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1. WO2020139994 - LEAKAGE DETECTION IN A FLAME SENSE CIRCUIT

Note: Text based on automatic Optical Character Recognition processes. Please use the PDF version for legal matters

[ EN ]

LEAKAGE DETECTION IN A FLAME SENSE CIRCUIT

Technical Field

[0001] The present disclosure pertains generally to flame sensing circuits and more particularly to leakage detection for flame sensing circuits.

Background

[0002] Flame sensing systems are widely used to detect flames in combustion systems, often using flame-sensing rods or the like. In many instances, when no flame is detected, the fuel to the combustion system is turned off to help prevent un burned fuel from being released in the combustion system. In many instances, flame sensing systems rely on the detection of flame sense signals produced by a flame sensing rod or the like that is exposed to the flame. The flame sense signals can be small and in some cases rivaled by parasitic leakage currents. When this occurs, there is a danger that the parasitic leakage currents may be misinterpreted as a flame sense signal, which may result in the flame sensing system falsely reporting a flame when no flame is actually present. What would be desirable is an improved flame sensing system that can reliably detect such leakage currents to help improve the accuracy and reliability of a flame sensing system.

Summary

[0003] The disclosure pertains to flame sensing circuits and more particularly to leakage detection for flame sensing circuits. A particular example of the disclosure is found in a flame detection system that includes a flame sensor for sensing a flame, where the flame sensor may draw a flame sense current when a flame is present. An amplifier may be operatively coupled to the flame sensor for amplifying the flame sense current and for drawing an amplified flame sense current from an amplifier output. A detection circuit may be operatively coupled to the amplifier output for detecting the amplified flame sense current.

[0004] The detection circuit may include a capacitor having a first end operatively coupled to the amplifier output and a first resistor having a first end operatively coupled to the amplifier output. The first resistor may have a first resistance value. A second resistor may have a first end operatively coupled to the amplifier output and the second resistor may have a second resistance value that is different from the first resistance value.

[0005] A microcontroller may be operatively coupled to a second end of the first resistor and a second end of the second resistor and the first end of the capacitor. The microcontroller may be configured to charge the capacitor through the first resistor from a first lower threshold voltage to a first upper threshold voltage, and then allow the amplified flame sense current to discharge the capacitor down to the first lower threshold voltage. The microcontroller may determine a first duty cycle for charging and discharging of the capacitor through the first resistor. The microcontroller may also charge the capacitor through the second resistor from a second lower threshold voltage to a second upper threshold voltage. Then the microcontroller may allow the amplified flame sense current to discharge the capacitor down to the second lower threshold voltage. Further, the microcontroller may determine a second duty cycle of the charging and discharging of the capacitor through the second resistor. The microcontroller may determine a leakage current condition in the flame detection system based at least in part on the first duty cycle, the second duty cycle, the first resistance value and the second resistance value. The microcontroller may also provide a shutdown signal to shut down the flame (e.g. close a gas valve that supplies fuel to the combustion system) when the leakage current condition is determined.

[0006] Another example of the disclosure is method for detecting a leakage current condition in a flame detection system. The method may include amplifying with an amplifier a flame sense current provided by a flame sensor, resulting in an amplified flame sense current. The method may supply the amplified flame sense current to the amplifier via charge storage device and charge the charge storage device with a first charging circuit that produces a first charging rate. The method further may include subsequently charging the charge storage device with a second charging circuit that produces a second charging rate, wherein the second charging rate may be different from the first charging rate. The method may determine a leakage current condition in the flame detection system based at least in part on a comparison of the charging of the charge storage device with the first charging circuit and the charging of the charge storage device with the second charging circuit. The microcontroller may also provide a shutdown signal to shut down the flame (e.g. close a gas valve that supplies fuel to the combustion system) when the leakage current condition is determined.

[0007] Another example of the disclosure is a flame detection system that includes a flame sensor for sensing a flame. The flame sensor may draw a flame sense current when a flame is present. An amplifier may be operatively coupled to the flame sensor for amplifying the flame sense current and drawing an amplified flame sense current from an amplifier output. A negative voltage supply generator may supply a negative supply voltage to the amplifier. A detection circuit may be operatively coupled to the amplifier output for detecting the amplified flame sense current. A microcontroller may be operatively coupled to the negative voltage supply generator and the detection circuit. The microcontroller may be configured to change the negative supply voltage from a nominal negative supply voltage to a boosted negative supply voltage. The microcontroller may also determine a leakage current condition in the flame detection system when the amplified flame sense current detected by the detection circuit changes by more than a threshold amount when the negative supply voltage is changed from the nominal negative supply voltage to the boosted negative supply voltage and provide a shutdown signal to shut down the flame when the leakage current condition is determined.

Brief Description of Drawings

[0008] The disclosure may be more completely understood in consideration of the following description of various illustrative embodiments of the disclosure in connection with the accompanying drawings, in which:

[0009] Figure l is a schematic diagram of an illustrative flame detection system that includes a flame detection circuit with circuitry for detecting current leakage;

[0010] Figure 2 is a timing diagram showing operation of the circuitry for detecting leakage in the flame sense circuit of Figure 1;

[0011] Figure 3 is a schematic diagram of a pulsed negative supply voltage useful for detecting leakage in a flame sense circuit such as the flame sense circuit of Figure 1;

[0012] Figure 4 is a schematic block diagram of an illustrative flame sense circuit;

[0013] Figure 5 is a flow diagram of an illustrative method for detecting a leakage current condition in a flame sensing circuit; and

[0014] Figure 6 is a flow diagram of another illustrative method for detecting a leakage current condition in a flame sensing circuit.

[0015] While the disclosure is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit aspects of the disclosure to the particular illustrative embodiments described.

On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the disclosure.

Description

[0016] The following description should be read with reference to the drawings wherein like reference numerals indicate like elements. The drawings, which are not necessarily to scale, are not intended to limit the scope of the disclosure. In some of the Figures, elements not believed necessary to an understanding of relationships among illustrated components may have been omitted for clarity.

[0017] All numbers are herein assumed to be modified by the term“about”, unless the content clearly dictates otherwise. The recitation of numerical ranges by endpoints includes all numbers subsumed within that range (e.g., 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, and 5).

[0018] As used in this specification and the appended claims, the singular forms “a”,“an”, and“the” include the plural referents unless the content clearly dictates otherwise. As used in this specification and the appended claims, the term“or” is generally employed in its sense including“and/or” unless the content clearly dictates otherwise.

[0019] It is noted that references in the specification to "an embodiment", "some embodiments", "other embodiments", etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic.

Moreover, such phrases are not necessarily referring to the same embodiment.

Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is contemplated that the feature, structure, or characteristic may be applied to other embodiments whether or not explicitly described unless clearly stated to the contrary.

[0020] The present system and approach may incorporate one or more processors, computers, controllers, user interfaces, wireless and/or wire connections, and/or the like, in an implementation described and/or shown herein. This description may provide one or more illustrative and specific examples or ways of implementing the present system and approach. There may be numerous other examples or ways of implementing the system and approach.

[0021] Referring to Figure 1, which is a schematic diagram of an illustrative flame detection system 100 that includes a flame detection circuit with circuitry for detecting current leakage. The illustrative flame detection system 100 includes a flame sensor 116, a flame amplifier 115, a flame detection circuit 101, an inverting amplifier 122 and a microcontroller 110. The flame sensor 116 may sense a presence of a flame and may draw a flame sense current when a flame is present. In some cases, the flame sensor 116 may include a flame rod. The flame sensor 116 may be positioned adjacent or in a flame. The flame amplifier 115 may be operatively coupled to the flame sensor 116 and may amplify the flame sense current, and may draw an amplified flame sense current Ifiame from an amplifier output 120.

[0022] The flame detection circuit 101 may be operatively coupled to the flame amplifier 115 output 120 for detecting the amplified flame sense current Iriamc. In the example shown, the flame detection circuit 101 may include a capacitor 102 having a first end operatively coupled to the amplifier output 120 at node 21. The capacitor 102 may have any suitable capacitance value. In the example shown, the capacitor 102 has a value of 100 nF and is discharged by Ifiame being pulled into amplifier output 120 (a negative amplified flame current). A voltage at the capacitor 102 shown as Vriamc on node 21 may be controlled to stay within a defined voltage range such as -50 mV to 50m V, although this is just an example. The flame detection circuit 101 may also include a first resistor 104 (Rl) that is operatively connected between node 21 and a first pin (FBI) of the microcontroller 110. The first resistor 104 may have a first resistance value such as 82.5 kohms, for example. The flame detection circuit 101 may also include a second resistor 105 (R2) that is operatively connected between node 21 and a second pin (FB2) of the microcontroller 110. The second resistor 105 may have a second resistance value, such as 120 kohms. The first resistor 104, the second resistor 105, the capacitor 102 and the voltage follower amplifier 106 may be considered as collectively forming flame detection circuit 101. The voltage follower amplifier 106 may amplify the Vriamc signal on node 21 and provide an amplified Vriamc signal to an inverting amplifier 122, which may further amplify the amplified Vriamc before being provided to an input pin of the

microcontroller 110. The input put of the microcontroller may be connected to an A/D converter to convert the analog flame sense signal to a digital flame sense signal suitable for processing by the microcontroller 110. In the example shown, the microcontroller 110 may provide a baseline value to the“+” input of the operational amplifier 108 of the inverting amplifier 122 as shown. The baseline value may provide a zero point on which to compare and amplify the amplified Vfiame signal provided by the flame detection circuit 101. In some cases, the baseline value may be ground, but it is contemplated that the baseline value may be any suitable value.

[0023] During operation, the microcontroller 110 may be configured to periodically assert the FBI pin 117 to VCC 112 and switch FB2 pin 103 to a tri-state (e.g. floating) in order to charge the capacitor 102 through the first resistor 104 from a first lower threshold voltage (e.g. -50mv) to a first upper threshold voltage (e.g.

+50mv), and then allow the amplified flame sense current Ifiame to discharge the capacitor 102 back down to the first lower threshold voltage (e.g. -50mv). The microcontroller 110 may determine a first duty cycle D1 of the charging of the capacitor 102 through the first resistor 104 and subsequent discharging of the capacitor 102.

[0024] The microcontroller 110 may also periodically assert the FB2 pin 103 to VCC 112 and switch FBI pin 117 to a tri-state in order charge the capacitor 102 through the second resistor 105 from a second lower threshold voltage (e.g. -50mv) to a second upper threshold voltage (+50mv) and then allow the amplified flame sense current Ifiame to discharge the capacitor 102 back down to the second lower threshold voltage (-50mv). The microcontroller may determine a second duty cycle D2 of the charging of the capacitor 102 through the second resistor 105 and subsequent discharge of the capacitor 102. In some cases, the first lower threshold voltage may be the same as the second lower threshold voltage, and the a first upper threshold voltage may the same as the a second upper threshold voltage, but this is not required.

[0025] The microcontroller 110 may be configured to determine a leakage current condition in the flame detection system 100 based at least in part on the first duty cycle Dl, the second duty cycle D2, the first resistance value R1 and the second resistance value R2, as further described below. The microcontroller 110 may provide a shutdown signal to shut down the flame (e.g. close a gas valve supplying fuel to the combustion system) when the leakage current condition is determined.

[0026] More specifically, the microcontroller 110 may be configured to determine the first duty cycle Dl by asserting the FBI pin 117 to VCC 112 and switch FB2 pin 103 to a tri-state (e.g. floating), and then monitoring a voltage at node 21 at the first end of the capacitor 102 and clocking how long it takes to charge the capacitor 102 through the first resistor 104 from the first lower threshold voltage (i.e. -50m V) to the first upper threshold voltage (ChargeRlTime). The microcontroller 110 may then switch the FBI pin 117 and the FB2 pin 103 to a tri-state (e.g.

floating), and clock how long it takes for the amplified flame sense current Ifiame to discharge the capacitor 102 back down to the first lower threshold voltage

(DischargeFCTime). DischargeFCTime may denote the flame current Ifiame discharge time. The first duty cycle D1 may be calculated by using the relation

ChargeRlTime/(ChargeRlTime+DischargeFCTime). The ChargeRlTime and DischargeFCTime may be averaged values taken over a plurality of charging and discharging cycles of the capacitor 102 to help reduce noise in the system.

[0027] The microcontroller 110 may also be configured to determine the second duty cycle D2 by asserting the FB2 pin 103 to VCC 112 and switch FBI pin 112 to a tri-state (e.g. floating), and then monitoring a voltage at node 21 at the first end of the capacitor 102 and clocking how long it takes to charge the capacitor 102 through the second resistor 105 from the second lower threshold voltage (i.e. -50m V) to the second upper threshold voltage (ChargeR2Time). The microcontroller 110 may then switch the FB2 pin 103 and the FB I pin 117 to a tri-state (e.g. floating), and clock how long it takes for the amplified flame sense current Ifiame to discharge the capacitor 102 back down to the second lower threshold voltage (DischargeFCTime).

DischargeFCTime may denote the flame current Ifiame discharge time. The second duty cycle D2 may be calculated by using the relation

ChargeR2Time/(ChargeR2Time+DischargeFCTime). The ChargeR2Time and DischargeFCTime may be averaged values taken over a plurality of charging and discharging cycles of the capacitor 102 to help reduce noise in the system.

[0028] When the first lower threshold voltage is the same as the second lower threshold voltage, and the first upper threshold voltage is same as the a second upper threshold voltage, the DischargeFCTime should be the same absent current leakage. Said another way, the ratio D1/D2 should be the same as the ratio R1/R2 absent current leakage. As such, a current leakage condition may be indicated when the ratio D1/D2 deviates from the ratio R1/R2 by more than a threshold amount.

[0029] In some cases, a single charge/discharge cycle may be executed using R1 to determine Dl, followed by a single charge/discharge cycle using R2 to determine D2. This may be repeated over time. In some cases, the past“N” Dl values may be

averaged to determine an average D1 value, where“N” is a positive integer.

Likewise, the past“N” D2 values may be averaged to determine an average D2 value. In some cases, two or more consecutive charge/discharge cycles may be executed using R1 to determine Dl, followed by two or more consecutive charge/dis charge cycles using R2 to determine D2.

[0030] In some cases, the microcontroller 110 may be configured to determine the first duty cycle Dl by asserting the FBI pin 117 to VCC 112 and switch FB2 pin 103 to a tri-state (e.g. floating), and then monitoring a voltage at node 21 at the first end of the capacitor 102 and clocking how long it takes to charge the capacitor 102 through the first resistor 104 from the first lower threshold voltage (i.e. -50m V) to the first upper threshold voltage (ChargeRlTime). The microcontroller 110 may then switch the FBI pin 117 and the FB2 pin 103 to a tri-state (e.g. floating), and clock how long it takes for the amplified flame sense current Ifiame to discharge the capacitor

102 back down to the first lower threshold voltage (DischargeFCTime). The microcontroller 110 may determine the second duty cycle D2 by asserting the FB2 pin

103 to VCC 112 and the FBI pin 112 to VCC 112, and then monitoring a voltage at node 21 at the first end of the capacitor 102 and clocking how long it takes to charge the capacitor 102 through the first resistor 104 and the second resistor 105 from the second lower threshold voltage (i.e. -50m V) to the second upper threshold voltage (ChargeRlR2Time). The microcontroller 110 may then switch the FB2 pin 103 and the FBI pin 117 to a tri-state (e.g. floating), and clock how long it takes for the amplified flame sense current Ifiame to discharge the capacitor 102 back down to the second lower threshold voltage (DischargeFCTime). In this example, R1 is used to determine the first duty cycle, while the parallel resistance of R1 and R2 is used to determine the second duty cycle.

[0031] In some cases, a negative voltage supply generator 118 may supply a negative supply voltage (Vee). This may be useful because the flame sensor 116 may draw a negative current, which produce a negative voltage. The negative supply voltage (Vee) may be provided to the flame amplifier 115, and in some cases the amplifier 106, the amplifier 108 and/or the microcontroller 110. In some cases, the microcontroller 110 may be configured to periodically change the negative supply voltage provided by the negative voltage supply generator 118 from a nominal negative supply voltage (e.g. -800mv) to a boosted negative supply voltage (-2200mv), and then back again. If there is no leakage in the flame sensing circuit, the detected flame current Ifiame should remain the same regardless of whether the negative supply voltage is set to the nominal negative supply voltage (e.g. -800mv) or the boosted negative supply voltage (-2200mv). The microcontroller 110 may determine a leakage current condition when the amplified flame sense current Iriamc detected by the detection circuit changes by more than a threshold amount when the negative supply voltage is changed from the nominal negative supply voltage to the boosted negative supply voltage.

[0032] In some cases, the microcontroller 110 may be configured to change the negative supply voltage from the nominal negative supply voltage to the boosted negative supply voltage for a period of time (e.g. 200 milliseconds, 300 milliseconds, 500 milliseconds, 1 second, 5 seconds or any other suitable time) before changing the negative supply voltage back from the boosted negative supply voltage to the nominal negative supply voltage. The microcontroller 110 may wait for a period of time (e.g.

1 second, 2 seconds, 5 seconds, 10 seconds, 60 seconds, or any other suitable time) before again changing the negative supply voltage from the nominal negative supply voltage to the boosted negative supply voltage before changing the negative supply voltage back from the boosted negative supply voltage to the nominal negative supply voltage.

[0033] In some cases, and as shown in Figure 1, the Vfiame voltage on node 21 may be interfaced to the microcontroller 110 by means of an operational amplifier 106 connected in a voltage follower configuration followed by an operational amplifier 108 connected in an inverting amplifier configuration 122. The gain of the inverting amplifier 122 may be defined by the ratio of resistors R4 and R3. In the example shown, the inverting amplifier 122 may receive a DC bias voltage from the microcontroller 110 on the line 114. The DC bias voltage can be used to translate the output of the flame detection circuit 101, that may track between negative and positive voltages, to an output signal Vout that is positive only and suitable for reading by an analog-to-digital converter (ADC) of the microcontroller 110. In some cases, the DC bias voltage on the line 114 is defined by‘ Vdac’, i.e., a microcontroller DAC output. Rather than providing a DC bias voltage from the microcontroller 110 on the line 114, it contemplated that a suitable voltage may be supplied by, for example, a simple voltage divider.

[0034] During use, the microcontroller 110 may track the output signal Vout 113 provided by the inverting amplifier 122 and compare the output signal Yout 113 to two thresholds that correspond to the Vfiame thresholds of, for instance, +50mV and -50mV at node 21. In some cases, these thresholds correspond to a lower threshold (e.g. the first lower threshold and/or the second lower threshold) and an upper threshold (e.g. the first upper threshold and/or the second upper threshold). The microcontroller 110 may track the output signal V0ut 113 and control feedback drive pins FBI and FB2 accordingly, so that node 21 stays within a desired range such as -50mV to +50mV as described herein.

[0035] Figure 2 is a timing diagram showing operation of the circuitry for detecting leakage in the flame sense circuit of Figure 1. The voltage Vriamc on node 21 of Figure 1 is illustrated at trace 30. In this example, the voltage Vfiame on node 21 is controlled to stay within a defined voltage range such as -50 mV to 50mV. A +/-50mV ripple is considered as a small working voltage, which can be advantageous to help reduce the impact of leakage currents on the flame sensing measurement, since a parasitic resistance from ground (or Vee) may result in a parasitic current that

can mimic or falsely contribute to the flame sense current Ifiame.

[0036] The microcontroller 110 may be configured to determine the first duty cycle D1 by asserting the FBI pin 117 to VCC 112 as shown at 32 and switch FB2 pin 103 to a tri-state (e.g. floating), and then monitoring a voltage Vriamc at node 21 at the first end of the capacitor 102 and clocking how long (ChargeRlTime) it takes to charge the capacitor 102 through the first resistor 104 from the first lower threshold voltage (i.e. -50m V) to the first upper threshold voltage (i.e. +50mV), as shown at 24. The microcontroller 110 may then switch the FBI pin 117 and the FB2 pin 103 to a tri-state (e.g. floating) as shown at 33, and clock how long (DischargeFCTime) it takes for the amplified flame sense current Ifiame to discharge the capacitor 102 back down to the first lower threshold voltage (i.e. -50mV) as shown at 25.

DischargeFCTime may denote the flame current Ifiame discharge time. The

ChargeRlTime plus the DischargeFCTime results in a period PI . The first duty cycle D1 may be calculated by using the relation

ChargeRlTime/(ChargeRlTime+DischargeFCTime). In some cases, the

ChargeRlTime and DischargeFCTime may be averaged values taken over a plurality of charging and discharging cycles of the capacitor 102 to help reduce noise in the system, but this is not required.

[0037] The microcontroller 110 may also be configured to determine the second duty cycle D2 by asserting the FB2 pin 103 to VCC 112 as shown at 34 and switch

FBI pin 112 to a tri-state (e.g. floating), and then monitoring the voltage Vfiame at node 21 at the first end of the capacitor 102 and clocking how long (ChargeR2Time) it takes to charge the capacitor 102 through the second resistor 105 from the second lower threshold voltage (i.e. -50m V) to the second upper threshold voltage (i.e.

+50mV), as shown at 26. In the example shown, the first lower threshold voltage is the same as the second lower threshold voltage (i.e. -50m V), and the first upper threshold voltage is same as the a second upper threshold voltage (i.e. +50mV), but this is not required. The microcontroller 110 may then switch the FB2 pin 103 and the FBI pin 117 to a tri-state (e.g. floating) as shown at 35, and clock how long (DischargeFCTime) it takes for the amplified flame sense current Iriamc to discharge the capacitor 102 back down to the second lower threshold voltage (i.e. -50m V), as shown at 27. The ChargeR2Time plus the DischargeFCTime results in a period P2. The second duty cycle D2 may be calculated by using the relation

ChargeR2Time/(ChargeR2Time+DischargeFCTime). In some cases, the

ChargeR2Time and DischargeFCTime may be averaged values taken over a plurality of charging and discharging cycles of the capacitor 102 to help reduce noise in the system, but this is not required, but this is not required. The DischargeFCTime should be the same whether the capacitor 102 was charged using R1 or R2 absent current leakage. Said another way, the ratio D1/D2 should be the same as the ratio R1/R2 absent current leakage. As such, a current leakage condition may be indicated when the ratio D1/D2 deviates from the ratio R1/R2 by more than a threshold amount.

[0038] In some cases, the microcontroller 110 may be configured to periodically change the negative supply voltage (Vee) provided by the negative voltage supply generator 118 of Figure 1 from a nominal negative supply voltage (e.g. -800mv) to a boosted negative supply voltage (-2200mv) and then back again, as shown at 36. If there is no leakage in the flame sensing circuit, the detected flame current Iriamc should remain the same regardless of whether the negative supply voltage is set to the nominal negative supply voltage (e.g. -800mv) or the boosted negative supply voltage (-2200mv). The microcontroller 110 may determine a leakage current condition when the amplified flame sense current Iriamc detected by the detection circuit changes by more than a threshold amount when the negative supply voltage (Vee) is changed from the nominal negative supply voltage to the boosted negative supply voltage. For example, a 100 kOhm leakage path may appear as an 8 uA flame current during a nominal Vee cycle but as 22 uA during the boosted Vee cycle, which can be detected.

[0039] In some cases, the microcontroller 110 may be configured to change the negative supply voltage from the nominal negative supply voltage to the boosted negative supply voltage for a period of time (e.g. 200 milliseconds, 300 milliseconds, 500 milliseconds, 1 second, 5 seconds or any other suitable time) before changing the negative supply voltage back from the boosted negative supply voltage to the nominal negative supply voltage. The microcontroller 110 may wait for a period of time (e.g.

1 second, 2 seconds, 5 seconds, 10 seconds, 60 seconds, or any other suitable time) before again changing the negative supply voltage from the nominal negative supply voltage to the boosted negative supply voltage before changing the negative supply voltage back from the boosted negative supply voltage to the nominal negative supply voltage.

[0040] Figure 4 is a schematic block diagram of an illustrative flame sense circuit. The illustrative flame detection circuit 100a includes a flame sensor 116a for sensing a flame, a flame amplifier 115a operatively connected to the flame sensor 116a, a negative voltage supply generator 118a, a flame sense detection circuit 101a operatively coupled to the flame amplifier 115a output, and a microcontroller 110a.

[0041] The flame sensor 116a may draw a flame sense current when exposed to a flame. The flame amplifier 115a may amplify the flame sense current and draw an amplified flame sense current from an amplifier output. The negative voltage supply generator 118a may supply a negative supply voltage to the flame amplifier 115a as shown. The flame sense detection circuit 101a may detect the amplified sense current.

[0042] The microcontroller 110a may be operatively coupled to the negative voltage supply generator 118a and the flame sense detection circuit 101a. The microcontroller 110a may further be configured to change the negative supply voltage provided by the negative voltage supply generator 118a from a nominal negative supply voltage to a boosted negative supply voltage, determine a leakage current condition in the flame detection system when the amplified flame sense current detected by the flame detection circuit 101a changes by more than a threshold amount when the negative supply voltage is changed from the nominal negative supply voltage to the boosted negative supply voltage. The microcontroller 110a may further provide a shutdown signal 107 to shut down the flame (e.g. close a gas valve that supplies fuel to the combustion system) when a leakage current condition is determined.

[0043] The microcontroller 110a may be configured to change the negative supply voltage from the nominal negative supply voltage to the boosted negative supply voltage for a period of time (e.g. 200 milliseconds, 300 milliseconds, 500 milliseconds, 1 second, 5 seconds or any other suitable time) before changing the negative supply voltage back from the boosted negative supply voltage to the nominal negative supply voltage. The microcontroller 110a may wait for a period of time (e.g. 1 second, 2 seconds, 5 seconds, 10 seconds, 60 seconds, or any other suitable time) before again changing the negative supply voltage from the nominal negative supply voltage to the boosted negative supply voltage before changing the negative supply voltage back from the boosted negative supply voltage to the nominal negative supply voltage.

[0044] Figure 5 is a flow diagram showing an illustrative method 500 for detecting a leakage current condition in a flame detection system. The method may include amplifying with an amplifier a flame sense current provided by a flame sensor, resulting in an amplified flame sense current as shown in block 510. The amplified flame sense current is supplied to the amplifier via charge storage device, as shown in block 520. A charge storage device is charged with a first charging circuit that produces a first charging rate, as shown in block 530, and then at least partially discharged via the amplified flame sense current. The charge storage device is subsequently charged by a second charging circuit that produces a second charging rate, and then at least partially discharged via the amplified flame sense current. The second charging rate is different from the first charging rate, as shown in block 540.

A leakage current condition may be determined in the flame detection system based at least in part on a comparison of the charging of the charge storage device with the first charging circuit and the subsequent discharge via the amplified flame sense current, and the charging of the charge storage device with the second charging circuit and the subsequent discharge via the amplified flame sense current, as shown in block 550. A shutdown signal may be provided to shut down the flame (e.g. close a gas valve supplying fuel to the combustion system) when the leakage current condition is determined, as shown in block 560.

[0045] The method 500 may optionally include a negative supply voltage that is selectively changed from a nominal negative supply voltage to a boosted negative supply voltage, and a leakage current condition may be determining in the flame

detection system when the sensed flame sense current changes by more than a threshold amount, as indicated at block 570.

[0046] Figure 6 is a flow diagram of another illustrative method 600 for detecting a leakage current condition in a flame sensing circuit. An amplifier may be operatively coupled to a flame sensor for amplifying a flame sense current of the flame sensor, as indicated at block 610. A negative voltage supply generator may be used for supplying a negative supply voltage to the amplifier, as indicated at block 620. The amplified flame sense current may be detected by a detection circuit, as indicated at block 630. A microcontroller may be configured to change the negative supply voltage from a nominal negative supply voltage to a boosted negative supply voltage, as indicated at block 640. A leakage current condition may be determined in the flame detection system when the amplified flame sense current detected by the detection circuit changes by more than a threshold amount when the negative supply voltage is changed from the nominal negative supply voltage to the boosted negative supply voltage, as indicated at block 650. A shutdown signal may be provided to shut down the flame (e.g. close a gas valve supplying fuel to the combustion system) when the leakage current condition is determined, as indicated at block 660.

[0047] Those skilled in the art will recognize that the present disclosure may be manifested in a variety of forms other than the specific embodiments described and contemplated herein. Accordingly, departure in form and detail may be made without departing from the scope and spirit of the present disclosure as described in the appended claims.