METHODS AND SYSTEMS FOR USING FLAME RECTIFICATION TO DETECT THE PRESENCE OF A BURNER FLAME

Abstract

Systems and methods for detecting the presence of a burner flame using flame rectification are shown and described. A conductive flame sensor is positioned to conduct electricity to a burner flame when the burner is lit. The flame provides a conductive path to the burner conductive body and when operatively connected to an alternating current source, half-wave rectifies the current flowing to the sensor. A flame sensing circuit provides an output signal that is conditioned for use as an input to a flame indicator and/or a controller that is configured to shut off gas flow to the burner when no flame is present after an attempt at igniting the burner.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No. 63/252,663, filed on Oct. 6, 2021, the entirety of which is hereby incorporated by reference.

FIELD

This disclosure relates to systems for detecting the presence of a burner flame using flame rectification, and more specifically, improved methods and systems using flame rods and hot surface igniters forming part of the rectified current pathway when a flame is present.

BACKGROUND

When operating combustion burners, it is desirable to provide some means of determining whether a flame is present to ensure that uncombusted combustion gas is not supplied to the burner and its surroundings and does not create an explosion hazard. One known device for detecting the presence of a flame is a “flame rod” or “flame rectification rod.”

A flame rod is a conductive rod with a ceramic insulator which serves as a first electrode and is positioned to contact the flame when the burner is ignited. The burner housing serves a second electrode. When it is present, and an excitation voltage is supplied to the flame rod, the flame provides a conductive pathway that allows current to flow from the flame rod to the burner housing. Conversely, when an excitation voltage is supplied and no flame is present, no current flows from the flame rod to the burner housing. A sensing circuit is typically connected to the flame rod to detect the presence of current from the flame rod to the burner housing so that an indication that a flame is or is not present may be provided. The combustion process typically produces soot or other deposits that foul the flame rod. The deposits act as an insulator, increasing the impedance of the flame rod and reducing the current to the burner at a given voltage. As a result, flame rods must be replaced or serviced at some frequency as their impedances reach too high a level. Thus, a need has arisen for an improved means of using flame rectification to detect the presence of a burner flame.

SUMMARY

In accordance with a first aspect of the present disclosure, a burner flame detection system is provided which comprises a conductive flame sensor comprising a conductive terminal and a flame sensing circuit comprising a flame detection signal output node. The conductive flame sensor conductive terminal is positioned proximal to a burner having a conductive body. The burner has an ignited state and an unignited state such that when the burner is in the ignited state, the burner and the conductive flame sensor are in electrical communication with one another. The flame sensing circuit is configured to supply an alternating current having a frequency of from about 24 kHz to about 300 KHz to the conductive flame sensor conductive terminal, and when the burner is in an ignited state and the alternating current is supplied to the conductive terminal, the flame sensing circuit generates a rectified current from the conductive flame sensor conductive terminal to the burner.

In accordance with a second aspect of the present disclosure, a method of determining if a burner is ignited is provided. The method uses a conductive flame sensor comprising a conductive terminal and positioned proximate a burner. The method comprises providing a flame sensing alternate current source operatively connected to the conductive terminal, the alternating current having a frequency of from about 24 kHz to about 300 kHz; and generating a rectified current from the conductive flame sensor to the burner when a source of the alternating current supplies the alternating current to the flame sensor conductive terminal, and the burner is in an ignited state

In accordance with a third aspect of the present disclosure, a burner flame detection system is provided which comprises a hot surface igniter comprising a conductive pattern connected to a conductive terminal and positioned proximal to a burner having a conductive body. The burner has an ignited state and an unignited state, such that when the burner is in the ignited state, the conductive terminal and the burner conductive body are in electrical communication with one another. In a preferred example, the burner flame detection system includes a flame sensing circuit configured to supply a flame sensing alternating current to the hot surface igniter conductive terminal, wherein when the burner is in the ignited state and the flame sensing alternating current is supplied to the hot surface igniter conductive terminal, the flame sensing circuit generates a rectified current from the hot surface igniter conductive terminal to the burner.

In accordance with a fourth aspect of the present disclosure, A method of determining if a burner having a conductive body is ignited is provided. The method comprises providing a conductive flame sensor having a conductive terminal and positioned proximate the burner and providing a flame sensing alternate current source operatively connected to the conductive terminal and having an alternating current with a frequency of from about 24 kHZ to about 300 kHz and generating a rectified current from the conductive flame sensor to the burner when the flame sensing alternating current source supplies flame sensing alternate current to the conductive flame sensor's conductive terminal, and the burner is in an ignited state.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic depicting a burner flame detection system in accordance with the present disclosure;

FIG. 2A is a schematic depicting a modification of FIG. 1 in which a hot surface igniter's heating conductive pattern is also used as a flame sensing conductive pattern;

FIG. 2B is a schematic depicting a modification of FIG. 1 in which a hot surface igniter includes both a heating conductive pattern and a separate flame sensing conductive pattern.

FIG. 3A is a schematic depicting a direct current source used to supply a DC voltage to the oscillator circuit of FIG. 1;

FIG. 3B is a schematic depicting an alternating current source used to supply a DC voltage to the oscillator circuit of FIG. 1;

FIGS. 4A-4D depict various examples of “third leg” hot surface igniters suitable for use as flame sensors in the burner flame detection system of FIG. 1;

FIGS. 5A-5C depict voltage versus time data for the emitter, collector, and base of the flame rod driver circuit bipolar junction transistor and for the flame rod driver circuit capacitor output node of FIG. 1 when the flame sensing circuit is subjected to an alternating current and a burner flame is not present;

FIGS. 6A-6C depict voltage versus time data for the emitter, collector, and base of the flame rod driver circuit bipolar junction transistor and for the flame rod driver circuit capacitor output node of FIG. 1 when the flame sensing circuit is subjected to an alternating current and a burner flame is present;

FIGS. 7A-7B are plots of simulated peak-to-peak and mean output voltage, respectively, versus frequency for a flame rod with a specified degree of fouling;

FIGS. 7C-7D are plots of simulated mean and peak-to peak input voltage, respectively, versus frequency for the flame rod with the specified degree of fouling of FIGS. 7A-7B; and

FIGS. 8A-8C are plots of simulated peak-to-peak output voltage, mean output voltage, and mean input voltage, respectively, versus frequency for the fouled flame rod of FIGS. 7A-7B one week later

Like reference numerals refer to like parts in the figures.

DESCRIPTION

The systems and methods herein use the property of “flame rectification” to determine whether a burner flame is present. In flame rectification, an active flame defines an electrical path from a flame sensor to a burner body. As is known to skilled artisans, based on Mollberg's flame model, a conductive path through a flame may be modeled as a high resistance (megaohms) resistor in series with a diode. Thus, when subjected to an alternating current, the flame conducts electricity when the AC signal is positive and acts as an open circuit when the AC signal is negative.

Flame rods are conductive rods that are typically used as flame rectification sensors. The rod is typically positioned inside the flame when the burner is lit. Over time, soot from the combustion process and other particulate matter accumulate on the flame rod and “foul it” causing it to diminish in its sensitivity. It has been found that a flame rod may be modeled as a capacitor. The accumulated deposits may be modeled as an insulator of varying thickness. As is known to those skilled in the art, the complex impedance of an RC circuit is a vector sum of a resistance and a “reactive capacitance”:

$\begin{array}{cc}{Z}^2=R^2+X_c^2& \left(1\right)\end{array}$ $\begin{array}{cc}{X}_c=\frac{1}{2\pi \mathrm{fC}}& \left(2\right)\end{array}$

where Z=complex (vector) impedance (ohms)

R=resistance (ohms)

f=frequency (sec⁻¹)

C=capacitance (farads)

X_c=capacitive reactance

As equations (1) an (2) suggest, as deposits accumulate on a flame rod, its capacitance decreases, which increases the contribution of the reactive capacitance Xc to the impedance Z. At a standard (US) AC frequency of 60 Hz, the contamination that develops on flame rods produces a significant impedance. As equations (1) and (2) also suggest, as the frequency f of an applied AC signal increases, the capacitive reactance Xc decreases, and the complex impedance approaches the resistance. It has been discovered that by sufficiently increasing the frequency of an AC excitation signal supplied to a flame rod, the sensitivity of the flame rod's impedance to the accumulation of soot or other deposits can be significantly diminished.

Hot surface igniters are a well-known means of igniting combustion gas. Silicon nitride hot surface igniters typically comprise two insulating tiles with a printed conductive, heat generating pattern printed on one of the inside faces of the two insulating tiles. When connected to a voltage source, the conductive, heat generating pattern generates heat. It has also been discovered that a hot surface igniter of this construction can also function as a flame rectifying sensor and be modeled as a capacitor. Relative to flame rods, hot surface igniters have the added advantage of generating combustion temperatures which allows them to burn off accumulated deposits and avoid the replacement cycles that are necessary for flame rods. Disclosed herein are circuits intended to generate a binary (ON/OFF) signal of a voltage range suitable for a commercial microcontroller based on the presence of a flame rectified signal generated when a flame rectification sensor is exposed to a flame.

Referring to FIG. 1, a burner flame detection system 20 is depicted. Burner flame detection system 20 comprises conductive flame sensor 22, and a flame sensing circuit 24. Flame sensor 22 is preferably a flame rectification sensor positioned with respect to burner 21 such that when burner 21 is lit, an electrical path exists from the flame sensor 22, through the flame to a conductive burner body comprising burner 21.

Flame sensing circuit 24 is designed to provide a signal at flame sensing circuit 24 flame detection signal output node 36 that is suitable for input to a flame presence indicator. The indicator is preferably visual and/or audible. The flame presence indicator may be a stand-alone indicator or may be integrated with a controller, such as a commercially available microcontroller. In preferred examples the controller is operatively connected to a gas valve that is operative to selectively supply combustion gas to burner 21. In certain preferred examples, flame sensing circuit 24 is designed to provide a binary signal (ON/OFF) at flame sensing circuit output node 36 even though the signal generated by flame sensor 22 is not a binary signal. In the example of FIG. 1, flame sensing circuit 24 is designed to provide a logical high indication (e.g., 3.3V or 5 V) when no flame is present and a logical low (e.g., 0 V) when a flame is present.

Flame sensor 22 is preferably a flame rod or a hot surface igniter. In certain examples in which flame sensor 22 is a hot surface igniter, the heating conductive pattern generates heat during an ignition operation and detects the presence of a flame during a flame detection operation. In other examples, the hot surface igniter includes a flame sensing conductive pattern separate from the heating conductive pattern so that heating and flame sensing can occur simultaneously.

The flame sensing circuit 24 receives an AC voltage signal at input node 38, which is an output from AC generating circuit 31. The AC signal generated by the AC generating circuit may be a sine wave but is preferably a square wave.

In preferred examples, and as shown in FIG. 1, the AC generating circuit 31 is a multi-vibrator oscillator circuit having a DC source 33 and which generates an AC square wave at output node 34.

In the example of FIG. 1, the multi-vibrator oscillator circuit is an astable oscillator circuit of the type known in the art. AC generating circuit 31 comprises four resistors 35a-35d, each having an input node connected to DC source 33. Resistors 35a and 35d are each connected to respective output nodes 41a and 41b. Output node 41a is connected to the collector of bipolar junction transistor (BJT) 37a and to capacitor 39b. Output node 41b is connected to the collector of BJT 37b and to capacitor 39a. Resistors 35b and 35c are connected to nodes 43a and 43b, respectively. Resistor 35b output node 43a is also connected to the base of BJT 37a, which is also connected to capacitor 39a. Resistor 35c output node 43c is also connected to the base of BJT 37b, which is also connected to capacitor 39b.

When a DC source 33 supplies a DC voltage to resistors 35a-35d, capacitors 39a and 39b will charge and discharge. As capacitors 39a and 39b charge and discharge, BJTs 37a and 37b will alternate being ON and OFF, causing the BJT 37a and 37b collector voltages to rise and fall, thereby recharging capacitors 39a and 39b. Capacitor 39c and resistor 35e serve as a high-pass filter that removes the DC output that would otherwise be observed at AC generating circuit 31 output node 34. DC source 33 provides a supply voltage of from about 10V to about 48V, preferably from about 12V to about 36 V, and more preferably about 24V. DC source 33 is shown in greater detail in FIG. 3A. The DC source 33 comprises a battery 92 with a ground terminal 100 and a positive terminal 93, which is connected to the AC generating circuit 31. In an alternate implementation for situations where DC is not available, an AC converting circuit of the type shown in FIG. 3B may be provided. The AC converting circuit 49 comprises 24V AC supply 95 having a positive terminal 96 and a ground terminal 100. Positive terminal 96 is connected to diode 97 which is connected to output node 91 which is connected to AC generating circuit 31. Ripple capacitor 98 is provided between the output node 91 and ground. When the voltage of AC supply 95 is positive and above the saturation voltage of capacitor 98, capacitor 98 charges. When the AC supply 95 voltage is below the saturation voltage of capacitor 98, capacitor 98 discharges, thereby smoothing the ripple caused by diode 97 half wave rectifying the current from AC supply 95.

Capacitors 39a and 39b—along with resistors 35b and 35c—are selected to achieve a desired frequency of the AC signal at output node 34 by adjusting the current flow across capacitors 39a and 39b. Resistors 35a and 35d are selected to achieve a desired rising edge time of the AC signal at output node 34 by adjusting the current through BJTs 37a and 37b. In preferred examples, AC generating circuit 31 is a balanced multivibrator with the resistances of resistors 35b and 35c being equal, the resistances of resistors 35a and 35d being equal, and the capacitances of capacitors 39a and 39b being equal.

In preferred examples, the component values of AC generating circuit 31 are selected to produce a square wave AC voltage signal at output node 34 having a frequency range of from 24 kHZ to 300 kHZ, more preferably 40 kHz to 200 kHz, and still more preferably from 70 kHZ to 100 kHZ, and more preferably from 80 kHz to 90 kHZ. In preferred examples, these frequencies yield a stable signal (logical high or low) at flame sensing circuit output node 36 when burner 21 is lit. It has been found that when flame sensor 22 is a flame rod and the AC signal at AC generating circuit output node 34 has a frequency in these ranges, the signal at flame sensing circuit output node 36 is stable when burner 21 is lit even when significant deposits have accumulated on the flame rod. Exemplary component values for AC generating circuit 31 for achieving he preferred frequencies referenced above are as follows:

TABLE 1
		More	Even more
Component	Preferred	preferred	preferred
Resistor 35a	8 kΩ-12 kΩ	9 kΩ-11 kΩ	9.5 kΩ-10.5 kΩ
Resistor 35b	80 kΩ-120 kΩ	90 kΩ-110 kΩ	95 kΩ-105 kΩ
Resistor 35c	80 kΩ-120 kΩ	90 kΩ-110 kΩ	95 kΩ-105 kΩ
Resistor 35d	8 kΩ-12 kΩ	9 kΩ-11 kΩ	9.5 kΩ-10.5 kΩ
Capacitor	460 pF-520 pF	470 pF-510 pF	480 pF-500 pF
39a/39b
Resistor 35e	8 kΩ-12 kΩ	9 kΩ-11 kΩ	9.5 kΩ-10.5 kΩ
BJT 37a/BJT	0.5 V-0.9 V	0.55 V-0.85 V	0.6 V-0.8 V
37b VBE
breakdown
voltage
BJT 37a/BJT	30 V-50 V	35 V-45 V	38 V-42 V
37b VCE
breakdown
voltage

Flame sensing circuit 24 comprises a flame sensor driver circuit 26, a signal conditioning circuit 28, and a load circuit 30. The flame sensor 22 includes a conductive terminal 32 that receives an output signal from an output node (not separately shown) of the flame sensor driver circuit 26. In the case of a flame rod, conductive terminal 32 is electrically connected to the flame rod body. In the case of a hot surface igniter, conductive terminal 32 is electrically connected to a heating conductive pattern or a flame sensing conductive pattern in the hot surface igniter.

Flame sensor driver circuit 26 provides a means of amplifying a DC offset introduced when the flame sensor 22 is subjected to an alternating current while a flame is present. The flame sensor driver circuit 26 comprises resistor 42 which is in series with flame sensor 22 and which has an input node 45 connected to capacitor 40 and resistor 44. Capacitor 40 has an input connected to input node 38. Input node 38 is connected to BJT 46 emitter 48 and AC generating circuit output node 34. BJT 46 emitter 48 is also the output node of the flame sensor driver detection circuit 24. Resistor 44 is connected to BJT base 60 and resistor 42 input node 45. BJT 46 collector 50 defines a flame sensor driver circuit flame detection output node that is connected to signal conditioning circuit 28. BJT 46 acts as a switch that supplies current from collector 50 to signal conditioning circuit 28 when a flame is present.

BJT 46 is a PNP BJT in which there is a positive offset from the emitter 48 to base 60 having a fixed voltage when there is a path for current flow through the base 60. When no flame is present, there is effectively an open circuit from flame sensor 22 to burner 21, and there is no path for current flow from emitter 48 to base 60. However, when a flame is present, and there is a path for current flow through the base to resistor 42, BJT 46 is turned ON, which allows current to flow from emitter 48 to collector 50. When there is no flame, collector 50 floats on the emitter 48 voltage during the positive AC cycle and is connected to ground during the negative AC cycle. Current flow through resistor 42 and flame sensor 22 produces a DC offset voltage at node 45 which causes the BJT 46 offset between emitter 48 and base 60 to exceed the threshold required to turn BJT 46 ON. Preferred examples of component values for flame sensor driver circuit 26 are as follows:

TABLE 2
		More	Even more
Component	Preferred	preferred	preferred
Capacitor 40	0.003 μF-0.014 μF	0.004 μF-0.013 μF	0.008 μF-0.011 μF
Resistor 42	80 kΩ-120 kΩ	90 kΩ-110 kΩ	95 kΩ-105 kΩ
Resistor 44	80 kΩ-120 kΩ	90 kΩ-110 kΩ	95 kΩ-105 kΩ
BJT 46	−440 V to −390 V	−430 V to −370 V	−420 V to −380 V
VCE
breakdown
voltage

When a flame is present, with BJT 46 ON current flows from the AC generating circuit output node 34 to diode 52, which is part of signal conditioning circuit 28. Signal conditioning circuit 28 includes an RC low pass filter and converts the input signal at diode 52 to a positive and less variable signal at signal conditioning circuit output node 66 when a flame is present. The reduced variability ensures that the load circuit 30 can more reliably supply a DC voltage having discrete binary values at load circuit output node 36, which is also the flame sensing circuit 24 output node.

Signal conditioning circuit 28 comprises diode 52 which his connected to a parallel combination of resistor 58 and capacitor 54. Resistor 58 and capacitor 54 define an RC low pass filter. When the AC signal at BJT 46 collector 50 is positive, diode 52 is forward-biased and allows current to pass. When the AC voltage at BJT 46 collector 50 is negative, diode 52 is reverse-biased and does not allow current to pass. Thus, resistor 58 and capacitor 54 only see positive voltages. When the AC voltage at BJT 46 collector 50 is more than the voltage of capacitor 54, capacitor 54 charges until reaching its peak voltage. When the AC voltage at BJT 46 collector 50 is below the capacitor voltage, capacitor 54 discharges. Thus, the capacitor 54 smooths the ripple created by the half-wave rectification provided by diode 52. Resistor 58 provides a path to ground to remove excess charge across capacitor 54. Resistor 58 input node 56 is connected to current limiting resistor 62.

Signal conditioning circuit 28 also comprises a current limiting resistor 62 which, along with Zener diode 64, is connected to signal conditioning circuit output node 66. Zener diode 64 is reverse biased and protects load circuit 30 against power surges because once it reaches its breakdown voltage, Zener diode 64 allows current to flow to ground, thus capping the signal conditioning circuit output node 66 at the breakdown voltage. It is generally desirable to maximize the voltage at the signal conditioning circuit output node 66. Thus, diode 52 is preferably selected to have a small voltage drop. It is also desirable to filter out the AC noise and have a faster response at signal conditioning circuit output node 66, therefore, capacitor 54 is selected to have a low capacitance. Resistor 62 is preferably selected to have a resistance that will protect against voltage surges at signal conditioning circuit output node 66, and Zener diode 64 is selected to have a breakdown voltage at the maximum desired voltage at signal conditioning circuit output node 66.

Preferred examples of component values for signal conditioning circuit 28 are shown in Table 3:

TABLE 3
		More	Even more
Component	Preferred	preferred	preferred
Diode 52 forward	0.6 V-1.5 V	0.8 V-1.4 V	0.9 V-1.3 V
voltage drop
Capacitor 54	0.003 μF-0.014 μF	0.004 μF-0.013 μF	0.008 μF-0.011 μF
Resistor 58	8 MΩ-12 MΩ	8.5 MΩ-11.5 MΩ	9 MΩ-11 MΩ
Resistor 62	0.7 MΩ-1.3 MΩ	0.8 MΩ-1.2 MΩ	0.9 MΩ-1.1 MΩ
Zener diode 64	10 V-14 V	11 V-13 V	11.5 V-12.5 V
breakdown
voltage

Signal conditioning circuit 28 is connected to load circuit 30. In the example of FIG. 1, the signal conditioning circuit 28 is connected to the gate of MOSFET 68. MOSFET 68 is preferably an n-type MOSFET in which the source 70 is connected to ground and the drain 72 is connected to node 77. Because it is an n-type MOSFET, the channel current flows from the drain to the source when the MOSFET is ON.

DC source 74 supplies direct current to load circuit 30. Load circuit 30 adjusts the voltage from the signal conditioning circuit 28 to match the input requirements of a controller connected to flame sensing circuit output node 36. In an alternate implementation, DC source 33 supplies both the AC generating circuit 31 and the load circuit 30. In certain examples, the voltage at signal conditioning circuit output node 66 ranges from 0-25V, preferably from 0-12V, and more preferably from 0-9V, depending on the frequency of the voltage signal at AC generating circuit output node 34.

The output from node 66 of the signal conditioning circuit 28 is not a current that flows into the load circuit 30, but rather a voltage that turns the MOSFET 68 ON and OFF. When MOSFET 68 is ON it acts like a low resistance resistor, providing a low impedance path to ground. As a result, without Zener diode 78 and resistor 80, the input voltage to a controller (i.e., the voltage at flame sensing circuit output node 36) would be approximately zero when a flame is present, and approximately the voltage of DC supply 74 (e.g., 24V) when no flame is present, which is too high for many commercially available microcontrollers. Zener diode 78 and resistor 80 form a parallel combination and are selected to match the microcontroller (not shown) input requirements. Zener diode 78 also protects the controller against excessive voltages by effectively capping the voltage at flame sensing circuit output node 36 at the breakdown voltage of Zener diode 78.

Using the following relationship, the resistance of resistor 80 can be determined based on a desired maximum input voltage to the controller:

$\begin{array}{cc}{R}_{80=\frac{R_{76}}{\left(\frac{V_{74}}{V_{80}}-1\right)}}& \left(3\right)\end{array}$

• • where, R₈₀=resistance of resistor 80 (ohms)
  • V₇₄=DC input voltage from DC source 74 (volts)
  • V₈₀=maximum controller input voltage (volts)

In an example where R₇₆ is 47 kΩ, and the maximum controller input voltage is 3.3V, equation (3) yields a resistance of 7.5 kΩ for R₈₀. Zener diode 78 may also be selected to have a breakdown voltage equal to the maximum input voltage of the microcontroller to protect it from surges. Preferred exemplary component values for the load circuit 30 are provided in Table 4:

TABLE 4
		More	Even more
Component	Preferred	preferred	preferred
MOSFET 68 (drain	2.5 Ω-5.5 Ω	3 Ω-5 Ω	3.5 Ω-4.5 Ω
to source resistance
when MOSFET is ON)
MOSFET 68 (gate	0.8 V-3.3 V	0.8 V-2.5 V	0.8 V-1.8 V
threshold voltage)
Resistor 76	35 kΩ-60 kΩ	40 kΩ-55 kΩ	45 kΩ-50 kΩ
Resistor 80	6 kΩ-10 kΩ	6.5 kΩ-9.5 kΩ	7 kΩ-9 kΩ
Zener diode 64	2.0 V-5.5 V	2.5 V-4 V	3.0 V-3.5 V
breakdown
voltage

As mentioned previously, in certain examples, a hot surface igniter may be used in place of a flame rod for flame sensor 22. The hot surface igniter comprises at least two ceramic insulating tiles having a heating conductive pattern disposed between them, such as by printing the pattern on one of the inner faces of one of the insulating tiles. In preferred examples, the ceramic tiles comprise silicon nitride. Examples of such silicon nitride igniters are shown in U.S. patent application Ser. No. 16/366,479, the entirety of which is hereby incorporated by reference.

In certain examples, silicon nitride igniters comprising only a heating conductive pattern are used as flame sensor 22, while in other examples hot surface igniters are provided which include both a heating conductive pattern and a separate flame sensing conductive pattern that is used as flame sensor 22. In the former case, the igniter operates in both a heating mode and a flame sensing mode. In the heating mode, the heating conductive pattern generates heat when a voltage is applied across its conductive terminals. In the flame sensing mode, a voltage is not applied across the terminals of the igniter. Instead, one of the terminals is connected to resistor 42 of flame sensor driver circuit 26 and the other is disconnected from ground so that the flame sensing conductive circuit acts as an electrode that conducts electricity to the burner flame when the burner 21 is lit. A schematic illustrating an implementation of this type of igniter is shown in FIG. 2A.

In FIG. 2A igniter 84 is represented as a resistor. The heating conductive pattern (not separately shown) is connected to a positive terminal 113a and a ground terminal 113b. Switches 86 and 88 are selectively openable to disconnect igniter 84 from an AC mains supply 82 and selectively closable to connect igniter 84 to mains AC supply 82, in which case AC mains supply acts as a source of ignition alternating current. AC mains supply 82 has a positive terminal 83 and a ground terminal 85. Switches 86 and 88 preferably define a single throw, double pole switch assembly in which one actuator simultaneously opens switches 86 and 88 or simultaneously closes switches 86 and 88.

In a heating mode, switches 86 and 88 are in a closed position and make electrical contact with positive igniter terminal 113a and ground igniter terminal 113b, respectively. In the heating mode, current flows from AC mains supply 82 positive terminal 83 through igniter 84 and to node 90 which connects igniter ground terminal 113b to AC mains ground terminal 85. In a flame sensing mode, switches 86 and 88 are in an open position so that the igniter 84 is disconnected from both AC mains supply 82 and ground. It has been discovered that disconnecting ground terminal 113b from ground is important when igniter 84 is in a flame sensing mode because otherwise current from the flame sensor driver circuit 26 may short to ground instead of flowing through the burner flame and to the body of burner 21.

Referring to FIG. 2B, an exemplary implementation of “third leg” hot surface igniters is provided. The phrase “third leg hot surface igniters” refers to igniters containing both a heating conductive pattern and a flame sensing conductive pattern. As illustrated in FIGS. 4A-4D, the heating conductive pattern (e.g., heating conductive patterns 116, 136, 156, 170 in FIGS. 4A-4D), can be sandwiched between insulating tiles of hot surface igniter 84. The flame sensing conductive pattern may be located between two outermost insulating tiles or on the outside of one of the outermost insulating tiles. The flame sensing conductive pattern is electrically isolated from the heating conductive pattern and includes a single conductive terminal 32 that is connected to the output of the flame sensing driver circuit resistor 42 (i.e., flame sensing driver circuit flame sensor output node) and the flame sensing conductive pattern. The heating conductive pattern is connected to positive terminal 92 and ground terminal 94. The positive terminal 92 is selectively connectable to AC mains supply 82 by selectively opening and closing switch 86, which is connected to positive terminal 83 of AC mains supply 82. Ground terminal 94 is connected to ground as is ground terminal 89 of AC mains supply 82, and igniter ground terminal 94 connects to AC mains ground terminal 89 at node 90. During a heating operation, switch 86 is closed to place positive terminal 92 of hot surface igniter 84 in electrical communication with positive terminal 83 of AC mains supply 82. During a flame sensing operation, if a heating operation is not concurrently in progress, switch 86 will remain open. The heating conductive pattern and flame sensing conductive pattern are electrically isolated from one another such that the igniter ground terminal 94 can remain connected to ground without shorting out the electrical the through the burner 21 flame when burner 21 is lit.

Referring to FIGS. 4A-4D several examples of third leg hot surface igniters 102, 120, 140, and 160 are shown. Each of igniters 102, 120, 140, and 160 includes at least two ceramic, insulating tiles with a heating conductive pattern disposed between the at least two tiles. Each of igniters 102, 120, 140, and 160 also includes a flame sensing conductive pattern disposed between the at least two ceramic insulating tiles or on an outer face of one of the at least two ceramic insulating tiles. The ceramic tiles are preferably formed from the compositions and have the dimensions of those described in U.S. patent application Ser. No. 16/366,479 (the '479 App). The heating conductive patterns described herein may be formed from the same materials and using the same processes described in the '479 App. The flame sensing conductive patterns may also be formed from the same materials and using the same processes as described in the '479 App., but need not have the same patterns as they act only as an electrode for conducting electricity to a flame when burner 21 is lit. They also need not be formulated to generate heat unless the same conductive pattern is used for heat generation and flame sensing.

As discussed in the '479 App., ceramic hot surface igniters used in the gas burner systems described herein are prepared by sintering ceramic compositions. In certain examples, post-sintering, the ceramic insulating tiles used to form the igniter (not including conductive ink circuit) have a room temperature resistivity that is no less than 10¹² Ω-cm, preferably no less than 10¹³ Ω-cm, and more preferably, no less than 10¹⁴ Ω-cm. In the same or other examples, the tiles have a thermal shock value in accordance with ASTM C-1525 of no less than 900° F., preferably no less than 950° F., and more preferably, no less than 1000° F.

In other examples, the conductive ink comprising the heating conductive pattern has a (post-sintering) room temperature resistivity of from about 1.4×10⁻⁴ Ω·cm to about 4.5×10⁻⁴ Ω·cm, preferably from about 1.8×10⁻⁴ Ω·cm to about 4.1×10⁻⁴ Ω·cm, and more preferably from about 2.2×10⁻⁴ Ω·cm to about 3.7×10⁻⁴ Ω·cm. In the case of a material with a constant cross-sectional area along its length, resistivity ρ at a given temperature T is related to resistance R at the same temperature T in accordance with the well-known formula:

R(T)=ρ(T)(l/A), where  (4)

• • ρ=resistivity of conductive circuit material (Ω-cm) at temperature T;
  • R=Resistance in ohms (Ω) at temperature T;
  • T=Temperature (° F. or ° C.);
  • A=cross-sectional area (cm²) of conductive ink circuit perpendicular to the direction of current flow; and
  • l=total length (cm) of the conductive ink circuit along the direction of current flow.

In the case of a cross-sectional area that varies along the length of the conductive circuit, the resistance may be represented as:

$\begin{array}{cc}R=\rho \left(T\right){\int }_0^L\frac{dl}{A}& \left(5\right)\end{array}$

where, L=total length of circuit along direction of current flow (cm), and the remaining variables are as defined for equation (4).

In certain examples, the ceramic bodies comprising the ceramic hot surface igniters described herein preferably comprise silicon nitride and a rare earth oxide sintering aid, wherein the rare earth element is one or more of ytterbium, yttrium, scandium, and lanthanum. The sintering aids may be provided as co-dopants selected from the foregoing rare earth oxides and one or more of silica, alumina, and magnesia. A sintering aid protective agent is also preferably included which also enhances densification. A preferred sintering aid protective agent is molybdenum disilicide. The rare earth oxide sintering aid (with or without the co-dopant) is preferably present in an amount ranging from about 2 to about 15 percent by weight, more preferably from about 8 to about 14 percent by weight, and still more preferably from about 12 to about 14 percent by weight of the ceramic body. Molybdenum disilicide is preferably present in an amount ranging from about 3 to about 7 percent, more preferably from about 4 to about 7 percent, and still more preferably from about 5.5 to about 6.5 percent by weight of the ceramic body. The balance is silicon nitride.

The conductive ink circuit is preferably printed onto the face of one of the ceramic tiles to yield a ceramic hot surface igniter (post-sintering) with heating properties that are tailored to the specific application for which the igniter is intended as well as to the voltage at which the igniter will operate. Listed below in Table 5 are some exemplary room temperature resistance (RTR) values for various applications.

TABLE 5
	Supply Voltage		Preferred RTR
Application	(Volts)	RTR Range (Ohms)	Range (Ohms)
HVAC	120	40-52	44-48
Oven	120	32-42	36-39
Hot Water Heater	120	32-42	36-39
Hot Water Heater	230	130-170	145-155

The conductive ink used for the heating conductive circuit may comprise tungsten carbide in an amount ranging from about 20 to about 80 percent, preferably from about 30 percent to about 80 percent, and more preferably from about 70 to about 75 percent by weight of the ink. Silicon nitride is preferably provided in an amount ranging from about 15 to about 40 percent, preferably from about 15 to about 30 percent, and more preferably from about 18 to about 25 percent by weight of the ink. The same sintering aids or co-dopants described for the ceramic body are also preferably included in an amount ranging from about 0.02 to about 6 percent, preferably from about 1 to about 5 percent, and more preferably from about 2 to about 4 percent by weight of the ink. In certain examples, the flame sensing conductive pattern comprises is formed from an ink of the same composition as the heating conductive pattern.

Referring to FIG. 4A, an exploded view of hot surface igniter 102 is provided. Igniter 102 comprises first ceramic insulating tile 104 and second ceramic insulating tile 106. First ceramic insulating tile 104 has an inner face 110a and an outer face 110b. Second ceramic insulating tile 106 has an outer face 108a and an inner face 108b. The inner faces 110a and 108b of the first ceramic insulating tile 104 and second ceramic insulating tile 106 face one another. Heating conductive pattern 116 is printed on inner face 108b of second insulating ceramic tile 106 and is connected to conductive terminal 32. Flame sensing conductive pattern 112 is printed on outer surface 100b of first insulating ceramic tile 104. The insulating ceramic tiles 104 and 106 are laminated as described in the '479 Application to create a unitary hot surface igniter structure.

Referring to FIG. 4B, hot surface igniter 120 comprises first ceramic insulating tile 122, second ceramic insulating tile 124, and third ceramic insulating tile 126. First ceramic insulating tile 122 includes an inner face 128b and an outer face 128a, and the flame sensing conductive pattern is printed on the inner face 128b. Conductive terminal 32 is connected to flame sensing conductive pattern 138.

Second insulating ceramic tile 124 includes a first face 130a that faces ceramic insulating tile 122, and a second face 130b that faces third insulating ceramic tile 126. Heating conductive pattern 136 is printed on the second face 130b of the second ceramic insulating tile 126 and faces the inner face 132a of third ceramic insulating tile 132a. The heating conductive pattern is connected to two terminals 134a and 134b for connection to a power source and ground, respectively. The three ceramic insulating tiles 122, 124, and 126 are laminated together using the techniques described in the '479 App. to create a unitary hot surface igniter structure.

In the example of FIG. 4C, hot surface igniter 140 includes three ceramic insulating tiles 142, 144, and 146. A flame sensing conductive pattern (not shown, but similar to the patterns 112 and 138 of FIGS. 4A and 4B) is connected to conductive terminal 32 and is printed on a first face 150a of ceramic insulating tile 144. Heating conductive pattern 156 is printed on a second face 150b of ceramic insulating tile 144. Both the flame sensing pattern and the heating conductive pattern 156 are sandwiched between ceramic insulating tiles 142 and 144. Ceramic insulating tile 142 has an outer face 148a and an inner face 148b. Inner face 148b faces the face 150a of ceramic insulating tile 144. Ceramic insulating tile 146 has an inner face 152a and an outer face 152b. Inner face 152a faces face 150b of ceramic insulating tile 144. The three ceramic tiles 142, 144, and 146 are laminated using the techniques in the '479 App. to create a unitary hot surface igniter.

In the example of FIG. 4D, hot surface igniter 140 includes a first ceramic insulating tile 162 and a second ceramic insulating tile 165. First ceramic insulating tile 162 has an outer face 164a and an inner face 164b. Second ceramic insulating tile 164 has an inner face 166a and an outer face 16b. However, in this example, both the heating conductive pattern 170 and the flame sensing conductive pattern 172 are printed on the same face 164b of first ceramic insulating tile 160 and face the inner face 166a of second ceramic insulating tile 164. The ceramic insulating tiles 162 and 164 are laminate using the techniques described in the '479 App. to create a unitary hot surface igniter.

Example 1

Simulation of Flame Sensor Driver Circuit

In this example, the operation of the flame sensor driver circuit 26 of FIG. 1 is illustrated both when burner 21 is lit and when it is not lit. The component values used for the simulation are as follows:

	Component	Value
	Flame Sensor Driver Circuit
	BJT 46 VCE breakdown	−400	v
	voltage
	Capacitor 40 Capacitance	40	μF
	Resistor 42	100	kΩ
	Resistor 44	100	kΩ
	Signal Conditioning Circuit
	Diode 52 forward voltage	1.1	V
	drop
	Capacitor 54	0.01	μF
	Resistor 58	10	M Ω
	Resistor 62	1	M Ω
	Zener diode 64 breakdown	12	V
	voltage

A 24 kHZ voltage signal is supplied to the emitter 48 of BJT 46, and voltages at node 45, BJT collector 50 and BJT base 60 are determined via simulation. FIG. 5A shows the input voltage signal 180 at emitter 48 and the voltage signal 182 at node 45. FIG. 5B shows the input voltage signal 180 and the voltage signal 184 at BJT base 60. FIG. 5C shows the input voltage signal 180 at emitter 48 and the voltage signal 190 at BJT collector 50.

When burner 21 is not lit, there is an open circuit between flame sensor 22 and burner 21. Because there is no current path through resistor 42, there is no current path available from BJT emitter 48 to BJT base 60. Thus, there is no emitter-base offset in BJT 46, and BJT remains OFF. As a result, the node 45 voltage floats on the emitter 48 voltage, and signals 180 and 182 are the same. During positive AC cycles at emitter 48, collector 50 is remains 0V and is essentially grounded. During negative AC cycles, collector 50 sees the voltage of emitter 48. Thus, as shown in FIG. 5C the collector 50 voltage signal 190 closely tracks the input voltage signal 180 during negative AC cycles, but during positive AC cycles, the collector 50 voltage is capped at zero. Because the voltage signal at collector 50 is never positive when burner 21 is not lit—and because of diode 52—no current flows to the signal conditioning circuit 28, and the signal conditioning circuit 28 output node 66 will see ground.

When burner 21 is lit, there is an active current path from flame sensor 22 to burner 21 which, in accordance with Mollberg's flame model, can be modeled as a relatively lower resistance resistor in series with a diode, the series combination of which is in parallel with a relative higher resistance resistor. As a result, positive current passes from the flame sensor 22 to the burner 21, but only a small negative leakage current passes from the burner 21 to the flame sensor 22.

When the positive current flows from the flame sensor 22 to the burner 21, the rectification effect of the flame causes a negative (time-varying) DC offset in the voltage at node 45 relative to node 38 because capacitor 40 (like capacitors in general) cannot pass DC. FIG. 6A shows the input voltage signal at node 38 and the voltage at node 45. The gap between the input signal 180 and the node 45 signal 194 at the cycle peaks represents this negative DC offset. A slightly lower offset is present between BJT emitter 48 and BJT base 60 than between BJT emitter 48 and node 45 because of the voltage drop across resistor 44 as shown by the gaps between the input voltage signal 180 and the BJT collector signal 198 in FIG. 6B. When BJT 48 is ON, the path from the emitter 48 to collector 50 is a very low impedance path. Thus, in FIG. 6C the BJT collector signal 202 closely tracks the input voltage signal 180 at node 38 (and emitter 48). However, only positive voltages will pass signal conditioning circuit diode 52.

As illustrated by FIGS. 5A-5C and 6A-6C, when burner 21 is lit, the signal conditioning circuit 28 sees the positive voltage cycles of AC generating circuit 31, but not the negative ones, and the values of the positive AC cycle voltages are smoothed and made more constant at signal conditioning circuit output node 66 because of the low-pass filtering of the RC network in signal conditioning circuit 28. As a result, the time-varying DC offset generated by the flame when burner 21 is lit is converted to something much closer to a binary ON/OFF signal, albeit one with values that may not be suitable for certain commercially available microcontrollers. Thus, load circuit 30 is provided to convert that signal to a DC voltage with a logical high and low that match the microcontroller input requirements.

Example 2

Determination of AC Frequency for Reducing the Effects of Flame Rod Fouling

As discussed previously, higher frequency AC voltages supplied top flame rod can reduce the impact of accumulated deposits on the impedance of the flame rod. They can also reduce the impact of the ceramic insulating tiles of a hot surface igniter on the igniter's impedance. In this example, the circuit of FIG. 1 is used with a flame rod serving as flame sensor 22, except that AC generating circuit 31 is replaced with an arbitrary wave form generator (AWG), and the load circuit is modified to eliminate resistor 80 and Zener diode 78 and to add an LED between resistor 76 and MOSFET 68. As the square wave frequency from the AWG is varied, the percentage of the time the LED is ON is assessed when a flame is present. The goal is to have the LED be on 100 percent of the time that burner 21 is lit. The component values are as follows:

	Component	Value
	Flame Sensor Driver Circuit
	BJT 46 VCE breakdown	−400	v
	voltage
	Capacitor 40 Capacitance	40	μF
	Resistor 42	100	kΩ
	Resistor 44	100	kΩ
	Signal Conditioning Circuit
	Diode 52 forward voltage	1.1	V
	drop
	Capacitor 54	0.01	μF
	Resistor 58	10	M Ω
	Resistor 62	1	M Ω
	Zener diode 64 breakdown	12	V
	voltage

A “Monster Mash” is a formula used to simulate extreme soiling conditions by mixing a wide variety of diverse food ingredients typically used in—and creating soils in—household ovens. In this example, the Monster Mash comprises cherry pie filling, tomato puree, egg yolks, whole milk mozzarella cheese, pasteurized cheese spread, lard, and tapioca. The Monster Mash is applied along 2.249 inches (57.13 mm) of a flame rod having a length of 3.249 inches (82.53 mm) and a diameter of 0.114 inches (2.89 mm). It is applied by running the flame rod through a volume of the Monster Mash applied to a flat surface to obtain a thin layer that is slightly translucent and smoothed around the flame rod until even. The flame rod is placed in a pan but supported above the surface of the pan to prevent burning and placed in an oven preheated to 375° F. for 7-8 minutes, until the Monster Mash is golden brown. The baked layer diameter of the flame rod is 0.127 inches (3.23 mm). Burner 21 is lit, the AWG frequency is varied from 10 Hz to 3 MHz, and the “on-rate” is measured. The peak to peak voltage at flame sensor driver circuit input node 38 is measured as is the mean voltage. The peak to peak voltage at signal conditioning circuit output node 66 is also measured as is the mean voltage.

FIGS. 7A-D show the results of a first set of runs. FIGS. 7A and B show that the peak-to-peak voltage and mean voltage at output node 66 do not respond to the burner being lit until the frequency of the input voltage at flame sensor driver circuit input node 38 (which is also the flame sensing circuit input node) reaches a transition at about 10 kHz. At lower frequencies, due to the fouling of the Monster Mash, the flame sensor driver circuit 26 mean voltage at output node 66 is expected to be about zero, as is observed below 10 KHz (FIG. 7B). At 10 KHz the signal conditioning circuit 28 output node 66 voltage switches rapidly between zero and 8V. At 700 kHZ, the output node 66 mean voltage varies between 3.5V and 8V. Above 700 kHZ the output node 66 voltage signal becomes less stable, and the output node 66 mean voltage varies by up to 2V (not shown in FIG. 7B). The LED is observed to be ON 100 percent of the time at frequencies between 200 kHZ and 400 kHZ. The minimum frequency that yields a consistent output is 24 kHz, which yields a 95 percent on rate.

FIGS. 8A-C show the results of a second set of runs performed a week after the first set of runs. FIGS. 8A and B show that the peak-to-peak voltage and mean voltage at output node 66 do not respond to the burner being lit until the frequency of the input voltage at flame sensor driver circuit input node 38 reaches a transition at about 2 kHz. In the 2 kHz transition, the signal conditioning circuit output node 66 voltage switches rapidly between 0 and 8V. The lack of a reliable peak-to-peak voltage signal at output node 66 below 2 kHz (FIG. 8A) is indicative of the presence of fouling.

The LED is ON 99 percent of the time once the frequency reaches 20 kHZ but is not consistently lit 100 percent of the time except when the frequency is in the range of 40 kHz-2 MHz. Because it is not rectified, the voltage at node 38 remains at zero or very close thereto during the entirety of the runs (FIG. 8C).

The differences in the runs of FIGS. 7A-7D compared to those in FIGS. 8A-8C are believed to be attributable to inconsistencies in applying the Monster Mash and due to the length of the tests. However, based on the data it is believed that a frequency range of 24 kHZ to 300 kHZ is preferred, 40 kHZ to 200 kHz is more preferred and that a frequency range of 70 kHZ to 100 kHZ is still more preferred.

Claims

1. A burner flame detection system comprising:

a conductive flame sensor comprising a conductive terminal and positioned proximal to a burner having a conductive body, the burner having an ignited state, and an unignited state, such that when the burner is in the ignited state, the burner and the conductive flame sensor are in electrical communication with one another; and

a flame sensing circuit comprising a flame detection signal output node, wherein the flame sensing circuit is configured to supply an alternating current having a frequency of from about 24 kHz to about 300 KHz to the conductive flame sensor conductive terminal, and when the burner is in the ignited state and the alternating current is supplied to the conductive terminal, the flame sensing circuit generates a rectified current from the conductive flame sensor conductive terminal to the burner.

2. The burner flame detection system of claim 1, further comprising a switch that is operable to operatively connect a source of the alternating current to the flame detection signal output node when the burner is in the ignited state.

3. The burner flame detection system of claim 1, wherein the flame sensing circuit comprises an input node, a conductive flame sensor driver circuit, and a signal conditioning circuit having a signal conditioning circuit output node.

4. The burner flame detection system of claim 3, wherein the conductive flame sensor driver circuit comprises a flame sensor driver circuit input node, a flame sensor driver circuit flame sensor output node, a flame sensor driver circuit flame detection output node, a capacitor, and a bipolar junction transistor (BJT) having a collector an emitter and a base, and the flame sensor driver circuit input node is connected to the BJT emitter and the capacitor.

5. The burner flame detection system of claim 1, wherein the alternating current has a square wave waveform.

6. The burner flame detection system of claim 1, wherein the conductive flame sensor is a hot surface igniter.

7. The burner flame detection system of claim 6, wherein the hot surface igniter comprises first and second insulating tiles and a heating conductive pattern located between the first and second insulating tiles, and the heating conductive pattern is connected to the conductive terminal.

8. The burner flame detection system of claim 7, wherein the hot surface igniter comprises a flame sensing conductive pattern.

9. The burner flame detection system of claim 1, wherein the conductive flame sensor is a flame rod.

10. A burner flame detection system, comprising:

a hot surface igniter comprising a conductive pattern connected to a conductive terminal and positioned proximal to a burner having a conductive body, the burner having an ignited state and an unignited state, such that when the burner is in the ignited state, the conductive terminal and the burner conductive body are in electrical communication with one another.

11. The burner flame detection system of claim 10, further comprising a flame sensing circuit configured to supply a flame sensing alternating current to the hot surface igniter conductive terminal, wherein when the burner is in the ignited state and the flame sensing alternating current is supplied to the hot surface igniter conductive terminal, the flame sensing circuit generates a rectified current from the hot surface igniter conductive terminal to the burner.

12. The burner flame detection system of claim 10, further comprising a flame detection signal output node and a switch that is operable to operatively connect a flame sensing alternate current source to the flame detection signal output node when the burner is in the ignited state.

13. The burner flame detection system of claim 11, wherein the flame sensing circuit comprises a flame sensor driver circuit having an input node, and a signal conditioning circuit having a signal conditioning circuit output node.

14. The burner flame detection system of claim 13, further comprising a flame sensing alternating current source having a frequency of from about 24 kHz to about 300 KHz connected to the flame sensor driver circuit input node.

15. The burner flame detection system of claim 14, wherein the flame sensing alternating current has a square wave waveform.

16. A method of determining if a burner is ignited using a conductive flame sensor comprising a conductive terminal and positioned proximate the burner, the method comprising:

providing a flame sensing alternate current source operatively connected to the conductive terminal, the alternating current having a frequency of from about 24 kHz to about 300 kHz; and

generating a rectified current from the conductive flame sensor to the burner when the flame sensing alternating current source supplies the alternating current to the conductive flame sensor conductive terminal, and the burner is in an ignited state.

17. The method of claim 16, wherein the conductive flame sensor is a flame rod.

18. The method of claim 16, wherein the conductive flame sensor is a hot surface igniter having a conductive pattern connected to the conductive terminal.

19. The method of claim 18, further comprising the step of selectively connecting the conductive terminal to a source of ignition alternating current.

20. The method of claim 18, wherein the conductive pattern is a first conductive pattern, the hot surface igniter comprises a second conductive pattern, and the second conductive pattern is electrically isolated from the first conductive pattern and connected to a source of ignition alternating current.

21. A method of determining if a burner having a conductive body is ignited, the method comprising:

providing a hot surface igniter positioned proximate the burner and having a conductive pattern connected to a conductive terminal;

providing a flame sensing alternate current source operatively connected to the conductive terminal; and

generating a rectified current from the hot surface igniter conductive pattern to the burner when the flame sensing alternating current source supplies flame sensing alternating current to the hot surface igniter conductive terminal, and the burner is ignited.

22. The method of claim 21, wherein the conductive terminal is a first conductive terminal, and the conductive pattern is connected to a second conductive terminal, the method further comprising the step of selectively connecting the first conductive terminal to a source of ignition alternating current and the second conductive terminal to ground.

23. The method of claim 22, further comprising selectively disconnecting the first conductive terminal from the source of ignition alternating current and selectively disconnecting the second conductive terminal from ground.

24. The method of claim 21, wherein the conductive pattern is a first conductive pattern, the hot surface igniter comprises a second conductive pattern, and the second conductive pattern is electrically isolated from the first conductive pattern and connected to a source of ignition alternating current.