Abstract:
A method of harvesting power from a heating, ventilation, and air conditioning (HVAC) system using an HVAC controller may include harvesting power from the HVAC system and measuring an electrical characteristic of the HVAC controller. The electrical characteristic may indicate whether the power harvested from the HVAC system is high enough to risk interfering with a normal operation of the HVAC system. The method may also include increasing the power harvested from the HVAC system until the electrical characteristic indicates that the HVAC controller is at risk of interfering with the normal operation of the HVAC system.

Description:
CROSS-REFERENCE To RELATED APPLICATIONS 
       [0001]    This patent application is a continuation of U.S. Ser. No. 13/267,871 filed on Oct. 6, 2011, which is incorporated by reference herein. U.S. Ser. No. 13/267,871 claims the benefit of U.S. Prov. Ser. No. 61/415,771 filed Nov. 19, 2010 and U.S. Prov. Ser. No. 61/429,093 filed Dec. 31, 2010, each of which is incorporated by reference herein. 
         [0002]    The subject matter of this patent specification also relates to the subject matter of the following commonly assigned applications: U.S. Ser. No. 12/881,430 filed Sep. 14, 2010; U.S. Ser. No. 12/881,463 filed Sep. 14, 2010; U.S. Ser. No. 12/984,602 filed Jan. 4, 2011; U.S. Ser. No. 12/987,257 filed Jan. 10, 2011; U.S. Ser. No. 13/033,573 and U.S. Ser. No. 29/386,021, filed Feb. 23, 2011; U.S. Ser. No. 13/034,666, U.S. Ser. No. 13/034,674 and U.S. Ser. No. 13/034,678 filed Feb. 24, 2011; U.S. Ser. No. 13/038,206 and U.S. Ser. No. 13/038,191filed Mar. 1, 2011; and U.S. Ser. No. 13/199,108, filed Aug. 17, 2011. Each of the above-referenced patent applications is incorporated by reference herein. The above-referenced patent applications are collectively referenced hereinbelow as “the commonly assigned incorporated applications.” 
     
    
     FIELD 
       [0003]    This invention relates generally to the monitoring and control of HVAC systems and/or for other systems for controlling household utilities, and/or resources. More particularly, embodiments of this invention relate facilitating power stealing or power harvesting in a control device such as a thermostat having a rechargeable battery. 
       BACKGROUND 
       [0004]    Thermostats having electronics, such as programmable thermostats, may rely on an independent power source, such as a disposable battery. However, a disposable battery eventually needs to be replaced by the user. Electronic thermostats can also be powered directly from an HVAC system transformer such as using a 24VAC “common” wire (“C-wire”) from the transformer, but only if one is available. When provided, the C wire has the particular purpose of supplying power for an electronic thermostat. However, many HVAC installations do not have a C-wire provided to the thermostat. For such cases, many electronic thermostats have been designed to extract power from the transformer from the circuit used to turn on and off the HVAC function which is called “power stealing,” “power sharing” or “power harvesting.” The thermostat “steals,” “shares” or “harvests” its power during the “OFF” or “inactive” periods of the heating or cooling system by allowing a small amount of current to flow through it into the load coil below its response threshold (even at maximum transformer output voltage). During the “ON” or “active” periods of the heating or cooling system the thermostat can be designed to draw power by allowing a small voltage drop across itself. Hopefully, the voltage drop will not cause the load coil to dropout below its response threshold (even at minimum transformer output voltage). Examples of thermostats with power stealing capability include the Honeywell T8600, Honeywell T8400C, and the Emerson Model 1F97-0671. Some thermostats are designed to be operable for a variety of power sources. For example, U.S. Pat. No. 6,886,754 discusses a thermostat operable from battery, common-wire or power-stealing depending upon the type of HVAC system the thermostat is installed with. 
         [0005]    However, a trade-off inherently exists between drawing enough power during power stealing so as to provide adequate power for thermostat operation and drawing too much power such that the power stealing causes false switching: where the HVAC function unintentionally turns on, or activates, or unintentionally turns off. Determining how much current to draw during power stealing is also complicated by the wide variety of HVAC systems with which the thermostat may be installed, as well as the desire to design a thermostat that is relatively easy to install. 
         [0006]    Some attempts have been made to decrease the power usage of the thermostat. For example, U.S. Pat. No. 7,755,220 discusses power stealing for a thermostat using a triac with FET control. The discussed electronic thermostat circuit topology attempts to minimize the current needed to control the thermostat outputs. However, for advanced and user-friendly functions that consume more power, greater amounts of power will need to be obtained through power stealing. 
       SUMMARY 
       [0007]    According to some embodiments a method is described for harvesting power from an HVAC triggering circuit that includes a call switch (e.g. a relay) to turn on and turn off an HVAC function (such as heating or cooling). A thermostat connected to the HVAC triggering circuit uses the harvested power. The method includes making at least one measurement that is at least partially influenced by the amount of electrical current flowing through the call switch; and controlling the amount of power the thermostat harvests from the HVAC triggering circuit based at least in part on the measurement so as to reduce a likelihood of inadvertently switching the HVAC function on or off 
         [0008]    According to some embodiments, a second measurement is also made when a different amount of electrical current is flowing through the call switch such that a relationship between harvested current and voltage drop can be estimated. The amount of power the thermostat harvests can include selecting from two or more predetermined levels of harvesting current drawn by the thermostat. According to some embodiments, subsequent measurements can be made and the amount of current the thermostat harvests from the HVAC triggering circuit can be adjusted according to the subsequent measurements. 
         [0009]    According to some embodiments, the described method occurs when the HVAC function is active, inactive, or both. 
         [0010]    According to some embodiments, the power harvesting causes a drop in voltage across the call switch, the thermostat controls the current draw such that the harvesting results in the drop in voltage across the call switch to be less than about 8V RMS . 
         [0011]    According to some embodiments, a thermostat is described for harvesting power from an HVAC triggering circuit that includes a call switch to turn on and turn off an HVAC function. The thermostat includes power stealing circuitry adapted and configured to harvest power from the HVAC triggering circuit; voltage measurement circuitry adapted and configured to make at least one electrical measurement that is at least partially influenced by the amount of electrical current flowing through the call switch; and processing and control circuitry adapted and configured to control an amount of power that the thermostat harvests from the HVAC triggering circuit based at least in part on the measurement so as to reduce a likelihood of inadvertently switching the HVAC function on or off. 
         [0012]    As used herein the term “HVAC” includes systems providing both heating and cooling, heating only, cooling only, as well as systems that provide other occupant comfort and/or conditioning functionality such as humidification, dehumidification and ventilation. 
         [0013]    As used herein the terms power “harvesting,” “sharing” and “stealing” when referring to HVAC thermostats all refer to the thermostat are designed to derive power from the power transformer through the equipment load without using a direct or common wire source directly from the transformer. 
         [0014]    As used herein the term “residential” when referring to an HVAC system means a type of HVAC system that is suitable to heat, cool and/or otherwise condition the interior of a building that is primarily used as a single family dwelling. An example of a cooling system that would be considered residential would have a cooling capacity of less than about 5 tons of refrigeration (1 ton of refrigeration=12,000 Btu/h). 
         [0015]    As used herein the term “light commercial” when referring to an HVAC system means a type of HVAC system that is suitable to heat, cool and/or otherwise condition the interior of a building that is primarily used for commercial purposes, but is of a size and construction that a residential HVAC system is considered suitable. An example of a cooling system that would be considered residential would have a cooling capacity of less than about 5 tons of refrigeration. 
         [0016]    As used herein the term “thermostat” means a device or system for regulating parameters such as temperature and/or humidity within at least a part of an enclosure. The term “thermostat” may include a control unit for a heating and/or cooling system or a component part of a heater or air conditioner. As used herein the term “thermostat” can also refer generally to a versatile sensing and control unit (VSCU unit) that is configured and adapted to provide sophisticated, customized, energy-saving HVAC control functionality while at the same time being visually appealing, non-intimidating, elegant to behold, and delightfully easy to use. 
         [0017]    It will be appreciated that these systems and methods are novel, as are applications thereof and many of the components, systems, methods and algorithms employed and included therein. It should be appreciated that embodiments of the presently described inventive body of work can be implemented in numerous ways, including as processes, apparata, systems, devices, methods, computer readable media, computational algorithms, embedded or distributed software and/or as a combination thereof. Several illustrative embodiments are described below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    The inventive body of work will be readily understood by referring to the following detailed description in conjunction with the accompanying drawings, in which: 
           [0019]      FIG. 1  is a diagram of an enclosure in which environmental conditions are controlled, according to some embodiments; 
           [0020]      FIG. 2  is a diagram of an HVAC system, according to some embodiments; 
           [0021]      FIGS. 3A-B  illustrate a thermostat having a user-friendly interface, according to some embodiments; 
           [0022]      FIG. 4  illustrates a thermostat having a head unit and a backplate (or wall dock) for ease of installation, configuration and upgrading, according to some embodiments; 
           [0023]      FIGS. 5A-B  are block diagrams showing a thermostat wired to an HVAC system, according to some embodiments; 
           [0024]      FIG. 6  is a block diagram showing thermostat electronics for adaptive power stealing, according to some embodiments; 
           [0025]      FIG. 7  is a flow chart showing steps in a two-level adaptive power-stealing thermostat, according to some embodiments; and 
           [0026]      FIG. 8  is a flow chart showing steps in a multi-level or continuous adaptive power-stealing thermostat, according to some embodiments. 
       
    
    
     DETAILED DESCRIPTION 
       [0027]    A detailed description of the inventive body of work is provided below. While several embodiments are described, it should be understood that the inventive body of work is not limited to any one embodiment, but instead encompasses numerous alternatives, modifications, and equivalents. In addition, while numerous specific details are set forth in the following description in order to provide a thorough understanding of the inventive body of work, some embodiments can be practiced without some or all of these details. Moreover, for the purpose of clarity, certain technical material that is known in the related art has not been described in detail in order to avoid unnecessarily obscuring the inventive body of work. 
         [0028]      FIG. 1  is a diagram of an enclosure in which environmental conditions are controlled, according to some embodiments. Enclosure  100 , in this example is a single-family dwelling. According to other embodiments, the enclosure can be, for example, a duplex, an apartment within an apartment building, a light commercial structure such as an office or retail store, or a structure or enclosure that is a combination of the above. Thermostat  110  controls HVAC system  120  as will be described in further detail below. According to some embodiments, the HVAC system  120  is has a cooling capacity less than about 5 tons. According to some embodiments, a remote device  112  wirelessly communicates with the thermostat  110  and can be used to display information to a user and to receive user input from the remote location of the device  112 . 
         [0029]      FIG. 2  is a diagram of an HVAC system, according to some embodiments. HVAC system  120  provides heating, cooling, ventilation, and/or air handling for the enclosure, such as a single-family home  100  depicted in  FIG. 1 . The system  120  depicts a forced air type heating system, although according to other embodiments, other types of systems could be used. In heating, heating coils or elements  242  within air handler  240  provide a source of heat using electricity or gas via line  236 . Cool air is drawn from the enclosure via return air duct  246  through filter  270 , using fan  238  and is heated heating coils or elements  242 . The heated air flows back into the enclosure at one or more locations via supply air duct system  252  and supply air grills such as grill  250 . In cooling, an outside compressor  230  passes gas such as Freon through a set of heat exchanger coils to cool the gas. The gas then goes to the cooling coils  234  in the air handlers  240  where it expands, cools and cools the air being circulated through the enclosure via fan  238 . According to some embodiments a humidifier  254  is also provided. Although not shown in  FIG. 2 , according to some embodiments the HVAC system has other known functionality such as venting air to and from the outside, and one or more dampers to control airflow within the duct systems. The system is controlled by algorithms implemented via control electronics  212  that communicate with a thermostat  110 . Thermostat  110  controls the HVAC system  120  through a number of control circuits. Thermostat  110  also includes a processing system  260  such as a microprocessor that is adapted and programmed to controlling the HVAC system and to carry out the techniques described in detail herein. 
         [0030]      FIGS. 3A-B  illustrate a thermostat having a user-friendly interface, according to some embodiments. Unlike many prior art thermostats, thermostat  300  preferably has a sleek, simple, uncluttered and elegant design that does not detract from home decoration, and indeed can serve as a visually pleasing centerpiece for the immediate location in which it is installed. Moreover user interaction with thermostat  300  is facilitated and greatly enhanced over conventional designs by the design of thermostat  300 . The thermostat  300  includes control circuitry and is electrically connected to an HVAC system, such as is shown with thermostat  110  in  FIGS. 1 and 2 . Thermostat  300  is wall mounted and has circular in shape and has an outer rotatable ring  312  for receiving user input. Thermostat  300  has a large frontal display area  314 . According to some embodiments, thermostat  300  is approximately 80 mm in diameter. The outer rotating ring  312  allows the user to make adjustments, such as selecting a new target temperature. For example, by rotating the outer ring  312  clockwise, the target temperature can be increased, and by rotating the outer ring  312  counter-clockwise, the target temperature can be decreased. Within the outer ring  312  is a clear cover  314  that according to some embodiments is polycarbonate. Also within the rotating ring  312  is a metallic portion  324 , preferably having a number of windows as shown. According to some embodiments, the surface of cover  314  and metallic portion  324  form a curved spherical shape gently arcing outward that matches a portion of the surface of rotating ring  312 . 
         [0031]    According to some embodiments, the cover  314  is painted or smoked around the outer portion, but leaving a central display  316  clear so as to facilitate display of information to users. According to some embodiments, the curved cover  314  acts as a lens that tends to magnify the information being displayed in display  316  to users. According to some embodiments central display  316  is a dot-matrix layout (individually addressable) such that arbitrary shapes can be generated, rather than being a segmented layout. According to some embodiments, a combination of dot-matrix layout and segmented layout is employed. According to some embodiments, central display  316  is a backlit color liquid crystal display (LCD). An example of information is shown in  FIG. 3A , which are central numerals  320 . According to some embodiments, metallic portion  324  has number of openings so as to allow the use of a passive infrared motion sensor  330  mounted beneath the portion  324 . The motion sensor as well as other techniques can be use used to detect and/or predict occupancy, as is described further in co-pending patent application U.S. Ser. No. 12/881,430, which is incorporated by reference herein. According to some embodiments, occupancy information is used in generating an effective and efficient scheduled program. According to some embodiments, proximity and ambient light sensors  370 A and  370 B are provided to sense visible and near-infrared light. The sensors  370 A and  370 B can be used to detect proximity in the range of about one meter so that the thermostat  300  can initiate “waking up” when a user is approaching the thermostat and prior to the user touching the thermostat. Such use of proximity sensing is useful for enhancing the user experience by being “ready” for interaction as soon as, or very soon after the user is ready to interact with the thermostat. Further, the wake-up-on-proximity functionality also allows for energy savings within the thermostat by “sleeping” when no user interaction is taking place our about to take place. 
         [0032]    According to some embodiments, for the combined purposes of inspiring user confidence and further promoting visual and functional elegance, the thermostat  300  is controlled by only two types of user input, the first being a rotation of the outer ring  312  as shown in  FIG. 3A  (referenced hereafter as a “rotate ring” input), and the second being an inward push on the upper cap  308  ( FIG. 3B ) until an audible and/or tactile “click” occurs (referenced hereafter as an “inward click” input). For further details of suitable user-interfaces and related designs, which are employed, according to some embodiments, see co-pending Patent Applications U.S. Ser. No. 13/033,573 and U.S. Ser. No. 29/386,021, both filed Feb. 23, 2011, and are incorporated herein by reference. 
         [0033]    According to some embodiments, the thermostat  300  includes a processing system  360 , display driver  364  and a wireless communications system  366 . The processing system  360  is adapted to cause the display driver  364  and display area  316  to display information to the user, and to receiver user input via the rotating ring  312 . The processing system  360 , according to some embodiments, is capable of maintaining and updating a thermodynamic model for the enclosure in which the HVAC system is installed. For further detail on the thermodynamic modeling, see U.S. patent Ser. No. 12/881,463 filed, which is incorporated by reference herein. According to some embodiments, the wireless communications system  366  is used to communicate with devices such as personal computers and/or other thermostats or HVAC system components. 
         [0034]      FIG. 4  illustrates a thermostat having a head unit and a backplate (or wall dock) for ease of installation, configuration and upgrading, according to some embodiments. As is described hereinabove, thermostat  300  is wall mounted and has circular in shape and has an outer rotatable ring  312  for receiving user input. Thermostat  300  has a cover  314  that includes a display  316 . Head unit  410  of round thermostat  300  slides on to back plate  440 . According to some embodiments the connection of the head unit  410  to backplate  440  can be accomplished using magnets, bayonet, latches and catches, tabs or ribs with matching indentations, or simply friction on mating portions of the head unit  410  and backplate  440 . According to some embodiments, the head unit  410  includes a processing system  360 , display driver  364  and a wireless communications system  366 . Also shown is a rechargeable battery  420  that is recharged using recharging circuitry  422  that uses power from backplate that is either obtained via power harvesting (also referred to as power stealing and/or power sharing) from the HVAC system control circuit(s) or from a common wire, if available, as described in further detail in co-pending patent application U.S. Ser. Nos. 13/034,674, and 13/034,678, which are incorporated by reference herein. 
         [0035]    Backplate  440  includes electronics  482  and temperature sensor  484  in housing  460 , which are ventilated via vents  442 . Wire connectors  470  are provided to allow for connection to HVAC system wires. Connection terminal  480  provides electrical connections between the head unit  410  and backplate  440 . Backplate electronics  482  also includes power sharing circuitry for sensing and harvesting power available power from the HVAC system circuitry. Based on features and configurations described in one or more of the commonly assigned incorporated applications, supra, the thermostat  300  is a multi-sensing network-connected self-learning device that is pleasing both to the eye and to the touch, and furthermore is advantageous in that it can provide its rich combination of capabilities and visually pleasing user interfaces without requiring a C-wire even though the requisite underlying device hardware can require instantaneous power draws greater than power-stealing can safely provide, at least by virtue of the use of a rechargeable battery (or equivalently capable onboard power storage medium) that will recharge during time intervals in which the hardware power usage is less than what power stealing can safely provide, and that will discharge to provide the needed extra electrical power during time intervals in which the hardware power usage is greater than what power stealing can safely provide. This can be contrasted with electronic thermostats that, if they were to provide a powerful display (for example), they would require the presence of a C-wire from the HVAC system (or line power from a household 110V source such as a wall plug), or else if they were indeed equipped to power-steal from the HVAC system, will have user interface displays (for example) that are made very low-power and less visually pleasing in order to keep the thermostat&#39;s instantaneous power usage within budget power-stealing levels at all times. Although applicable in a wide variety of other scenarios, the preferred embodiments described herein can be particularly advantageous when used in a device such as the multi-sensing network-connected self-learning thermostat  300  in that the relative amount of time spent in a battery-discharging state can be decreased in many cases, by virtue of the capability of safely detecting, without damage to equipment or unintentional call relay activation, whether a higher average amount of power stealing can be achieved, and in turn the statistical amount of decreased battery-discharging can provide advantages such as increased design margins, extensions in the average service life of the rechargeable battery, and increased statistical device reliability. 
         [0036]      FIGS. 5A-B  are block diagrams showing a thermostat wired to an HVAC system, according to some embodiments.  FIG. 5A  shows an adaptive power stealing thermostat  510  wired for control to an HVAC system having two power transformers  560  and  562 . A two-transformer HVAC system is commonly found in residences and light commercial buildings in which an existing heating system was subsequently upgraded or had had an air conditioning system installed. Heat power transformer  560  converts 110 volt AC power to 24 volt AC power for the heating control circuit  564 . Similarly, cooling power transformer  562  converts 110 volt AC power to 24 volt AC power for the cooling control circuit  566 . Note that the 110 or 24 volt levels could be different, depending on the location of the building and/or what types of power is available. For example, the 110 volts could be 220 or 240 volts in some geographic locations. 
         [0037]    Relay  570  controls the gas valve for the HVAC heating system. When sufficient AC current flows through the gas valve relay  570 , gas in the heating system is activated. The gas valve relay  570  is connected via a wire to terminal  584 , which is labeled the “W” terminal, on thermostat  510 . Relay  572  controls the fan for the HVAC heating and cooling systems. When sufficient AC current flows through the fan relay  572 , the fan is activated. The fan relay  572  is connected via a wire to terminal  582 , which is labeled the “G” terminal on thermostat  510 . Contactor (or relay)  574  controls the compressor for the HVAC cooling system. When sufficient AC current flows through the compressor contactor  574 , the compressor is activated. The contactor  574  is connected via a wire to terminal  580 , which is labeled the “Y” terminal, on thermostat  510 . The heat power transformer  560  is connected to thermostat  510  via a wire to terminal  592 , which is labeled the “Rh” terminal. The cooling power transformer  562  is connected to thermostat  510  via a wire to terminal  590 , which is labeled the “Rc” terminal. Further details of HVAC wiring, switching, and power stealing that can be used in combination many of the embodiments described herein are discussed in co-pending applications U.S. Ser. No. 13/034,666, U.S. Ser. No. 13/034,674 and U.S. Ser. No. 13/034,678 which are incorporated herein by reference. 
         [0038]    Note that the thermostat  510  can also be wired to control an HVAC system having a single power transformer. In this case, relays  570 ,  572  and  574 , which control the gas valve, fan and the compressor, respectively, are all attached to a single transformer. The power transformer is connected to thermostat  510  via a single return wire (e.g. labeled “R”, “Rc” or “Rh”). 
         [0039]      FIG. 5B  shows a common alternative to a relay or contactor in HVAC systems that use a micro controller. Instead of using a relay or contactor to turn on the HVAC function (which in this case is the cooling function) a voltage drop across a resistor  578  is detected by microcontroller  576 . The micro controller  576  then activates relays or other switches according to a timing program or other program. For example, controller  576  can activate the compressor using a relay driver  577  to activate relay  579 . 
         [0040]    In power stealing by thermostat  510  when the HVAC system is in-active (i.e., the HVAC function is not being called for by the thermostat) the thermostat must harvest power from one or more of the control circuits  564  and/or  566  such that the relay or microcontroller does not “notice” the current being drawn. 
         [0041]      FIG. 6  is a block diagram showing thermostat electronics for adaptive power stealing, according to some embodiments. In the example shown, power is being harvested from the HVAC cooling circuit, although according to other embodiments the power can harvested from one or more other control circuits, such as the heating and/or fan control circuit. In accordance with the basic functionality of a thermostat, there is a switch (not shown in  FIG. 6 ) in the thermostat  510  that is disposed between nodes  580  and  590 , this switch being “open” when the HVAC cooling circuit is “inactive” (i.e., the thermostat is not calling for the HVAC cooling function) and being normally “closed” when the HVAC cooling circuit is “active” (i.e., the thermostat is calling for the HVAC cooling function). The power stealing circuit of  FIG. 6  is capable of carrying out power stealing during both the “inactive” and “active” time periods. During an “inactive” time period, i.e. when the switch (not shown) between nodes  580  and  590  is open, alternating current from the HVAC control circuit flows into the circuit of  FIG. 6  across terminals Y ( 580 ) and Rc ( 590 ) where it is first rectified, preferably full-wave, by rectifier  610 . The rectified signal passes through a slew rate limiter  612 , which is used to smooth out any current spikes. Such current spikes, if not reduced or eliminated, passing the circuitry including capacitor  630  could have undesirable effects such as activating the HVAC control relay or burning fuses. After the slew rate limiter  612  the rectified signal passes through anti surge resistor  614  and then on to high voltage buck converter  632 , which steps down the voltage to a level suitable for use by digital circuitry within the thermostat. Capacitor(s)  630  are used as a charge reservoir in between alternating current cycles when performing power stealing from the “inactive” HVAC circuit. During an “active” time period, the capacitor(s)  630  are still used as a charge reservoir, although the mechanism by which charge flows into the charge reservoir is somewhat different. In particular, during the “active” time period, i.e. when the switch (not shown) between nodes  580  and  590  is closed (or, more specifically, is effectively closed), power stealing from the HVAC control circuit is carried out by momentarily opening the switch (not shown) between nodes  580  and  590  for brief intervals, which will in turn supply a corresponding brief interval of incoming current that will serve to re-charge the capacitor(s)  630 . The time during which the switch (not shown) between nodes  580  and  590  is closed is kept short enough that the HVAC relay, contactor, or microcontroller (e.g. relay  574  or microcontroller  576  in  FIGS. 5A and 5B , respectively) do not “notice” and therefore the HVAC function is not interrupted. 
         [0042]    According to some embodiments, a voltage measurement is made by an analog-to-digital converter  620  across a voltage divider made up of resistors  622  and  624  as shown in  FIG. 6 . The voltage measurement by ADC  620 , as is described in greater detail herein, is used to select an appropriate level of current to draw during power stealing. 
         [0043]    Following the high voltage buck converter  632 , the signal passes through a low-dropout voltage regulator (LDO)  634  followed by a current controller  640 , battery charger  642  and on to other system electronics  650 . Battery charger  642  also charges the re-chargeable battery  644 . According to some embodiments, the circuitry shown within the dashed line is located on the head unit of the thermostat  510  while the remaining circuitry is located on the backplate. The current controller  640  is used to select from multiple levels of current to the battery charger  642 . According to some embodiments, the current control circuitry can selectively draw 0 mA, 8 mA, 20 mA, 40mA or 100 mA depending on conditions. According to some embodiments 100 mA is only drawn when there is a common wire (“C-wire”) power supply available. Current controller  640  can thus be used to draw an amount of current during power stealing that has been determined to be safe, based on voltage measurements made by ADC  620 , in terms of not undesirably activating the HVAC function when that function is inactive (i.e., not undesirably tripping the call relay), and not interrupting the HVAC function when that function is active (i.e., not undesirably un-tripping the call relay). According to some embodiments, two or more of the functional blocks shown in  FIG. 6  can be implemented in on the same physical chip. For example, according to some embodiments, the current controller  640  and battery charger  642  are implemented in single chip. According to some embodiments, the post-regulating LDO  634  is not necessary, and the buck converter output  632  can connect directly to the current control input  640 . 
         [0044]      FIG. 7  is a flow chart showing steps in a two-level adaptive power-stealing thermostat, according to some embodiments. In step  710  the voltage is measured when no current is being drawn by the thermostat, and the HVAC function (e.g. cooling or heating) is in-active (i.e. not being called for). According to the embodiments associated with  FIG. 6 , the voltage is measured by ADC  620  as described. However, according to other embodiments, the voltage can be measured at other locations depending upon the particular design of the power-stealing circuitry, as will be apparent to those skilled in the art. In step  712  the voltage is measured when the thermostat is drawing 20 mA of current. In the embodiments associated with  FIG. 6 , the current draw is controlled by current controller  640 . However, according to other embodiments, the current can be controlled using other means and depending upon the particular design of the power-stealing circuitry, as will be apparent to those skilled in the art. At step  714 , if the difference in voltage AV as measured in steps  710  and  712  is greater than a first threshold voltage then in step  722  the current controller  640  is programmed to draw 20 mA during power-stealing when the HVAC function is in-active. If the AV does not exceed V 1 , then in step  716  the current controller  640  is programmed to draw 40 mA during power-stealing when the HVAC function is inactive. In step  718 , if 40 mA is being drawn, the voltage is periodically measured (such by ADC  620  in  FIG. 6 ), a second voltage drop ΔV 2  is calculated between the voltage levels when no current is being drawn and when 40 mA is being drawn. In step  720 , since 40 mA is twice 20 mA, we expect the voltage difference ΔV 2  to be about twice ΔV, so the threshold V 2  is set to be about twice V 1 . Similarly, we expect the ΔV 2  to be less than a second threshold, V 2 . However, steps  718  and  720  are periodically performed, according to some embodiments, as a safety check. For example other variations in the system can affect the voltage drop, such as variations in the overall supply current (e.g. the 110VAC supply at transformer  562  in  FIG. 5 ). If, unexpectedly, ΔV 2  is greater than a second threshold, V 2 , then in step  722 , the current controller is re-programmed to draw only 20 mA. According to some embodiments, threshold V 1  could be 4V and threshold V 2  could be 8V. 
         [0045]      FIG. 8  is a flow chart showing steps in a multi-level or continuous adaptive power-stealing thermostat, according to some embodiments. The steps shown in  FIG. 7  are for a relatively simple method in which there are only two-levels of current to choose between, namely 20 mA and 40 mA, based on the voltage measurements when different known currents are being drawn. According to some embodiments, more than two levels of power stealing can be implemented as shown in  FIG. 8 . In steps  810  and  812 , just as in steps  710  and  712  of  FIG. 7 , a voltage is measured while drawing two amounts of current to establish a ΔV for a ΔI. For example, I 1  can be 0 mA as in step  710 , but according to some embodiments I 1  and I 2  can be any two different known levels of current and the measured ΔV is used to determine an estimated value in step  816  for the resistance R for the HVAC relay, contactor (such as shown in  FIG. 5A ) or detector circuit (such as shown in  FIG. 5B ) using the relationship V=IR. Using the value for R, an appropriate current draw for in-active power stealing (I inactive ) can be set which will not (or is extremely unlikely to) activate the HVAC function. 
         [0046]    According to some embodiments, the threshold value for R is not calculated, but instead a threshold ΔV is used which should not be exceeded for a known ΔI. Further, the tolerable values for ΔV depend greatly on the thermostat hardware design and particularly on the power stealing circuitry design. Preferably, the appropriate current draw is empirically determined for different measured ΔV or calculated R values by testing carried out on a wide variety of HVAC systems in which the thermostat is likely to be installed. Because of the difference in hardware designs and also the difference in locations and method of measurement, it is difficult to compare empirically determined threshold values for different thermostats. However, it has been found for residential and light commercial HVAC systems, according to some embodiments, that the equivalent voltage drop across the HVAC relay or relay equivalent (e.g. relay  574  in  FIG. 5A  or resistor  578  in  FIG. 5B , in the case of power stealing from a cooling control circuit) should be less than about 8 V RMS . Even more preferably, for residential and light commercial HVAC systems, it has been found that the equivalent voltage drop across the HVAC relay or relay equivalent should be less than about 6 V RMS  for power stealing when the HVAC function is inactive. According to a preferred embodiment, the equivalent voltage drop across the HVAC relay or relay equivalent is maintained at less than 5.5 V RMS  for power stealing when the HVAC function is inactive. 
         [0047]    In step  816 , the voltage drop is periodically measured to check if it is greater than expected, or greater than the predetermined threshold. In step  818 , if the voltage drop is greater than the expected range, then in step  820  the current draw for power stealing is decreased. On the other hand, in step  822  if the voltage drop is lower than an expected range, then greater amounts of power can be safely harvested from the HVAC control circuit without risk of triggering the HVAC relay or relay equivalent. In this case, in step  824 , the current draw for power stealing is increased. According to some embodiments, an additional step  819  can be added in cases where it is determined that even drawing current at the lowest rate still causes too great of a voltage drop. In such cases the user is alerted that to continue use of the thermostat either a common wire needs to be added, or some other wiring change such as adding a resistor on the furnace control board. 
         [0048]    Although many of the embodiments have been described with respect to controlling power harvesting while the HVAC function is in-active, according to some embodiments the same or similar techniques are used to control the amount of power harvested when the HVAC function is active, so as to reduce the likelihood of inadvertently switching the HVAC function off. For example, according to some embodiments, the control takes into account one or more factors such as the voltage drop, the rate at which the capacitor(s)  630  charges, and the voltage the capacitor(s)  630  reaches for a fixed short period of open-switch time. 
         [0049]    Although the foregoing has been described in some detail for purposes of clarity, it will be apparent that certain changes and modifications may be made without departing from the principles thereof. It should be noted that there are many alternative ways of implementing both the processes and apparatuses described herein. Accordingly, the present embodiments are to be considered as illustrative and not restrictive, and the inventive body of work is not to be limited to the details given herein, which may be modified within the scope and equivalents of the appended claims.