Abstract:
A flame rod drive signal generator and system for producing a flame rod drive signal for a flame rod of a combustion system. In one illustrative embodiment, the flame rod drive signal generator may include a voltage source, an input signal having a frequency, an LC oscillator and a drive mechanism. The drive mechanism may be powered by the voltage source, and may have an output coupled to the LC oscillator. The drive mechanism may receive the input signal, and produces a current in the LC oscillator that has a frequency that is related to the frequency of the input signal. The LC oscillator may then provide a flame rod drive signal to a flame rod that has an amplitude that is larger than the amplitude of the voltage source. In some cases, a controller may monitor the amplitude of the flame rod drive signal and adjust the frequency, duty cycle, or both, of the input signal to achieve a desired amplitude of the flame rod drive signal. Alternatively, or in addition, the controller may monitor an ionization current produced by the flame rod when the flame rod is subject to a flame.

Description:
FIELD 
     The present invention relates generally to flame sensing circuits, and more particularly, to flame rod drive signal generators and systems. 
     BACKGROUND 
     Commercial and residential buildings and structures can include many building system components. In some cases, the building components may be gas-fired building components such as furnaces, boilers, water heaters, deep fryers, as well as many other types of gas-fired building components. Gas-fired building components often include a combustion system acting as the heating system for the component. One example combustion system may include a gas source, a gas valve to regulate the gas source, a burner, an ignition system to ignite the burner when desired, and a controller to control the operation of the combustion system. 
     In some combustion systems, a flame rod may be provided to sense the presence of the flame, indicating that the gas burner is ignited. In this case, the presence of the flame may be detected by an ionization current in the flame rod. To detect the ionization current, the controller may apply an alternating current voltage between the flame sensing rod and the base of the flame (i.e. ground). The ions in the flame may provide a high resistance current path between the flame rod and the ground. Because the surface of the flame base is larger than the flame rod, more electrons may flow in one direction than the other, resulting in a relatively small direct current (DC) offset current. When a flame is present, this DC offset may be detected by the controller, which may indicate that a flame is present. The controller may then control the operation of the combustion system according to the presence of the flame. For example, when the flame is present, the controller may further open and/or leave open the gas valve and/or air flow dampers. If there is no flame present the controller may close the gas valve or take other action. 
     In many cases, the drive signal for the flame rod may need to be a relatively high-voltage AC signal, such as 100 Volts, 200 Volts or the like. However, in many cases, the control system may only have a relatively low voltage power source available, such as 24 Volts, 5 Volts or the like. As such, the control system may need to boost the low voltage into a high voltage source to generate the flame sensing signal. In some cases, to boost the relatively low voltage source, a DCDC step-up circuit may be used. In this case, the DCDC step-up circuit may be able to generate a high voltage DC power source, which may then be chopped to generate the desired high-voltage AC signal for the flame rod. However, this method can add significant cost to the control system. Therefore, there is a need for alternative control systems that can generate a relatively high voltage AC signal to drive a flame rod. 
     SUMMARY 
     The following summary is provided to facilitate an understanding of some of the innovative features unique to the present invention and is not intended to be a full description. A full appreciation of the invention can be gained by taking the entire specification, claims, drawings, and abstract as a whole. 
     The present invention relates generally to flame sensing circuits, and more particularly, to flame rod drive signal generators and systems. In one illustrative embodiment, a flame rod drive signal generator is provided for producing a flame rod drive signal for a flame rod of a combustion system. The flame rod drive signal generator may include a voltage source, an input signal having a frequency, an LC oscillator and a drive mechanism. The drive mechanism may be powered by the voltage source, and may have an output that is coupled to the LC oscillator. The drive mechanism may receive the input signal, and produces a current in the LC oscillator that has a frequency that is related to the frequency of the input signal. The LC oscillator may provide a flame rod drive signal that has an amplitude that is larger than the amplitude of the voltage source, and in some cases, significantly larger. A controller may monitor the amplitude of the flame rod drive signal and adjust the frequency of the input signal to achieve a desired amplitude of the flame rod drive signal. The controller may also monitor an ionization current produced by the flame rod when the flame rod is subject to a flame. 
    
    
     
       BRIEF DESCRIPTION 
       The invention may be more completely understood in consideration of the following detailed description of various illustrative embodiments of the invention in connection with the accompanying drawings, in which: 
         FIG. 1  is a schematic diagram of an illustrative flame rod drive signal circuit for a combustion system; 
         FIG. 2  is an illustrative graph showing waveforms for the illustrative flame rod drive signal circuit of  FIG. 1 ; 
         FIG. 3  is an illustrative graph of voltage versus frequency for the flame rod drive signal circuit of  FIG. 1 ; 
         FIG. 4  is a schematic diagram of another illustrative flame rod drive signal circuit; 
         FIG. 5  is a schematic diagram of another illustrative flame rod drive signal circuit; 
         FIG. 6  is an illustrative graph showing waveforms of the illustrative flame rod drive signal circuit of  FIG. 5 ; and 
         FIG. 7  is a schematic diagram of an illustrative flame sensing circuit. 
     
    
    
     DETAILED DESCRIPTION 
     The following description should be read with reference to the drawings wherein like reference numerals indicate like elements throughout the several views. The detailed description and drawings show several embodiments, which are meant to be illustrative of the claimed invention. 
       FIG. 1  is a schematic diagram of an illustrative flame rod drive signal circuit  10  for a combustion system. In the illustrative embodiment, the flame rod drive signal circuit  10  includes a push-pull drive stage and an oscillation network. The push-pull drive stage may have an input and an output. The input of the push-pull drive stage may be connected to a voltage source  11 , shown as Vdc, having a first voltage such as 24V, 5V or some other suitable voltage. The oscillation network may include an input and an output. The input of the oscillation network may be connected to the output of the push-pull drive stage and the output of the oscillation network may provide a flame rod drive signal, shown as FlameDriveVoltage, having a second voltage. In some cases, the second voltage may be greater than the first voltage, and sometimes substantially greater such as 100V, 200V or any other suitable flame rod drive voltage. In some cases, a flame rod (shown as  54  in  FIG. 7 ) may be connected to the output of the oscillation network to receive the flame rod drive signal. 
     Voltage source  11  may provide a first voltage to the flame rod drive signal circuit  10 . In some cases, voltage source  11  may be provided as part of a building control system controller. In some cases, the building system controller may be a controller for a gas-fired building component. In the illustrative embodiment, voltage source  11  may be a direct current (DC) voltage source. In one example, voltage source  11  may be a rectified 24-volt AC signal. In the illustrative example, voltage source  11  may range from about 25 volts to 40 volts, as desired. However, it is contemplated that any suitable voltage source  11  may be used for the circuit, as desired. 
     In the illustrative embodiment, push-pull drive stage may include a pair of transistors  16  and  18 , resistors  12  and  14 , and a diode  20 . Push-pull drive stage may include a first input, at node A, connected to the voltage source  11 , and a second input, connected to a pulse width modulation (PWM) input signal. Push-pull drive stage may also include an output, designated as node C in  FIG. 1 . 
     In the illustrative embodiment, transistor  16  and  18  are shown as bipolar junction transistors (BJTs). However, it is contemplated that any suitable type of transistor or switching device may be used, such as field effect transistors (FETs). In the illustrative embodiment, transistors  16  and  18  are shown as NPN transistors. However, it is contemplated that transistors  16  and  18  may be PNP transistors, or a combination of NPN and PNP transistors, depending on the circuit configuration and design. In this configuration, as BJT transistors, transistors  16  and  18  may include a collector terminal, a base terminal, and an emitter terminal as shown. 
     Transistor  16  may be configured to have its collector terminal connected to node A, or the voltage source  11 . Base terminal of transistor  16  may be connected to node B. Emitter terminal of transistor  16  may be connected to node C. Node B may be connected to node A via resistor  14 . Node C may be connected to an anode terminal of diode  20  with the cathode connected to node B. 
     Transistor  18  may be configured to have its collector terminal connected to node B. Base terminal of transistor  18  may be connected to the PWM input signal via resistor  12 , and emitter terminal of transistor  18  may be connected to ground as shown. 
     In the illustrative embodiment, the PWM input signal may be provided by a controller, such as, for example, a microcontroller or microprocessor. In the illustrative embodiment, the PWM signal may be a logic signal having a logic high state and a logic low state. In the logic high state, the PWM signal may be about 5 volts. In the logic low state, the PWM signal may be about 0 volts. When the PWM signal is logic high, transistor  18  may be turned on. When PWM signal is logic low, transistor  18  may be turned off. 
     In some cases, the controller may be able to control the PWM signal frequency and/or duty cycle. In some cases, as will be discussed below in further detail, the frequency and/or duty cycle of the PWM signal may control, at least in part, the amplitude of the flame rod drive output signal, provided at node D. 
     In one operational example, when the PWM signal is logic low, transistor  18  may be turned off. In this state, the voltage at the emitter of transistor  16 , at node C, may be a positive voltage. In some cases, the voltage at node C may be about one or two diode drops below the first voltage provided by the voltage source  11 . As such, current may flow through the load (i.e. LC oscillator including series connected inductor  22  and capacitor  24 ). In other words, in this state, current may be “pushed” through the load. 
     When the PWM signal is a logic high, transistor  18  may be turned on. In this state, the voltage at node C may be about 0 volts. As such, current may flow from the load to node C and through transistor  18 . In other words, in this state, current may be “pulled” through the load. It is to be understood that the foregoing push-pull drive stage, that includes transistors  16  and  18 , is merely illustrative and that any equivalent circuit or any similar type of circuit may be used, as desired. 
     The oscillation network load may include an input and an output. The input of the oscillation network may be connected to node C, or the output of the push-pull drive stage. The output of the oscillation network may be at node D, which may be correspond to the flame rod drive signal provided to the flame rod. In the illustrative embodiment, the oscillation network may amplify the voltage provided at the input of the oscillation network to a second voltage at the output of the oscillation network. 
     As illustrated, the oscillation network may include a LC oscillator that includes a series connected inductor  22  and capacitor  24 . In the illustrative embodiment, inductor  22  is connected between node C and node D, and capacitor  24  is connected between node D and ground. 
     The oscillation network may have a resonant frequency. The resonant frequency may be based on the inductance of inductor  22  and the capacitance of capacitor  24 , as indicated in the following equation: 
               f   r     =     1     2   ⁢   π   ⁢     LC               
In one example, the inductance of inductor  22  may be about 68 milliHenries (mH) and the capacitance of capacitor  24  may be about 4700 picofarads (pf). In this example, using the above equation, the resonant frequency may be about 8.9 kiloherts (kHz). However, this is merely illustrative and it is contemplated that any suitable inductance and capacitance may be used, depending on the desired resonant frequency for the circuit.
 
     In operation, the oscillation network may amplify the voltage at node C according to the frequency and/or duty cycle of the PWM input signal provided by the controller. To determine the flame rod drive signal provided by the LC oscillator, the frequency of the PWM signal is needed. In one example, the frequency of the PWM signal may be about 8.33 kilohertz (kHz), which is less than the resonant frequency of the oscillation network. In some cases, having the frequency of the PWM signal to be offset from the resonant frequency of the oscillation network may be desirable, but it is not required. Knowing the frequency of the PWM signal, the angular frequency (ω) may be determined according to the following equation:
 
ω=2πf
 
In the illustrative example, with a frequency of about 8.33 kHz, the angular frequency ω is about 52 Kilo-Radians/Second.
 
     Next, to determine the amplitude of the flame rod drive signal, the inductance reactance (X L ) and capacitance reactance (X C ) may be determined. The inductance reactance and capacitance reactance may be determined according to the following equations: 
               X   L     =     ω   ⁢           ⁢   L                   X   C     =     1     ω   ⁢           ⁢   C             
where L is the inductance of inductor  22  and C is the capacitance of capacitor  24 . In the illustrative example, with an inductance of 68 mH and a capacitance of 4700 pf, X L  is about 3.5 kΩ and X C  is about 4 kΩ.
 
     To illustrate the basic calculation, and neglecting parasitic parameters such as the series resistance of the inductor, the core loss of the inductor, the leakage resistance of the capacitor, etc. (which may or may not be neglected in practice, depending on the application), the impedance (Z) of the LC oscillator may be calculated according to the following equation:
 
 Z=|X   L   −X   C |
 
Using the illustrative inductance and capacitance values, Z may be about 500 ohms.
 
     Next, the current (i) flowing through the inductor  22  and capacitor  24  may be determined using the source voltage (v) of the circuit in the following equation: 
             i   =     v   Z           
In one cases, the illustrative source voltage (i.e. voltage at node C) may be about 25 volts. Using this illustrative voltage and the illustrative impedances, the current i may be about 49.7 milliamps (mA).
 
     Then, with the example current, the inductor voltage (V L ) and the capacitor voltage (V C ) may be determined according to the following equations:
 
V L =iX L  
 
V C =iX C  
 
In the given example, V L  may be about 177 volts and V C  may be about 202 volts. Thus, the voltage at node D, or V C  in this case, is 202 Volts.
 
     In the illustrative embodiment, increasing the illustrative source voltage (i.e. voltage at node C) may increase the flame rod drive signal output voltage. Additionally, as will be discussed in  FIG. 3 , increasing the frequency closer to the resonant frequency may increase the voltage at the flame rod drive signal output. In some cases, the illustrative embodiment may generate a relatively high flame rod drive signal voltage, such as, for example, between 50 volts and 400 volts, and may be an alternating current (AC) signal which is ideal for driving a flame rod. 
     In one illustrative example, resistor  12  may be about 3.3 kΩ and resistor  14  may be about 10 kΩ. However, it is contemplated that any suitable resistance may be used for resistors  12  and  14 , as desired. 
     In some cases, transistors  16  and  18  may be configured to withstand only the relatively low voltage level of voltage source  11  instead of the relatively high voltage levels of the flame rod drive output signal. This may reduce the cost of the overall system. Additionally, in some embodiments, transistors  16  and  18  may be operated in the off or saturated stated. In this case, the power consumption of the transistors  16  and  18  may be relatively low. In some cases, and due to the relatively high voltage of the oscillation network, it may be desirable to have relatively high voltage components in the oscillation network. For example, capacitor  24  may be a film capacitor rated at 160 VAC or higher. 
       FIG. 2  is an illustrative graph  90  showing waveforms of the illustrative flame rod drive signal circuit  10  of  FIG. 1 . The illustrative graph shows waveforms of the voltage over a period of time at the emitter of transistor  16  (node C)  92 , at the collector of transistor  18  (node B)  94 , at the flame rod drive voltage output signal (node D)  96 , and at the PWM input signal from the controller  98 . 
     As illustrated in the waveforms, the voltage of waveform  92  at the emitter of transistor  16  and waveform  94  of the collector of transistor  18  may be similar. In some cases, the illustrative voltages may be about one diode drop apart. Additionally, these voltages may be about one or two diode drops below the voltage source  11  (not shown) when transistor  18  is turned off. 
     The illustrative PWM waveform  98  may alternate between a logic high state (about 5 volts) and a logic low state (about 0 volts). As discussed previously, when the PWM waveform  98  is logic low, the voltage at the emitter of transistor  16  and the collector of transistor  18  may be relatively high, which, in the illustrative case, may be about 25 volts. However, the voltage of waveforms  92  and  94  may be dependant upon the PWM signal that is provided to the flame rod drive signal circuit. When the PWM signal is logic high, the voltage at the emitter of transistor  16  and the collector of transistor  18  may be relatively low, which, in the illustrative case, may be about 0 volts. 
     In the illustrative embodiment, with the example inductance, capacitance, and frequency, the flame rod drive signal waveform  96  may be a generally sinusoidal signal having a relatively large amplitude. In the illustrative embodiment, the voltage may range from about −185 volts to about 180 volts. However, the illustrated flame rod drive signal waveform  96  is merely illustrative and it is contemplated that any suitable flame rod drive signal may be used, as desired. 
       FIG. 3  is an illustrative graph  110  of voltage versus frequency for the flame rod drive signal circuit of  FIG. 1 . In the illustrative graph  110 , the resonant frequency, shown at  112 , may be the frequency at which the flame rod drive signal waveform  96  peaks. From the illustrative example above, the resonant frequency may be about 8.9 kilohertz. However, any suitable resonant frequency may be used, depending on the inductance and capacitance values of the oscillation network, as desired. 
     As discussed previously, the voltage provided by the flame rod drive signal may be determined by the sum of X C  and X L . To the right of the resonant frequency, or at a frequency greater than the resonant frequency, X L  may be greater than X C . To the left of the peak, or at a frequency less than the resonant frequency, X C  may be greater than X L . By varying the PWM frequency along this curve, the voltage produced by the LC oscillator may be increased or decreased to a desire value. 
     In the illustrative embodiment, parasitic capacitance may be present in the flame rod drive signal circuit. To help reduce the effect of any parasitic capacitance, it may be desirable to operate at a frequency lower than the resonant frequency, such as in a region designated by reference numeral  114 . The effect of the parasitic capacitance may be reduced because, as stated above, X C  increases as the frequency decreases. As such, at lower frequencies, the effect of parasitic capacitance will make up a smaller percentage of the overall capacitance value, in essence, reducing the parasitic capacitance effect when the frequency is reduced. 
       FIG. 4  is a schematic diagram of another illustrative flame rod drive signal circuit  30 . The illustrative flame rod drive signal circuit  30  is similar to the flame rod drive signal circuit described above with reference to  FIG. 1 , with the addition of diode  32 . With the circuit shown in  FIG. 1 , when the PWM frequency is lower than the resonant frequency, transistor  16  may be reversely biased for some time in each cycle, allowing current flow from node C to Vdc. While a BJT can work in this condition, adding diode  32  to provide a current path, may improve the overall efficiency of the drive circuit. If a MOSFET is used as  16 , then the diode  32  may not help in this regard. As illustrated, diode  32  may have an anode connected to the emitter of transistor  16  and a cathode connected to the collector of transistor  16 . 
       FIG. 5  is a schematic diagram of another illustrative flame rod drive signal circuit  40 . The illustrative flame rod drive signal circuit  40  may be similar to that described above with reference to  FIG. 1 , with the modification of swapping the position of inductor  22  and capacitor  24 . In the illustrative embodiment, inductor  22  may be grounded, whereas in  FIG. 1 , capacitor  24  was grounded. In the illustrative embodiment, the swapping of inductor  22  and capacitor  24  may produce a waveform with sharper rising and falling edges, as shown in  FIG. 6 . Additionally, the phase of the flame rod drive output signal may be 180 degrees offset relative to the flame rod drive output signal of the embodiment of  FIG. 1 . If desired, diode  32  may be added to flame rod drive signal circuit  40 , similar to  FIG. 4 . 
       FIG. 6  is an illustrative graph  100  showing waveforms of the flame rod drive signal circuit of  FIG. 5 . The illustrative graph  100  shows waveforms of the voltage over a period of time at the emitter of transistor  16  (node C)  102 , the collector of transistor  18  (node B)  104 , the flame rod drive voltage output signal (node D)  108 , and the PWM input signal from the microcontroller  106 . 
     As illustrated in the waveforms, the voltage of waveform  102  at the emitter of transistor  16  and waveform  104  of the collector of transistor  18  may be similar. In some cases, the illustrative voltages may be about one diode drop apart. Additionally, these voltages may be about one or two diode drops below the voltage source  11  (not shown), when the transistor  18  is turned off. 
     The illustrative PWM waveform  98  may alternate between a logic high state (about 5 volts) and a logic low state (about 0 volts). As discussed previously, when the PWM waveform  108  is at a logic low, the voltage at the emitter of transistor  16  and the collector of transistor  18  may be relatively high, which, in the illustrative case, may be about 25 volts. When the PWM signal is at a logic high, the voltage at the emitter of transistor  16  and the collector of transistor  18  may be relatively low, which, in the illustrative case, may be about 0 volts. 
     In the illustrative embodiment, the flame rod drive signal waveform  106  may be a generally sinusoidal signal having a relatively large amplitude. In the illustrative case, the voltage may range from about −160 volts to about 160 volts. However, the illustrated flame rod drive signal waveform  106  is merely illustrative and it is contemplated that any suitable flame rod drive signal may be used, as desired. In the illustrative embodiment, waveform  106  may include one or more spikes  109  when the PWM waveform switches between the logic high and logic low states. 
       FIG. 7  is a schematic diagram of an illustrative flame sensing circuit  50 . In the illustrative embodiment, the flame sensing circuit  50  may include a flame rod drive signal circuit  10  (see  FIG. 1 ) having a flame rod drive signal output (node D). As illustrated, the illustrative flame sensing circuit  50  may also include a voltage sensing network  56 , a ripple filter  58 , and/or a bias element  78 . In  FIG. 7 , the flame rod drive signal circuit  10  is shown as the flame rod drive signal circuit of  FIG. 1 , however, it is contemplated that the embodiments of  FIGS. 4  and/or  5  may be used, if desired. 
     In the illustrative embodiment, flame sensing circuit  50  may be connected to a flame rod  54 . Flame rod  54  may be provided in or adjacent to a flame in a combustion system to detect the presence of a flame on a burner in the combustion system. When a flame is present, the flame rod may have a corresponding DC offset current. When no flame is present, the flame rod may have no or little DC offset current. It is contemplated that any suitable flame rod may be used, as desired. 
     In the illustrative embodiment, a microcontroller  52  may be connected to the circuit  50  to provide one or more inputs to the circuit  50  and/or receive one or more outputs from the circuit  50 . In the illustrative embodiment, the microcontroller  52  may have a first output to provide a PWM input signal to the flame rod drive signal circuit  10  to switch transistor  16  on and off, as discussed above. Microcontroller  52  may also have a second output connected to the flame sensing circuit  50  to provide a bias input to bias element  78 , if desired. Microcontroller  52  may have a first input to receive an AmplitudeSense_AD output signal indicating the amplitude of the flame rod drive signal provided to the flame rod  54 . Microcontroller  52  may also have a second output connected to the flame sensing circuit  50  to receive a Flame_AD signal indicating the presence of the flame in the combustion system. In some cases, microcontroller  52  may use the two received signal to adjust the frequency and/or duty cycle of the PWM to achieve a desired flame rod drive signal amplitude, and/or to adjust the voltage of the bias signal. 
     Voltage sensing network  56  may be able to sense the amplitude of the flame rod drive signal provided to the flame rod  54 . In the illustrative embodiment, voltage sensing network  56  may have an input connected to node D, which is at the output of the flame rod drive signal circuit  10 , and an output (node G) connected to the first input of the microcontroller  52 . The voltage sensing network  56  may provide an output signal to the microcontroller  52  indicative of the amplitude of the flame rod drive signal. In response, and in some cases, microcontroller  52  may adjust the frequency and/or duty cycle of the PWM to adjust the amplitude of the flame rod drive signal on node D. 
     In the illustrative embodiment, the voltage sensing network  56  may include a voltage divider. As illustrated, the voltage divider may include resistors  62  and  64 . In some cases, a diode  60  may be provided to help prevent current backflow, and a capacitor  66  may be provided to help filter the output. In the illustrative example, anode of diode  60  may be connected to node D and cathode of diode may be connected to resistor  62 , which may be connected to node G as shown. Resistor  64  may be connected between node G and ground. Capacitor  66  may be connected between node G and ground. Node G may be connected to the microcontroller  52  as the output of the voltage sensing network  56 . In this configuration, microcontroller  52  may be able to sense the voltage across resistor  64 , indicating the amplitude of the flame rod drive signal. 
     In one example, resistor  62  may be about 470 kΩ, resistor  64  may be about 30 kΩ, and capacitor  66  may be about 0.22 microfarads. However, it is contemplated that any suitable resistance and/or capacitance values may be used, as desired. In some cases, the voltage across the voltage sensing network may be relatively large. In this case, the resistor  62  and capacitor  66  may be high voltage components. However, any suitable components may be used, as desired. 
     Ripple filter  58  may be configured to filter and sense the DC offset current of the flame rod  54 . In the illustrative embodiment, ripple filter  58  may include an input connected to node E and an output connected to node F, which may be connected to the second input of microcontroller  52 . In the illustrative example, the ripple filter  58  may be a 2-pole low-pass filter that may be able to filter out the AC component of the flame rod drive signal. In the illustrative example, the 2-pole low-pass filter may include resistors  70  and  72  and capacitors  74  and  76 . Resistor  70  and capacitor  74  may form a first low pass filter and resistor  72  and capacitor  76  may form a second low pass filter. In one example, resistors  70  and  72  may be about 470 kΩ, and capacitors  74  and  76  may be about 22 nanofarads. However, it is contemplated that any suitable resistances and capacitances may be used, as desired. 
     In addition, and in some cases, the second pole of the ripple filter  58  including resistor  72  and capacitor  76  may be removed depending on if the microcontroller  52  has enough processing power to perform a comparable filtering function. For example, and during operation, microcontroller  52  may be able to sense the flame signal (node F) in synchronization with the flame rod drive signal (node D) and remove the ripple to get the flame current signal. In some cases, microcontroller may use the amplitude and other properties of the AC component of the ripple to diagnose the flame sensing circuit  50  and check the condition of the parts or portions of the flame sensing circuit  50 . 
     In some cases, with the circuit parts or portions in good working condition, the AC component amplitude may be estimated or measured. These amplitude data may be stored in a non-volatile memory of the controller. During normal operation, the AC component may be monitored. If the AC component becomes too high or too low compared to the stored value, an error message may be reported. The AC component amplitude may be used to help identify the possible faulty part or portion of the circuit. For example, many diagnostics uses are described in U.S. application Ser. No. 11/276,129, entitled CIRCUIT DIAGNOSTICS FROM FLAME SENSING AC COMPONENT, which is incorporated herein by reference. 
     In some cases, a bias element  78  and bias signal may be provided to adjust the flame sensing signal to a suitable voltage level for the microcontroller  52  to sense. For example, and in some cases, the bias element  78  may be connected to the second output of microcontroller  52 . In such a case, resistor  78  may be the bias element, and may be about 200 kΩ, however, any suitable resistance may be used, as desired. 
     The bias signal can be a voltage, such as a DC voltage, or a PWM signal provided by the microcontroller  52 . In some cases, if the bias signal is a PWM signal, the bias may be changed to increase the flame sensing dynamic range, and DC leakage may be detected and compensated to improve flame sensing accuracy and robustness, such as described in U.S. application Ser. No. 10/908,463, entitled DYNAMIC DC BIASING AND LEAKAGE COMPENSATION, which is incorporated herein by reference. 
     In some cases, flame sensing circuit  50  may include a decoupling capacitor  68 . The decoupling capacitor  68  may help reduce or eliminate DC coupling in the circuit  50 . In one example, the decoupling capacitor  68  may have a capacitance of about 4700 pf However, any suitable capacitance may be used, as desired. 
     In the illustrative embodiment, a resistor  80  may be provided in series with the flame rod  54  for safety. In some cases, resistor  80  may be provided to limit the current in case a human being touches the rod while it carries a high AC voltage. In one case, the resistance of resistor  80  may be about 200 kΩ. However, any suitable resistance may be used, as desired. In some cases, resistor  80  may not be provided. 
     In some cases, the flame rod drive signal voltage (node D) may be changed to increase the dynamic range of the flame sensing circuit  50 , such as described in U.S. application Ser. No. 10/908,467, entitled ADAPTIVE SPARK IGNITION AND FLAME SENSING SIGNAL GENERATION SYSTEM, which is incorporated herein by reference. 
     The control algorithm may be similar to that of the flame sensing methods described in U.S. application Ser. No. 10/908,465, entitled LEAKAGE DETECTION AND COMPENSATION SYSTEM; U.S. application Ser. No. 11/276,129, entitled CIRCUIT DIAGNOSTICS FROM FLAME SENSING AC COMPONENT; U.S. application Ser. No. 10/908,467, entitled ADAPTIVE SPARK IGNITION AND FLAME SENSING SIGNAL GENERATION SYSTEM; and U.S. application Ser. No. 10/908,463, entitled DYNAMIC DC BIASING AND LEAKAGE COMPENSATION, which are incorporated herein by reference. 
     Having thus described the preferred embodiments of the present invention, those of skill in the art will readily appreciate that yet other embodiments may be made and used within the scope of the claims hereto attached. Numerous advantages of the invention covered by this document have been set forth in the foregoing description. It will be understood, however, that this disclosure is, in many respect, only illustrative. Changes may be made in details, particularly in matters of shape, size, and arrangement of parts without exceeding the scope of the invention. The invention&#39;s scope is, of course, defined in the language in which the appended claims are expressed.