Abstract:
A thermostat and related methods are described for controlling one or more functions, such as heating and cooling in an HVAC. According to some embodiments the thermostat includes a switching circuit for controlling an HVAC function, where closing the switching circuit activates the HVAC function. The thermostat may also include power harvesting circuitry adapted and arranged to harvest power from the HVAC system, where during times when the HVAC function is active the switching circuit opens for a time interval. The power harvesting circuitry may harvest power from the HVAC system during the time interval, and the time interval may be short enough that the HVAC function remains activated without interruption during the time interval.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application claims priority to and is a continuation of U.S. patent application Ser. No. 13/034,678 entitled “Thermostat Battery Recharging During HVAC Function Active and Inactive States” filed on Feb. 24, 2011. 
     U.S. patent application Ser. No. 13/034,678 claims the benefit of the following commonly assigned applications: U.S. Prov. Ser. No. 61/415,771 filed Nov. 19, 2010; U.S. Prov. Ser. No. 61/429,093 filed Dec. 31, 2010. 
     The subject matter of this patent application 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. 13/034,674 filed on Feb. 24, 2011; and U.S. Ser. No. 13/034,666 filed on Feb. 24, 2011. Each of the above-referenced patent applications is incorporated by reference herein. 
    
    
     COPYRIGHT AUTHORIZATION 
     A portion of the disclosure of this patent document may contain material that is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. 
     BACKGROUND 
     This invention generally relates to control systems for heating, ventilation and air conditioning (HVAC) systems. More particularly, embodiments of this invention relate to thermostats having jumper-free designs and/or isolation circuitry. 
     As is known, for example as discussed in the technical publication No. 50-8433, entitled “Power Stealing Thermostats” from Honeywell (1997), early thermostats used a bimetallic strip to sense temperature and respond to temperature changes in the room. The movement of the bimetallic strip was used to directly open and close an electrical circuit. Power was delivered to an electromechanical actuator, usually relay or contactor in the HVAC equipment whenever the contact was closed to provide heating and/or cooling to the controlled space. Since these thermostats did not require electrical power to operate, the wiring connections were very simple. Only one wire connected to the transformer and another wire connected to the load. Typically, a 24 VAC power supply transformer, the thermostat, and 24 VAC HVAC equipment relay were all connected in a loop with each device having only two external connections required. 
     When electronics began to be used in thermostats the fact that the thermostat was not directly wired to both sides of the transformer for its power source created a problem. This meant either the thermostat had to have its own independent power source, such as a battery, or be hardwired directly from the system transformer. Direct hardwiring a “common” wire from the transformer to the electronic thermostat may be very difficult and costly. However, there are also disadvantages to using a battery for providing the operating power. One primary disadvantage is the need to continually check and replace the battery. If the battery is not properly replaced and cannot provide adequate power, the electronic thermostat may fail during a period of extreme environmental conditions. 
     Since many households did not have a direct wire from the system transformer (such as a “common” wire), some thermostats have been designed to derive power from the transformer through the equipment load. The methods for powering an electronic thermostat from the transformer with a single direct wire connection to the transformer is called “power stealing” or “power sharing.” The thermostat “steals,” “shares” or “harvests” its power during the “OFF” 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” periods of the heating or cooling system the thermostat draws 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. However, these systems do not have power storage means and therefore always rely on power stealing or must use disposable batteries. 
     U.S. Pat. No. 4,174,807 discusses the use of a rechargeable battery in an autocycling control circuit for heating and/or air conditioning systems. However, the battery is only used in the event of a power failure and then only to keep the digital clock and timing sequence program from being disrupted. 
     U.S. Pat. No. 6,566,768 and U.S. Pat. No. 5,903,139 discuss the use of power stealing in combination with a capacitor used to store power. The discussed systems, however lack the ability to power steal from HVAC systems having two power transformers, such as the case when an existing HVAC heating system is subsequently upgraded to add air conditioning and a second power transformer is installed. Additionally, the discussed systems do not have the capability to charge a battery. 
     SUMMARY 
     According to some embodiments a thermostat is provided for controlling one or more HVAC functions in an HVAC system. The thermostat includes a rechargeable battery; charging circuitry adapted and arranged to recharge the battery; and control circuitry adapted and arranged to control the one or more HVAC functions using power from the rechargeable battery. According to some embodiments, the thermostat also includes power harvesting circuitry adapted and arranged to harvest power from the HVAC system in cases where no common wire is available to the thermostat, and to supply power to the charging circuit for recharging the battery. 
     The power harvesting circuitry is preferably adapted and arranged to automatically select harvesting power from a power source among two or more available power sources. According to some embodiments, the power harvesting circuitry is adapted and arranged to harvest power from a circuit of the HVAC functions during times when the HVAC functions are active or inactive. During power harvesting from a circuit of an HVAC function when the HVAC function is active, the power harvesting circuitry, according to some embodiments, repeatedly charges and discharges one or more capacitive and/or magnetic elements to store electrical energy for use by the charging circuit and/or other operations in the thermostat. The power harvesting circuitry can includes solid state switching components and circuitry adapted and arranged to quickly open and close one or more circuits controlling the one or more HVAC functions. 
     According to some embodiments, the charging circuitry is adapted to charge the battery such that longer cycle life is preferred over higher charge capacity, such as controlling the ratio of charge current to total capacity, and/or limiting the float voltage. 
     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. 
     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. 
     As used herein the term “thermostat” includes any device, instrument and/or system for controlling at least some aspect of an HVAC system. While it is very common for a thermostat to control an HVAC system primarily based on temperature, the term includes controlling devices, for example, that control an HVAC system based on other parameters such as humidity. 
     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). 
     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. 
     As used herein the term “common wire” when referring to HVAC systems refers to a direct wire from an HVAC power transformer that is in addition to the power or return wire to the transformer. Thus, power can be drawn from a circuit including the common wire and the power or return wire without risk of switching on or off relays, switches and/or contactors for operating various HVAC systems since those switching means are not in series in such a circuit. 
     As used herein the term “silent” or “silently” when referring to thermostat operation and/or control means that any sound made by the thermostat is generally inaudible to the human ear at a range of greater than 1 meter. 
     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 
       The inventive body of work will be readily understood by referring to the following detailed description in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a diagram of an enclosure with an HVAC system, according to some embodiments; 
         FIG. 2  is a diagram of an HVAC system, according to some embodiments; 
         FIG. 3  is a block diagram of some circuitry of a thermostat, according to some embodiments; 
         FIGS. 4A-C  schematically illustrate the use of auto-switching connectors being used to automatically select a source for power harvesting, according to some embodiments; 
         FIG. 5  is a schematic of a half-bridge sense circuit, according to some embodiments; 
         FIGS. 6A-B  are schematics showing the high voltage buck, bootstrap LDO and battery LDO power circuitry, according to some embodiments; and 
         FIG. 7  shows a battery charging circuit and rechargeable battery, according to some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     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. 
       FIG. 1  is a diagram of an enclosure with and HVAC system, 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. 
       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 such as hydronic, in-floor radiant heating, heat pump, etc. 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 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. 
     Thermostat  110  controls the HVAC system  120  through a number of control circuits. In particular, there are often separate control systems for heating and cooling. The heating system can include a low voltage, for example 24 VAC, operated gas valve which controls the flow of gas to the furnace; the cooling system includes a contactor having a low-voltage coil and high-voltage contacts which control energizing of the compressor; and the circulation system includes a fan relay having a low-voltage coil and high-voltage contacts which control energizing of the fan which circulates the conditioned air. The electrical power for energizing such low-voltage operated devices is provided either by a single transformer  260  for both heating and cooling, or by two separate transformers  260  for heating and  262  for cooling. Often, a single transformer is provided when the heating and cooling system is installed as a complete unit. If the cooling system is added to an existing heating system, sometimes an additional transformer is used. 
     According to some embodiments, the thermostat  110  is split into two parts: a head unit  270  and a backplate  286 . The head unit  270  contains a main processor  272 , storage  274  (such as flash storage), local area wireless networking  276 , and display and user interface  278 . Also included are environmental sensors such as temperature, humidity and/or pressure. A rechargeable battery  282  and power management subsystems  284  are also included as will be described in further detail herein. The head unit  270  is removable by the user and can be connected to a computer for configuration. 
     The backplate  286  installs on the wall and interfaces with the HVAC wiring  264 . Backplate  286  provides power to the head unit  270  and also facilitates control of the attached HVAC systems, which in  FIG. 2  is heating and cooling but could include other functions such as humidification, dehumidification and ventilation. According to some embodiments, backplate  286  also include a cellular wireless interface. Components within backplate  286  will be described in further detail herein. 
       FIG. 3  is a block diagram of some circuitry of a thermostat, according to some embodiments. Circuitry  300 , according to some embodiments, is a backplate of a thermostat. A number of HVAC wires can be attached using HVAC terminals  312 . One example of which is the W1 terminal  314 . Each terminal is used to control an HVAC function. According to some embodiments, each of the wires from the terminals W1, W2, Y1, Y2, G, O/B, AUX and E is connected to a separate isolated FET drive within  310 . The common HVAC functions for each of the terminals are: W1 and W2 heating; Y1 and Y2 for cooling; G for fan; O/B for heatpumps; and E for emergency heat. Note that although the circuitry  300  is able control 8 functions using the isolated FET drives  310 , according to some embodiments, other functions, or fewer functions can be controlled. For example circuitry for a more simply equipped HVAC system may only have a single heating (W), and single cooling (Y) and a fan (G), in which case there would only be three isolated FET drives  310 . According to a preferred embodiment, 5 FET drives  310  are provided, namely heating (W), cooling (Y), fan (G), auxiliary (AUX) and compressor direction (O/B). Not shown are the circuit returns such as RH (return for heat) and RC (return for cooling). According to some embodiments the thermostat can control a humidifier and/or de-humidifier. Further details relating to isolated FET drives  310  are described in co-pending U.S. patent application Ser. No. 13/034,674, entitled “Thermostat Circuitry for Connection to HVAC Systems,” filed on even date herewith which is incorporated herein by reference. 
     The HVAC functions are controlled by the HVAC control general purpose input/outputs (GPIOs)  322  within MCU  320 . MCU  320  is a general purpose microcontroller such as the MSP430 16-bit ultra-low power MCU available from Texas Instruments. MCU  320  communicates with the head unit via Head Unit Interface  340 . The head unit together with the backplate make up the thermostat. The head unit has user interface capability such that it can display information to a user via an LCD display and receive input from a user via buttons and/or touch screen input devices. According to some embodiments, the head unit has network capabilities for communication to other devices either locally or over the internet. Through such network capability, for example, the thermostat can send information and receive commands and setting from a computer located elsewhere inside or outside of the enclosure. The MCU detects whether the head unit is attached to the backplate via head unit detect  338 . 
     Clock  342  provides a low frequency clock signal to MCU  320 , for example 32.768 kHz. According to some embodiments there are two crystal oscillators, one for high frequency such as 16 MHz and one for the lower frequency. Power for MCU  320  is supplied at power input  344  at 3.0 V. Circuitry  336  provides wiring detection, battery measurement, and buck input measurement. A temperature sensor  330  is provided, and according to some embodiments and a humidity sensor  332  are provided. According to some embodiments, one or more other sensors  334  are provided such as: pressure, proximity (e.g. using infrared), ambient light, and pyroelectric infrared (PIR). 
     Power circuitry  350  is provided to supply power. According to some embodiments, when the thermostat is first turned on with insufficient battery power, a bootstrap power system is provided. A high voltage low dropout voltage regulator (LDO)  380  provides 3.0 volts of power for the bootstrap of the MCU  320 . The bootstrap function can be disabled under MCU control but according to some embodiments the bootstrap function is left enabled to provide a “safety net” if the head unit supply vanishes for any reason. For example, if the head-unit includes the re-chargeable battery  384  and is removed unexpectedly, the power would be lost and the bootstrap function would operate. The input to this Bootstrap LDO  380  is provided by connectors and circuitry  368  that automatically selects power from common  362  (highest priority), cool  366  (lower priority); or heat (lowest priority)  364 . 
     In normal operation, a 3.0 volt Primary LDO  382  powers the backplate circuitry and itself is powered by VCC Main. According to some embodiments, high voltage buck  360  is provided as a second supply in the backplate. The input to this supply is the circuitry  368 . According to some embodiments, the high voltage buck  380  can supply a maximum of 100 mA at 4.5 v. According to some embodiments, the VCC main and the Primary LDO  382  can be powered by a rechargeable battery (shown in  FIG. 7 ) in cases where there is no alternative power source (such as the high voltage buck or USB power, for example). 
       FIGS. 4A-C  schematically illustrate the use of auto-switching connectors being used to automatically select a source for power harvesting, according to some embodiments. The connectors  362 ,  364 , and  366  are connectors as shown in  FIG. 3 . For further details regarding preferred automatically switching connectors, see co-pending U.S. patent application Ser. No. 13/034,666, entitled “Thermostat Wiring Connector” filed on even date herewith and incorporated herein by reference. The connector  362  is used for connection to an HVAC “C” (common) wire and includes two switched pairs of normally closed secondary conductors  410  and  412 . The connector  366  is used for connection to an HVAC “Y” (cooling) wire and includes one switched pair of normally closed secondary conductors  454 . The connector  364  is used for connection to an HVAC “W” (heating) wire. Note that although not shown in  FIGS. 4A-C , one or more additional pairs of switched secondary conductors can be provided with any of the connectors  362 ,  366  and  365 , such as could be used for the purpose of electronically detecting the presence of an HVAC system wire to the connector. Power harvesting circuitry  460  is used to supply power to the thermostat and is also connected to the Rc wire  462  (or according to other embodiment the Rh wire). For example, the power harvesting circuitry  460  can include the HV buck  360  and Bootstrap LDO  380  as shown in and described with respect to  FIGS. 3 and 6A -B. 
       FIG. 4A  shows the case of the switches  454 ,  410  and  412  when no C wire and no Y wire is attached. In this case all of the switches  454 ,  410  and  412  are closed and the power harvesting circuitry  460  is connected at input  464  with the W wire via circuit paths  420 ,  422  and  426 .  FIG. 4B  shows the case of the switches  454 ,  410  and  412  when no C wire is attached but there is a Y wire attached. In this case switches  410  and  412  are closed but switch  454  is opened due to the presence of the Y wire. In this case the power harvesting circuitry  460  is connected at input  464  with the Y wire via circuit paths  424  and  428 .  FIG. 4C  shows the case of the switches  454 ,  410  and  412  when both C and Y wires are attached. In this case all the switches  454 ,  410  and  412  are open and the power harvesting circuitry  460  is connected at input  464  with the C wire via circuit path  430 . Note that the case of a connection of C and W wires and no Y wire is not shown but that in this case the W wire would not be connected to circuitry  420  since switch  410  would be open. Thus, through the use of circuitry and the connectors shown, the power harvesting circuitry is automatically switched so as to use connections to C, Y and W wires in decreasing order of priority. Preferably, the C wire is the highest priority as this ordinarily provides the best power source, if available. Note that according to some embodiments, the Y and W priorities are reversed to make W higher priority than Y. 
       FIG. 5  is a schematic of a half-bridge sense circuit, according to some embodiments. Circuit  500  provides voltage sensing, clipped to 3.0 volts, for presence detection and current sensing. At inputs  502 ,  504  and  506  are the 24 VAC waveforms from three of the HVAC circuits. In the case shown in  FIG. 5 , inputs  502 ,  504  and  506  are for HVAC W1, HVAC Y1 and HVAC G, respectively. The sense input bias buffer  550  is provided as shown. Note that a voltage divider is used in each case that takes the voltage from 24 volts to approximately 4 volts. Clamp diodes  520   a ,  520   b  and  520   c  ensure that the voltage goes no higher or lower than the range of the microcontroller  320  (shown in  FIG. 3 ). The Sense outputs  530 ,  532  and  534  are connected to the microcontroller  320  so that the microcontroller  320  can sense the presence of a signal on the HVAC lines. The circuits are repeated for the other HVAC lines so that the microcontroller can detect signals on any of the HVAC lines. 
       FIGS. 6A-B  are schematics showing the high voltage buck, bootstrap LDO and battery LDO power circuitry, according to some embodiments.  FIG. 6A  shows the input  464  from the connector selected power, which corresponds to input  464  to power circuitry  460  in  FIG. 4 . The diodes  632  are used to rectify the AC power signal from the HVAC power transformer wire that is selected by the connector circuitry shown in  FIG. 4 . When the thermostat is installed in a building having two HVAC power transformers, such as may be the case when an existing HVAC heating-only system is upgraded to add an HVAC cooling system. In such cases, there are two power wires from the HVAC system, often called “Rh” the power wire directly from the heating system transformer, and “Rc” the power wire directly from the cooling transformer. Input  462  is from a terminal connected to the Rc wire. According to some embodiments, the Rc and Rh terminals are switched using automatic switching or other jumperless design, as shown and described in co-pending U.S. patent application Ser. No. 13/034,674, entitled “Thermostat Circuitry for Connection to HVAC Systems,” filed on even date herewith and which is incorporated herein by reference. 
     Rectified input  624  is input to the high voltage buck circuit  610 , according to some embodiments. In buck circuit  610 , which corresponds to high voltage buck  360  in  FIG. 3 , the voltage on the input capacitors  612 ,  614  and  616  of high voltage buck  610  can be measured by the MCU  320  (of  FIG. 3 ) at node  620 , allowing the MCU to momentarily open the W1 or Y1 contacts during an “enabled” or “on” phase in order to recharge the buck input capacitors  612 ,  614  and  616  and continue power harvesting. According to some embodiments, the same HVAC circuit (e.g. heating or cooling) is used for power harvesting, whether or not there is more than one HVAC function in the system. According to some other embodiments, when the thermostat is used with an HVAC system having two circuits (e.g. heating and cooling), the system will power harvest from the non-activated circuit. In cases where a common wire is available from the HVAC power transformer, the system preferably does not power harvest at all from the heating and cooling circuits. According to some embodiments, the step down converter  630  is a high efficiency, high voltage 100 mA synchronous step-down converter such as the LTC3631 from Linear Technology. According to some embodiments, inductor  642  is a 100 uH power inductor such as the MOS6020 from Coilcraft. According to some embodiments, one or more other types of elements in addition to or instead of input capacitors  612 ,  614  and  616  are used to store electrical energy during power harvesting when the HVAC function is active (or “on”). For example, magnetic elements such as inductors and/or transformers can be used. 
     In order to control the HVAC functions, the HVAC function wire is shorted to the return or power wire. For example, in the case of heating, the W wire is shorted to the Rh (or R or Rc depending on the configuration). In the case of cooling the Y wire is shorted to the Rc (or R or Rh depending on the configuration). By shorting these two wires, the 24 VAC transformer is placed in series with a relay that controls the HVAC function. However, for power harvesting, a problem is that when these wires are shorted, there is no voltage across them, and when open, there is no current flow. Since Power=Voltage*Current, if either quantity is zero the power that can be extracted is zero. According to some embodiments, the power harvesting circuitry allows power to be taken from the two wires in both the states of HVAC—the HVAC “on” and the HVAC “off”. 
     In the HVAC “off” state, some energy can be harvested from these two wires by taking less energy than would cause the of the relay to turn on, which would cause the HVAC function to erroneously turn on. Based on testing, it has been found that HVAC functions generally do not turn on when (0.040 A*4.5V)=0.180 watts is extracted at the output. So after the input diodes, capacitors, and switching regulator, this allows us to take 40 mA at 4.5 volts from these wires without turning on the HVAC system. 
     In the HVAC “on” state, the two wires must be connected together to allow current to flow, which turns on the HVAC relay. This, however, shorts out the input supply, so our system does not get any power when the HVAC “on” switch is closed. To get around this problem, the voltage is monitored on the capacitors  612 ,  614  and  616  at the input switching power supply node  620 . When the voltage on these capacitors “C in ” drops close to the point at which the switching power supply would “Drop out” and lose output regulation, for example at about +8 Volts, the HVAC “on” switch is turned off and C in , is charged. During the time that C in , is charging, current is still flowing in the HVAC relay, so the HVAC relay stays on. When the C in , capacitor voltages increases some amount, for example about +16 Volts, the HVAC “on” switch is closed again, C in  begins to discharge while it feeds the switching regulator, and current continues to flow in the HVAC relay. Note that C in , is not allowed to discharge back to the HVAC “on” switch due to input diodes  632 . When the voltage on C in  drops to about +8 Volts the HVAC “on” switch is turned off and the process repeats. This continues until the system tells the HVAC “on” switch to go off because HVAC is no longer needed. According to some embodiments, the ability of the HVAC “on” switch to turn on and off relatively quickly is provided by circuitry  450  as shown in and described with respect to FIG. 4 of co-pending U.S. patent application Ser. No. 13/034,674, entitled “Thermostat Circuitry for Connection to HVAC Systems,” filed on even date herewith, which is incorporated herein by reference. 
     According to some embodiments, one or more alternative power harvesting techniques are used. For example, rather than having the HVAC “on” switch turn on when the voltage on C in  reaches a certain point, it the system might turn off the “HVAC “on” switch for a predetermined period of time instead. According to some embodiments, power harvesting is enhanced by synchronizing the power harvesting with the AC current waveform. 
       FIG. 6B  is a schematic of high voltage low dropout voltage regulators used to provide bootstrap power and battery, according to some embodiments. The bootstrap LDO circuitry  680 , and battery LDO circuitry correspond to the bootstrap LDO  380  and battery LDO  382  in  FIG. 3  respectively. Rectified input  624  is input to bootstrap circuit  680 . According to some embodiments, regulator  670  is low-dropout linear regulator such as the TPS79801 from Texas Instruments. The output power  690  is provided to the backplate at 3.0V. The bootstrap disable signal  680  can be used to disable the bootstrap power unit, as shown. The input  660  comes from VCC main, which can be, for example, from the rechargeable battery. According to some embodiments, the low dropout regulator  662  is a low quiescent current device designed for power-sensitive applications such as the TLV70030 from Texas Instruments. 
       FIG. 7  shows a battery charging circuit  700  and rechargeable battery, according to some embodiments. The charger  710  is used to charge the lithium-ion battery  750 . In general, li-ion battery capacity depends on what voltage the battery is charged to, and the cycle life depends on the charged voltage, how fast the battery is charged and the temperature during which the battery is charged. Ordinarily, Li-ion batteries are charged at about 4.2V. In some cases the charging voltage is even higher in an attempt to gain greater capacity, but at the expense of decreased cycle life. However, in the case of the rechargeable battery  750  for use with a wall-mounted thermostat, a greater cycle life is preferred over capacity. High capacity is generally not needed since charging power is available via the power harvesting circuitry, and greater cycle life is preferred since user replacement may be difficult or unavailable. Thus, according to some embodiments, a low charging speed, low final float voltage and reduced charging temperature range is preferred. According to some embodiments, a final float voltage of between 3.9V and 4.1V is used. According to some embodiments a final float voltage of less than 4.0V is used, such as 3.95V. According to some embodiments, the ratio of charge current to total capacity “C” is also controlled, such as charging the battery to 0.2 C (0.2 times the rated capacity) to provide better cycle life than a higher ratio. According to some embodiments, using a lower charging current aids in avoiding unintended tripping of the HVAC relay. 
     According to some embodiments, charger  710  is a USB power manager and li-ion battery charger such as the LTC4085-3 from Linear Technology. Backplate voltage  720  is input to charger  710 . The circuitry  730  is used to select the charging current. In particular the value of resistor  732  (24.9 k) in parallel with resistor  734  (16.9 k) in combination with the inputs Double Current  738  and High Power  728  are used to select the charging current. If High Power  728  and Double Current  738  are both set to 0, then the charging current is 8.0 mA; if the High Power  728  is set to 0 and Double Current  738  is set to 1, then the charging current is 19.9 mA; if the High Power  728  is set to 1 and Double Current  738  is set to 0, then the charging current is 40.1 mA; and if the High Power  728  and Double Current  738  are both set to 1, then the charging current is 99.3 mA. Resistor  736  is used to set the default charge current. In the case shown, a 220 k resistor set the default charge current to 227 mA. According to some embodiments, a charge temperature range of 0-44 degrees C. is set via the Thermistor Monitoring Circuits. 
     According to some embodiments, the thermostat is capable of being powered by a USB power supply. This could be supplied by a user, for example, by attaching the thermostat via a USB cable to a computer or another USB power supply. In cases there a USB power supply is available, it is selected as the preferred power source for the thermostat and can be used to recharge the rechargeable battery. According to some embodiments, a charge current of about 227 mA is used when a USB supply source is available; a charge current of about 100 mA is used when an HVAC common wire is present; and a charge current of between about 20-40 mA is used when power is harvested from an HVAC heating and/or cooling circuit. 
     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.