U.S. patent application number 13/104433 was filed with the patent office on 2012-11-15 for flame sense circuit for gas pilot control.
This patent application is currently assigned to International Controls and Measurements Corporation. Invention is credited to Bashkim Racaj.
Application Number | 20120288806 13/104433 |
Document ID | / |
Family ID | 47142083 |
Filed Date | 2012-11-15 |
United States Patent
Application |
20120288806 |
Kind Code |
A1 |
Racaj; Bashkim |
November 15, 2012 |
Flame Sense Circuit for Gas Pilot Control
Abstract
A flame sense circuit for a gas burner control employs a flame
rod for flame rectification. An AC source applies an AC wave
through a capacitor to the flame rod, and the flame rod is also
connected via a resistive voltage divider to a DC voltage source. A
voltage divider node is connected to a voltage follower, which
impedance-matches an ADC input of a microprocessor controller. The
flame sense circuit senses conditions of no-flame, and flame rod in
contact with or leaking to ground, and when flame is present can
quantify sensed flame as good, weak and marginal.
Inventors: |
Racaj; Bashkim; (North
Syracuse, NY) |
Assignee: |
International Controls and
Measurements Corporation
|
Family ID: |
47142083 |
Appl. No.: |
13/104433 |
Filed: |
May 10, 2011 |
Current U.S.
Class: |
431/18 ;
431/78 |
Current CPC
Class: |
F23N 5/123 20130101;
F23N 2229/12 20200101 |
Class at
Publication: |
431/18 ;
431/78 |
International
Class: |
F23N 5/12 20060101
F23N005/12 |
Claims
1. A flame sense circuit arrangement for detecting and quantifying
a sensed flame in a burner of a fuel-fired appliance, comprising: a
gas burner having a chassis in electrical contact with a point of
reference voltage; a source of AC voltage connected between said
point of reference voltage and one electrode of a capacitor, the
capacitor having a second electrode coupled to a first electrical
node A; a resistive voltage divider arrangement having a first
resistance connected between said first node A and a second node B,
and a second resistance connected between said second node B and a
source of DC voltage; a flame rod having a conductor connected with
said first node A and having one end thereof positioned to be in
contact with the flame in said burner when flame is present; a
voltage follower amplifier having a high impedance input coupled to
said second node B and capable of receiving a net DC voltage
component, and having a low impedance output; a processor
arrangement coupled to said low impedance output and capable of
providing indications corresponding to respective flame conditions
including flame absence, flame rod in contact with ground, and a
plurality of sensed flame quantity levels; and an indicator device
coupled to said processor arrangement and adapted to provide
perceptible indicia corresponding to said conditions of flame
absence, flame rod in contact with ground, and said plurality of
flame quality levels.
2. The flame sense circuit arrangement according to claim 1,
comprising a low-value protective resistor in series between said
flame rod and said capacitor.
3. The flame sense circuit arrangement according to claim 1,
wherein said first resistance has a value on the order of about 20
megohms to 50 megohms, and said second resistance has a value on
the order of about 10 megohms.
4. The flame sense circuit arrangement according to claim 1,
wherein said capacitor has a capacitance value on the order of
about 2 nf.
5. The flame sense circuit arrangement according to claim 1,
wherein said voltage follower amplifier is formed of an operational
amplifier having a (+) input terminal coupled to said second node
B, a (-) input terminal, and an output terminal that is coupled to
said (-) input terminal and to an input of said processor
arrangement.
6. The flame sense circuit arrangement according to claim 5, said
voltage follower amplifier having an output impedance on the order
of 10 Kilohms.
7. The flame sense circuit arrangement according to claim 5,
comprising a second capacitor coupled between the second node B and
said point of reference voltage and adapted to filter net DC
voltage to the (+) input terminal of said operational
amplifier.
8. The flame sense circuit arrangement according to claim 1,
wherein said processor arrangement includes an analog-to-digital
converter having an input coupled to said indicator device.
9. The flame sense circuit arrangement according to claim 8,
wherein said microprocessor is programmed to give a flame-absent
indication when the voltage value is at a first predetermined
voltage above a reference level, a grounded flame rod indication
when said voltage value is at another predetermined voltage below
said first predetermined voltage, and to provide one or more flame
present indications when said voltage value is between said
reference level and said second predetermined voltage.
10. The flame sense circuit arrangement according to claim 9,
wherein said microprocessor is programmed to provide a plurality of
sensed flame quantity indications when said voltage value is within
each of a plurality of zones between said second predetermined
voltage and said reference level.
11. The flame sense circuit arrangement according to claim 1,
wherein said flame rod is a pilot flame igniter electrode, and said
circuit arrangement further includes a spark coil secondary
connected in series with said flame rod and said first node A, and
a threshold breakdown switch device connected between said first
node A and a chassis ground.
12. The flame sense circuit arrangement according to claim 10,
wherein said indicator device includes one or more LEDs which
provide coded visual signals corresponding respectively to each of
said conditions.
Description
BACKGROUND OF THE INVENTION
[0001] This present invention relates to control devices for
controlling gas fired appliances, such as furnaces and water
heaters, and is more particularly concerned with a flame-sense
arrangement that operates according to the principle of flame
rectification.
[0002] The present invention is also concerned with a gas appliance
control that employs an intermittent ignition system for the pilot
light flame of the appliance in which the igniter or spark feature
can also serve as a flame rod of the flame-sensor arrangement.
[0003] Flame rectification refers to a method of flame detection,
using the property of a flame in which a plasma or ionized region
within the flame serves as a unidirectional conductor, so that a
current can flow from an interior of the flame towards the outside
of the flame. Typically, a flame rod or conductive probe penetrates
the flame envelope, and serves as anode, while a grounded pilot gas
jet on the burner chassis serves as cathode. The flame then behaves
as a diode connected in series with a large-value resistor, i.e.,
with a resistance value in the tens of megohms. The actual
resistance value within the flame depends on the quality of the
flame, and on the condition of the flame rod.
[0004] When an AC voltage is applied across the flame, e.g., using
the thermostat power of 12 to 24 volts AC, the flame rectifies
this, resulting in a net direct current on the order of about one
micro-ampere (or smaller). This net direct current is then passed
through a series of voltage divider resistances which have values
on the order of several tens of megohms, and this develops a
detectible voltage level. This can be used as an output voltage and
applied to a later stage of the control. Any AC ripple is filtered
out, i.e., diverted to ground, and a buffer amplifier can feed the
voltage signal to a control input of another stage downstream. A
voltage within a given range can be taken to mean that there is
flame present in the burner. A voltage outside that range, i.e., at
one or another higher levels, can mean that there is no flame
present or can mean that there is leakage to ground from the flame
rod. A voltage that is within one given voltage range or another
can mean the presence of flame at the pilot gas burner, or can also
mean that the flame is of marginal, poor, or good quality. A
voltage that is outside that range, i.e., above it at a first or
second high voltage level, can mean that flame is absent or that
there is an electrical problem with the flame rod.
[0005] The controller can attempt to re-light the pilot flame,
based on the output of the flame detector, or in some cases can
shut off the gas valve and go into a lockout condition. Typically,
the intermittent-operating igniter will apply spark voltage for a
brief interval to attempt to ignite the pilot flame, and the flame
sense circuit will sample flame quality during the interval between
successive ignition intervals.
[0006] Up until now, these arrangements could only detect the
presence or absence of flame, but have not been able to quantify or
evaluate the condition of flame when it is present, even though
that information would be useful during control installation or for
troubleshooting. Previous flame rectification detectors have been
unable to identify grounded flame rod conditions or conditions of
leakage to ground, although that information also would be useful
in installation of the control or in troubleshooting.
[0007] Examples of flame quality issues that would be useful, if
they could be distinguished and detected, include a coated or
contaminated flame rod; small pilot flame; or a waving pilot flame
(i.e., not enveloping the flame rod), each of which can result in a
weak flame current. Other installation or trouble shooting
conditions include grounding due to mis-wiring during installation,
ceramic insulator body breakdown, i.e., the insulator holding the
igniter/flame rod becoming cracked or contaminated with the rod
then coming into electrical contact with the chassis. Other quality
issues on installation can occur if there is lead wire damage due
to exposure to excessive temperatures, or contamination forming a
conductive link to the chassis, i.e., build-up or coating on the
ceramic body resulting in leakage current leakage from flame rod to
ground.
[0008] Ideally, the flame sense arrangement should not only be able
to detect presence/absence of flame, but should also be able to
identify grounded flame rod conditions, and flame strength or
quality conditions, and provide optical or other perceptible
signals that correspond to the quality of the pilot light flame.
There can be optical codes or indications for marginal flame, week
flame, good flame, as well as whether flame is present. To date,
this has not been achieved.
OBJECTS AND SUMMARY OF THE INVENTION
[0009] Accordingly, it is an object of the present invention to
provide a flame rectification flame-sense circuit that overcomes
the above-mentioned drawback(s) of the prior art, and that is
capable of providing outputs corresponding to flame presence, flame
absence, flame rod grounding to chassis, and flame strength or
quality.
[0010] It is another object to provide a flame rectification flame
sense circuit that achieves these objects using a simple and
straightforward circuit arrangement, and which does not require a
large number of components to do so.
[0011] In accordance with one aspect of the present invention, A
flame sense circuit arrangement is provided for detecting and
evaluating quality of a flame, e.g., the pilot burner flame, in a
burner of a fuel-fired appliance, in which the gas burner has a
chassis in electrical contact with a point of reference voltage,
i.e., which can be considered "ground" or zero volts. A source of
AC voltage is connected between such point of reference voltage
(e.g., ground) and one electrode of a capacitor, the capacitor
having a second electrode coupled to a first electrical node, i.e.,
node "A" in FIG. 1. A resistive voltage divider arrangement having
a first resistance is connected between said first node "A" and a
second node "B". A second resistance is connected between the
second node "B" and a source of DC voltage, e.g., +5 VDC. The flame
rod has a conductor connected with the first node "A" and has one
end thereof positioned to be in contact with the flame in the
burner when flame is present. A voltage follower amplifier has a
high impedance input coupled to the second node and is capable of
receiving a net DC voltage component. The amplifier has a low
impedance output to supply an output voltage level to the input of
the processor arrangement from the low impedance output. The
processor arrangement is capable of providing indications
corresponding to respective flame conditions including flame
absence, flame rod in contact with ground, and a plurality of flame
quality levels. These can be different series of coded pulses, each
of which corresponds to a flame condition, and the technician or
installer can look those up in an installation guide. An indicator
device (e.g., an LED or a cluster of LEDs) is or are coupled to the
processor arrangement and can be adapted to provide perceptible
indicia corresponding to said conditions of flame absence, flame
rod in contact with ground, and the plurality of flame quality
levels. A low-value protective resistor in series between the flame
rod and the capacitor serves to prevent excessive currents in the
event of a grounded flame rod. This can be a resistance of about 10
kilohms, and can be between the capacitor and the first node, or
between the first node and the flame rod.
[0012] Favorably, the first resistance in the voltage divider has a
value on the order of about 20 megohms to 50 megohms, and the
second resistance has a value on the order of about 10 megohms. The
capacitor may have a capacitance value on the order of about 2
nf.
[0013] Again, in a preferred mode the voltage follower amplifier is
formed of an operational amplifier having a (+) input terminal
coupled to the second node B, a (-) input terminal, and an output
terminal that is coupled to the (-) input terminal and to an input
of the processor arrangement. The voltage follower amplifier having
an output impedance on the order of 5 to 10 Kilohms. A second
capacitor may be coupled between the second node "B" and the point
of reference voltage (e.g., ground) and is adapted to divert AC
ripple from the (+) input terminal of the operational amplifier.
The processor arrangement can also include an analog-to-digital
converter input coupled to the output of the voltage follower
amplifier, and is suitably programmed to provide respective signals
to the indicator device depending on the voltage value at the
output of the voltage follower amplifier.
[0014] In the control circuitry, a microprocessor is programmed to
give a "flame-absent" indication when the voltage value is at a
first predetermined voltage at or above a reference level (e.g.,
five volts), a "grounded flame rod" indication when the voltage
value is at another predetermined voltage (e.g., 4 volts) below the
first predetermined voltage, and one or more flame present
indications when the voltage value is between the reference level
(ground) and the second predetermined voltage (4 volts).
[0015] The microprocessor controller is programmed to provide a
plurality of flame quality indications when said voltage value is
within each of a plurality of respective zones between the second
predetermined voltage and the reference level (e.g., ground).
[0016] In a favorable gas appliance arrangement, the flame rod is a
pilot flame igniter electrode, and a spark coil secondary is
connected in series with the flame rod and the first node "A". A
threshold breakdown neon switch device connected between said first
node "A" and a chassis ground.
[0017] The indicator device includes one or more LEDs which provide
a coded visual signal corresponding respectively to each of the
flame conditions. These can be used also as an alarm to alert the
homeowner to a service condition, so that repair or service can be
obtained before a breakdown occurs.
[0018] The above and many other objects, features, and advantages
of this invention will be more fully appreciated from the ensuing
description of certain preferred embodiments, which are to be read
in conjunction with the accompanying Drawing.
BRIEF DESCRIPTION OF THE DRAWING
[0019] FIG. 1 is a general schematic view of a flame sense circuit
arrangement according to a preferred embodiment of this
invention.
[0020] FIG. 2 is chart showing thr relation of output voltage to
flame quality with this embodiment.
[0021] FIG. 3 is schematic view showing the flame sense circuit of
this embodiment of this invention combined with spark or
ignition.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0022] With reference now to the Drawing, FIG. 1 shows a general
arrangement of a flame sense circuit 10, for monitoring the quality
of flame of a gas burner pilot 12. The pilot 12 has a chassis 14,
which is electrically grounded with a flame 15, when present,
extending upward, and with a flame rod 16, which is electrically
isolated, extending into the envelope of the flame 15.
[0023] In the flame sense circuit 10, there is an AC voltage source
18, e.g., 12-volt or 24-volt AC 60 Hz thermostat power, coupled
between a ground point and one terminal of a capacitor C1. The
other terminal of the capacitor C1 is coupled to a junction point
or node A, which is in turn connected to the flame rod 16.
Favorably, this capacitor has a capacitance value of about 2.2
nanofarads, and there may be a protective resistor R1 between the
capacitor C1 and the flame rod 16 to limit current in the event
that the flame rod touches the chassis and grounds. The resistor R1
favorably has a resistance value of about 10 Kilohms. In this
embodiment, the resistor R1 is situated between the capacitor C1
and the node A, but could alternatively be situated between the
node A and the flame rod 16. The flame itself may act as a
unidirectional conductor of low conductance, and is represented
here in dash lines as a diode. When the alternating current is
applied from the source 18 through the capacitor C1, the flame
conducts a small current (on the order of one micro-ampere, or
less) in the direction from the flame rod 16 to the grounded
chassis 14. This results in a net negative voltage accumulating
onto the upper plate of the capacitor C1, as illustrated. The net
dc voltage level at the capacitor C1 depends on the position, size
and quality of the flame 15.
[0024] A resistive voltage divider 20 extends from the node A to a
source of DC ("battery") voltage Vdd, typically about +5 volts. The
divider 20 includes a pair of resistors R2 and R3 connected in
series between node A and a second node B, and a third resistor R4
connected between the node B and the DC voltage source Vdd. The
resistors R2 and R3 are each in the megohm range, i.e., each 10
M.OMEGA. to 22 M.OMEGA. and resistor R4 preferably has a value of
about 10 M.OMEGA..
[0025] A second capacitor C2 is coupled between the second node B
and a reference voltage to shunt AC ripple. The node B2 is also
connected to a voltage follower circuit, here formed of an
operation amplifier 22 or OpAmp. A net DC level then appears on the
(+) input of the OpAmp 22. The OpAmp is configured with the output
terminal coupled to the (-) input, and there is an output resistor
R5 at the output terminal. This resistor R5 has a resistance value
of about 4.7 to 10 K.OMEGA., and is selected to match the input
impedance of an input of an analog-to-digital functionality 24 of a
microprocessor controller. This stage then converts the analog
voltage from the OpAmp to digital form and supplies it to a
controller stage 26 in the microprocessor controller. This stage
interprets the DC level that appears node B and at the OpAmp
output, in accordance with a program or algorithm based on the
voltage chart shown in FIG. 2, explained just below. The
microprocessor controller then sends a flash code to a visible
indicator LED 28 to indicate the flame status, i.e., flame absent,
flame marginal, flame weak, flame good, or flame rod grounded. The
code can be a number of flashes in a group, e.g., with a single
flash indicating good flame, two flashes meaning weak flame, three
meaning marginal flame, no flashes meaning no flame detected, and a
solid ON condition meaning flame rod grounded. Other flash codes
for these and other conditions are possible, using e.g.,
combinations of short and long flashes.
[0026] The power supply for the OpAmp 22 may be regulated to 6.5 V
DC which enables a complete output range of 0 VDC to 5 Vdc at the
(+) input to the ADC functionality, and can provide better
resolution between flame condition zones, as explained below in
reference to FIG. 2.
[0027] It can be seen that when flame is absent, or not in contact
with the flame rod, then no net current flows through the flame
rod, and the capacitor is not pulled negative. Under that
condition, the voltage waveform at point A is a superposition of an
AC signal and a small positive DC component from the voltage
divider 10. The voltage at the node B is pulled up to battery
voltage Vdd, e.g., +5 volts, and that voltage also appears at the
input of the ADC functionality 24. This is in the no-flame zone of
FIG. 2. Resistive conduction between the flame rod and chassis
ground will not change the symmetry between positive and negative
half cycles of the AC waveform but only reduces its amplitude. This
results in an output voltage being in a Resistive Conduction zone
that is shared with the No Flame zone, as shown in FIG. 2. If there
is flame rod contact with the chassis, then node A appears is at
ground, and the voltage at the node B provides the divided voltage:
V.sub.out=Vdd.times.(R2+R3)/(R2+R3+R4), which in this example would
be about +4 volts. A narrow zone appears here to allow for
resistance of the flame rod being non-zero, and allowing for
voltage ripple. The flame threshold would be at a voltage below
that level. Where flame is present, and flame is in contact with
the flame rod, the rectification effect of the flame will pull the
net voltage of the capacitor C1 negative. The effect is smaller for
marginal and weak flame conditions, and larger for normal flame
conditions, so there are "marginal", "weak" and "good" flame
conditions defined for voltage ranges as indicated in FIG. 2. The
voltage output will be in a Flame zone, which can be resolved
further into sub zones: Good, Weak, Marginal. A
Flame-Below-Threshold zone is also shown in FIG. 2, and is used as
a stability or buffer zone, such as when sensed flame is too low
and is considered to be below threshold, with the system thus being
unstable. This is considered the same as No Flame, for control
purposes.
[0028] The flame sensing circuit 10 can also be used for sensing
flame through the spark transformer or ignition coil as shown in
FIG. 3, where the control circuit is an intermittent pilot gas
ignition control. Here spark is applied intermittently, and flame
is sensed during the spark pause interval. Here the gas pilot 12 is
shown with the flame rod 16 mounted on the chassis 14, with a tip
of the flame rod facing a grounded spark electrode 30. A ceramic
insulator body 32 mounts the flame rod 16 to the chassis 14. A
spark transformer or ignition coil secondary 34 is coupled to the
flame rod 16 with also serves as the high voltage spark electrode.
A switching spark gap 36 connects at a node C to the other end of
the secondary 34, and conducts to ground if the voltage V on the
node C exceeds some threshold, e.g., 90 volts. The flame sense
circuit 10 is also joined to the flame rod 16 (through the ignition
coil secondary 34) at node C, and the spark gap 36 diverts spark
voltage away from the flame sense circuit. The spark transformer or
ignition coil 34 can be energized intermittently, e.g., at one
second intervals, and the flame sense operation can be carried out
during the pause intervals between ignition attempts.
[0029] The indicator LED 28 can provide flash codes associated with
the respective flame zones shown in FIG. 2. Pulses or flashes at a
steady heart-beat rate can mean Good Flame, two flashes and a pause
can mean Weak, one flash and a pause can mean Marginal, and no
flashes can mean no flame, or flame below threshold. A steady ON
can mean grounded flame rod. The circuit can (with only minor
modification) be used to measure and indicate or display flame
current. This can be the equivalent of indicating or displaying the
flame current that a micro-ammeter shows when in series with the
flame rod.
[0030] While the invention has been described with reference to
specific preferred embodiments, the invention is certainly not
limited to those precise embodiments. Rather, many modifications
and variations will become apparent to persons of skill in the art
without departure from the scope and spirit of this invention, as
defined in the appended claims.
* * * * *