Abstract:
An electronic thermostat circuit has improved power stealing for controlling an AC control device. The circuit comprises a source of AC control power coupled to an electronic switch means having an electronic switch means control input. The electronic switch means controls the AC control device. The diode bridge controls the electronic switch means by a DC control signal applied to the diode bridge means DC connection. The amplifier means has an amplifier input for controlling the state of the amplifier and an amplifier output for generating the DC control signal. An isolated gate FET means is electrically coupled to the amplifier input for controlling the state of the amplifier. The isolated gate FET means is powered by the current derived from the source of AC control power by power stealing. The digital signal controls the state of the AC control.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to an electronic thermostat output control circuit to improve power stealing, and more particularly to a circuit topology to minimize the power needed to control thermostat outputs.  
       BACKGROUND OF THE INVENTION  
       [0002]     Electronic thermostats can be used to control the temperature in a building space as part of the building&#39;s heating, ventilation and air conditioning (“HVAC”) system. Thermostats typically receive temperature information from one or more temperature sensors. While simple thermostats react to the sensor inputs using only analog circuits, most modem thermostats run a microcontroller program or algorithm. The algorithm determines when the various devices controlled by the thermostat should be turned on or off based on the temperature data. Thus, thermostats function as switches to control devices such as furnaces, blowers, motors, and compressors. Rather than switching the full device load current, the controlled devices are typically switched through intermediate controls such as relays or solid state switches.  
         [0003]     A thermostat is typically placed in or near the space where it is to control the temperature. A minimal number of wires are run from the thermostat&#39;s location to the location or locations of the various devices controlled by the thermostat. The most common configuration is to run a single control wire for each device with a common return wire for all of the device controls. Typically the control power for this circuit is 24 VAC provided by a control transformer.  
         [0004]     Thermostat electronics circuitry can be powered by a local power source such as a battery. In addition, parts of the control circuitry can be powered by trickling some small amount of current from the control circuit using “power stealing”, that is drawing a relatively small amount of power from a device control line without actually switching the device on. One problem is to minimize the amount of power stealing so as to avoid a false switching of the device being controlled by that line. Another problem is that even with power stealing, there can still be significant battery drain by the output electronic circuits in the thermostat that controls the switching of each device controlled by the thermostat.  
         [0005]     Accordingly there is a need for an electronic thermostat circuit topology that can minimize the current needed to control thermostat outputs.  
       SUMMARY OF THE INVENTION  
       [0006]     An electronic thermostat circuit has improved power stealing for controlling an AC control device. The circuit comprises a source of AC control power coupled to an electronic switch means having an electronic switch means control input. The electronic switch means controls the AC control device. A diode bridge means has an AC connection and a DC connection. The AC connection is electrically coupled to the electronic switch means control input. The diode bridge controls the electronic switch means by a DC control signal applied to the diode bridge means DC connection. An amplifier means has an on state and an off state. The amplifier means is electrically coupled to the diode bridge means DC connection. The amplifier means has an amplifier input for controlling the state of the amplifier and an amplifier output for generating the DC control signal. An isolated gate FET means is electrically coupled to the amplifier input for controlling the state of the amplifier. The isolated gate FET means is further electrically coupled to a digital input signal. The digital input signal controls the state of the DC amplifier means. The isolated gate FET means is powered by the current derived from the source of AC control power by power stealing. The digital signal controls the state of the AC control. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]     The advantages, nature and various additional features of the invention will appear more fully upon consideration of the illustrative embodiments now to be described in detail in connection with the accompanying drawings. In the drawings:  
         [0008]      FIG. 1  shows the inventive circuit topology;  
         [0009]      FIG. 2  shows a typical thermostat switch circuit with power stealing (prior art);  
         [0010]      FIG. 3  shows a typical thermostat with a plurality of output circuits (prior art);  
         [0011]      FIG. 4  shows an exemplary embodiment of the inventive circuit topology; and  
         [0012]      FIG. 5  is a schematic diagram of output circuit with high current load (prior art). 
     
    
       [0013]     It is to be understood that the drawings are for the purpose of illustrating the concepts of the invention and are not necessarily drawn to scale.  
         [0014]     In the schematic diagrams, unless otherwise stated, an upward pointing triangle represents a DC power supply, a sideways triangle (left or right) represents a circuit connection and a downward pointing triangle represents an electronic circuit common. A circle with a slash represents an electrical terminal (such as a binding point comprising a screw and threaded plate to hold an attached captive wire) and can be used as a circuit connection.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0015]      FIG. 1  shows the inventive thermostat output circuit topology. Circuit  101  shows the inventive thermostat output for controlling a device turned on or off by AC control  110 . Each system device  113 , such as a furnace or compressor, is controlled by a respective AC control  110 . AC control  110  can be a relay coil or solid state switch or other type of AC operated system device control. AC power source  109  provides the AC power to switch a device control represented by AC control  110 .  FIG. 1  shows a single thermostat output for controlling one system device  113 . An electronic switch with a control input, such as TRIAC D5 controlled by gate  115  can switch AC control  110 . Typical thermostats comprise a plurality of such outputs, for controlling system devices  113  such as fans, compressors, furnaces, etc.  
         [0016]     Thermostat microcontroller  102  signals the thermostat output to change the on or off condition of the system device  113  as controlled by AC control  110 . Isolated gate FET Q 1  is controlled by microcontroller (“μC”)  102  via a connection made at the Gate of Q 1  at connection point  112 . Connection point  112  can be a wire such as a trace on a PC board or a connector. Connection points are shown by left or right pointing arrows. Because Q 1  is an isolated gate FET, such as an enhancement mode FET, the DC current supplied by DC source +V  111  to maintain the Gate of switch Q 1  in its switched state is on the order of microamps, or more typically nanoamps. When Q 1  is switched on by μC  102  it provides control current through R 1  to cause amplifier switch  103  to be on. Amplifier  103  can be a composite Darlington transistor topology or equivalent DC electronic switch as known in the art. Amplifier switch  103  requires only a small current on the order of microamps to cause it to turn on. Thus it can be seen that the Q 1  drain current is on the order of tens of microamps. Resistor R 1  can further limit the control current provided by FET Q 1  to only that current needed to reliably turn on amplifier switch  103 . Diodes D 1  through D 4  are wired in a diode bridge  114  configuration. By its connection to the DC connection of bridge  114 , when amplifier switch  103  is turned on, AC current can flow through the AC connection of bridge  114  and the gate  115  of TRIAC D 5 , thus powering TRIAC D 5  on and energizing the respective control load  110  via connections  104  and  105 . Note that connections such as  104  and  105  can be interchangeably represented on the schematic diagrams as side facing arrows or a circle with a slash line representing a screw terminal.  
         [0017]     Except for the tens of microamps supplied by FET Q 1  to control amplifier switch  103 , all other DC and AC power for controlling TRIAC D 5  and the respective control load  110  comes from AC power source  109 . It can now be seen that by using this thermostat output circuit topology, power source +V  111  requires only tens of microamps per output channel to energize a particular system device  113  via its respective AC control  110 . And, when the system device  113  is to be controlled to its off state, virtually no current is required from +V  111  in holding FET Q 1  off, where both the Q 1  gate current and Q 1  drain current are near zero.  
         [0018]     Another technique of thermostat power stealing was described in “Power Supply for Electronic Thermostat”, U.S. Pat. No. 6,205,041 issued Mar. 20, 2001. U.S. Pat. No. 6,205,041 is incorporated by reference herein.  FIGS. 2-3  show typical electronic thermostat output configurations using power stealing.  
         [0019]      FIG. 2  is a simplified diagram showing electronic thermostat power stealing. AC power source  109  activates AC control  110  when switch  202  closes. AC control  110  is electrically referenced to AC common  108 , while the electronic circuit of the thermostat is referenced to thermostat electronics common  106 . Switch  202  can be a mechanical switch, relay contact, solid state switch contact, or a semiconductor switch, such as a TRIAC. Power stealing can occur when switch  202  is open. One such way to power steal is through diode  203 , passing a rectified current to regulator  204  with a return current path through electronics common  106 . (Standard filter capacitors are not shown for simplicity.) In this way potential +V  111  can be created by power stealing. Most thermostats typically include a plurality of controlled outputs  201 . Since power stealing from each controlled output  201  occurs when switch  202  is open, diodes  204 - 206  permit any of the other controlled output sections to contribute power to regulator  204 . Thus only one of “N” switches need be open to permit power stealing via regulator  204 . In the rare case that all switches are closed, a battery can power +V  111  until at least one of the switches opens.  
         [0020]      FIG. 3  shows how a typical thermostat  315  can control a plurality of system devices. In this example, the terminals power system devices via AC controls following conventions of the art including terminals, “G”  104 , fan motor control  110 ; “W”  306 , furnace control  310 ; “Y”  307 , compressor control  311 ; “Out 1 ”  308 , load control  312 ; and “Out 2 ”  313 , load control  2   313 . AC power source  109  powers the AC controls via switches  202  and  302 - 305 . As in the previous diagrams, thermostat electronics  315  can be referenced to electronics common  106  connected to terminal R in the AC circuit. It can be seen in this diagram that a more conventional DC power supply could be built using AC common terminal “C”  314  if it is available at the thermostat. But, most traditional HVAC control systems do not wire the AC common “C” back to the thermostat, thus there is a need for improved power stealing techniques.  
         [0021]     Example:  
         [0022]      FIG. 4  shows an advantageous embodiment of the inventive circuit topology  101  of  FIG. 1 . It is to be understood that these component values and component types are merely exemplary values and types that were used in a particular embodiment of the inventive circuit topology. For example, any suitable P channel enhancement mode FET can be used as Q 11 , or any suitable NPN or PNP transistors can be used as Q 12  and Q 13 . Similarly, the values of resistors and capacitors can be varied in other embodiments.  
         [0023]     It is to be noted that a particular embodiment of the exemplary output circuit topology  101  of  FIG. 4  has the following component values: 
    Q 11  BSS84, P channel enhancement mode FET     Q 13  MMBTA05LT1     Q 12  MMBTA55LT1     D 11  MMBD1204     D 14  MMBD1205     D 13  T405-600B     R 53  2.2 Meg Ohms     R 54  2.2 kilo Ohms     R 55  1 Meg Ohms     R 60  100 kilo Ohms     R 61  150 kilo Ohms     R 56  4.7 kilo Ohms     R 58  150 Ohms     C 21 , C 33  0.1 micro Farads    
 
         [0038]     The operation of the circuit of  FIG. 4  when the components have the particular values as set forth above will now be described. The improved power stealing circuit topology  101  comprises FET Q 11  coupled to a PC  102  output control line “P 9 _ 4 ”. When P 9 _ 4  is “HIGH” or logic level  1 , the potential near +V causes the gate-source voltage of Q 11  to be above a level that turns Q 11  on, therefore both the gate and drain current are near zero. When μC  102  output control line P 9 _ 4  goes “LO” or to logic level  0 , a very small current, typically on the order of nano Amps, flows through R 53  and R 55  causing a Q 11  gate-source voltage that turns Q 11  on. Once on, a current on the order of tens of microamps flows through R 60 , R 61 , and the base of Q 13 , turning on composite Darlington transistor amplifier switch  103 . Amplifier switch  103 , comprising Q 13 , Q 12 , and resistors R 56  and R 54 , conducts causing a DC current in the DC connection to the bridge comprising dual diode packages D 11  and D 14  (equivalent to the  FIG. 1  diode bridge comprising diodes D 1 -D 4 ). The DC current flow in the bridge causes an AC current to flow through the bridge AC connection from terminal G to the gate of TRIAC D 13 , thus energizing TRIAC D 13  and the corresponding AC control connected to terminal G. As previously discussed, a typical electronic thermostat comprises a plurality of output control circuits  101 .  
         [0039]     It can now be seen that each thermostat output circuit  101  of the improved circuit topology draws only tens of microamps at most from supply +V. This is particularly advantageous because +V is supplied by a combination of power stealing and battery power. Since many thermostats use non-rechargeable batteries it is important to minimize the power drawn by each output circuit  101 . (By contrast,  FIG. 5  shows an output circuit topology where control currents, and thus loading is far higher on the order of milliamps.) Moreover, if too much power is drawn by power stealing, one or more AC controls might be inadvertently activated by power stealing, rather than an actual “ON” command.  
         [0040]     It should be noted that while the exemplary circuits show Q 1  or Q 11  as a “P channel” enhancement mode FET in a high side switch configuration, the connection to a microcontroller output could also be accomplished by a low side isolated gate “N channel” FET using a pull up resistor. Current saving performance may be different in embodiments using an N channel switch.