Abstract:
An electrical circuit for controlling the activation of a fan includes optically coupled isolators. The circuit is electrically connected in series to a thermostat including a bimetal actuated contact with an anticipator across the contact. Power to the circuit is provided by voltage developed across the system relay coil in series with the thermostat. A zener diode establishes a threshold voltage consistent with the operation of the system relay coil and the thermostat. The optically coupled isolators provide the required electrical isolation needed to protect the thermostat from a power control circuit of an electronically controlled fan.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to thermostats, and more particularly to controlling a fan with a thermostat. 
     Known thermostats employ a resistive element to reduce or prevent overshooting a temperature setting. The resistive element is typically referred to as an anticipator. Thermostats without anticipators sense a temperature one or two degrees above a temperature setting and then open a switch contact. For instance, in an unanticipated thermostat set for turning on a furnace, the switch contact will remain closed so the furnace continues to run until the temperature rises one or two degrees above a temperature set point. The temperature rise above the temperature setting is caused by the delay in heating the thermal mass of a bimetal heat sensing element, located in the thermostat, above the set point temperature, and this excess temperature rise is known as overshoot. When the temperature rises above the temperature set point, the switch contacts open turning the furnace off. Then as the room temperature decreases, the temperature has to drop one or two degrees below the temperature set point before the switch contacts are closed to turn on the furnace. This temperature drop below the temperature set point is known as undershoot. Typically the anticipator functions to minimize undershooting and overshooting the thermostat&#39;s temperature set point. 
     The anticipator “anticipates” when the room temperature approaches the temperature set point of the thermostat. An anticipator is a resistive heating element. When a thermostat is turned “on” for heating, a current is applied to the anticipator. The current flow through the anticipator heats the anticipator which is electrically connected to a bimetal switch. The bimetal switch deflects with temperature changes to open or close the contacts. When the room temperature decreases below the thermostat set point, the bimetal switch contracts and closes the contacts turning on the furnace. The anticipator “heats” the bimetal switch to sense a higher temperature within the thermostat compared to the room temperature. By adding internal heat, the anticipator reduces the amount of room temperature differential required to turn off the furnace. 
     However, in such a system employing an anticipator, the connection of the anticipator across the contacts of the thermostat does not allow the voltage across controlled elements, such as a relay coil, to go to zero during the off state. Instead, a finite voltage remains across the relay coil when the thermostat is in the off state. Lower resistance of the relay coil compared to the anticipator, results in a lower voltage across the relay coil. In known systems, the relay coil applies electrical power to a heating or cooling system. However when the thermostat is in the off state, there exists a finite voltage across the relay coil less than an amount of voltage required to energize and close the relay coil. It would be desirable to coordinate the finite voltage across the relay coil to that voltage required for the activation of a variable speed fan. If would be further desirable for an electronic circuit to provide an interface to electrically couple the different operating voltage requirements between a relay coil and the variable speed fan. 
     BRIEF SUMMARY OF THE INVENTION 
     In an exemplary embodiment, an electric circuit is connected in series to a thermostat including a bimetal actuated contact with an anticipator across the contact. The electrical circuit including a pair of optically coupled isolators to allow a variety of input control options. The options include the selection of different air flow rates. Either of the pair of optically coupled isolators maybe connected to the thermostat. Power to the electric circuit is provided by voltage developed across the system relay coil in series with the thermostat, e.g., current flows from a system control transformer though the thermostat contacts and through the relay coil. The voltage generated across the relay coil is used to power the electric circuit to electronically control a fan. 
     The electrical circuit includes in one embodiment, a first input terminal and a second input terminal, which are connected to one optically coupled isolator. Connected in series to both the first and the second input terminals is a first zener diode that establishes a threshold voltage consistent with the operation of the system relay coil and the thermostat. In addition, a second zener diode connected in series to the first input terminal rectifies the AC voltage to a half-wave rectified voltage. Connected to the second input terminal is a fall-wave bridge rectifier that rectifies an AC voltage to a DC voltage. A purpose of the first zener diode in series with the first terminal is to generate a half-wave rectified voltage for the optically coupled isolator to differentiate the input voltage from the first input terminal from the input voltage from the second input terminal. In addition, the first zener diode in series with the first terminal protects the electrical circuit from transient voltages, e.g., electro-static discharge voltage. In addition, the optically coupled isolators provide the electrical isolation to protect the thermostat, including a ground reference control circuit, from high voltage circuits that provide power to the electronically driven fan. 
     More particularly, the fan control is electrically connected to an electrically erasable programmable read-only memory (EEPROM), which is programmed to control a plurality of fan modes based on combinations of inputs from the thermostat. As a result, a cost-effective and reliable electrical circuit including optically coupled isolators to control fan activation is provided . 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic illustration of an exemplary embodiment of the invention. 
     FIG. 2 is an exemplary system including the electric circuit shown in FIG.  1   
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a schematic illustration of an exemplary embodiment of an electrical circuit  10 . Electrical circuit  10  includes input terminals  12 ,  14 , and  16 , and return line  22 . Connected to input terminal  12  at node  20  is full-wave bridge rectifier  18 . Full-wave bridge rectifier  18  is connected at node  24  to resistors  26 ,  28 ,  30 , and  32  that are connected in series and connected to return line  22  at node  50 . At node  34 , full-wave bridge rectifier  18  is connected to zener diode  36 . Zener diode  36  is connected to node  40  by its anode  38 . Zener diode  36  is connected to shunt resistor  42  at node  40 . In addition, Shunt resistor  42  is provided current from fall-wave bridge rectifier  18  at nodes  40  and  44 . Node  44  and node  49  share a common line. Full-wave bridge rectifier  18  is connected to node  49  and therefore connected to shunt resistor  42  at node  44 . In addition, optically coupled isolator  52  is connected in parallel to shunt resistor  42  at nodes  40  and  44 . The output of optically coupled isolator  52  is connected to node  58 . Load resistor  60  is connected to node  58 . 
     Input terminal  14  is connected to full-wave bridge rectifier  62  at node  64 . Full-wave bridge rectifier  62  is connected to resistors  68 ,  70 ,  72 , and  74  that are connected in series at node  75 . Node  76  connects full-wave bridge rectifier  62  to zener diodes  77  and  78 . Zener diode  77  and zener diode  78  are configured so their cathodes face each other. The anode of zener diode  77  is connected to input line  16 , while the anode of zener diode  78  is connected to node  84 . Shunt resistor  86  is connected between nodes  84  and  88 . In addition, Shunt resistor  86  is provided current from fall-wave bridge rectifier  62  at nodes  84  and  88 . Node  88  and node  90  share a common line. Full-wave bridge rectifier  62  is connected to node  90  and therefore connected to shunt resistor  86  at node  88 . In addition, optically coupled isolator  92  is connected in parallel to shunt resistor  86  at nodes  84  and  88 . The output-of optically coupled isolator  92  is connected to node  102 . Node  102  connects optically coupled isolator  92  and resistor  98  to transistor  100 . Connected to the output of transistor  100  at node  104  is a network comprising resistor  106  and resistor  108 . In one embodiment, resistor  108  is connected to memory (not shown). 
     In an exemplary embodiment, input terminal  12  and return line  22  are electrically connected in parallel to a relay coil (not shown) controlled by a thermostat (not shown). In one embodiment, to control operation of a fan, a thermostat including a set of contacts (not shown) are electrically connected to an anticipator _(not shown), and the contacts are connected to input terminal  14  and return line  22 . In an alternative embodiment, a system control relay (not shown) is connected to input terminal  14  and return line  22 , and provides 24 VAC to input terminal  14 . In a further alternative embodiment, the system control relay is connected to input terminal  16  and return line-  22 , and provides 24 VAC td input terminal  16 . In a still further embodiment, one termination of a secondary winding of a transformer (not shown) is connected to input terminal  14  through the thermostat contacts, and the second termination of said secondary winding is connected to return line  22  to compete the electric circuit. In another embodiment, one termination of a secondary winding of the transformer is connected to input terminal  16  through the thermostat contacts, and the second termination of the secondary winding of the transformer is connected to return line  22  to compete the electric circuit. 
     Current flows through both the anticipator and the relay coil, fan speed is controlled through a microcomputer (not shown). Electrical power is provided to electrical circuit  10  by the relay coil connected in parallel though input terminal  12 . In one embodiment, the voltage signal input to terminal  12  is 24VAC. Input terminal  12  is electrically connected to full-wave bridge rectifier  18  at node  20 . Furthermore, full-wave bridge rectifier  18  is connected to return line  22  through a plurality of resistors  26 ,  28 ,  30 , and  32  connected to node  24 . In one embodiment, resistors  26 ,  28 ,  30 , and  32  are surface mount resistors. The AC voltage at input terminal  12  is applied through node  20  to full-wave bridge rectifier  18 , where it is rectified to a DC voltage. 
     The DC voltage generated by full-wave bridge rectifier  18  is limited at node  34  by zener diode  36  and the input of LED  54  included in optically coupled isolator  52 . Zener diode  36  blocks current to the input of optically coupled isolator  52  and resistor  42  when the input voltage at input terminal  12  is below the operating voltage of zener diode  36 . In order for current to flow through LED  54  of optically coupled isolator  52 , an absolute voltage, positive or negative voltage, at input terminal  22  and return  22  must exceed the voltage required to activate fall-wave bridge rectifier  18  and zener diode  36 . As the voltage increases the current increases such that optically coupled isolator  52  is activated. Resistor  42  serves to decrease any current presented to optically coupled isolator  52 . In addition, resistor  42  assists to provide a predictable input operating voltage threshold for optically coupled isolator  52 . In another embodiment, electrical circuit  10  does not include resistor  42 . In one embodiment, optically coupled isolator  52  is a PC367NT manufactured by SHARP Microelectronics of the Americas, Camas, Wash. 98607. 
     Optically coupled isolator  52  is electrically connected in parallel to resistor  42  at nodes  40  and  44 . Optically coupled isolator  52  includes a light emitting diode (LED)  54  and a transistor  56 . In a preferred embodiment, optically coupled isolator  52  is activated when the voltage across LED  54  is at least 1.2 volts and the forward current through LED  54  is at least 0.5 mA. When LED  54  is activated, an optical signal is transmitted to transistor  56 . The optical signal generates a current in the base of transistor  56  which biases transistor  56  so it is turned on. When transistor  56  is on, current flows from the collector. In one embodiment, if the forward current through LED  42  is 0.5 mA, the resulting collector current produced in transistor  56  will be 2.5 mA when the voltage across the collector-to-emitter is 5V. 
     In one embodiment, the signal at node  58  is inverted with respect to the signal input to transistor  56 . The output of the signal from transistor  56  is taken from its collector at node  58 . Connected to node  58  is resistor  60 , which serves to pull-up the voltage at node  58  to a value approximately at Vcc when transistor  56  is turned off. As the voltage increases at input terminal  12  and return line  22 , the current increases to LED  54  and turns on transistor  56 . When transistor  56  is turned on, the voltage at node  58  decreases. Resistor  60  also serves to determine a threshold operating voltage at the input to optically coupled isolator  52  and to set the response time of transistor  56 . In one embodiment, resistor  60  has a value of 15,000 ohms. This embodiment does not require a fast response time as the input is limited to a line frequency approximately between 50-60 Hz. However, as the value of resistor  60  increases, the turn off response time of transistor  56  increases. A microcomputer (not shown) is connected to node  58 . 
     The output signal at node  58  is then transmitted to the microcomputer which determines fan activation. The term microcomputer, as used herein, refers to microprocessors, microcontrollers, CPUs, reduced instruction set circuits (RISC), application specific integrated circuits (ASICs), logic circuits, and any other circuit or processor capable of executing a series of instructions or software programs. 
     The second portion of electrical circuit  10  is connected at node  50 . The second portion of electrical circuit  10  has input terminal  14  and input terminal  16  as inputs. As mentioned above, a secondary winding of a transformer (not shown) is connected to input terminal  14  through the thermostat contacts. In another embodiment, the secondary winding of the transformer is connected to input terminal  14  through other controlling contacts. The alternate termination of the secondary winding of the transformer is connected to return line  22  to compete the electric circuit. In an alternative embodiment, the transformer (not shown) is connected to input terminal  16  to produce an alternate control operation. In a further alternative embodiment, the transformer is connected to both input terminal  14  and input terminal  16 , where terminal  14  overrides the connection to terminal  16 . The transformer provides a 24 AC voltage to the input of terminal  14 . Full-wave bridge rectifier  62  is electrically connected to input terminal  14  at node  64 . Furthermore, full-wave bridge rectifier  62  is connected to return line  22  at node  67 . Further connected between node  67  and node  50  is a plurality of resistors  68 ,  70 ,  72 , and  74 . In one embodiment, resistors  68 ,  70 ,  72 , and  74  are surface mount resistors. The 24AC signal is transmitted to fall-wave bridge rectifier  62 , where it is rectified to produce a DC voltage. 
     The DC voltage generated by full-wave bridge rectifier  62  is limited at node  76  by zener diode  78  and the input of LED  94  included in optically coupled isolator  92 . Zener diode  78  blocks current to the input of optically coupled isolator  92  and resistor  86  when the input voltage across input terminals  14  and  16  and return line  22  is below the operating voltage of zener diode  78 . In one embodiment, in order for current to flow through LED  94  of optically coupled isolator  92 , absolute voltage, positive or negative voltage, at input terminal  14  and return  22  must exceed the voltage required to activate full-wave bridge rectifier  62  and zener diode  78 . In another embodiment, when input terminal  16  is connected, the absolute voltage at input terminal  16  and return  22  must exceed the voltage required to activate full-wave bridge rectifier  62  and zener diode  78 . As the voltage increases the current increases such that optically coupled isolator  92  is activated. Resistor  86  serves to decrease any current presented to optically coupled isolator  92 . In addition, resistor  86  assists to provide a predictable input operating voltage threshold for optically coupled isolator  92 . In another embodiment, electrical circuit  10  does not include resistor  86 . In one embodiment, optically coupled isolator  92  is a PC367NT manufactured by SHARP Microelectronics of the Americas, Camas, Wash. 98607. 
     Optically coupled isolator  92  is electrically connected in parallel to resistor  86  at nodes  84  and  88 . Optically coupled isolator  92  includes a light emitting diode (LED)  94  and transistor  96 . In a preferred embodiment, optically coupled isolator  92  is activated when the voltage across LED  84  is at least 1.2 volts and the forward current through LED  94  is at least 0.5mA. When LED  94  is activated, an optical signal is transmitted to transistor  96 . The optical signal generates a current in the base of transistor  96  which biases transistor  96  so it is turned on. When transistor  96  is on, current flows from the collector. In one embodiment, if the forward current through LED  94  is 0.5mA, the resulting collector current produced in transistor  96  will be 2.5 mA when the voltage across the collector-to-emitter is 5V. 
     Optical coupled isolator  92  is electrically connected to resistor  98  and transistor  100  at node  102 . In one embodiment, resistor  98  biases the base of transistor  100  to conduct current. The output of transistor  96  supplies current to resistor  98  to remove the bias current from transistor  100  and turn transistor  100  off. In one embodiment, transistor  100  is a PNP transistor configured as a voltage gain stage. Node  104  is connected to transistor  100  and resistors  106  and  108 . The voltage at node  104  is pulled up to a voltage approximately equal to Vcc when transistor  100  is on. The output signal from transistor  100  is taken from the collector of transistor  100  at node  104 . When transistor  100  is turned off, the voltage at node  104  is pulled low by resistor  106 . This current signaling mode requires little movement of voltage at node  102 ; and therefore, speeds up operation of a signal input to at least one of terminal  14  and terminal  16 . Faster operation of electric circuit  10  facilitates serial digital communication between a microcomputer (not shown) and an electrically erasable programmable read only memory (EEPROM)(not shown) connected to resistor  108 . 
     In one embodiment, resistor  108  is connected to a microcomputer (not shown) and memory (not shown). In one embodiment, the memory is an electrically erasable programmable read only memory (EEPROM) (not shown). Resistor  108  allows the microcomputer to override the signal at node  104  and directly communicate with the EEPROM. The result is the ability to program the EEPROM with data affecting fan operation from terminal  14  or terminal  16  and for the microcomputer to read and correct data in the EEPROM. The EEPROM is programmed to control different modes of the fan based on combinations of inputs to terminals  12 ,  14  and  16 . 
     In one embodiment diode  77 , connected between input terminal  16  and node  76 , is a zener diode. Zener diode  77  provides the half-wave rectification of the AC voltage input to terminal  16  which is distinguished from the fall-wave rectification of the AC voltage input to terminal  14 . In one embodiment, diode  77  is a zener diode of minimal power rating, and voltage rating in excess of 24 VAC, and a breakdown voltage of 47 volts. Zener diode  77  protects itself from transient voltages, and electronic static discharge voltage on input line  16 . 
     FIG. 2 is an exemplary system including the electric circuit  10  shown in FIG. 1 including inputs  12 ,  14 ,  16 , a return line  22 , a first output  58 , and a second output  108 . Electric circuit  10  is electrically connected in parallel to a bimetal thermostat relay coil and a transformer. The transformer is electrically connected in series to an anticipator circuit located within a thermostat. First output  58  is electrically connected to a microcomputer. The microcomputer is configured to control fan speed. The microcomputer is electrically connected to a memory. Second output  108  is configured to connect to the memory. 
     While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.