Abstract:
A ground wire adaptor for operatively connecting a low-voltage control unit (e.g. a thermostat) requiring a three-wire connection to a three-wire system-under-control (e.g. a HVAC system) using a two-wire conductor. A ground terminal and an output control signal of the control unit are interconnected such that the output control signal, in the form of a half-wave rectified signal, is superimposed on the ground. The ground wire adaptor detects the half-wave rectified signal superimposed on the ground connection and generates a control output signal to be provided to the system-under-control responsive to the presence of the half-wave rectified signal (i.e. the control signal from the control unit). In another aspect of the invention the ground wire adaptor can be used to connect an ‘N’ function control unit to an ‘N’ function system-under-control using a two-wire conductor.

Description:
FIELD OF INVENTION 
     The present invention relates to the field of low-voltage control systems. In particular, to a ground wire adaptor for use in a low-voltage control system. 
     BACKGROUND 
     The use of low-voltage (e.g. less than equal 48V) control systems to operate domestic heating, ventilation and air conditioning (HVAC) systems is common.  FIG. 1  is a schematic representation of a typical prior art two-wire control system  100 . In their simplest form these control systems historically have consisted of a low-voltage alternating cycle (AC) source  105  (e.g. a transformer) located proximate to a HVAC component  110  (e.g. a furnace), a control unit  115  (e.g. a thermostat) located in the living area of a home and a two-wire conductor  120  interconnecting the transformer  105 , the thermostat  115  and the furnace  110 . The conductor  120  is arranged to supply AC current from the transformer  105  to the thermostat  115  via a first wire  122 . The thermostat  115  switches the AC current typically using mechanical means  125  (e.g. a mercury bulb switch). The switched output of the thermostat  115  is connected via a second wire  124  to the furnace  110  to provide control (i.e. to signal a demand for heat). A current return path is provided between the furnace  110  and the transformer  105 . 
     With the advancement of electronics technology and a desire for greater energy efficiency, the analog thermostats having mechanical switching means are being replaced by digital thermostats some of which include solid-state switching means. Where it is desired to replace an analog thermostat with a digital thermostat in a home having a two-wire conductor as described above, an issue exists with regard to providing a ground reference for the digital thermostat. Normally when no heat is being demanded the output of the thermostat would be in an open circuit state and therefore no current return path to the transformer would exist to provide a ground reference for the thermostat. 
     Manufactures of digital thermostats have typically addressed the lack of ground reference using one of two approaches. The first approach is to provide the digital thermostat with an independent power source (e.g. disposable dry-cell batteries) that powers the control logic and only switching the AC current for control signal purposes. This solution is not desirable in some situations (e.g. when the home is unattended for long periods of time) as a failure of the independent power source (e.g. when the batteries are discharged) causes failure of the thermostat. The second approach is to require that the two-wire be replaced (or alternatively supplemented) with a conductor having at least three wires. In some situations replacing the conductor is not practical or is too costly. In particular, replacement of the conductor is not a viable alternative when a replacement thermostat is being marketed to a homeowner who wants to do the installation himself. 
     Another related issue arises when additional HVAC equipment (e.g. heat-pump, air conditioning, humidifier) as added to existing HVAC equipment and the thermostat is replaced with a new thermostat having additional control capability for the added equipment. Typically at least one independent wire is required for each piece of equipment in addition to one wire for supplying AC current. In existing homes the existing conductor, even when it contain more than two wires, may not have sufficient independent wires for all of the equipment. One solution is to replace (or supplement) the existing conductor but, as discussed above, in some situations replacing the conductor is not practical or is too costly. 
     What is needed is a mechanism to allow the use of a low-voltage control unit (e.g. thermostat) requiring at least a given number (‘N’) of independent connections (i.e. wires), to control one or more pieces of equipment, with an interconnecting conductor having less than ‘N’ wires. 
     SUMMARY OF INVENTION 
     A ground wire adaptor for operatively connecting a low-voltage control unit (e.g. a thermostat) requiring a three-wire connection to a three-wire system-under-control (e.g. a HVAC system) using a two-wire conductor. A ground terminal and an output control signal of the control unit are interconnected such that the output control signal, in the form of a half-wave rectified signal, is superimposed on the ground. The ground wire adaptor detects the half-wave rectified signal superimposed on the ground connection and generates a control output signal to be provided to the system-under-control responsive to the presence of the half-wave rectified signal (i.e. the control signal from the control unit). In another aspect of the invention the ground wire adaptor can be used to connect an ‘N’ function control unit to an ‘N’ function system-under-control using a two-wire conductor. 
     In accordance with one aspect of the present invention, there is provided a ground wire adaptor for connecting an environmental control, having a power terminal, a ground terminal, a control output terminal and a conductive element connected between the ground terminal and the control output terminal, to a heating/ventilating/air-conditioning (HVAC) equipment, having a first transformer output terminal, a second transformer output terminal and a control input terminal, via a two wire conductor, having a first wire and a second wire, the ground wire adaptor comprising: a first transformer input terminal connected to the first transformer output terminal for receiving a low-voltage alternating current and connected to the power terminal via the first wire; a second transformer input terminal connected to the second transformer output terminal for providing a ground return path; an EC terminal connected to the ground terminal via the second wire for providing a ground path; an asymmetrically-resistive electrical network connected between the EC terminal and the second transformer input terminal, providing a relatively lower resistance to current flow from the EC terminal to the second transformer input terminal and a relatively higher resistance to current flow from the second transformer input terminal to the EC terminal, for providing ground path continuity; a HE terminal connected to the control input terminal for providing a control signal; a control logic unit for sensing voltage on the EC terminal and responsive to detecting a negative half-cycle providing a control signal; a switching device for connecting, responsive to the control signal from the control logic unit, the first transformer terminal to the HE terminal to provide the control signal to the HVAC equipment; wherein a low-voltage alternating current half-cycle control signal is provided at the control output terminal of the environmental control when a function is being demanded and further wherein responsive to a control signal at the control input terminal, the HVAC equipment provides the demanded function. 
     In accordance with another aspect of the present invention, there is provided a ground wire adaptor for providing a plurality of control signals to a heating/ventilating/air-conditioning (HVAC) equipment, having a first transformer output terminal, a second transformer output terminal and a plurality of control input terminals, and for connecting to both ends of a two wire conductor, having a first wire and a second wire, connecting a first location and a second location, the ground wire adaptor comprising: a multi-function environmental control, at the first location, having: a power terminal connected to the first wire for receiving a low-voltage alternating current; an output terminal connected to the second wire for establishing a ground reference for the multi-function environmental control; a control logic unit for continuously generating a control bit stream having repeated frames of N+1 bits, a first synchronizing bit of each successive frame alternating between values ‘0’ and ‘1’, each of the subsequent N bits in each frame taking on a value ‘1’ when a corresponding HVAC equipment function is being demanded and taking on a valve ‘0’ when the corresponding HVAC equipment function is not being demanded; and a switching device connected between the power terminal and the output terminal for receiving the control bit stream and responsive to each bit in the bit stream operating into a non-conductive mode when the bit has value ‘0’ and operating into a conductive mode when the bit has value ‘1’ thereby generating a control signal at the output terminal; and an adaptor module, at the second location, having: a first transformer input terminal connected to the first transformer output terminal for receiving a low-voltage alternating current and connected to the power terminal via the first wire; a second transformer input terminal connected to the second transformer output terminal for providing a ground return path; a EC terminal connected to the output terminal via the second wire for providing a ground path and for receiving the control signal; an asymmetrically-resistive electrical network connected between the EC terminal and the second transformer input terminal, providing a relatively lower resistance to current flow from the EC terminal to the second transformer input terminal and a relatively higher resistance to current flow from the second transformer input terminal to the EC terminal, for providing ground path continuity; a plurality of HE terminals, each connected to a corresponding control input terminals; a plurality of switching devices each one for connecting the first transformer terminal to a corresponding HE terminal responsive to a function control signal; a control logic unit for sensing voltage on the EC terminal and for: detecting the presence or absence of negative half-cycles and associating a ‘0’ value with the absence of a negative half-cycle and associating a ‘1’ value with the presence of a negative half-cycle; detecting successive frames by identifying the synchronizing bit and associating each of the subsequent N bits in each frame with a corresponding function control signal; asserting a function control signal to each of the switching devices to enter a conductive mode of operation when the corresponding function control signal has value ‘1’; and de-asserting the function control signal to each of the switching devices to enter the conductive mode of operation when the corresponding function control signal has value ‘0’; wherein a function control signal is provided at each of the HE terminals of the adaptor module when a corresponding function is being demanded and wherein, responsive to the function control signal at the corresponding control input terminal, the HVAC equipment provides the demanded corresponding function. 
     Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art or science to which it pertains upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The present invention will be described in conjunction with drawings in which: 
         FIG. 1  is a schematic representation of a typical prior art two-wire control system. 
         FIG. 2  is a schematic representation of an exemplary ground wire adaptor in an exemplary environment in which it can be used. 
         FIG. 3  is a schematic representation of an exemplary ground wire adaptor in an alternative environment in which it can be used. 
         FIG. 4  is a schematic representation of an alternative exemplary ground wire adaptor in an exemplary environment in which it can be used. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 2  is a schematic representation of an exemplary ground wire adaptor  200  in an exemplary environment in which it can be used. The ground wire adaptor  200  can be used in conjunction with HVAC equipment  910  and an environmental control  950 . The HVAC equipment  910  can be any of the well know HVAC equipment types such as, for example, a furnace (i.e. a heating system), an air conditioning unit, a heat pump and combinations of these and similar devices that are controllable using switched low-voltage (e.g. less than or equal 48V) control circuits. The environmental control  950  can be any of the well know environmental control unit types such as, for example, a thermostat, a humidistat or other similar device that provides control of the HVAC equipment  910  by switching a low-voltage control circuit. 
     The environmental control  950  has three connection terminals: a power terminal  951 , a ground terminal  952 , and a control output terminal  953 . The environmental control  950  provides a low-voltage signal from the control output terminal  953  when the HVAC equipment  910  is to be activated (e.g. when heat is demanded). The environmental control  950  further has a half-wave rectifier  954 , a control logic unit  955  and a switching device  956 . The output of the half-wave rectifier  954  is used to power the control logic unit  955 . The switching device  956  provides switching between the power terminal  951  and the control output terminal  953  responsive to a control signal from the control logic unit  955 . The switching device  956  can be any well-known power switching device such as, for example, a mechanical relay or a solid-state switching device. 
     The HVAC equipment  910  includes a transformer  915  that provides a low-voltage AC output current. The HVAC equipment  910  has three connection terminals: a first transformer terminal  911 , a second transformer terminal  912 , and a control input terminal  913 . The HVAC equipment  910  is activated (e.g. begins generating heat) when a low-voltage signal is applied to the control input terminal  913 . In an alternative configuration (not illustrated), the transformer  915  can be independent of the HVAC equipment  910  and terminals  911  and  912  are connected to the respective terminals of the transformer  915 . 
     The ground wire adaptor  200  comprises a control logic unit  210 , a switching device  215 , an environmental control (EC) terminal  204 , an HVAC equipment (HE) terminal  203 , a first transformer terminal  201 , a second transformer terminal  202 , and an asymmetrically-resistive electrical network  220 . The switching device  215  provides switching between the first transformer terminal  201  and the HE terminal  203  responsive to a control signal from the control logic unit  210 . The switching device  215  can be any well-known power switching device such as, for example, a mechanical relay or a solid-state switching device. The asymmetrically-resistive electrical network  220  comprises a diode  222  (i.e. a rectifier) and a resistor  224  connected in parallel between the EC terminal  204  and the second transformer terminal  202  for providing a current return path for the environmental control  950  and a ground reference to second terminal  912  of the transformer  915 . The asymmetrically-resistive electrical network  220  has a substantially zero resistance to current flowing from EC terminal  204  to the second transformer terminal  202  as the current flows through the diode  222  and a relatively higher resistance to current flow in the opposite direction as the current flows through resistor  224 . Current flow in the opposite direction (i.e. from the second transformer terminal  202  to the EC terminal  204 ) results in a negative voltage, measured from EC terminal  204  to second transformer terminal  202 , that is proportional to the magnitude of the current flow. The control logic unit  210  applies a voltage sensing function to a signal on the EC terminal  204  and, responsive to the signal, provides a control signal to drive open or closed the switching device  215 . 
     A two-wire conductor  920 , having a first wire  921  and a second wire  922 , interconnects the environmental control  950  and the ground wire adaptor  200 . The two-wire conductor  920  can be a pre-existing conductor from the location of the environmental control  950  (typically in a living area of a home) to the location of the HVAC equipment  910  with the ground wire adaptor  200  being co-located with the HVAC equipment  910 . In an alternative embodiment the ground wire adaptor  200  can be located proximate an end of the two-wire conductor distal from the environmental control  950  and the HVAC equipment can be non-co-located with the ground wire adaptor  200  (i.e. can be elsewhere). The first wire  921  interconnects the power terminal  951  with the first transformer terminal  201  and the second wire  922  interconnects the ground terminal  952  and the EC terminal  204  respectively of the environmental control  950  and the ground wire adaptor  200 . A resistor  957  interconnects the ground terminal  952  and the control output terminal  953  of the environmental control  950 . In an alternative arrangement (not illustrated) an active device such as, for example, a transistor can be used to interconnect the ground terminal  952  and the control output terminal  953  of the environmental control  950  replacing resistor  957 . The transistor and associated passive components (e.g. providing transistor drive and switching-current limiting) can be incorporated into the environmental control  950 . 
     A three-wire conductor  930 , having a first wire  931 , a second wire  932  and a third wire  933 , interconnects the ground wire adaptor  200  and the HVAC equipment  910 . The first wire  931  interconnects the first transformer terminal  201  and the first transformer terminal  911  of respectively the ground wire adaptor  200  and the HVAC equipment  910 . The second wire  932  interconnects the second transformer terminal  202  and the second transformer terminal  912  of respectively the ground wire adaptor  200  and the HVAC equipment  910 . The third wire  933  interconnects the HE terminal  203  and the control input terminal  913  of respectively the ground wire adaptor  200  and the HVAC equipment  910 . In an alternative configuration the first wire  931 , a second wire  932  and a third wire  933  can be independent conductors not arranged in the three-wire conductor  930  but still interconnected as described earlier in this paragraph and proving equivalent function. 
     Waveform  810  illustrates an exemplary substantially sinusoidal voltage signal output by the transformer  915  and received at the power terminal  951 . Waveform  810  enters the rectifier  954  and waveform  820  containing only positive half-cycles results and is provided to the control logic unit  955 . 
     Waveform  830  illustrates a voltage signal output at the ground terminal  952  and received at the EC terminal  204  when switching device  956  is in an open (i.e. off) mode of operation. Waveform  830  comprises a substantially zero (0) voltage signal superimposed with positive half-cycles of substantially minor voltage magnitude compared to the voltage output by the transformer  915 . The positive half-cycles have a voltage magnitude substantially equal to the forward voltage drop of diode  222  (typically less than 1 V). 
     Waveform  835  illustrates a voltage signal output at the ground terminal  952  and received at the EC terminal  204  when switching device  956  is in a conductive (i.e. on) mode of operation. Waveform  835  comprises positive half-cycles of substantially minor voltage magnitude (e.g. less than 1 V) compared to the voltage output by the transformer  915  and negative half-cycles of more significant voltage magnitude. The amplitude ratio between the negative peak voltage at EC terminal  204  and the corresponding peak at the transformer  915  is substantially equal to the ratio of the resistance of resistor  224  to the resistance of resistor  957 . For example, when resistor  224  has a resistance value of 2 kΩ and resistor  957  has a resistance value of 10 kΩ, the ratio is 1/5. In the same example, when the transformer  915  has an output of 24 VAC RMS (i.e. approximately 33.94 V peak) the negative peak at the EC terminal  204  is substantially 6.79 V (i.e. 33.94 V/5). The control logic unit  210  uses a voltage sensing mechanism to differentiate between waveform  830  and waveform  835 . The voltage sensing mechanism can, for example, comprise a resistor divider connected between the EC terminal  204  and a reference voltage from an analog to digital (A/D) converter included in the control logic unit  210 , wherein the output of the resistor divider is sampled by the A/D converter input. Alternatively the voltage sensing mechanism can comprise any well-known analog circuit including an operational amplifier or voltage comparator arranged for comparing a voltage to a reference voltage level. When waveform  830  is detected, switching device  215  is driven into the open (i.e. off) mode of operation resulting in no demand for the HVAC equipment  910 . When waveform  835  is detected, switching device  215  is driven into the conductive (i.e. on) mode of operation resulting in a waveform similar to  810  being presented at control input terminal  913  signifying a demand for the HVAC equipment  910  to operate. The ground wire adaptor  200  embodiment of  FIG. 2  provides for an environmental control  950  having one control output to be operatively connected to a two wire voltage supply (i.e. the transformer  915 ) and to a HVAC equipment  910  having a control input, using only two wires while providing a ground reference to the control unit  950  even when the control output of the environmental control  950  is not asserting a demand for operation of the HVAC equipment  910 . 
       FIG. 3  is a schematic representation of the exemplary ground wire adaptor  200  in an alternative environment in which it can be used. In the alternative environment the HVAC equipment  910  has multiple (‘N’) input control signal terminals, each one for receiving a control signal for a separate function (e.g. heat, cool, fan) in the HVAC equipment  910 . The environmental control  950  has a corresponding number (‘N’) of switching devices for providing corresponding control signals to the HVAC equipment  910 . The ground wire adaptor  200  is connected and operates in the same way as described above with reference to  FIG. 2  to provide a first control signal from the environmental control  950  to the HVAC equipment  910  and to provide a ground reference to the environmental control  950 . The remaining control signals (i.e. 2 through N) are provided by interconnecting each of the remaining (i.e. 2 through N) switching devices in the environmental control  950  to a corresponding input control signal terminals on the HVAC equipment  910  using individual wires for each signal. The ground wire adaptor  200  embodiment of  FIG. 3  provides for an environmental control  950  having N separate control output to be operatively connected to a two wire voltage supply (i.e. the transformer) and to an N input HVAC equipment  910  using only N+1 wires while providing a ground reference to the control unit  950  even when the control output of the environmental control  950  is not asserting a demand for operation of the HVAC equipment  910 . 
       FIG. 4  is a schematic representation of an alternative exemplary embodiment of the ground wire adaptor  200  in an exemplary environment in which it can be used. An HVAC equipment  910  has multiple (numbered ‘1’ through ‘N’) input control signal terminals  913 , each one for receiving a control signal for a separate function (e.g. heat, cool, fan) in the HVAC equipment  910 . The alternative embodiment of the ground wire adaptor  200  comprises an adaptor module  260  and a multi-function environmental control  250 . The multi-function environmental control  250  provides for the encoding of a controls signal for each of the ‘1’ through ‘N’ functions of the HVAC equipment  910  based on parameters, including sensed environmental parameters (e.g. temperature and humidity), and can be located distal (i.e. remote) from the adaptor module  260 . The adaptor module  260  provides similar functions and comprises components substantially as described above with reference to the ground wire adaptor  200  and  FIG. 2  except as otherwise specified below. 
     The multi-function environmental control  250  has two connection terminals: a power terminal  251  and a control output terminal  252 . A two-wire conductor  920 , having a first wire  921  and a second wire  922 , interconnects the environmental control  250  and the adaptor module  260 . The two-wire conductor  920  can be a pre-existing conductor from the location of the multi-function environmental control  250  (typically in a living area of a home) to the location of the adaptor module  260 , with the adaptor module  260  typically being co-located with the HVAC equipment  910 . The first wire  921  interconnects the power terminal  251  with the first transformer terminal  201  and the second wire  922  interconnects the control output terminal  252  and the EC terminal  204  respectively of the multi-function environmental control  250  and the adaptor module  260 . 
     The multi-function environmental control  250  further comprises a half-wave rectifier  254 , a control logic unit  255  and a switching device  256 . The output of the half-wave rectifier  254  is used to power the control logic unit  255 . The multi-function environmental control  250  uses the control output terminal  252  to establish a ground reference and further provides a low-voltage signal from the control output terminal  252  when any of the N functions of the HVAC equipment  910  are to be activated (e.g. when heating, cooling or the fan is demanded). Waveform  840  illustrates an exemplary voltage signal at the control output terminal  252  and received at the EC terminal  204 . The waveform  840  consists of positive half-cycles of substantially minor voltage magnitude compared to the voltage output by the transformer  915  interspersed between each of one of a train of N+1 negative half-cycles numbered ‘0’ through ‘N’. The 0 th  negative half-cycle is a synchronizing (i.e. framing) bit that alternates between a substantially zero (0) voltage value and a more significant voltage magnitude (hereinafter referred to as values ‘0’ and ‘1’ respectively). Each of the remaining negative half-cycles (1 through N) in the train independently take on a value of ‘0’ or ‘1’ responsive to the control logic unit  255  demanding the corresponding function in the HVAC equipment. The train of N+1 negative half-cycles (and the corresponding interspersed positive half-cycles) is repeated continually, one train after another. The train of negative half-cycles is generated by switching device  256  being driven alternately between an open and a conductive mode of operation responsive to a control signal received from the control logic unit  255 . Waveform  850  illustrates an exemplary control signal from the control logic unit  255  to the switching device  256 . The control signal can, for example, be a serial digital bit stream. 
     The adaptor module  260  comprises a control unit  210 , an asymmetrically-resistive electrical network  220 , and a plurality of switching devices  215  each one corresponding to one of the N HVAC equipment  910  functions, an environmental control (EC) terminal  204 , a plurality of HVAC equipment (HE) terminals  203 , a first transformer terminal  201 , and a second transformer terminal  202 . Control logic unit  210  decodes the waveform  840  to extract N individual HVAC function demand control signals. The control logic unit  210  synchronizes (i.e. frames) the train of half-cycles in waveform  840  using the synchronizing bit (i.e. the 0 th  half-cycle). The synchronizing bit is identified by the fact that it alternates between values ‘0’ and ‘1’ with each successive train while the other (1 through N) half-cycles tend to remain in one value or the other for longer periods of time as they represent HVAC equipment  910  function demands. When the synchronizing bit has been identified, the individual function demand control signals (1 through N) can be derived from the subsequent N consecutive negative half-cycles. The logic control unit  210  can alternately drive open or conductive (i.e. off or on) each of the corresponding switching devices  215  responsive to the corresponding HVAC function demand control signals (1 through N). In a preferred embodiment, for each function demand control signal (1 through N) the logic control unit  210  can wait until the function demand control signal is found to have the same value (i.e. ‘0’ or ‘1’) for a pre-determined number (e.g. two) of successive trains before responding to the function demand control signal in order to mitigate sensitivity to electrical noise, transients and mis-synchronization. The HVAC equipment  910  activates an associated function (e.g. heating, cooling) responsive to each low-voltage function control signal that is asserted. The ground wire adaptor  200  embodiment of  FIG. 4  provides for an environmental control  250  having support for control of N separate HVAC equipment functions to be connected to a two wire voltage supply (i.e. the transformer) and to an N input HVAC equipment  910  using only two wires. 
     In the above description, reference has been made to negative half-cycles. It will be understood that by configuring the half-wave rectifier and voltage sensing accordingly (e.g. reversing the polarities of half-wave rectifier  954  and diode  222 ), positive half-waves can be substitutes for the negative half-cycles in the above described embodiments while maintaining the features and functionality of the ground wire adaptor  200 . 
     It will be apparent to one skilled in the art that numerous modifications and departures from the specific embodiments described herein may be made without departing from the spirit and scope of the present invention.