Patent Publication Number: US-6985080-B2

Title: Flame sense circuit and method with analog output

Description:
FIELD OF THE INVENTION 
   The invention relates generally to flame sense circuits, and more particularly to analog flame sense circuits that utilize the flame rectification method for sensing flame. 
   BACKGROUND OF THE INVENTION 
   Many consumer and commercial appliances, including furnaces, water heaters, ovens, etc., include gaseous fuel burners. These appliances typically operate by providing a controlled gaseous fuel flow valve and an ignition source for igniting the flow of gaseous fuel in the burner housing. To ensure safety of operation, these appliances typically also include a flame sensor that is used to detect the presence or absence of flame in the burner housing. The output of this flame sensor may be used by the appliance controller or other circuitry to control the flow of gaseous fuel through the gaseous flow valve, to control the ignition source (such as where electronic spark, hot surface, etc. ignition are used), and to control a purge fan if one is provided. Such controls are necessary to prevent a condition where gaseous fuel is continued to be delivered to the burner housing without being combusted. If such a case were allowed to continue, the accumulation of unburned gaseous fuel in the burner assembly could result in a potentially explosive condition. Further, such control also allows for the diagnosis of potential problems and the identification of the need for cleaning or maintenance on the burner based upon the quality of the flame sensed therein. 
   While various methods of flame sensing are known in the art, including optical and pyrometer type sensors, a preferred method of sensing flame in consumer and commercial appliances such as those identified above and others is known as the flame rectification method for sensing flame. Indeed, many gas control safety standards written for such applications by, e.g. the American Gas Association now the Canadian Standards Association, specify that the flame rectification methodology of flame sensing be employed. The phenomenon of flame rectification is well known in the art. Specifically, it is known that the outer cone of a flame is ionized and can conduct electricity. Under the principle of flame sensing by flame rectification, two electrodes of different size are placed in contact with this outer envelope of the flame. These two differently sized electrodes are then connected to a circuit that supplies an AC voltage thereacross. In this configuration, the current that flows through the flame tends to flow only in one direction, from the smaller electrode to the larger electrode. 
   Recognizing that the presence of a flame will allow essentially DC current to flow therethrough, various circuits have been developed that allow for the sensing of both the presence and quality of the flame. These circuits may be broadly classified in one of two technology areas. The first area, to wit analog circuits, employ junction field effect transistors (JFETs). In such analog circuits, a JFET is configured as an amplifier and produces a negative voltage that is somewhat proportional to the flame current. Essentially, the JFET transistors are used to provide a high impedance buffer from the flame sense circuit to the appliance control electronics. 
   Unfortunately, such prior analog circuits do not provide an accurate measure of the flame current, and are particularly sensitive to normal variations of the component parameters. Two such parameters of a JFET that have a significant impact on the effectiveness of such circuits are the input to output gain and the gate turnoff threshold. Further, these parameters have wide variations with normal production and temperature tolerances. In such conventional circuits, these variations produce inaccuracies in the flame sense circuit. Even when JFETs are specifically manufactured and selected in production for a narrower range of these parameters, the remaining variations still significantly affects the circuit performance. As a result, these analog circuits suffer from poor accuracy. 
   The second class of flame sense circuits utilizing the flame rectification methodology includes digital circuits. Unfortunately, the typical digital flame sense circuit also uses a JFET transistor. In these digital circuits, the time required for the flame current to charge a capacitor at the input terminal of the JFET is measured. The voltage pulse width at the output terminal of the JFET is somewhat proportional to the flame current. While such digital circuits have been designed to reduce the poor performance effects of the JFET transistors in the analog circuits, the digital circuits still suffer from poor accuracy. Additionally, their added complexity also increases the system cost, reduces reliability, and does not allow for a straightforward measurement of the flame current with common laboratory instruments. Further, while the digital circuits utilize various algorithms in an attempt to compensate for the JFET transistor inaccuracies, the algorithms cannot accurately adapt for all of the various transistor inaccuracies, appliance parameters, specific electrode sizes, type of gas, etc. in a cost-effective reliable circuit that may reliably be employed for such appliances. 
   There exists, therefore, a need in the art for a simple, reliable, and accurate flame sense circuit that not only provides reliable detection of the presence of a flame, but also provides a simple method of determining the strength and/or quality of the flame. 
   BRIEF SUMMARY OF THE INVENTION 
   In view of the above, it is an objective of the present invention to provide a new and improved flame sense circuit. More particularly, it is an object of the present invention to provide a new and improved flame sense circuit that utilizes the property of flame rectification to detect the presence and quality of a flame. Still further, it is an objective of the present invention to provide such a flame sense circuit in a simplified analog manner that utilizes the property of flame rectification which occurs when a flame bridges two asymmetrically sized electrodes that are energized by a source of alternating current (AC). 
   Preferably, a circuit constructed in accordance with the teachings of the present invention utilizes electrodes that preferably include a small flame probe or an igniter, and a larger burner. This asymmetry causes a net flow of electric current, i.e. essentially a direct current (DC), from the small electrode to the large electrode. Preferably, the flame sense circuit of the present invention detects this DC current (typically approximately one microampere), and converts it to an easily useable voltage. The circuitry of the present invention provides an output voltage signal that is proportional to this flame sense current and positive in magnitude. An appliance control circuit can easily use this voltage signal for determining the magnitude of the flame current, and consequently the status of the flame. The output voltage signal is not sensitive to normal variations of component parameters, nor does it require complex digital circuitry for operation. The out put voltage signal can easily be measured with common instruments during product development, validation, and servicing. Preferably, the circuit of the present invention utilizes a configuration of resistors, capacitors, bipolar junction transistors (BJTs), and voltage sources. Such a circuit is simple and only requires discreet components, rather than integrated circuits. Such a circuit produces an output voltage signal that is proportional to the flame current and positive in magnitude and is not sensitive to normal variations of transistor parameters, therefore producing a more accurate representation of the flame current. 
   In one embodiment of the present invention, an AC voltage source generates a flame current through a capacitor, a resistor, and the gas flame. Flame rectification causes this to be a substantially DC current in a direction flowing from the flame sense probe across the flame to the burner. This DC current causes a net charge to build up on the capacitor, i.e. a DC voltage. The net DC voltage on this capacitor further causes a sense current to flow through an additional resistor. This sense current has a pulsed waveform at a frequency of the AC voltage source. A two-pole low-pass filter comprising two resistors and two capacitors converts this pulsed current into a DC voltage. A DC voltage source adds a positive bias voltage to all components of this low pass filter. 
   The resistors discussed thus far have all preferably had large resistances in the megaohm range, for example ranging from approximately 5 to 33 megaohms. However, since such values are not suitable for direct connection to appliance control electronics that have a low effective impedance, bipolar junction transistors are used in a high-gain emitter-follower amplifier configuration that converts the high impedance voltage into a low impedance voltage on an output resistor. This analog output voltage is inversely proportional to the flame current. A higher voltage is produced for small flame currents and a lower voltage for high flame currents. Small flame current is indicative of a weak flame and possible system problems. A high flame current is indicative of a strong flame and a well-functioning system. 
   A control circuit would typically compare this output voltage against reference values to determine the status of the flame. If there is no flame, or if a flame was established and then lost, the control circuit would immediately turn off the gas supply to the burner. Since this flame sense circuit of the present invention provides a critical safety function, it must not be sensitive to environmental conditions and must fail in a safe manner. All the components in the circuit can be readily chosen to withstand the normal extremes of temperature, humidity, shock, and vibration. An important advantage of this circuit is that the output voltage is not sensitive to normal variations in the parameters of the transistors. Furthermore, if any of the components fail either short or open circuit, the output voltage would go to an abnormally high or low level, indicating a fault condition. 
   Advantageously, all of the component values of the circuit of the present invention, including the AC and DC voltage sources, can be chosen from a wide range of possible values suitable for optimum circuit operation. Furthermore, the transistors forming the high-gain emitter-follower amplifier can be replaced either with an integrated Darlington transistor, or an integrated circuit amplifier having high impedance and high gain characteristics such as an operation amplifier. Furthermore, a single transistor with sufficiently high gain may also be used in place of the transistors. Additionally, the two-pole filter may be replaced with only a single-pole filter with the values of the resistor and capacitor adjusted accordingly to achieve desired performance, recognizing that a longer flame failure recognition response time may be the result. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings incorporated in and forming a part of the specification illustrate several aspects of the present invention, and together with the description serve to explain the principles of the invention. In the drawings: 
       FIG. 1  is a simplified single line schematic illustration of a flame sense circuit constructed in accordance with the teachings of the present invention; 
       FIG. 2  provides a graphical illustration of the output voltage versus the flame current for the embodiment of the flame sense circuit illustrated in  FIG. 1 ; 
       FIG. 3  is a simplified single line schematic illustration of an alternate embodiment of a flame sense circuit constructed in accordance with the teachings of the present invention; 
       FIG. 4  is a simplified single line schematic illustration of yet a further embodiment of a flame sense circuit constructed in accordance with the teachings of the present invention; and 
       FIG. 5  is a simplified single line schematic illustration of a still further embodiment of a flame sense circuit constructed in accordance with the teachings of the present invention. 
   

   While the invention will be described in connection with certain preferred embodiments, there is not intent to limit it to those embodiments. On the contrary, the intent is to cover all alternatives, modifications and equivalents as included within the spirit and scope of the invention as defined by the appended claims. 
   DETAILED DESCRIPTION OF THE INVENTION 
   Turning now to the drawings, and with specific reference to  FIG. 1 , there is illustrated an embodiment of a flame sense circuit  10  of the present invention. In this circuit  10 , an AC voltage source  12  is used to supply AC voltage, e.g. 120 volts AC, to the circuit. This AC voltage is provided through capacitor  14  and resistor  16  to excite a flame sense probe  18 , which may be a flame probe, a gas igniter, etc. As may be seen in this  FIG. 1 , the flame probe  18  is small compared with the burner  20  which is used in this embodiment as the other electrode to provide the flame sensing. However, one skilled in the art will recognize that this other electrode  20  may be provided as a separate piece from the burner as is desired. 
   As a result of the asymmetrically sized electrodes  18 ,  20 , when flame  22  is present, a substantially DC current will flow from electrode  18  to electrode  20 . The direction of the flame current is illustrated by arrow  24 . This substantially DC flame current causes a net charge to develop on capacitor  14 . As result of this net DC voltage on capacitor  14 , a substantially DC sense current will flow through resistor  28  in the direction illustrated by the arrow  26 . This essentially DC sense current, however, has a pulsed waveform at the frequency of the AC source  12 . This is because the flame  22  is actually a poor or leaky rectifier. In a preferred embodiment of the present invention, a two-pole low-pass filter, consisting of resistor  30  and capacitor  32 , and resistor  34  and capacitor  36 , converts this pulsed sense current into a DC voltage on capacitor  36 . 
   While the voltage resulting from the sense current would tend to be negative, a DC bias is provided to ensure that the sense voltage is a positive value. This bias may be provided by DC voltage source  38 , which provides a positive bias voltage to all of the components of the low pass filter (resistor  30 , capacitor  32 , resistor  34 , and capacitor  36 ). As will be recognized by one skilled in the art from this description, the DC voltage source  38  may comprise simply a resistor and a Zener diode to provide the proper bias. While the magnitude of the DC bias may vary, in one embodiment of the present invention, the bias voltage is set at 15 Vdc. 
   Since the flame current flowing from electrode  18  to electrode  20  is in the microampere range, the resistance values of the resistors discussed to this point are all relatively large so that a voltage of sufficient magnitude may be generated. Indeed, in one embodiment the values are as follows: resistor  16  is 10 megaohms; resistor  28  is 33 megaohms; resistor  30  is 5.1 megaohms, and resistor  34  is 5.1 megaohms. However, such large resistances are not suitable for direct connection to the appliance&#39;s control electronics. This is because such electronics typically have a low effective input impedance. Therefore, the circuit  10  of the present invention provides what may be thought of as a translation of the high impedance voltage generated by the sense current to a relatively low impedance voltage suitable for coupling to the appliance&#39;s control electronics. 
   In one embodiment of the present invention illustrated in  FIG. 1 , this translation is performed via the bi-polar junction transistors (BJTs)  40 ,  42  that are configured to form a high-gain emitter-follower amplifier  44 . This amplifier  44  converts the high impedance voltage on capacitor  36  into a relatively low impedance voltage on resistor  46  for coupling to the appliance&#39;s control electronics. In one embodiment of the present invention, the value of resistor  46  is approximately 50 kΩ. 
   As may be seen in  FIG. 2 , the analog output voltage represented by trace  48  is inversely proportional to the flame current. That is, a higher voltage is produced for small flame currents, and a lower voltage for large flame currents. A small flame current is indicative of a weak flame and possible system problems, while a high flame current is indicative of a strong flame and a well functioning system. As such, the appliance&#39;s control electronics can monitor the output voltage, and compare that voltage to an internal reference voltage to determine the status of the flame, and thereby the status of the system. If there is no flame, or a flame was established and then lost, the control circuit would immediately turn off the gas supply to the burner to prevent the development of a hazardous condition. Further, if a weak flame is sensed, the system electronics may provide indication that servicing of the burner is required, may institute a self clean operation, or may simply log this information for subsequent retrieval by maintenance personnel. 
   As indicated above, the accuracy and reliability of prior flame sense circuits were adversely affected by the various parameters of the JFET transistors typically used therein. The circuit of the present invention suffers from no such accuracy or reliability problems, and is, in fact, not sensitive to normal variations in the parameters of the bipolar junction transistors (BJTs)  40 ,  42  used to form the high-gain emitter-follower amplifier  44 . Furthermore, if any of the components of the embodiment of the present invention illustrated in  FIG. 1  fail either open circuit or short circuit, the output voltage would go to an abnormally low or high level, which will be interpreted by the control electronics that a fault condition in the sensing circuit exists. The control circuit may then execute a controlled shut down of the system. 
   An alternate embodiment of the flame sense circuit of the present invention is illustrated in FIG.  3 . As may be seen from an examination of this alternate embodiment, only a single bi-polar junction transistor (BJT)  50  is used in place of the emitter-follower pair of transistors  40 ,  42  illustrated in  FIG. 1  to provide the high-impedance, high-gain characteristics necessary for proper system operation. The higher the gain of the single transistor  50  the better for use with standard control circuitry. Depending on the input characteristics of the control circuitry, a single BJT  50  having a gain of approximately 100 or higher may be used. Indeed, there are single transistors that have gains up over 600 or 700 that are preferred for operation in the embodiment of the flame sense circuit illustrated in FIG.  3 . The lower the gain of transistor  50 , the higher the impedance of resistor  46  should be so that the smaller amount of gain of the single transistor  50  multiplied by the higher value of resistance of resistor  46  reflected into the low pass filter will still allow the flame sense circuit of the present invention to function in relation to the relatively high impedance sense circuitry to which this circuit supplies its output. In further embodiments of the present invention, this transistor  50  may be replaced by an integrated Darlington transistor, or an integrated circuit amplifier having the high-impedance and high-gain characteristics such as an operational amplifier  60  discussed above and illustrated in FIG.  4 . 
     FIG. 5  illustrates yet a further embodiment of a flame sense circuit constructed in accordance with the teachings of the present invention. As may be seen in this schematic illustration, only single-pole filter is used having resistor  30  and capacitor  32 . In this configuration, the values of the resistor  30  and capacitor  32  may be varied to provide similar performance as the circuits discussed above, recognizing that a longer flame failure recognition response time may result. Other circuit modifications will be apparent to those skilled in the art in view of the foregoing description. For example, a resistor may be added in series with capacitor  14  to account for different system characteristics. Further, resistors  16  and  28  can be comprised of series combinations of resistors to withstand increased voltage and to provide operational redundancy. Additionally, all of the component values, including the AC and DC sources, can be chosen from a wide range of possible values selected to optimize circuit operation for different applications. 
   All references, including publications, patent applications, and patents, cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein. 
   The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention. 
   Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.