Abstract:
A power supply circuit for an electronic thermostat includes a half wave rectifier, at least one voltage dropping resistor and a gated rectifier. The gated rectifier preferably conducts only when the voltage at its output drops below the voltage at its gate. The voltage at the gate is defined by gating circuitry downstream of the voltage dropping resistors. The intermittent conduction of the silicon controlled rectifier produces less current flow through voltage dropping resistors within the power supply circuit. This causes less heat dissipation by these resistors within the thermostat.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to wall mounted electronic thermostats and, in particular, to power supplies for such thermostats. 
     Wall mounted electronic thermostats for controlling temperatures in rooms are well known. These devices are commonly powered by batteries or by low voltage alternating current sources, typically 24 volts AC. The 24 volt AC power must be reduced to a lower level DC voltage for use in supplying power to the various component parts of the thermostat. This has heretofore been accomplished by various power supply circuits, which employ resistors to reduce the voltage level. These voltage dropping resistors dissipate heat which in turn can impact the sensing of room temperature by the thermostat. These resistors also produce additional loads on the transformer, which provides the stepped down 24 volts AC to the thermostat. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide an electronic thermostat with a power supply circuit that substantially reduces the amount of heat produced by the power supply circuit in stepping down the voltage through the voltage dropping resistors. 
     The above and other objects are achieved by a power supply circuit, which includes a half wave rectification of the 24 volt AC signal. Resistors downstream of the half wave rectification significantly drop the voltage of the half wave signal. In accordance with the invention, a gated rectifier is located downstream of the voltage dropping resistors. The gated rectifier is preferably a silicon controlled rectifier. The silicon controlled rectifier is operative to conduct only when the output voltage from the silicon controlled rectifier drops below a voltage at the gate of the silicon controlled rectifier. The voltage at the gate of the silicon controlled rectifier is defined by certain gating circuitry. The current flowing through the silicon controlled rectifier when it is conducting preferably charges a capacitor connected to the output of the silicon controlled rectifier. The voltage preferably defined by the capacitor at the output of the silicon controlled rectifier is the voltage that is applied to the various elements of the electronic thermostat. The current flowing through the voltage dropping resistors drops off when the silicon controlled rectifier does not conduct. This eliminates the dissipation of heat in these resistors that would otherwise occur if current were constantly flowing through the silicon controlled rectifier. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects and advantages of the present invention will be apparent from the following description in conjunction with the accompanying drawings, in which: 
     FIG. 1 illustrates a wall mounted electronic thermostat having a back end portion with electrical connections to an alternating current power source; 
     FIG. 2 illustrates the electrical connections of the back end portion of the electronic thermostat to an alternating current source. 
     FIG. 3 illustrates a power supply circuit connected to the back end portion of the thermostat and, moreover, connected to various components within the electronic thermostat needing a particular level of DC voltage; and 
     FIG. 4 is a detailed illustration of the power supply circuit of FIG.  3 . FIG. 5A illustrates voltage levels at certain points within the power supply circuit of FIG. 4; 
     FIG. 5B illustrates the current at one point within the power supply circuit of FIG. 4; 
     FIG. 5C illustrates the current at another point within the power supply circuit if FIG.  4 ; 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, a programmable electronic thermostat  10  is seen to comprise a main body portion  12  and a back end portion  14 . The back end portion  14  is normally attached to a wall via slots  16  and  18 . The main body portion  12  preferably mounts to the back end portion  14  after it is attached to the wall. The back end portion  14  includes a terminal block  20 , having a terminal connection to 24 volt AC power source. This power source is usually located in the system being controlled by the thermostat. This system may be a furnace or an air conditioning system. The terminal block also includes several terminals for receiving or sending signals to the system being controlled. 
     Referring to the main body portion  12 , it is seen that the front of this main body portion includes an LCD display  22  and a series of depressable buttons on a keypad  24  for entering information into the thermostat. A hinged door  26  is normally closed so as to allow for viewing of the LCD display  22  and access to the keypad such as  24 . 
     FIG. 2 illustrates electrical wiring between the terminal block  20  and the system being controlled. A step down transformer  28  within the system  27  being controlled by the thermostat provides 24 volt AC power to terminals  30  and  31  of the terminal block  20 . It is to be appreciated that the step down transformer  28  may not necessarily be located within the system  27  that is to be controlled by the thermostat  10 . 
     Referring now to FIG. 3, a schematic depiction of various components of the electronic thermostat  10  contained within the main body portion  12  are shown relative to the terminal block  20 . In particular, a power supply circuit  32  receives the 24 volt AC power from the terminals  30  and  31  of the terminal block  20 . The power supply circuit  32  provides a low voltage, V cc , to the display  22  as well as the keypad  24  within the main body portion  12 . This power supply voltage is also provided to a microprocessor  34  as well as an electrical erasable programmable memory  36  associated with the microprocessor. It is to be understood that the microprocessor  34  controls what is displayed on the display  22  as well as reads entries from the keypad  24 . This is facilitated by a control bus (not shown) for sending and receiving information from the microprocessor to the display  22  and the keypads  24 . 
     Referring now to FIG. 4, the power supply circuit  32  is illustrated in detail. The circuit begins with a diode D 1 , which acts to half wave rectify the 24 volt AC signal from the terminal  30 . Resistors R 1  and R 2  drop the rectified voltage from the diode D 1 . The resulting voltage at a point  38  is applied to two different current paths within the power supply circuit. The first current path is through a resistor R 3  before splitting into a current path through capacitor C 2  to ground as well as a current path through a resistor R 4  to the input side of a silicon controlled rectifier Q 1 . The capacitor C 2  in combination with the resistances R 3  and R 4  filters the voltage from point  38  before applying it to the input of the silicon controlled rectifier Q 1 . 
     The second current path from point  38  is through a resistor R 5  before partially splitting into a path through capacitor C 3  and a path to a point  40 . The capacitor C 3  normally charges to the voltage at the point  38  less any voltage drop occurring across R 5 . 
     The voltage stored in capacitor C 3  defines the voltage relative to ground across a zener diode Z 1 . The zener diode Z 1  is also connected through a resistor R 6  to a gate of the silicon controlled rectifier Q 1 . 
     The difference in the voltage at the gate of Q 1  from the voltage at point  40  is merely the voltage drop across the resistor R 6 . This voltage drop is preferably small if the resistor R 6  is appropriately sized so as to limit the current flow to the gate of Q 1  when the zener Z 1  is not conducting or to ground through the zener Z 1  if the zener is conducting. A capacitor C 4  connected across the gate and the cathode output of the silicon controlled rectifier prohibits any false triggering of the gate of the silicon controlled rectifier Q 1  due to quick surges in current through the resistor R 6 . 
     The cathode output of the silicon controlled rectifier is connected to a resistor R 7  that is in turn connected to a zener diode Z 2 . The zener diode Z 2  protects the cathode output of the silicon controlled rectifier from dropping below the voltage rating of this zener diode. A capacitor C 5  connected across the resistor R 7  and the zener Z 2  normally defines the voltage, V cc , which is provided to the various elements of the thermostat  10 . The voltage V cc  is also the voltage at the output of the silicon controlled rectifier Q 1 . 
     It is to be noted that a particular embodiment of the power supply circuit of FIG. 3 has the following component values: 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 R 1   
                 50 Ohms 
               
               
                   
                 R 2   
                 50 Ohms 
               
               
                   
                 R 3   
                 250 Ohms 
               
               
                   
                 R 4   
                 250 Ohms 
               
               
                   
                 R 5   
                 4.7 K Ohms 
               
               
                   
                 R 7   
                 .1 K Ohms 
               
               
                   
                 R 8   
                 470 Ohms 
               
               
                   
                 C 2   
                 10 microfarads 
               
               
                   
                 C 3   
                 10 microfarads 
               
               
                   
                 C 4   
                 .001 microfarad 
               
               
                   
                 C 5   
                 100 microfarads 
               
               
                   
                 Z 1   
                 4.7 volts 
               
               
                   
                 Z 2   
                 5.6 volts 
               
               
                   
                   
               
             
          
         
       
     
     The diode D 1  is preferably an IN4007. The silicon controlled rectifier Q 1  is preferably a sensitive gate rectifier with a two hundred volt reverse breakdown. The particular silicon controlled rectifier used in the preferred embodiment is a PO102BL available from STMicroelectronics. 
     The operation of the circuit of FIG. 4 when the components have the particular values as set forth above will now be described relative to FIGS. 5A,  5 B and  5 C. In this regard, FIG. 5A illustrates the zener diode voltage V z  versus time at the point  40  of FIG.  4 . FIG. 5A also illustrates the voltage V cc  versus time at the cathode output of the silicon controlled rectifier Q 1 . FIG. 5B illustrates the anode to cathode current I Q  through the silicon controlled rectifier Q 1  versus time FIG. 5C illustrates the current I R  through the dropping resistors R 1  and R 2  versus time. 
     At time t 0  the half wave rectifier diode D 1  begins to conduct thereby initiating a rectified half wave voltage cycle. Because the silicon controlled rectifier Q 1  is pinched off and not conducting, all the current I R  initially flowing through R 2  is charging capacitors C 2  and C 3 . Capacitor C 2  will define the voltage applied to the anode input of the silicon controlled rectifier Q 1 . Capacitor C 3  will be used to establish the voltage across the zener diode Z 1 . This voltage will hereinafter be referred to as V z . 
     At time t 1 , V cc  is sufficiently below V z  so that current flowing through the resistor R 6 , to the gate of the silicon controlled rectifier Q 1  causes Q 1  to conduct. The anode to cathode current I Q  through the silicon controlled resistor Q 1  quickly goes from zero to nearly minus twenty milliamperes shortly thereafter, as shown in FIG.  5 B. At this time, V cc  begins to rise as capacitor C 4  is charged up by the current I Q . At the same time V z  begins to dip a little as the gate of Q 1  draws current from capacitor C 3 . The current I Q  through the silicon controlled rectifier Q 1  is now falling as capacitor C 5  is charged up from the capacitor C 2  thereby causing the charge on C 2  to become depleted. V cc  rises and then falls off a little as the charge on capacitor C 2  is depleted. 
     At time t 2 , the current I R  through the dropping resistors R 1  and R 2  has dropped back to zero and the capacitors C 2  and C 3  are both charged up. The current through the dropping resistors R 1  and R 2  stays at zero until t 4  when the cycle repeats, as shown in FIG.  5 C. Because the current through resistors R 1  and R 2  is primarily charging the capacitors C 2  and C 3 , the amplitude of the current pulse is shorter and the width of the current pulse is substantially narrower than it would be for a linear pass transistor power supply. This reduced current pulse reduces the heat generated in the dropping resistors, R 1  and R 2 . 
     At time t 3 , voltages V cc  and V z  are sufficiently close so that the current to the gate of the silicon controlled rectifier Q 1  is pinched off thereby causing the silicon controlled rectifier Q 1  to stop conducting, as shown in FIG. 5B. V cc  is falling at this time because the circuits of the thermostat are drawing power from the capacitor C 4  while it is not being charged because the silicon controlled rectifier Q 1  is not conducting. The voltage V z  levels off because there is not any current to the gate of the silicon controlled rectifier Q 1 , which would discharge capacitor C 3 . V z  rises again at time t 4  when the next half wave cycle of current through diode D 1  charges capacitor C 3 . The silicon controlled rectifier Q 1  however remains off until V cc  falls to a point where the current through R 6  again flows to the gate of the silicon controlled rectifier Q 1  at t 5  and the cycle repeats. 
     It is to be appreciated from the above that a particular embodiment of the invention has been described. Alterations, modifications and improvements by those skilled in the art are intended to be a part of this disclosure even though not expressly stated herein and are intended to be within the scope of the invention. Accordingly, the foregoing description of by way of example only and the invention is to be limited only by the following claims and equivalents thereto.