistive network to control a regulated voltage output by the second voltage regulator. The power converter may include an error amplifier that controls an output current of the power converter. The current-limiting output from the second voltage regulator may drop in voltage when the second voltage regulator drops out, thereby causing the power converter to use less current. Optimizing the amount of power stolen may include reducing an amount of power output by the power converter when the current-limiting output indicates that the second voltage regulator is dropping out. Optimizing the amount of power stolen may include reducing an amount of power output by the power converter when the current-limiting output indicates that the second voltage regulator is not dropping out. The method may also include decoding the plurality of current levels measured by the current monitor to determine a message sent from the external system. The method may also include encoding a message to be sent to the external system in the plurality of voltage levels. The method may also include receiving a command from a smart-home device to send the message to the external system. The method may also include using the power stolen by the power converter to power a system processor. The method may also include using the power stolen by the power converter to charge a rechargeable energy-storage device. The method may also include using the power stolen by the power converter to power a system processor. The external system may include an HVAC system comprising a boiler. 
     A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings. Also note that other embodiments may be described in the following disclosure and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    illustrates an embodiment of a block diagram of a HVAC control system that includes a stand device and an actuator device. 
         FIG.  2    illustrates an exploded front view of an embodiment of an actuator device. 
         FIG.  3 A  illustrates a front view of an embodiment of a thermostat attached to a stand device. 
         FIG.  3 B  illustrates a side view of an embodiment of a thermostat attached to a stand device. 
         FIG.  4    illustrates a detailed view of the wired communication channel between the HVAC system and the actuator, according to some embodiments. 
         FIG.  5    illustrates some of the operating characteristics of one communication protocol. 
         FIG.  6    illustrates a block diagram of a system for stealing power from a wired communication channel without interfering with a communication protocol, according to some embodiments. 
         FIG.  7    illustrates a circuit-level implementation of an input circuit, according to some embodiments. 
         FIG.  8    illustrates a circuit-level implementation of a first power stealing communication circuit, according to some embodiments. 
         FIG.  9    illustrates a circuit-level implementation of the current monitor circuit, according to some embodiments. 
         FIG.  10    illustrates a circuit-level implementation of a first stage of a power stealing communication circuit, according to some embodiments. 
         FIG.  11    illustrates a circuit-level implementation of a second stage of a power stealing communication circuit, according to some embodiments. 
         FIG.  12    illustrates a simplified circuit diagram of the system power converter, according to some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, for purposes of explanation, numerous specific details are set forth to provide a thorough understanding of the various embodiments of the present invention. Those of ordinary skill in the art will realize that these various embodiments of the present invention are illustrative only and are not intended to be limiting in any way. Other embodiments of the present invention will readily suggest themselves to such skilled persons having the benefit of this disclosure. It will be apparent to one skilled in the art that the present invention may be practiced without some or all of these specific details. In other instances, well known details have not been described in detail in order not to unnecessarily obscure the present invention. 
     In addition, for clarity purposes, not all of the routine features of the embodiments described herein are shown or described. One of ordinary skill in the art would readily appreciate that in the development of any such actual embodiment, numerous embodiment-specific decisions may be required to achieve specific design objectives. These design objectives will vary from one embodiment to another and from one developer to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming but would nevertheless be a routine engineering undertaking for those of ordinary skill in the art having the benefit of this disclosure. 
     An actuator device (referred to as an “actuator”) may be connected with HVAC control wires. Such HVAC control wires may control operation of various HVAC components, including: a furnace, a boiler, a fan, an air conditioner, and/or a multi-stage heating or cooling system. The actuator unit may open and close circuits in order to control operation of components of the HVAC system. So as not to require an external power connection, the actuator unit may steal power from the HVAC system over the same wires used for communication with and control of the HVAC system. In order to not disrupt the communication protocol, the actuator may include a first voltage regulator that regulates an output voltage according to a transmit signal provided by the actuator. A current monitor can be placed in series with the first voltage regulator to measure the input current and decode a received signal. The output of the current monitor can be fed into a second voltage regulator that operates on the edge of its dropout mode. By receiving the transmit input provided to the first voltage regulator, the second voltage regulator can follow the output of the first regulator and provide a current limit signal to a system power converter. As the current limit signal increases proportional to an amount of current shunted through a resistive path, the system power converter can increase the amount of current stolen from the communication signal at the output of the current monitor to power the actuator and/or charge a rechargeable energy-storage device. As the current stolen by the system power converter reaches a maximum, the second regulator may enter its dropout mode, causing the system power converter to decrease the amount of current it steals. This feedback loop allows the system to steal the maximum out of current possible without interfering with the communication protocol. 
     Embodiments detailed herein are focused on various communication aspects between the actuator and the HVAC system. Such aspects can improve the efficiency and reliability with which the actuator can communicate and receive power from the HVAC system. It is to be appreciated that while one or more embodiments are described further herein in the context of a typical HVAC system used in a residential home, such as a single-family residential home, the scope of the present teachings is not so limited. More generally, intelligent thermostat systems according to one or more of the embodiments are applicable for a wide variety of enclosures having one or more HVAC systems including, without limitation, duplexes, townhomes, multi-unit apartment buildings, hotels, retail stores, office buildings, and industrial buildings. Further, it is to be appreciated that while the terms user, customer, installer, homeowner, occupant, guest, tenant, landlord, repair person, and/or the like may be used to refer to the person or persons who are interacting with the thermostat or other device or user interface in the context of one or more scenarios described herein, these references are by no means to be considered as limiting the scope of the present teachings with respect to the person or persons who are performing such actions. 
     Additionally, the communication between the HVAC system and the actuator is merely provided by way of example, and not meant to be limiting. The 2-wire communication techniques coupled with power stealing circuitry described herein can be used in any device communicating by way of a wired communication channel. Therefore, the actuator and the boiler control described below can be readily substituted for any master/slave communication devices where one device powers the other over the same channel. This includes any smart home device, such as hazard detectors, security system sensors, smart doorbells, cameras, child monitors, wearable technology, smart home assistants, microphones, speakers, smart appliances, televisions, and so forth. 
     It is to be appreciated that “smart home environments” may refer to smart environments for homes such as a single-family house, but the scope of the present teachings is not so limited, the present teachings being likewise applicable, without limitation, to duplexes, townhomes, multi-unit apartment buildings, hotels, retail stores, office buildings, industrial buildings, and more generally any living space or work space having one or more smart hazard detectors. 
     While embodiments detailed herein are focused on communication between actuators and HVAC systems, it should be understood that the embodiments detailed herein may be applicable to other smart home devices and/or sensor devices. For instance, aspects of the communication circuits described below may be applied to thermostats, smoke detectors, carbon monoxide detectors, doorbells, home assistants, video cameras, remote temperature sensors, or other smart devices that may be installed in a home, office, or other location. 
       FIG.  1    illustrates an embodiment of a block diagram of an HVAC control system  100  that includes an actuator  110 , according to some embodiments. Specifically, the HVAC control system  100  may include the actuator  110 , a mounting plate  120 , a thermostat  140 , and a stand device  150 . The actuator  110  may be attached to a mounting plate  120 . The mounting plate  120  may facilitate the actuator device  110  being attached to a surface (e.g., a wall) and/or allowing HVAC control wires to be routed into a back of the actuator device  110  through a wired communication channel  162 . For example, a pair of HVAC control wires may be exposed through a surface of a wall. The mounting plate  120  may allow the actuator  110  to be secured to the surface of the wall while allowing HVAC control wires to be passed through a rear surface of the actuator  110 . The actuator device  110  may be connected via one or more HVAC control wires to the HVAC system  130  based on the existing wiring of the HVAC system  130 . 
     In some embodiments, the HVAC system  130  may include a boiler-based HVAC system. For example, the HVAC system  130  may include a boiler device that includes a control interface with a wired communication port that may be coupled to, for example, the wired communication channel  162  with the actuator  110 . As described below, the boiler device can provide power through the wired communication channel  162  to the actuator  110 . The actuator  110  may include a rechargeable energy storage device  180 , such as a super capacitor or a rechargeable battery. A power-stealing and communication circuit  182  in the actuator  110  can “steal” power from the boiler device through the wired communication channel  162  to provide operating power to the actuator  110  and/or recharge the energy storage device  180 . Additionally, the actuator  110  and the boiler device may communicate with each other over the wired communication channel  162  using a 2-wire communication channel protocol, such as OpenTherm®. 
     The actuator device  110  may include a display  111 , a user interface  112 , a processing system  113 , a wireless interface  114 , and a control interface  115 . The display  111 , which may include one or more LEDs or other forms of lighting elements, an active display, an LCD display, a color display, and/or the like, may present information to a user regarding the operation of the actuator device  110 , the status of the wired communication channel  162 , the status of a wireless communication channel  160 , and/or any other operational information. The display  111  may include a “dead front” display; such a display may appear to be a blank surface that is difficult to identify as a display when the one or more lighting elements are inactive. When active, the lighting elements may be visible on the blank surface. The user interface  112  may include one or more buttons or other forms of user input devices that allow a user to provide input directly to the actuator  110 . For instance, the user interface  112  may be used to engage one or more components of the HVAC system  130  without a user being required to interact with the thermostat  140 . The processing system  113  may include one or more processors that send/receive information via a wireless interface  114  to the thermostat  140 . The processing system  113  may receive input from the user interface  112 , output information that is presented via the display  111 , and control actuation of various HVAC components and functions via the control interface  115 . The wireless interface  114  may use one or more wireless communication protocols, such as: Wi-Fi® (IEEE 802.11), IEEE 802.15.4, Bluetooth®, Z-Wave®, ZigBee®, Thread®, or any other wireless communication protocol to communicate with the thermostat  140 . Control interface  115  may open and close circuits that include HVAC control wires based on instructions from processing system  113  to control HVAC system  130 . 
     The thermostat  140  may wirelessly communicate with the actuator  110  via the wireless communication channel  160 . The thermostat  140  may transmit instructions, through one of the wireless communication protocols described above to instruct the actuator  110  to activate or deactivate one or more components of the HVAC system  130 . The thermostat  140  may be removably coupled with a stand  150 . The thermostat  140  may communicate via a wireless network (e.g., a Wi-Fi WLAN) with the Internet  141 . Via the Internet  141 , the thermostat  140  may transmit data to and receive data from a cloud-based server system  142 . The cloud-based server system  142  may maintain a user account that stores data related to the thermostat  140  and may permit a user to remotely control and/or view data related to the thermostat  140 . For example, a user may communicate with the cloud-based server system  142  to modify a setpoint schedule implemented by the thermostat  140  or may provide a real-time setpoint that is used to immediately control the HVAC system  130  by the thermostat  140  through the actuator  110 . 
     The stand  150  may be placed on a surface and may include a power system  151  that powers the thermostat  140 . The power system  151  may be connected to a power outlet (e.g., 120 V, 230 V) and may provide a constant voltage to thermostat  140 . The stand  150  may have one or more on-board temperature sensors, such as a temperature and/or humidity sensor  152 , that provides temperature and/or humidity measurements to thermostat  140 . 
       FIG.  2    illustrates an exploded view of an actuator  110 , according to some embodiments. The actuator  110  may include a rotatable cover assembly  205 , a chassis  210 , an HVAC wiring connector cover  215 , cover fastener assemblies  220  ( 220 - 1 ,  220 - 2 ), a light boot assembly  230 , a battery contact  240 , a battery spring  241 , a spring cap  242 , a battery spring  243 , a spring cap  244 , a battery contact  245 , a support  250 , a printed circuit board (PCB)  255 , a backplate  260 , batteries  265 ; a light pipe assembly  270  (which may include multiple light pipes and the structure to which the light pipes are attached), a battery holder tab  293 , a button  291 , and a cover leash  292  The rotatable cover assembly  205  may be designed to be facing away from a surface to which the backplate  260  is mounted. As such, the rotatable cover assembly  205 , when removably attached with the chassis  210 , may be the component of the actuator  110  that is most visible to a user. The rotatable cover assembly  205  may be removable by a user, for example, by pulling on the edges of the rotatable cover assembly  205 . Removing the rotatable cover assembly  205  can allow a user to access the HVAC wiring connectors and/or the battery compartment of the chassis  210 . The PCB  255  may have components such as a wireless interface (e.g., wireless interface  114 ) and a processing system (e.g., processing system  113 ) mounted to it. 
       FIGS.  3 A and  3 B  illustrate views of a thermostat  140  attached to a stand  150 .  FIG.  3 A  illustrates a front view of the thermostat  140  attached to the stand  150 , while  FIG.  3 B  illustrates a side view of the thermostat  140  attached to the stand  150 . The stand  150  may permit the thermostat  140  to be powered and to be supported on a flat surface (e.g., table, shelf, desk, floor, etc.). The thermostat  140  may be cylindrical and may have an outer ring that rotates. Ring  306  may match a contour of the stand  150 . 
       FIG.  4    illustrates a detailed view of the wired communication channel  162  between the HVAC system  130  and the actuator  110 , according to some embodiments. In this example, the HVAC system  130  may include a boiler having a boiler control  310 . The boiler control  310  may include a 2-wire interface for communicating with the actuator  110 . In some embodiments, the boiler control  310  can pass a simple voltage signal through one wire to the actuator  110 . The actuator  110  can control the operation of a relay that connects two of the wires in the wired communication channel  162 . For example, closing the relay may cause two of the wires in the wired communication channel  162  to be shorted together, which may in turn cause the boiler control  310  to activate a function of the HVAC system  130 , such as turning on the boiler, causing the boiler to circulate heated fluid through the smart-home environment, and so forth. 
     In some embodiments, more complex communication protocols may be used over the wired communication channel  162 . Most 2-wire communication protocols communicate by virtue of controlling the state of the voltage and/or current on the wired communication channel  162 . Protocols may govern the rise/fall times, voltage thresholds for high and low states, current thresholds for high and low states, frequency of transactions, and so forth. Therefore, the voltage, current, and/or timing of any signals that are propagated on the wired communication channel  162  may need to be tightly controlled to prevent interference with the communication signal. 
     The specificity of most 2-wire communication protocols presents a technical problem for using the wired communication channel  162  to provide operating power for the actuator  110 . In some embodiments, the actuator  110  may be co-located with the boiler control  310  and may receive direct line power through additional power wires that can be included in the wired communication channel  162 . However, other embodiments may support configurations where the actuator  110  is not co-located with the boiler control  310 . Instead, the actuator  310  can be located in another part of the smart-home environment, such as on a different floor, in a different room, more than 15 feet away, etc. In these configurations, the actuator  110  might only have access to wires specifically dedicated to communication and not have access to wires specifically dedicated to providing line power. 
     Prior to this disclosure, the actuator  110  would require an external power source, such as a line voltage connection, a USB cable, a plug into a wall outlet, and so forth. However, requiring an external power source would cause users to need to wire the actuator  110  to both the boiler control  310  and to the external power source separately. This not only led to wiring errors due to the number of connections that needed to be made, but often prevented users from locating the actuator  110  in an ideal position, opting instead to locate the actuator  110  close to the external power source. From a technical perspective, the actuator  110  was completely dependent upon the external power source, thus, when power outages occurred, the actuator  110  would lose power and the user would not be able to control the HVAC system  130  without installing backup batteries. 
     This technical problem can be further understood by examining one of the more popular 2-wire protocols that can be used to communicate between the boiler control  310  and the actuator  110 .  FIG.  5    illustrates some of the operating characteristics of the OpenTherm® communication protocol. This protocol is based on two current signaling levels that are set by the boiler control  310  and two voltage signaling levels that are set by the actuator  110 . Graph  501  in  FIG.  5    illustrates the different voltage/current levels that can be used to transmit encoded digital signals over the wired communication channel  162 . Note that the voltage/current levels are specific to the OpenTherm® communication protocol, and that other protocols may use different voltage/current levels that would be well within the scope of this invention. In this protocol, the standard specifies low and high signal level ranges for both line voltage and line current, where transitions between signal levels correspond to binary data encodings. In some embodiments, the boiler control  310  may include a current source that can be used to set the line current on the wired communication channel  162 . When outputting a low-current level, the line current can reside in state  502  or  506  in graph  501 . When outputting a high-current level, the line current can reside in state  504  or  508  in graph  501 , corresponding to between 17 mA and 23 mA. The actuator  110  can read the value of the current that is set by the boiler control  310 , and interpret the state transitions to decode a binary message from the boiler control  310 . 
     In a similar fashion, the actuator  110  can control the voltage on the wired communication channel  162  in order to send encoded binary communications to the boiler control  310 . For example, the actuator  110  may include a current sink that can be used to set the voltage on the wired communication channel  162 . A low-voltage level can fall within state  502  or state  504  in graph  501  corresponding to a voltage range of between 0 V and 8 V. A high voltage-level can fall within state  506  or  508  on graph  501  corresponding to a voltage range of between 15 V and 18 V. This system allows the wired communication channel  162  to function by sending current signals from the boiler control  310  to the actuator  110 , and voltage signals in the reverse direction from the actuator  110  to the boiler control  310 . 
     In some embodiments, the physical idle state of the line may be a low-power consumption state using low voltage and low current, such as state  502 . Some embodiments may hold the line current low when the voltage is used by the actuator  110  for signaling to reduce power consumption. Other embodiments may use state  508  in a high-power mode that allows the current to also be in a high state during voltage transitions. 
     These current and/or voltage ranges and requirements present a technical problem when it comes to powering the actuator  110  using the wired communication channel  162 . Specifically, draining any power from the wired communication channel  162  would normally affect either the line current, the line voltage, or both. Interfering with any of these parameters may cause the boiler control  310  and/or the actuator  110  to misinterpret a communication signal, to detect a false communication signal, or to miss a communication signal altogether. For example, some communication protocols may use Manchester encoding or other binary encodings that detect signal transitions representing encoded binary data. Graph  521  illustrates an example of such encoding where high-to-low transitions are interpreted as a logical “1” and low-to-high transitions are interpreted as a logical “0”. Assuming that the actuator  110  begins stealing power at time  522  in graph  521 , this may cause the voltage level to decrease below a lower threshold of the communication protocol, which may in turn appeared to be a logical “0” to the boiler control  310 . This may result in inadvertently activating or deactivating functions of the HVAC system  130 , which may result in user discomfort, wasted energy, or other similar problems. 
     Additionally, graph  511  illustrates the timing requirements for rise and fall times of signals on the wired communication channel  162 . In this example, a maximum rise time  512  and a maximum fall time  514  may be approximately 50 μs with a typical value of around 20 μs. Simply tacking on a traditional power stealing circuit to the wired communication channel  162  would necessarily add additional capacitance to the channel, which will necessarily affect the rise and fall times of signals on the wired communication channel  162 . Transition times that take too long may cause the actuator  110  and/or the boiler control  310  to miss a signal transition. These requirements and considerations illustrate that any power-stealing circuit that is designed to steal power from the boiler control  312  over the wired communication channel  162  should not interfere with the communication signal levels or dynamic timing characteristics of a 2-wired communication protocol. 
     The embodiments described herein solve these technical problems and improve the technologies of HVAC communication, device powering, discrete circuit design, battery charging, and low-power communication. These embodiments provide a method to supply a boiler control  310  (master) with power from an actuator or boiler control unit (slave), by maximizing its supplied current while providing master/slave communication without interfering with the chosen protocol.  FIG.  6    illustrates a block diagram of a system for stealing power from a wired communication channel  162  without interfering with a communication protocol, according to some embodiments. This simplified block diagram illustrates the functions of different circuit blocks that can be implemented using a variety of different discrete circuit components, microcontrollers, and other digital/analog circuit elements. Some sample circuit implementations are also described in greater detail below. 
     The boiler controller  310  may be connected to the actuator  110  through a 2-wire connection using, for example, the OpenTherm communication protocol. Although the example of the boiler controller  310  is used for illustrative purposes in  FIG.  6   , other embodiments may extend to other types of systems. Therefore, the boiler control  310  may be replaced with any external system that communicates using a defined communication protocol where power is harvested or stolen from the communication wire connectors. The boiler control  310  may be replaced with any external system, such as an environmental system for controlling an environmental parameter, and other smart-home device, a smart appliance, a computer system, and electrical system, and so forth. 
     At a top level, some embodiments may be divided into a first stage  620  and the second stage  622 . The first stage  620  may include a switchable OT voltage regulator  602  that acts as a voltage clamp. Specifically, the voltage regulator  602  of the first stage  620  can switch to multiple voltage levels while maintaining a constant current. For example, the voltage regulator  602  of the first stage  620  may support at least two—and in some embodiments more—different output voltage levels that can be programmed to correspond to the voltage level requirements of the particular communication protocol. An input from a controller on the actuator  110 , such as a microprocessor or microcontroller, can provide a transmit input  612  to the voltage regulator  602 . This transmit input  612  can be used to switch and/or control the voltage level output of the voltage regulator  602 . Thus, the first state  620  can use the voltage regular  602  to translate the transmit input  612  to the appropriate voltage level required for the communication protocol. Additionally, the first stage  620  can feed the vast majority of the current received from the wired communication channel into the second stage  622 . In some embodiments, the voltage regulator  602  of the first stage  620  may require just a fraction of the received current, in some cases as little as between 1 μA and 20 μA to maintain regulation. Additionally, the first stage  620  shields the voltage transitions of the voltage regulator  602  from the parasitic capacitance of the rest of the circuit that may be added in the subsequent discussion. 
     The power-stealing communication circuit  182  of the actuator  110  may additionally comprise a second stage  622  in some embodiments. The second stage  622  may include a current monitor circuit  606  and a second voltage regulator  608 . The current monitor  606  can produce a signal proportional to the current received by the wired communication channel. This signal can be compared to thresholds that are defined according to the specification of the communication protocol, and the digital receive signal  604  can be routed to a controller for the actuator  110 , such as a microprocessor or microcontroller to interpret the digital communication. In some embodiments, the current monitor  606  may include a resistor and a high-side current sense amplifier that is fed into an analog-to-digital converter (ADC) or one or more comparators to decode the received signal  604 . The current can be measured effectively by clamping the voltage using the first voltage regulator  602  and ensuring that nearly all of the current received from the boiler control  310  is routed through the current monitor  606 . In one example, a small resistance value (e.g., 1 ohm) can be placed in series between the first stage  620  and the second stage  622 , and the resulting voltage drop across the small resistance value can be measured using the ADC/comparators to read the current value transmitted by the boiler control  310 . Depending on the range of the current value, the current monitor  606  can generate a binary 0/1 encoding to be sent to or determined by the controller of the actuator  110 . Placing the current monitor circuit  606  in the location depicted in  FIG.  6    minimizes the current diverted from the measured signal. This also simplifies compensation with the comparison thresholds used to generate the receive signal  604 . 
     Prior to this disclosure, diverting current for the power stealing function was very difficult to compensate for when generating the received signal  604  and sending a clear transmit signal  612  and led to communication interference. Specifically, stealing current away from the receive signal  604  to charge/power the actuator  110  would necessarily change the current of the received signal  604 , possibly causing the processor of the actuator  110  to misinterpret the digital communication. To compensate for this, the second stage  622  may include a second voltage regulator  608  that provides the system power input and isolates the power stealing function from the communication function. 
     In some embodiments, the second voltage regulator  622  may either follow a scaled-down reference from the first stage  620  or the transmit input  612  can also be routed into the second voltage regulator  608  as an input. This allows the second voltage regulator  608  to know the operating voltage range of the first voltage regulator  602 . This also allows the power-stealing communication circuit  182  to steal the maximum amount of power when the voltage level set by the first voltage regulator  602  switches to a higher voltage output. Without providing this real-time feedback of the output voltage of the first voltage regulator  602 , the second stage  622  would have to operate at a constant level compatible with the lowest voltage output, thereby wasting the increased power that could be stolen or fed into the system power converter during high voltage output intervals. Allowing the second stage regulator to track the first stage regulator maximizes the power output from the current monitor  606  into the system power converter  610 , which maximizes the current stolen by the actuator  110  to power its internal systems and/or charge an energy storage element. 
     The system power converter  610  converts the output of the second stage  622  into a system voltage used by the actuator  110  as the system power  614 . The system power converter  610  may include a voltage regulator that supports an input current limiter  616  that is controlled by the second voltage regulator  608  of the second stage  622  through a current limit output/signal  624 . The current limit output/signal  624  may indicate a dropout condition in the second voltage regulator  608 . When the second voltage regulator  608  enters a dropout condition where it can no longer provide a regulated output because the power converter  614  is stealing a maximum amount of current, the current limit signal  624  can cause the input current limit of the system power converter  610  to decrease, thereby relaxing the dropout condition. With enough load demand, the system power converter  610  will therefore always pull the maximum current amount out of the second stage  622 , keeping the second regulator  608  operating at the edge of its dropout condition. Some embodiments may further include an energy storage circuit in the system power block  614  that can max out the load demand. For example, this energy storage circuit may include a charger for a secondary battery, a super capacitor, and/or the like. 
     Based on the design describe above, an input current that is varied by the boiler control  310  can be traced through the power-stealing communication circuit  182  depicted in  FIG.  6   . If the boiler control  310  increases the current on the wired communication channel, the first voltage regulator  602  can maintain the current transmit voltage and cause the additional current to flow into the current monitor  606  of the second stage  622 . This also lifts the second voltage regulator  608  out of its dropout condition, thereby allowing the system power converter  610  to increase its current limit. This increase will continue until all of the available current feeds into the system power converter  610  and the second voltage regulator  608  again enters the dropout condition. The current limit signal  624  may reflect the current dropout state of the second voltage regulator, and the current limiter  616  of the system power converter  610  will adjust the input current pulled by the system power converter  610  accordingly. If the actuator  110  changes the voltage on the wired communication channel from, for example, a low voltage to a high voltage, this will change the output of the first stage  620  and the second stage  622  simultaneously. Consequently, the second voltage regulator  608  will enter its dropout condition, as it requires additional current to charge its input to the new target voltage. This in turn may reduce the input current of the system power converter  610  during this transient period. 
     If the system power converter  610  cannot consume all of the available current, the second stage  622  can sink any residual current after it flows through the current monitor  606  and the receive input  604  controls the proper current amount. For example, the second voltage regulator  608  can sink any residual current through a resistive pathway  625  to ground. Because this current is simply converted to heat, the embodiments described above provide a distinct technical advantage by limiting the amount of wasted current, unnecessary heat generation, and maximizing the power used by the actuator  110 . Using both the first voltage regulator  602  and the second voltage regulator  608  also minimizes capacitance constraints. For example, the protection provided by the first voltage regulator  602  can allow a large input capacitance to be used on the system power converter  610  without slowing the rise/fall times that are mandated by the communication protocol. 
       FIG.  7    illustrates a circuit-level implementation of an input circuit, according to some embodiments. In this implementation, a two-wire protocol can be used, and the current/voltage signals can be transmitted across a pair of HVAC wires  704  that run between the boiler control  310  and the actuator  110 . The HVAC wires  704  may be polarity-agnostic wires connected to an input port on the actuator  110 . The input port  702  may include a plurality of wiring connectors that are configured to receive a plurality of wires. Any type of physical wiring connector may be used. Once received, the signals from the wires  704  can be run through a bridge rectifier  708  to rectify the signal. In some embodiments, a diode bridge rectifier  708 - 1  may be used. In some embodiments, a FET bridge rectifier  708 - 2  may instead be used. The FET bridge rectifier  708 - 2  may result in more efficient power stealing by removing the diode voltage drops from the diode bridge rectifier  708 - 1 . The FET bridge rectifier  708 - 2  may work with voltages below the V gs(max)  of the FETs. For voltages that are likely to exceed the V gs(max)  of the FETs, the FET V gs  can be clamped to prevent damage the FETs using the Zener-resistor network to clamp the gates and sources of the FETs in the alternate FET bridge rectifier  708 - 3 . Some embodiments may also include a fuse  706  to protect against over-current anomalies. The resulting signal in the circuit in  FIG.  7    is fed into the first/second stage  801  of the circuit described below. 
       FIG.  8    illustrates a circuit-level implementation of a first power stealing communication circuit  182 , according to some embodiments. The input  701  from the circuit of  FIG.  7    can first be fed into a current monitor  814  that will be described in greater detail below in  FIG.  9   . The first voltage regulator of the first stage described above can be implemented using a pair of Zener diode clamps  804 . A first diode clamps  804 - 1  can be selected to be within the lower voltage range of the communication protocol (e.g., less than 8 V for OpenTherm), and a second diode clamp  804 - 2  can be selected to be within the upper voltage range of the communication protocol. The diode clamp to be activated can be selected using the transmit control  612  provided from the processor of the actuator  110 . 
     A transistor  812  can act as a shunt regulator that is controlled by the voltage dictated by the Zener diode clamps  804 . The output  850  is sent to a low-dropout (LDO) voltage regulator to generate the regulated voltage necessary for the microprocessor to run (e.g., 1.8 V). The first and second stage voltage regulators from  FIG.  6    can be implemented as shunt regulators using the PNP bipolar transistor  812  and the selected Zener diode clamps  804  for base voltage regulation. The dropout voltage signal for an input-current regulated DC/DC converter can be generated by a shunt resistor in the collector path of the transistor  812  of the second stage. For both voltage regulators, the Zener diodes  804  can be switched according to the transmit input  612  to provide the correct input voltage supply levels. This embodiment does not require a secondary DC/DC converter. Instead, the LDO described below has a very low current output by design to ensure that the clamps  804  are able to continuously regulate voltage. 
     In some embodiments, a low-power microcontroller can also be included within the power domain of the power stealing circuit. This low-power microcontroller can have an operating current of less than 2 mA. This low level of power usage ensures that the low-power microcontroller will always operate below the current threshold of the communication protocol. Therefore, the low-power microcontroller can always be guaranteed operate so long as the boiler control  310  provides a current signal to the actuator  110 . The low-power microcontroller can monitor the power stealing circuitry and provide keep alive messages for the communication channel. Some embodiments may also provide an alternate power path that bypasses the circuit described above in  FIG.  8    to provide auxiliary power when not receiving a current from the boiler control  310 . For example, the actuator  110  may include backup batteries and/or a USB interface that can be used to provide external or auxiliary power in addition to the power stolen from the boiler control  310 . 
       FIG.  9    illustrates a circuit-level implementation of the current monitor circuit  814 , according to some embodiments. The current monitor  814  may include a resistor  904  (e.g., 1 ohm) through which the input current flows in series. The value of the resistor  904  should be selected to be a relatively small value such that only a small amount of voltage drop budget is used by the current monitor  814 . In this embodiment, a current-sense amplifier  906  can be used to provide precision current measurement, the output of which can be sent to the microcontroller  902  for the actuator  110 . The voltage differential across the resistor  904  can be fed as positive and negative inputs into the current-sense amplifier  906 . The amplified signal can then be sent to an onboard ADC on the processor  902  and decoded to read the binary data encoded in the current flowing into the actuator  110 . In other embodiments, the signal can be decoded before reaching the processor  902  using voltage references and/or comparator circuits. Depending on the particular communication protocol used, the processor  902  can then detect level transitions (e.g., between 9 mA and 17 mA) to decode the binary data. 
       FIG.  10    illustrates a circuit-level implementation of a first stage of a power stealing communication circuit  182 , according to some embodiments. This circuit may correspond to the first stage  620  depicted in  FIG.  6    above. The input  710  may be received from the input circuit of  FIG.  7   . The first stage  620  can be implemented using a low-voltage, adjustable precision shunt regulator  1004 , such as the TLV431x available from Texas Instruments®. This shunt regulator provides a 3-terminal adjustable voltage reference with a selectable input. The regulator  1004  can drive a bipolar transistor  1014  to implement the regulation function. The regulated voltage can be selected using the transmit input  612  from the microcontroller. The transmit input  612  can be used to drive a FET  1002 . When the FET  1002  is off, the voltage divider using resistor  1012  will determine the voltage for the regulator  1004 . When the FET is on, the voltage divider may then connect resistor  1010  in parallel with resistor  1012 , changing the value of the voltage divider, and changing the value of the voltage regulated by the regulator  1004 . The values for resistor  1010  and resistor  1012  can be selected based on the desired voltage and the particular brand of regulator  1004  used in the circuit. The output from the first stage  620  can be fed into the current monitor  814  depicted above in  FIG.  8   , and then fed into a second stage  622  described below. 
       FIG.  11    illustrates a circuit-level implementation of a second stage of a power stealing communication circuit  182 , according to some embodiments. This circuit may correspond to the second stage  622  depicted in  FIG.  6    above. A second low-voltage, adjustable precision shunt regulator  1102  may also be included in the second stage  622 . This regulator  1102  operates in much the same fashion as the first shunt regulator  1004  in  FIG.  10   . For example, the transmit input  612  can selectively connect a resistor  1112  in parallel with a second resistor  1110  to adjust feedback to the regulator  1102  and control its output. When the transmit input  612  is in a first state, the voltage divider includes only resistor  1110 , while when the transmit input  612  is in a second state, the voltage divider includes resistor  1110  in parallel with resistor  1112 . This allows the second stage  622  to follow the first stage  620  simultaneously when the transmit input  612  changes. 
     During operation, the load output of the current monitor  814  will initially go high. This will raise the voltage on the top input of the regulator  1102  and of the transistor  1104 , causing the transistor  1104  to begin conducting. When the transistor  1104  begins conducting, most of the current received from the current monitor  814  will be shunted through the shunt resistor  1114 . As the current through the shunt resistor  1114  increases, the voltage at node  1118  will increase. Note some embodiments may include a diode  1116  to add a minimum voltage drop at node  1118 . The voltage at node  1118  is fed into the current limiter  616  of the system power converter  610 . As will be described below, as this input to the current limiter  616  increases, the system power converter  610  will increase the amount of current it steals from the second stage  622 . Specifically, the system power converter  610  will continue increasing the amount of current that it steals until the voltage at node  1118  begins to decrease. 
     When a maximum amount of power is stolen from the second stage  622  by the system power converter  610 , the regulator  1102  will begin to drop out and no longer regulate a voltage sufficient to cause the transistor  1104  to conduct. As the transistor  1104  stops conducting (i.e., drops out), the voltage on node  1118  sent to the current limiter  616  will drop. This in turn will cause the current limiter  616  to decrease the amount of current stolen by the system power converter  610 . As the current stolen by the system power converter  610  decreases, the regulator  1102  will come out of its dropout mode and again regulate a voltage to cause the transistor  1104  to conduct. This will again cause the voltage on node  1118  to rise, and allow the system power converter  610  to increase the amount of current it pulls at its input. This cycle will continue, keeping the regulator  1102  operating on the edge of its dropout mode. This forms a feedback loop that ensures that the system power converter  610  steals as much power as possible from the second stage  622  without affecting the current/voltage used in the 2-wired communication protocol. This feedback loop process may also be referred to as optimizing an amount of power stolen, where the stolen power can be used to power the actuator and/or charge a rechargeable energy device. 
       FIG.  12    illustrates a simplified circuit diagram of the system power converter  610 , according to some embodiments. The system power converter  610  may use a PWM step-down DC/DC converter  1202 , such as LT1936 from Linear Technology®. The Vin input can deliver the current from the second stage  622  to the internal regulator of the converter  1202 . The SHDN pin may be used to put the converter  1202  into a shutdown mode. This feature can be disabled by tying the SHDN pin to the Vin pin. Similarly, the COMP can deactivate an internal compensation network by tying the COMP pin to GND. The VC pin is used to compensate the control loop of the converter  1202  by tying an external RC network from this pin to GND. An amplifier and comparator monitor the current flowing between the Vin and SW pins, turning the switch off when this current reaches a level determined by the voltage at the VC pin. An error amplifier measures the output voltage through an external resistor divider tied to the FB pin and servos the VC pin. If the error amplifier&#39;s output increases, more current is delivered to the output; if it decreases, less current is delivered. Thus, connecting the feedback loop from the second stage  622  to the VC pin of the converter  1202  in the system power converter  610  can control the amount of current pulled by the Vin input of the converter  1202 . The bank of capacitors  1206  represents the power output that can be sent to the rest of the actuator system. 
     In the foregoing description, for the purposes of explanation, numerous specific details were set forth in order to provide a thorough understanding of various embodiments of the present invention. It will be apparent, however, to one skilled in the art that embodiments of the present invention may be practiced without some of these specific details. In other instances, well-known structures and devices are shown in block diagram form. 
     The foregoing description provides exemplary embodiments only, and is not intended to limit the scope, applicability, or configuration of the disclosure. Rather, the foregoing description of the exemplary embodiments will provide those skilled in the art with an enabling description for implementing an exemplary embodiment. It should be understood that various changes may be made in the function and arrangement of elements without departing from the spirit and scope of the invention as set forth in the appended claims. 
     Specific details are given in the foregoing description to provide a thorough understanding of the embodiments. However, it will be understood by one of ordinary skill in the art that the embodiments may be practiced without these specific details. For example, circuits, systems, networks, processes, and other components may have been shown as components in block diagram form in order not to obscure the embodiments in unnecessary detail. In other instances, well-known circuits, processes, algorithms, structures, and techniques may have been shown without unnecessary detail in order to avoid obscuring the embodiments. 
     Also, it is noted that individual embodiments may have been described as a process which is depicted as a flowchart, a flow diagram, a data flow diagram, a structure diagram, or a block diagram. Although a flowchart may have described the operations as a sequential process, many of the operations can be performed in parallel or concurrently. In addition, the order of the operations may be re-arranged. A process is terminated when its operations are completed, but could have additional steps not included in a figure. A process may correspond to a method, a function, a procedure, a subroutine, a subprogram, etc. When a process corresponds to a function, its termination can correspond to a return of the function to the calling function or the main function. 
     The term “computer-readable medium” includes, but is not limited to, portable or fixed storage devices, optical storage devices, wireless channels and various other mediums capable of storing, containing, or carrying instruction(s) and/or data. A code segment or machine-executable instructions may represent a procedure, a function, a subprogram, a program, a routine, a subroutine, a module, a software package, a class, or any combination of instructions, data structures, or program statements. A code segment may be coupled to another code segment or a hardware circuit by passing and/or receiving information, data, arguments, parameters, or memory contents. Information, arguments, parameters, data, etc., may be passed, forwarded, or transmitted via any suitable means including memory sharing, message passing, token passing, network transmission, etc. 
     Furthermore, embodiments may be implemented by hardware, software, firmware, middleware, microcode, hardware description languages, or any combination thereof. When implemented in software, firmware, middleware or microcode, the program code or code segments to perform the necessary tasks may be stored in a machine readable medium. A processor(s) may perform the necessary tasks. 
     In the foregoing specification, aspects of the invention are described with reference to specific embodiments thereof, but those skilled in the art will recognize that the invention is not limited thereto. Various features and aspects of the above-described invention may be used individually or jointly. Further, embodiments can be utilized in any number of environments and applications beyond those described herein without departing from the broader spirit and scope of the specification. The specification and drawings are, accordingly, to be regarded as illustrative rather than restrictive. 
     Additionally, for the purposes of illustration, methods were described in a particular order. It should be appreciated that in alternate embodiments, the methods may be performed in a different order than that described. It should also be appreciated that the methods described above may be performed by hardware components or may be embodied in sequences of machine-executable instructions, which may be used to cause a machine, such as a general-purpose or special-purpose processor or logic circuits programmed with the instructions to perform the methods. These machine-executable instructions may be stored on one or more machine readable mediums, such as CD-ROMs or other type of optical disks, floppy diskettes, ROMs, RAMs, EPROMs, EEPROMs, magnetic or optical cards, flash memory, or other types of machine-readable mediums suitable for storing electronic instructions. Alternatively, the methods may be performed by a combination of hardware and software.