Abstract:
A new circuit and associated methods are disclosed for stealing power from HVAC circuit to supply relays and control circuits in an electronic thermostat, and protecting against damage to the relays from over-current condition. If a common connection is available, the circuit can obtain DC power always, if not available, the circuit can still obtain DC power when one of the relays is turned on, and the obtained power can be used to keep turning on the relay, making it possible to use economical and smaller form factor non-latching type relays or solid state relays, without wasting the limited battery charge. Compared with existing power stealing thermostat circuits, the disclosed circuit is advantageous due to its simplicity and no possibility of inadvertently turning on or off the HVAC.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     FEDERALLY SPONSORED RESEARCH 
       [0001]    Not applicable 
       SEQUENCE LISTING OR PROGRAM 
       [0002]    Not applicable 
       BACKGROUND OF THE INVENTION 
       [0003]    1. Field of the Invention 
         [0004]    This invention relates to the control of HVAC systems and/or for other systems for switching on and off one or more AC load. More particularly, embodiments of this invention relate facilitating power stealing or power harvesting in a control device such as a thermostat having a limited battery charge, and protection against circuit damage from short circuit and other over-current conditions. 
         [0005]    2. Description of the Related Art 
         [0006]    Conventional thermostats are typically battery powered and use mechanical latching relay to connect and disconnect wires labeled “Y”, “W” and/or “G” with “R” which supply 24V AC voltage. The configuration assumed for a thermostat is shown in  FIG. 4 . Although the relay needs substantial current to turn on, with a mechanical latching relay the current is only needed for a short amount of time during transitioning from off to on state or vice versa. 
         [0007]    However, latching relays require two, set and reset coils, and are more bulky and expensive than non-latching type mechanical relays, which only need one coil, or solid state relays (SSR). To maintain non-latching relays and SSRs in conducting state, a substantial current must be flowed constantly. This power consumption due to this current is often much larger than the thermostat control circuit power consumption. 
         [0008]    Some thermostat devices have been designed to “steal” power from the voltage potential between the 24VAC power source connection “Rc”, “Rh” or “R” wire and one of the HVAC control wires (load), such as U.S. Pat. No. 8,110,945 and U.S. Pat. No. 5,903,139.  FIG. 1  is the simplified circuit diagram of such prior art power stealing thermostats. However, if too much power is consumed by the thermostat control circuit, AC, heat or fan can be inadvertently turned on. U.S. Patent Application US20120199660 describes an elaborate scheme to reduce the possibility this can happen, but still cannot completely eliminate such possibility. 
         [0009]    Compared with mechanical relays, SSRs have advantages such as small form factor, silent operation, and high reliability due to no moving parts. However, they are more easily damaged if current higher than the rated current is flowed. Even mechanical relays can be damaged by over-current. Prior art such as U.S. Pat. No. 5,864,458 address this issue by using a combination of PTC type fuse and switches. However, because they are connected in series with the load, there is a trade-off between voltage drop on the fuse and the level of protection. Also, the PTC fuse needs to heat up before the protection mode is triggered, and this process is often too slow to save SSR from damage due to over-current. 
       BRIEF SUMMARY OF THE INVENTION 
       [0010]    I have discovered in accordance with this invention, a simple yet effective circuit that can be built using readily available components, to generate from the 24VAC power source in HVAC system, very stable 3V DC power to supply thermostat control circuits and relays, without substantially affecting the functionality of the HVAC system and without the possibility of inadvertently turning on or off the HVAC functions, and at the same time, providing over current protection function that is much faster acting than PTC fuses, to allow protection of SSRs and other circuit components from damage due to over current. 
         [0011]    According to the preferred embodiment shown in  FIG. 2   a , a circuit is described that controls a PNP bipolar junction transistor (BJT) Q 1  based on a feedback circuit that detects the positive voltage drop across the collector-emitter of Q 1 , and controls the base of Q 1  such that the voltage drop is no higher than a desired DC voltage, such as 3V. This circuit is connected in series with AC relays that turns on or off HVAC functions. When higher than the desired voltage is applied, the base of Q 1  is pulled down by the feedback circuit such that Q 1  becomes conducting enough to allow the extra voltage to be passed to the AC load to turn on HVAC function. The feedback circuit itself is also connected in parallel with the BJT and in series with the AC load. Such feedback circuit is readily available at low cost, preferably implemented as part number TLVH431 from Texas Instruments. By the parasitic diode internal to the BJT Q 1 , the circuit will conduct current when collect-emitter voltage is negative, but a second shunt diode D 1  can be added if the parasitic diode has unspecified characteristics. As a result, the collector-emitter voltage of Q 1  will be exactly 3V during the positive half cycle of the 24V AC, and ˜0V during the negative half cycle. 
         [0012]    A diode and a large capacitor generate a stable DC supply from the half-wave rectified collector-emitter voltage of Q 1 . The feedback circuit allows stable voltage to be generated regardless of how much current is consumed by the AC load. 
         [0013]    Because the thermostat typically uses 24VAC (rms) or above, the AC load originally sees voltage swinging from approximately +34V to −34V, and with the proposed circuit, the AC loads still see voltage swinging from approximately +31V to −34V, i.e. 95% or more of the original voltage swing, allowing proper functionality of the AC loads to be retained. 
         [0014]    Unlike other power stealing thermostats, there is no possibility of false switching because if the power stealing circuit uses too much power, it will only make the AC loads see higher (than 95%) voltage swing, making it more reliable to turn on/off HVAC functions, until diode D 3  turns on and the thermostat control circuit and the relays receive supplemental power from battery  109 . 
         [0015]    The insertion of the BJT Q 1  not only allows power stealing, but also allows it to function in place of a current sensing resistor that must be placed in series with the load to detect over-current in other over current protection schemes, such as U.S. Pat. No. 8,035,938. This is done by adding a scaled down PNP BJT Q 2  with similar temperature characteristics as Q 1 , and connecting their base and emitter to form a current mirror. The collector current in Q 2  will then be proportional to the load current, with a constant, substantially temperature independent scaling factor. This current is flowed through resistor R 4  to generate voltage drop in the 3V DC domain which supplies microprocessor  103  and comparator  102  with low propagation delay. If the voltage on R 4  rises above a threshold, a microprocessor interrupt is generated by the comparator  102  and the interrupt routine in the microprocessor  103  turns off all relays  104 ˜ 106  to break the circuit. Because no additional current sensing resistor is needed, not only will there be more voltage to be applied to AC load, but also no need to design additional heat sink element for such current sensing resistor. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         [0016]    In the accompanying drawings: 
           [0017]      FIG. 1  is a view showing the configuration of a power stealing thermostat circuit in the related art; 
           [0018]      FIG. 2   a  is a view showing the configuration of a power stealing thermostat circuit with over-current protection circuit according to a preferred embodiment of the present invention; 
           [0019]      FIG. 2   b  is a view showing the configuration of a power stealing thermostat circuit according to a second embodiment of the present invention; 
           [0020]      FIG. 2   c  is a view showing the configuration of a power stealing thermostat circuit according to a third embodiment of the present invention; 
           [0021]      FIG. 3  is a graph of waveforms illustrating the operation of the invention; and 
           [0022]      FIG. 4  is a view showing the assumed external connections of thermostat circuit shown in  FIGS. 1 ,  2   a , and  2   b.    
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0023]    Embodiments of active current surge limiters are described below. It should be emphasized that the described embodiments are merely possible examples of implementations, and are set forth for clear understanding of the principles of the present disclosure, and in no way limit the scope of the disclosure. 
         [0024]    The preferred embodiment of the invention is shown in  FIG. 2   a , which shows an internal configuration of a thermostat  400  in  FIG. 4  with basic function to turn on heat, AC and fan by connecting the AC power source  401  with AC loads  402  using solid state relays. Terminal  207  corresponds to either terminal “R”, “Rc” or “Rh” in a conventional thermostat and is typically connected to one end of a 24V AC power source. Terminal  208  corresponds to terminal “C” in a conventional thermostat and is connected to the opposite end of the 24V AC power source. 
         [0025]    A high voltage, high wattage bipolar junction transistor Q 1 , preferably DXT2014P5 from Diodes Inc., is connected among terminal R, one end of relays  204 ˜ 206  and output terminal of  201  as shown. The wattage of Q 1  is determined by the desired DC voltage to be generated (3V in this embodiment) times the maximum possible AC current from terminal R, times half. A heat sink may have to be attached to Q 1 . In place for bipolar junction transistor for Q 1 , an equivalent field effect transistor may also be used. 
         [0026]    Voltage reference and feedback circuit  201 , preferably implemented using part number TLVH431 from Texas Instruments, is powered by bias current generated by R 1 . The value of R 1  should be chosen small enough to supply  201  as well as the base current of BJT Q 1 . The output terminal of  201  is connected to the base terminal of Q 1 . The resistor divider formed by R 2  and R 3  generates a feedback voltage and is connected to the input terminal of  201 , such that desired collector-emitter voltage of Q1*R3/(R2+R3)=reference voltage in  201 . The size of R 2  and R 3  is determined by the reference input current requirement of circuit  201 . The ground terminal of  201  is connected to the collector terminal of Q 1 . 
         [0027]    Solid state relays  204 ˜ 206  are connected between collector terminal of Q 1  and each type of AC loads  402 . The control terminals of  204 ˜ 206  are connected to microprocessor  203 , such that firmware program running in  203  can turn on and off each of the relay  204 ˜ 206 . 
         [0028]    The diode D 1  is connected between collector and emitter of Q 1  and may be omitted because the Q 1  inherently includes this diode, in the form of what is called “body diode” of the transistor; but is beneficial to be included to allow more AC voltage to be applied to the AC load. 
         [0029]    The diode D 2 , preferably RB056L-40TE25 from Rohm Semiconductor, is connected between collector of Q 1  and Vss of the microprocessor. D 2  should be chosen to have much lower reverse leakage current than the sleep current of the microcontroller  203 , such that when HVAC is not turned on, the leakage will not degrade the battery life. At the same time, D 2  should have low forward voltage drop such that the collector-emitter voltage of Q 1  can be chosen as small as possible. 
         [0030]    A large, preferably 220 uF capacitor C 2  is connected between Vdd and Vss of the microprocessor  203 . The size of this capacitor is determined by the current requirement of the microprocessor  203  and other circuits that use the generated DC supply, such as comparator  202 , radio transceivers, and the relays  204 ˜ 206 . 
         [0031]    A second PNP BJT Q 2 , preferably BC857B from NXP Semiconductor, is connected as shown in the figure with base and emitter terminals tied to the base and emitter terminals of Q 1 , respectively. The nominal current of Q 2  is chosen to be smaller than Q 1  and they have similar temperature characteristics, such that when both are in linear region, their collector currents are related with a fixed, temperature independent radio. 
         [0032]    A low power comparator and a reference  202 , preferably MIC842HYC5 from Micrel Inc., is connected between Vdd and Vss. The input of  202  is connected to net  210 . The output of  202  is full swing digital signal, and is connected to the interrupt input of  203 . 
         [0033]    The collector of Q 2  is connected to resistor R 4 . The other end of R 4  is shared with the Vss of the comparator reference. The value of R 4  is chosen such that the voltage on  210  exceeding the comparator reference voltage indicates the collector current of Q 1  exceeding the rated current of any of the relay  204 ˜ 206 . 
         [0034]    The microcontroller  203  contains a firmware program that enables the interrupt, and includes an interrupt service routine (ISR) that is run whenever the comparator output indicates over current condition. The ISR turns off all relays  204 ˜ 206 , and then notify user of the over-current condition through LED, sound, or through wireless signals. 
         [0035]    A capacitor C 1  is connected between the collector of Q 1  and common terminal  208 , typically labeled “C”. When terminal  208  is connected, current flows through C 1  and Q 1 , allowing the circuit to generate stable 3V DC supply without wasting much power because the voltage and current in C 1  are substantially out of phase. C 1  is chosen to sustain at least 34V voltage and with 24VAC applied, allowing sufficient current to flow to maintain 3V on the capacitor C 2 . The preferred size of C 1  is found to be 10 uF to 15 uF in this preferred embodiment. 
         [0036]    A second embodiment of the invention is shown in  FIG. 2   b . A high current shunt regulator  220 , which can be implemented using a variety of methods including Zener diode and feedback circuit similar to those in TL431 from Texas Instruments, is connected between terminal R and one end of relays  224 ˜ 226 . The shunt regulator  220  tries to maintain the voltage across it at a fixed value, for example 3V, by adjusting its impedance. As a result, if terminal C is connected, or any of the relays  224 ˜ 226  is turned on and corresponding terminal is connected, current will flow from R, and during the positive half cycle, 3V appears across  220  and during the negative half cycle 0V appears across  220  because of D 4 . D 5  and C 4  generates a DC voltage from this waveform to supply microprocessor  223  which in turn supplies relays  224 ˜ 226 . When there is not enough current flowing from terminal R,  228  starts to conduct, and battery  229  provides supplemental DC power. Otherwise,  238  may use the generated power to charge battery  239 . When terminal C is connected, the capacitor C 3  allows enough current to flow from terminal R to supply microprocessor  223  and indirectly relays  224 ˜ 226 . 
         [0037]    A third embodiment of the invention is shown in  FIG. 2   c . Everything is similar to the second embodiment except that the generated DC supply shares a different terminal with the AC power source. This configuration is sometimes necessary if the relays  234 ˜ 236  are not electrically isolated. However, to implement the over-current protection, along with NPN power transistors, Vdd referenced voltage references are needed, and these components are not as commonly available as their Vss referenced counterparts. 
         [0038]    A high current shunt regulator  230 , which can be implemented using a variety of methods including Zener diode and feedback circuit similar to those in TL431 from Texas Instruments, is connected between terminal R and one end of relays  234 ˜ 236 . The shunt regulator  230  tries to maintain the voltage across it at a fixed value, for example 3V, by adjusting its impedance. As a result, if terminal C is connected, or any of the relays  234 ˜ 236  is turned on and corresponding terminal is connected, current will flow from R, and during the positive half cycle, 3V appears across  230  and during the negative half cycle 0V appears across  230  because of D 7 . D 8  and C 6  generates a DC voltage from this waveform to supply microprocessor  233  which in turn supplies relays  234 ˜ 236 . When there is not enough current flowing from terminal R,  238  starts to conduct, and battery  239  provides supplemental DC power. Otherwise,  238  may use the generated power to charge battery  239 . When terminal C is connected, the capacitor C 5  allows enough current to flow from terminal R to supply microprocessor  233  and indirectly relays  234 ˜ 236 . 
         [0039]    For purposes of explaining the operation of the invention as embodied in the  FIG. 2   a  circuit,  FIG. 3  shows voltages at various points for two cycles of the AC waveform present between terminals “R” and “C”. For purposes of explaining the invention, the waveform is not drawn to scale, but the voltage levels are marked. Each diode D 1  and D 2  is assumed to be ideal, i.e. with zero forward bias voltage and zero reverse leakage current. Terminals R and C are assumed to be connected to a 24V RMS sine wave AC power source, which is typically used by HVAC systems, so the peak AC voltage is approximated +/−34V. Relay  204  is presumed to be turned on (conducting) with zero impedance.  301  is the waveform of terminal R and Y, measured against terminal C, during the negative half cycle of the AC power source.  302  is the waveform of terminal R measured against terminal C during the positive half cycle, and  303  is the waveform of terminal Y measured against terminal C during the positive half cycle. Because the voltage drop on Q 1  is 3V only during the majority of positive half cycle, Y terminal swings from −34V to +31V, while R terminal swings from −34V to +34V. Therefore, the Y terminal receives higher than 95% of the voltage available from the source, allowing it to control HVAC function properly.  305  is the waveform on net “Vdd” measured against net “Vss”. During the positive half cycle, D 2  conducts making it 3V. During the negative half cycle, D 2  is reverse biased, and the capacitor C 2  provides charge, and this waveform will see slight droop until the next positive cycle.  304  is the waveform on net  210  measured against net “Vss”. The peak voltage of  304  is substantially proportional to the peak current flowing through the relay  204  because when  304  reaches near its peak, both Q 1  and Q 2  are in the linear operation region and forms a current mirror, and since the total current flowing through R 2 , R 3 ,  201 , D 1 , C 2 ,  202 ,  203  and  209  is negligibly small compared with the current flowing in Q 1  and the relay  204 . Therefore, comparator  202  can detect over-current condition by comparing waveform  304  against a fixed reference voltage.