Abstract:
A gas-fired appliance is disclosed that includes a gas valve powered by a power source. A plurality of switching units, each responsive to a condition of the appliance, are arranged in series between the gas valve and the power source. Each switching unit includes an emitter, such as an emitter of an optocoupler, that is used to monitor the opened or closed status of a switch in the switching unit. When at least one switch opens, power to the gas valve is reduced preventing the gas valve from operating. However, because this reduced power is provided to each subsequent switching unit, each optocoupler can be monitored regardless of the opened or closed status of previous switching units. As such, one or more open switches can be detected simulataneously.

Description:
RELATED APPLICATIONS 
       [0001]    This patent application claims the benefit of U.S. provisional patent application No. 60/968,424, filed on Aug. 28, 2007, the entirety of which is hereby incorporated by reference. This patent application also incorporates by reference the entire contents of co-pending U.S. patent application Ser. No. ______, filed on ______, 2008, entitled “APPLIANCE HAVING LOAD MONITORING SYSTEM” (Attorney Docket No. 010121-8165-00). 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates generally to appliances, such as gas-fired appliances, having safety limit strings, and more particularly to appliance controllers that monitor the state of the switches in a safety limit string to detect one or more simultaneous conditions in the appliance. 
       BACKGROUND 
       [0003]    Safety limit strings are known that include a plurality of switches arranged in series, each switch corresponding to an operating condition. Such safety limit strings are placed between a power source and a gas valve. When a fault condition is encountered, the related switch is opened and power is disconnected. 
         [0004]    Systems and methods of monitoring such safety limit string in order to diagnose the specific fault condition are also known. Such systems generally include an electrical contact located before each switch and are monitored by a circuit, which may include a controller. When a switch in the safety limit string is opened in response to a fault condition, the controller can identify the open switch by detecting the last electrical contact in the safety limit string to receive power from the power source. 
       SUMMARY 
       [0005]    Among other deficiencies in some known safety limit string systems, previous systems are unable to detect and identify multiple fault conditions occurring simultaneously. Because the supply of electrical power is terminated when a first open switch is encountered, any subsequent open switches in the safety limit string are not detected. 
         [0006]    One embodiment of the invention provides a gas-fired appliance that includes a first and a second switching unit placed in series between a power source and the gas valve. The first switching unit includes a switch circuit and a leakage circuit arranged in parallel. The leakage circuit includes a resistor and an emitter. When a switch opens in response to a condition in the appliance, current travels through the leakage circuit and a signal is emitted by the emitter. A receiver is positioned to receive any signals emitted from the emitter and communicate the signal to a microcontroller. The second switching unit can include similar components. 
         [0007]    In some embodiments, the resistor reduces the current through the circuit and, therefore, reduces the available power. The resistor in some such embodiments is selected such that when any one of the switches is open, the available power is insufficient to operate the gas valve. 
         [0008]    In some embodiments, the controller is configured to associate a signal received through the emitter with a condition in the appliance. 
         [0009]    Some embodiments include a plurality of optocouplers each including an emitter and a receiver. 
         [0010]    In some embodiments, a safety monitoring system is provided wherein a controller monitors the status of a plurality of optocouplers to detect a plurality of operating conditions. The receiver of the each optocoupler is connected to the microcontroller and each emitter is included in the leakage circuit of one of a plurality of switching units. The plurality of switching units is connectable in series between a power source and a load. 
         [0011]    In some embodiments, the invention provides a safety string including a plurality of normally closed and normally open switches connected in series with and coupling power to a gas valve. A plurality of detection circuits includes a resistor, having a relatively high resistance, connected in series with an optocoupler. The detection circuits are connected across the switches. An output of each optocoupler is coupled to a microcontroller or other programmable device (e.g., microprocessor, digital signal processor, etc.). When a switch opens, due to a fault condition, power to the gas valve is removed, and the optocoupler associated with the switch provides an indication to the microcontroller of which switch is open regardless of the state of the other switches. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0012]      FIG. 1  is a block diagram of a gas fired water heater. 
           [0013]      FIG. 2  is a schematic representation of a control system for the gas fired water heater in  FIG. 1 . 
           [0014]      FIG. 3  is a schematic representation of a safety system capable of being used in the gas water heater of  FIG. 1 . 
           [0015]      FIG. 4  is a functional illustration showing the flow of current in the safety system of  FIG. 3 , where all switches in the safety limit string are closed. 
           [0016]      FIG. 5  is a functional illustration showing the flow of current in the safety system of  FIG. 3 , where multiple switches in the safety limit string are open. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purposes of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein are meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless specified or limited otherwise, the terms “mounted,” “connected,” “supported,” and “coupled” and variations thereof are used broadly and encompass both direct and indirect mountings, connections, supports, and couplings. Further, “connected” and “coupled” are not restricted to physical or mechanical connections or couplings. 
         [0018]      FIG. 1  shows one construction of a gas-fired water heater  100 . Water heater  100  includes inlet pipe  101 , which supplies unheated water to tank  103 , and outlet pipe  105 , which removes heated water from tank  103 . Igniter  119  ignites gas burner  117  in combustion chamber  111  to heat the water. Gas valve  115  controls the flow of gas from gas inlet pipe  113  to burner  117 . Blower  109  provides air from air inlet pipe  107  to combustion chamber  111 . Vent  121  subsequently releases the air through air outlet pipe  123 . The operation of water heater  100  is monitored and controlled by controller  200 . 
         [0019]    Although the constructions referred to herein describe a gas-fired water heater, the invention could be embodied in other gas-fired appliances such as, for example, a boiler, a furnace, and an oven. Other constructions of the invention could also be embodied in non-gas-fired systems, such as an electric water heater, that include type of electric load other than an electrically operated gas valve. 
         [0020]      FIG. 2  shows one construction of controller  200  in greater detail. Microcontroller  201  is connected to user input device  221 , user display/output device  223 , electronically-controlled gas valve  215 , and various other input sensors and controlled devices. Input sensors may include, for example, temperature sensor  209  which detects the temperature of the water in tank  103  and water level sensor  211  which detects the volume of water in tank  103 . Controlled devices may include, for example, water pump  213  and igniter  219 . 
         [0021]    Safety limit string  300  is interposed between power source  203  and gas valve  215 . Safety limit string  300  includes a plurality of normally open or normally closed switches arranged in series. All switches in safety limit string  300  should be closed before the gas valve can be sufficiently energized (i.e., opened). The switches are linked to various safety controls  207 ; for example, pressure switches connected in safety limit string  300  ensure proper blower air intake (blower  109 ) and exhaust pressures (vent  121 ). If a problem is detected, one of the switches opens (e.g., when a blower pressure is too low), power to the gas valve is reduced, and the gas valve closes. 
         [0022]      FIG. 3  provides a more detailed view of one construction of the safety limit string  300 . A plurality of switching units ( 311 ,  321 , and  331 ) are arranged in series between a 24 VAC power source  203  and a gas valve  215 . Switching unit  311  includes two circuits arranged in parallel—a switch circuit and a leakage circuit. The switch circuit includes a switch  312  of relatively low resistance. The leakage circuit includes a resistor  313  having a relatively large resistance and the emitter of an optocoupler  315 . The receiver of optocoupler  315  is connected to the microcontroller  201 . Similar components in switching units  321  and  331  are labeled with similar reference characters. 
         [0023]    An optocoupler (such as  315 ,  325 , and  335 ) typically includes an emitter and a receiver. Referring to optocoupler  315  in  FIG. 3 , the emitter includes a light source such as LEDs  314 . The receiver includes a light detector such as phototransistor  316 . When current passes through the emitter, light is generated and detected by the receiver. Because the receiver is not conductively connected to the emitter, the circuit containing the emitter is separate from the circuit including the receiver. By connecting microcontroller  201  to the receiver of optocoupler  315 , microcontroller  201  can determine when current is passing through the emitter without interfering with the safety limit string  300 . As discussed in detail below, this construction allows current to continue through subsequent switching units so that the microcontroller  201  is able to detect multiple open switches at the same time. 
         [0024]    Because the switch circuit in this construction is less resistant than the leakage circuit, little or no current flows through the leakage circuit if switch  312  is closed. Microcontroller  201  monitors optocoupler  315  and is configured to associate this condition with a closed switch  312 . If switch  312  is open, current flows through the leakage circuit and the microcontroller  201  detects this current through optocoupler  315 . 
         [0025]    In some optocouplers (such as  315 ,  325 , and  335 ), the amount of current detected on the receiver (e.g., the phototransistor  315 ) is proportional to the amount of current on the emitter (e.g., the LEDs  314 ); however, if the current on the emitter is below a certain threshold, no current is detected on the emitter. As such, in some constructions, components are selected such that when switch  312  is closed, no current is detected at optocoupler  315 . In these constructions, the receiver of optocoupler  315  is connected to a digital input pin on microcontroller  201  and provides a high or low logic signal indicative of the status of switch  312 . 
         [0026]    In other constructions, the receiver of optocoupler  315  may detect a relatively small current even when switch  312  is closed. In such constructions, microcontroller  201  and associated circuitry on the receiver side of optocoupler  315  are configured to associate a current in excess of a predetermined threshold with an open switch. This comparison can be implemented by various methods including connecting the receiver of optocoupler  315  to a voltage or current comparator circuit that compares the detected current or voltage to a reference current or voltage. Such a comparator circuit is further configured to provide a high or low logic signal to microcontroller  201  indicative of the status of switch  312 . 
         [0027]    Alternatively, the receiver side of optocoupler  315  can be connected to an analog-to-digital converter on microcontroller  201 . Microcontroller  201  can be configured to compare the value at the analog-to-digital converter to a predetermined threshold or can adaptively associate switches into “open” and “closed” groupings depending on the relative voltage or current detected at the corresponding optocoupler. 
         [0028]      FIG. 3  shows an AC circuit construction in which optocoupler  315  includes two LEDs  314  (one for each direction in the alternating current) and a corresponding photodiode  316 . Such optocoupler integrated circuits are commercially available in the PS2505 Multi Photocoupler Series produced by NEC Electronics, Inc. These components may include one or more optocouplers on the same IC. DC optocouplers are also available which include a single LED for each phototransistor. Still other optocoupler configurations utilize photodiodes instead of phototransistors. 
         [0029]    In an example construction, switch  312  is a pressure switch monitoring air intake from blower  109 , switch  322  is a pressure switch monitoring exhaust pressure from vent  121 , and switch  332  is a bimetallic temperature switch configured to open if the temperature of the water in tank  103  exceeds a high-limit. It will be understood by those having ordinary skill in the art that safety limit string  300  may include various combinations of these and other switches and need not be assigned as in this construction. 
         [0030]      FIG. 4  illustrates the current flow through safety limit string  300  when all switches are closed. The flow of current is represented by the heavy dotted line. When all switches in safety limit string  300  are closed, current flows from power source  203  through low resistance switches  312 ,  322 , and  332  and provides enough power to open gas valve  215 . In this condition, microcontroller  201  can regulate gas flow by opening or closing gas valve  215 . Microcontroller  201  can also confirm correct operation of blower  109  and vent  121  by monitoring optocouplers  315  and  325  respectively and can verify that the high-limit temperature has not been exceeded by monitoring optocoupler  335 . 
         [0031]      FIG. 5  illustrates the current flow through safety limit string  300  when switch  322  is closed, but switches  312  and  332  are open. Resistors  313 ,  323 , and  333  in this construction have a high enough resistance such that when any one switch in the safety limit string  300  is open, the current through safety limit string  300  is reduced and the power is insufficient to energize (i.e., open) gas valve  215 . Conversely, resistors  313 ,  323 , and  333  have a low enough resistance such that when all of the switches in the safety limit string  300  are open, enough power remains such that the microcontroller  201  can detect current at optocouplers  315 ,  325 , and  335 . 
         [0032]    Current flows through the leakage circuit in switching unit  311  and is detected by microcontroller  201  through optocoupler  315 . Microcontroller  201  is configured to associate this condition with an insufficient intake pressure from blower  109 . Current continues to switching unit  321  and passes through the switch circuit. Little or no current is directed through the leakage circuit and, as such, is not detected by microcontroller  201  through optocoupler  325 . Microcontroller  201  is configured to associate this condition with a sufficient exhaust pressure at vent  121 . Current then passes through the leakage circuit of switching unit  331  and is detected by microcontroller  201  through optocoupler  335 . Microcontroller  201  is configured to associate this condition with a water temperature in tank  103  that exceeds the high-limit threshold. Finally, current arrives at gas valve  215 . However, resistors  313  and  333  have reduced the current such that the available power is insufficient to operate the gas valve  215 . Consequently, gas valve  215  remains closed and microcontroller  201  is aware of the adverse safety conditions. 
         [0033]    It should be understood that the constructions described above are exemplary and other configurations and designs are possible. For example, although the above constructions describe an AC circuit, DC circuits might also be constructed. Furthermore, terms such as “resistor” and “emitter” are used broadly. Unless otherwise specified, the term “resistor,” for example, may refer to a single discrete component or it may refer to an arrangement of multiple components that together introduce resistance into a circuit. As such, additional components may be added to the describe circuit constructions without departing from the intended scope. Likewise, unless otherwise specified, the term “emitter,” for example, may refer to any device that emits a signal. Various features and advantages of the invention are set forth in the following claims.