Abstract:
A thermostat and related methods is provided for controlling an HVAC system having one or two separate transformers for supplying power to the HVAC system. The thermostat includes isolation circuitry housed within the thermostat to safely connect to the HVAC control wires and power wire(s) whether the HVAC system has one or two separate transformers without the use of removable jumpers or manual rewiring. The thermostat can include a processor that sends DC signals for turning on and turning off each of the HVAC functions, and an isolator adapted to electrically isolate the processor from the control wires and power wire(s). The isolator can include a transformer, such as a low cost Ethernet transformer. The circuitry can include one or more field effect transistors adapted and arranged so as to open or close an electrical connections between the control and power wires, thereby turning on or off the associated HVAC function. According to some embodiments, the Rc and Rh terminals are permanently connected using a fuse.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application 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. 13/034,678 entitled “Thermostat Battery Recharging,” filed on even date herewith; and U.S. Ser. No. 13/034,666 entitled “Thermostat Wiring Connector,” filed on even date herewith. 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 jumperless designs and/or isolation circuitry. 
     In many single-stage heating and cooling systems, the heating system includes a low-voltage 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 or by two separate transformers. If the heating and cooling system is installed as a complete unit, generally a single transformer is provided. Such a single transformer has the required volt-ampere output to operate all the low-voltage operated devices. If the cooling system is added to an existing heating system, sometimes an additional transformer is used. 
     For example, in a system originally designed to provide heating only, a fan relay is generally not provided since the fan is generally controlled directly by a thermal switch on the furnace. Therefore, in systems for providing heating only, the only electrical load on the transformer is often the gas valve. When the cooling system is subsequently added, the electrical load on the transformer increases due to the addition of the fan relay and the contactor. If the existing transformer does not have the sufficient volt-ampere output to operate all the low-voltage operated devices, an additional transformer is often added. Additionally, even if the additional transformer may not be necessary, it may nevertheless be installed so as to simplify the electrical wiring involved in the installation of the add-on cooling system. 
     It is desirable that a thermostat for controlling a single-stage heating and cooling system be constructed so as to enable it to be readily usable with either the single-transformer or two-transformer power source. A common approach is to electrically isolate the secondary windings of the two transformers from each other using a removable wire jumper. For example, see U.S. Pat. No. 4,049,973. U.S. Pat. No. 4,898,229 discusses a thermostat with integral means for detecting out-of-phase connection of a two-transformer power source, wherein an LED is used to indicate the out-of-phase connection to an installer. The installer is instructed to reverse the polarity of one of the two transformers if the LED is energized. U.S. Pat. No. 5,127,464 discusses a thermostat providing electrical isolation between connected heating and cooling transformers. However, the design nevertheless relies on a removable jumper to be manually inserted in the case where there is only a single HVAC transformer. 
     Thermostats in residential and light commercial buildings are often used to control multiple conditioning functions, such as heating, cooling, ventilating, etc. Often, a thermostat is designed such that the user must manually actuate a mechanical switch to change functions. For example, many thermostats have a mechanical switch with which the user can select from heating, cooling or fan functions. Some thermostat designs make use of mechanical relays for switching between functions such as heating, cooling, etc. Furthermore, many thermostat designs make use of relays for controlling each HVAC function, such that the relay within the thermostat is actuated each time the heating turns on or off, and each time the cooling turns on or off. However, it has been found that some users find the audible sound from actuating relays within the thermostat to be undesirable. Mechanical relays have a number of other disadvantages over solid state switching including larger size, reliability over time, and slower speed of switching. 
     SUMMARY 
     According to some embodiments a thermostat is provided for controlling an HVAC system having one or two transformers for supplying power to the HVAC system. The HVAC system has at least one control wire for controlling one or more HVAC functions and one or two electrical power return wires each of which is electrically connected to one of the one or two transformers. The thermostat includes a plurality of wiring terminals for making electrical connections to the control wire and to the power return wires, and circuitry connected to the terminals. The terminals and circuitry are adapted and arranged such that at least one control wire and the one or two power return wires can be connected whether the HVAC system has one or two transformers without the use of removable jumpers or manual rewiring. According to some embodiments, the terminals are each adapted to accept an electrical power return wire and each include a switch that automatically disrupts an electrical connection between the other terminal when an electrical return wire is connected to the terminal. 
     According to some embodiments, the thermostat also includes one or more solid state switching components adapted and arranged to provide switching so as to turn on and off each of the HVAC functions, an isolator, and a processor adapted and configured to send DC signals for turning on and turning off each of the HVAC functions using the solid state switching components. The isolator is preferably adapted to electrically isolate the processor from the solid state switching components such that the solid state switching components do not reference ground. The isolator can include a transformer, such as a low cost Ethernet transformer, and/or a capacitor. The circuitry can include one or more field effect transistors. Circuitry connected to the solid state switching components can be adapted and arranged such that the switching is left in an open state in the event of a failure condition within the thermostat. 
     According to some embodiments, a thermostat is provided for controlling an HVAC system having one or two transformers for supplying power to the HVAC system, with the thermostat including two power return wiring terminals for making electrical connections to one or two power return wires, and a switching circuit for electrically connecting the two power return wiring terminals in cases where the HVAC system has only one power transformer and electrically disconnecting the two power return wiring terminals in cases where the HVAC system two power transformers. One or more detection circuits can be adapted and arranged to detect whether the HVAC system has one or two power transformers. The switching circuit can connect or disconnect the two power return wiring terminals based on the detection circuits and/or input from a user. According to some embodiments, the two power return wiring terminals can also be electrically connected using one or more mechanical switches. 
     According to some embodiments, a thermostat and associated method is provided for controlling an HVAC system having multiple HVAC functions including a first HVAC function and a second HVAC function. The thermostat includes circuitry housed within the thermostat adapted and configured to silently and automatically switch between controlling the first and second HVAC functions, and to silently control the first HVAC function and the second HVAC function. 
     According to some embodiments a thermostat is provided for controlling an HVAC system having one or more HVAC functions. The thermostat includes one or more solid state switching components adapted and arranged to provide switching on and off each of the one or more HVAC functions, and current sensing circuitry adapted and arranged to sense current passing through the one or more solid state switching components. The current sensing circuitry can measure a voltage differential across at least one of the solid state switching components, or measure a voltage differential across a thermistor in series with the solid state switching components. The current sensing can be used for detection of a fault condition such as a wire connection fault. 
     According to some embodiments, a thermostat is provided for controlling an HVAC system having multiple HVAC functions including a first HVAC function and a second HVAC function, the thermostat comprising circuitry housed within the thermostat adapted and configured to silently and automatically switch between controlling the first and second HVAC functions, and to silently control the first HVAC function and the second HVAC function. 
     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 “jumper” refers to a short length of conductor that is designed to be manually inserted or removed, such as by a user or installer, to close a break in or bypass part of an electrical circuit. The terms “jumperless” and “jumper-free” refer to a circuit or design that avoids the need for manual insertion and/or removal of a jumper during setup, installation, and/or configuration. 
     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 “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; 
         FIG. 4  is a schematic of solid-state electronic AC switch with a transformer isolated control input, according to some embodiments; 
         FIG. 5  is a schematic of a half-bridge sense circuit, according to some embodiments; 
         FIGS. 6A-B  illustrate a jumperless thermostat connected to two different HVAC systems, according to some embodiments; 
         FIGS. 7A-B  illustrate a jumperless thermostat connected to two different HVAC systems, according to some embodiments; and 
         FIGS. 8A-B  illustrate a jumperless thermostat connected to two different HVAC systems, according to some alternate 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 an 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 by 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 is 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. 
       FIG. 3  is a block diagram of some circuitry of a thermostat, according to some embodiments. According to some embodiments, the thermostat is made up of two main units: (1) a back plate that includes connector terminals for connection to the HVAC system wires, power harvesting circuitry, HVAC control circuitry and other components; and (2) a head unit that includes a user interface, additional sensors, wireless communication and other components. Circuitry  300 , according to some embodiments, is some of the circuitry included in the back plate of the thermostat. Note that other circuitry may be included in the back plate that is not shown. For example, the back plate can include wireless communication capability, according to some embodiments. A number of HVAC wires can be attached using HVAC terminals  312 . One example of which is the W 1  terminal  314 . Each terminal is used to control an HVAC function. According to some embodiments, each of the wires from the terminals W 1 , W 2 , Y 1 , Y 2 , 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: W 1  and W 2  heating; Y 1  and Y 2  for cooling; G for fan; O/B for heatpumps; and E for emergency heat. Note that although the circuitry  300  is able control  10  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). Further detail of the isolated FET drive circuitry, according to some embodiments is provided in  FIG. 4 . According to some embodiments, greater or lesser numbers of FET drives  310  can be provided in accordance with the application. For example, humidification and dehumidification can be controlled using one or more additional FET drives. Not shown are the circuit returns such as RH (return for heat) and RC (return for cooling). 
     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. 
       FIG. 4  is a schematic of solid-state electronic AC switch with a transformer isolated control input, according to some embodiments. Sub-circuit  400  controls a bidirectional power switch, which is an AC switch between terminals  442  and  444 , by sending a control signal across an isolation barrier  430  as a high frequency AC signal. The control signal is rectified and filtered and applied to the gates of two N-channel MOSFETs  424  and  425 . The switch is on when the DC gate to source voltage of the MOSFETs  424  and  425  is above the threshold voltage of the MOSFETs. Both MOSFETs  424  and  425  see essentially the same gate to source voltage. Additional circuitry is added to turn the switch off quickly shortly after the control signal is stopped. 
     Inputs  401   a  and  401   b  are a logic level clock signal from the MCU, and are preferably differential signals. Inputs  401   a  and  401   b  generate the frequency that is coupled across the isolation component. According to some embodiments, inputs  401   a  and  401   b  are not a fixed frequency, but rather a spread spectrum. Input  402  enables the AND gates  403 . AND gates  403  are AND logic gates that generate a buffered AC signal for driving the transformer  432 . An example of a suitable logic component for AND gates  403  is a dual buffer/driver such as the SN74LVC2G08 from Texas Instruments. 
     AC coupling capacitor  404 , this component prevents DC current from flowing in the transformer, which would reduce efficiency and could hurt operation due to transformer saturation. Resistors  405   a  and  405   b  work in conjunction with stray capacitances to round the sharp edges of the clock signals, limit instantaneous currents, and damp resonant circuits. This reduces EMI. 
     It should be noted that other topologies of driver circuits could be used for  401 - 405  above, according to other embodiments. The embodiment shown in  FIG. 4  has been found to reduce drive power requirements to a very low level. 
     Transformer  432  includes primary winding  406  and secondary winding  407 . The transformer  432  provides isolation, such that the switch could be many volts different potential from the control circuitry. According to some embodiments, transformer  432  is an Ethernet transformer. Ethernet transformers have been found to work well with a very low cost. According to the other embodiments, other styles of transformers could be used. According to some embodiments, coupled inductors such as LPD3015 series from Coilcraft are used. According to some embodiments, the transformer  432  is replaced with capacitors, as this is an alternative way to get AC energy across a boundary  430 . 
     The transformer  432  has a turns ratio of 1:1 primary winding  406  to secondary winding  407 , although other windings ratios can be used according to other embodiments. With ±3 volts across the primary of the transformer, a 1:1 ratio transformer generates about +6 volts of gate to source voltage on the FETs  424  and  425 . The topology shown is a modified push pull. According to other embodiments, other topologies including forward, flyback, and push pull could be used. Resistors  409   a  and  409   b  work in conjunction with stray capacitances to round the sharp edges of the clock signals, limit instantaneous currents, and damp resonant circuits. This reduces EMI. 
     AC coupling capacitor  410  accumulates a DC voltage across it in normal operation which is approximately the output gate to source voltage divided by 2. This capacitor  410  allows the transformer  432  to be used more effectively than if it was not there. If the output voltage is half what it should be, this capacitor  410  is likely shorted. 
     Bottom diode  411  is on for half the cycle, and enables the capacitor  410  to charge to half the output voltage. Top diode  412  is on for the other half of the cycle, and basically peak detects the voltage on the capacitor  410  with the voltage across the transformer, resulting in a rectified output voltage across capacitor  419 . 
     Circuit  450  is used to enable a fast turn off characteristic. When the voltage on the Switch Gate is rising with respect to the Switch Source, capacitor  413  charges up through diode  414 . When the voltage on the Switch Gate drops with respect to the Switch Source, this capacitor  413  pulls down on the emitter of NPN  416  which turns on  416 , which turns on  417 , which discharges  419  (as well as the capacitances of the MOSFETs  424  and  425 ) and quickly turns off the switch. This fast turn off characteristic may be useful in an energy harvesting application such as described in greater detail in co-pending U.S. patent application Ser. No. 13/034,678 entitled “Thermostat Battery Recharging” filed on even date herewith, and which is incorporated herein by reference. Capacitor  415  may be helpful in EMI immunity tests. Resistor  418  prevents PNP  417  from turning on due to leakage currents. 
     Resistor  420  discharges the gate source capacitance voltage and tends to turn off the switch, and to hold it off when no control signal is present. Gate resistor  422  prevents the FETs  424  and  425  from oscillating due to their follower topology. Zener diode  423  prevents the gate to source voltage from going too high, which could damage the FETs  424  and  425 . 
     FETs  424  and  425  are the main switching elements in the circuit  400 . FETs  424  and  425  tend to be on when the gate to source voltage is above the threshold voltage of the FETs, and tend to be off when the gate to source voltage is less than the threshold voltage. As this is a bidirectional AC Switch, two FETs are used because available FETs have a drain to source body diode, and if only one FET were used the switch would be “On” due to the body diode for half of the AC cycle. 
     Note that the with the circuit of  FIG. 4 , the left side of barrier  430  is digital logic controlled by the MCU and is ground referenced, while the right side of barrier  430  is a floating solid state (using FETs) switch that is does not reference ground. The floating no-ground reference nature of the FET drive advantageously enables connection to two-transformer systems with shorted (preferably with a fuse) Rc and Rh wires. If the isolation was not present, and the right side was ground referenced, when one circuit was “on” and the other was “off” the “on” circuit would take power from the “off” circuit. Thus the design as shown in  FIG. 4  allows for solid state switching of the HVAC circuits having either one or two power transformers without the need for removable jumpers during installation. 
     According to some embodiments, the circuitry  450  provides for the connection between terminals  442  and  444  to be open very quickly when the control signal is received from the driver circuit. According to some embodiments the fast turn-off circuitry  450  is used for isolated FET drives for HVAC wires used for power harvesting, such as W (heating) and Y (cooling), but is omitted from other isolated FET drives that are not used for power harvesting, such as for Aux, G (fan), and O/B (compressor direction). 
     Additionally, the circuitry shown in  FIG. 4  provides for a failsafe “open,” in that when there is no control signal being received the state for any reason, the connection between terminals  442  and  444  is in an open state. This is an important advantage over thermostat designs that use bi-stable relays for opening and closing the control circuit. Fast shut off and failsafe open features allow for safe wiring of the thermostat in HVAC system having two power transformers, such as shown in  FIG. 6 , without the need for a jumper wire to be manually removed. 
     According to some embodiments, the thermostat carries out current sensing through the HVAC control circuit by detecting the voltage across the FETs  424  and  425 . Unlike most thermostats, that use mechanical relays having virtually no measurable voltage drop to open and close the HVAC control circuit for the HVAC function, the thermostat as described herein uses solid state switching which has enough voltage drop so as to allow for current measurements. In the case of  FIG. 4 , the voltage measurement is made across the FETs  424  and  425  (or terminals  442  and  444 ). The current measurement made in this fashion, according to some embodiments is used to detect faults such as a common wire plugged in to the wrong terminal (such as a “Y” or “W” terminal). According to some embodiments a positive temperature coefficient thermistor  460  is used to detect current by measuring voltage drop, and in the case of wiring faults the thermistor also acts to limit current. 
       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 when the switches are closed. 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 Rc, HVAC Rh and HVAC Y, 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  illustrate a jumperless thermostat connected to two different HVAC systems, according to some embodiments.  FIG. 6A  shows jumperless thermostat  610  wired for control to an HVAC system having two power transformers  660  and  662 . As discussed elsewhere herein, 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  660  converts 110 volt AC power to 24 volt AC power for the heating control circuit  664 . Similarly, cooling power transformer  662  converts 110 volt AC power to 24 volt AC power for the cooling control circuit  666 . 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. 
     Relay  670  controls the gas valve for the HVAC heating system. When sufficient AC current flows through the gas valve relay  670 , gas in the heating system is activated. The gas valve relay  670  connected via a wire to terminal  684 , which is labeled the “W” terminal, on thermostat  610 . Relay  672  controls the fan for the HVAC heating and cooling systems. When sufficient AC current flows through the fan relay  672 , the fan is activated. The fan relay  672  connected via a wire to terminal  682 , which is labeled the “G” terminal on thermostat  610 . Contactor (or relay)  674  controls the compressor for the HVAC cooling system. When sufficient AC current flows through the compressor contactor  674 , the fan is activated. The contactor  674  connected via a wire to terminal  680 , which is labeled the “Y” terminal, on thermostat  610 . The heat power transformer  660  is connected to thermostat  610  via a wire to terminal  692 , which is labeled the “Rh” terminal. The cooling power transformer  662  is connected to thermostat  610  via a wire to terminal  690 , which is labeled the “Rc” terminal. 
     Thermostat  610  includes three isolated FET drives  630 ,  632  and  634  for switching open and close the AC current to each of the relays  670 ,  672  and  674 . Note that according to some embodiments, each of the FET drives  630 ,  632  and  634  are of the design of sub-circuit  400  as shown and described with respect to  FIG. 4 , and also correspond to the isolated FET drives  310  in  FIG. 3 . Although only three isolated FET drives are shown in  FIGS. 6A-B , according to some embodiments other numbers of isolated FET drives are provided depending on the number of expected controllable components in the HVAC system where the thermostat is intended to be installed. For example, according to some embodiments, 5 to 10 isolated FET drives can be provided. 
     Drive  630  includes a switching portion  640  for opening and closing the AC current between terminal  680  and terminal  690 , thereby controlling the compressor contactor  674  of the HVAC cooling system. The drive portion  640  is controlled by and isolated from, via a transformer, driver circuit  650 . The MCU  620  controls driver circuit  650 . Drive  632  includes a switching portion  642  for opening and closing the AC current between terminal  682  and terminal  690 , thereby controlling the fan relay  672  of the HVAC heating and cooling systems. The drive portion  642  is controlled and isolated from, via a transformer, driver circuit  652 . The MCU  620  controls driver circuit  652 . Drive  634  includes a switching portion  644  for opening and closing the AC current between terminal  684  and terminal  692 , thereby controlling the gas valve relay  670  of the HVAC system. The drive portion  644  is controlled by and isolated from, via a transformer, driver circuit  654 . The MCU  620  controls driver circuit  654 . Note that although the drive portions  640 ,  642  and  644  are isolated from the driver circuits  650 ,  652  and  654  respectively by a transformer, other isolation means could be provided as described with respect to  FIG. 4 . Note that due to the design of thermostat  610 , the terminals  690  and  692  (i.e. the Rc and Rh terminals) are permanently shorted without the use of a removable jumper. According to some embodiments, a safety fuse  636  is provided. 
       FIG. 6B  shows jumperless thermostat  610  wired for control to an HVAC system having a single power transformer  668  that converts 110 volt AC power to 24 volt AC power for the control circuit  664 . In this case, relays  672  and  674 , which control the fan and the compressor, respectively, are both attached to transformer  668 . The power transformer  668  is connected to thermostat  610  via a wire to terminal  692 , which is labeled the “Rh” terminal. Note that since thermostat  610  is designed with a short between terminals  690  and  692 , the power transformer  668  could alternatively be connected to thermostat  610  via a wire to terminal  690  (the Rc terminal). Additionally, no jumper needs to be installed or removed by a user or installer when using thermostat  610  with either a one transformer HVAC system as shown in  FIG. 6B  or a two transformer HVAC system as shown in  FIG. 6A . However, in cases where the thermostat is connected to two transformers via terminals  690  and  692 , depending on the relative phases of the power circuits, voltages of 48 to 54 VAC can generate voltages as high as about 80 volts within the thermostat, and therefore the components drive portions  640 ,  642  and  644  should be designed accordingly. For example, according to some embodiments, when thermostat  610  is designed with a short between terminals  690  and  692  as shown in  FIGS. 6A and 6B , the exposed components are designed such that up to 100 volts can be tolerated. According to some embodiments, other designs, such as shown in  FIGS. 7A-B  and  8 A-B, can be used to avoid relatively high peak voltages as described. 
       FIGS. 7A-B  illustrate a jumperless thermostat connected to two different HVAC systems, according to some embodiments.  FIG. 7A  shows jumperless thermostat  710  wired for control to an HVAC system having two power transformers  760  and  762 . As discussed elsewhere herein, a two-transformer HVAC system is commonly found in residences and light commercial building in which an existing heating system was subsequently upgraded or had had an air conditioning system installed. Heat power transformer  760  converts 110 volt AC power to 24 volt AC power for the heating control circuit  764 . Similarly, cooling power transformer  762  converts 110 volt AC power to 24 volt AC power for the cooling control circuit  766 . 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. 
     Relay  770  controls the gas valve for the HVAC heating system. When sufficient AC current flows through the gas valve relay  770 , gas in the heating system is activated. The gas valve relay  770  connected via a wire to terminal  784 , which is labeled the “W” terminal, on thermostat  710 . Relay  772  controls the fan for the HVAC heating and cooling systems. When sufficient AC current flows through the fan relay  772 , the fan is activated. The fan relay  772  connected via a wire to terminal  782 , which is labeled the “G” terminal on thermostat  710 . Contactor (or relay)  774  controls the compressor for the HVAC cooling system. When sufficient AC current flows through the compressor contactor  774 , the fan is activated. The contactor  774  connected via a wire to terminal  780 , which is labeled the “Y” terminal, on thermostat  710 . The heat power transformer  760  is connected to thermostat  710  via a wire to terminal  792 , which is labeled the “Rh” terminal. The cooling power transformer  762  is connected to thermostat  710  via a wire to terminal  790 , which is labeled the “Rc” terminal. 
     Thermostat  710  includes three isolated FET drives  730 ,  732  and  734  for switching open and close the AC current to each of the relays  770 ,  772  and  774 . Note that according to some embodiments, each of the FET drives  730 ,  732  and  734  are of the design of sub-circuit  400  as shown and described with respect to  FIG. 4 , and also correspond to the isolated FET drives  310  in  FIG. 3 . Although only three isolated FET drives are shown in  FIGS. 7A-B , according to some embodiments other numbers of isolated FET drives are provided depending on the number of expected controllable components in the HVAC system where the thermostat is intended to be installed. For example, according to some embodiments, 5 to 10 isolated FET drives can be provided. 
     Drive  730  includes a switching portion  740  for opening and closing the AC current between terminal  780  and terminal  790 , thereby controlling the compressor contactor  774  of the HVAC cooling system. The switching portion  740  is controlled by and isolated from, via a transformer, driver circuit  750 . The MCU  720  controls driver circuit  750 . Drive  732  includes a switching portion  742  for opening and closing the AC current between terminal  782  and terminal  790 , thereby controlling the fan relay  772  of the HVAC heating and cooling systems. The drive portion  742  is controlled and isolated from, via a transformer, driver circuit  752 . The MCU  720  controls driver circuit  752 . Drive  734  includes a switching portion  744  for opening and closing the AC current between terminal  784  and terminal  792 , thereby controlling the gas valve relay  770  of the HVAC system. The drive portion  744  is controlled by and isolated from, via a transformer, driver circuit  754 . The MCU  720  controls driver circuit  754 . Note that although the drive portions  740 ,  742  and  744  are isolated from the driver circuits  750 ,  752  and  750  respectively by a transformer, other isolation means could be provided as described with respect to  FIG. 4 . 
     Two normally-closed switches  716  and  726  are provided between the Rc terminal  790  and the Rh terminal  792 . Switch  716  is automatically opened when the presence of a wire connected to the Rc terminal  790  is detected, and switch  726  is opened automatically when the presence of a wire connected to Rh terminal  792  is detected. According to some embodiments, the switches  716  and  726  are provided using a connector as described in 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. In particular, the switches  726  and  716  can correspond to the switched pairs of secondary conductors  750  in  FIGS. 7C and 746  in  FIG. 7D  in that co-pending patent application. Since, in the case shown in  FIG. 7A  there are wires connected to both Rc and Rh terminals  790  and  792 , both switches  716  and  726  are opened and the Rc and Rh terminals  790  and  792  are not electrically connected to each other. Two fuses,  712  and  722  can also be provided for added safety. 
       FIG. 7B  shows jumperless thermostat  710  wired for control to an HVAC system having a single power transformer  768  that converts 110 volt AC power to 24 volt AC power for the control circuit  764 . In this case, relays  772  and  774 , which control the fan and the compressor, respectively, are both attached to transformer  768 . The power transformer  768  is connected to thermostat  710  via a wire to the Rh terminal  792 . Since a wire is connected to Rh terminal  792 , the switch  726  is open, and since no wire is connected to Rc terminal  790 , the switch  716  is closed. Thus an electrical connection exists between the Rc and Rh terminals  790  and  792  as all of the circuitry in thermostat  710  that would be connected to the Rc terminal, such as drives  730  and  732  are connected to the Rh terminal. Note that a similar configuration would result if the user attaches the wire  764  into the Rc terminal  790  instead of the Rh terminal  792 . In that case, switch  716  could be closed, but switch  726  would be open. 
       FIGS. 8A-B  illustrate a jumperless thermostat connected to two different HVAC systems, according to some alternate embodiments.  FIG. 8  shows jumperless thermostat  810  wired for control to an HVAC system having two power transformers  860  and  862 . As discussed elsewhere herein, a two-transformer HVAC system is commonly found in residences and light commercial building in which an existing heating system was subsequently upgraded or had had an air conditioning system installed. Heat power transformer  860  converts 110 volt AC power to 24 volt AC power for the heating control circuit  864 . Similarly, cooling power transformer  862  converts 110 volt AC power to 24 volt AC power for the cooling control circuit  866 . 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. 
     Relay  870  controls the gas valve for the HVAC heating system. When sufficient AC current flows through the gas valve relay  870 , gas in the heating system is activated. The gas valve relay  870  connected via a wire to terminal  884 , which is labeled the “W” terminal, on thermostat  810 . Relay  872  controls the fan for the HVAC heating and cooling systems. When sufficient AC current flows through the fan relay  872 , the fan is activated. The fan relay  872  connected via a wire to terminal  882 , which is labeled the “G” terminal on thermostat  610 . Contactor (or relay)  874  controls the compressor for the HVAC cooling system. When sufficient AC current flows through the compressor contactor  874 , the fan is activated. The contactor  874  connected via a wire to terminal  880 , which is labeled the “Y” terminal, on thermostat  810 . The heat power transformer  860  is connected to thermostat  810  via a wire to terminal  892 , which is labeled the “Rh” terminal. The cooling power transformer  862  is connected to thermostat  810  via a wire to terminal  890 , which is labeled the “Rc” terminal. 
     Thermostat  810  includes switching circuits  830 ,  832  and  834  for switching open and close the AC current to each of the relays  870 ,  872  and  874  under the control o MCU  820 . According to some embodiments, the circuits  830 ,  832  and  834  could be relays. According to other embodiments, switching circuits  830 ,  832  and  834  could be implemented using isolated FET drives such as shown in  FIGS. 6A-B  and  7 A-B. Although only three switching circuits are shown in  FIGS. 8A-B , according to some embodiments other numbers of switching circuits are provided depending on the number of expected controllable components in the HVAC system where the thermostat is intended to be installed. For example, according to some embodiments, 5 to 10 switching circuits can be provided. 
     According to some embodiments, thermostat  810  includes two auto detection circuits  840  and  842  to detect whether an AC signal is being applied to terminals  890  and  892  respectively. According some embodiments, a half-bridge sense circuit such as shown and described with respect to  FIG. 5 , is used for each of the auto detection circuits  840  and  842 . Also provided is a switching circuit  836  for opening and closing a connection between the terminals  890  and  892  depending on whether the thermostat  810  is installed with an HVAC system having one or two power transformers. Switching circuit  836  can be implemented using a relay, but solid state switching such as using FETs could be used according to some embodiments. 
       FIG. 8B  shows jumperless thermostat  810  wired for control to an HVAC system having a single power transformer  868 . In this case, relays  872  and  874 , which control the fan and the compressor, respectively, are both attached to transformer  868 . The power transformer  868  is connected to thermostat  810  via a wire to terminal  892 , which is labeled the “Rh” terminal. Auto detection using  840  and  842  is carried out while the switching circuit  836  is open. If AC signals are detected on both terminals  890  and  892 , then it is assumed that there are two separate HVAC power transformers, such as shown in  FIG. 8A . Accordingly the switching circuit  836  is left open. If AC signals are detected on only one of the terminals  890  and  892 , then it is assumed that there is only a single HVAC power transformer such as shown in  FIG. 8B . Accordingly the switching circuit  836  is closed. Additionally, no jumper needs to be manually installed or removed when using thermostat  810  with either a one transformer HVAC system as shown in  FIG. 8B  or a two transformer HVAC system as shown in  FIG. 8A . By providing an auto-detection capability, the thermostat  810  advantageously does not need to query so as to be easier to install and avoids problems associated with user errors. 
     According to some embodiments, user input is used to control switching circuit  836  instead of, or in addition to using auto detection circuits  840  and  842 . According to such embodiments, user input is provided via a user interface such as button on the head unit of thermostat  810  (not shown), and in response, the MCU  820  opens or closes the switching circuit  836 . For example, during installation, a user or installer may be queried whether the HVAC system has one or two power transformers. If the user indicates there are two HVAC power transformers than the switching circuit  836  is opened and if the user indicates there is only one HVAC power transformer then switching circuit  836  is closed. 
     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.