U.S. patent number 3,902,839 [Application Number 05/422,693] was granted by the patent office on 1975-09-02 for electronic pilot ignition and flame detection circuit.
This patent grant is currently assigned to Johnson Service Company. Invention is credited to Russell B. Matthews.
United States Patent |
3,902,839 |
Matthews |
September 2, 1975 |
**Please see images for:
( Certificate of Correction ) ** |
Electronic pilot ignition and flame detection circuit
Abstract
An electronic pilot ignition and flame detection circuit for use
in a fuel ignition system including a spark ignition circuit
operable when energized to generate sparks for igniting gas
emanating from a pilot source, a switching circuit including a
normally de-energized relay and a normally non-conducting silicon
controlled rectifier which controls the relay, and a pilot flame
sensing circuit operable as a pulse generating circuit for sensing
the pilot flame and providing pulses of a first amplitude for
enabling the silicon controlled rectifier to effect energization of
the relay whenever the pilot gas is ignited to cause the spark
ignition circuit to be de-energized and to prepare an energizing
path for a main burner gas valve solenoid. The flame sensing
circuit provides pulses of a lower amplitude whenever the pilot
flame is extinguished to preclude enabling of the silicon
controlled rectifier, such that the relay is disabled causing the
energizing path for the gas valve solenoid to be interrupted and
the spark ignition circuit to be re-energized. Alternatively, the
gas valve solenoid may be substituted in the circuit for the relay
coil to be controlled directly by the silicon controlled rectifier.
In such a case a separate self extinguishing pilot relight circuit
controls the pilot ignition.
Inventors: |
Matthews; Russell B. (Goshen,
IN) |
Assignee: |
Johnson Service Company
(Milwaukee, WI)
|
Family
ID: |
23675955 |
Appl.
No.: |
05/422,693 |
Filed: |
December 7, 1973 |
Current U.S.
Class: |
431/46; 431/51;
431/25; 431/59 |
Current CPC
Class: |
F23N
5/123 (20130101); F23N 5/203 (20130101); F23N
2239/04 (20200101); F23N 2229/02 (20200101); F23N
2227/22 (20200101); F23N 2227/30 (20200101); F23N
2231/12 (20200101); F23N 2227/36 (20200101) |
Current International
Class: |
F23N
5/20 (20060101); F23N 5/12 (20060101); F23q
009/08 () |
Field of
Search: |
;431/25,46,51,59 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Dority, Jr.; Carroll B.
Attorney, Agent or Firm: Johnson, Dienner, Emrich &
Wagner
Claims
I claim:
1. In an automatic fuel ignition system including a pilot source
for establishing a pilot flame and valve means operable when
energized to supply a gaseous fuel to burner apparatus, an
electronic control circuit for monitoring the pilot flame and
controlling the energization of said valve means, said control
circuit comprising pilot flame sensing means including sensing
electrode means located in the proximity of the pilot source for
sensing said pilot flame, and pulse generating means controlled by
said pilot flame sensing means to provide pulse outputs of a first
amplitude whenever the pilot flame is established and to provide
pulse outputs of a second amplitude whenever the pilot flame is
extinguished, and switching means responsive to pulses at said
first amplitude to effect energization of said valve means, said
switching means being disabled whenever pulses at said second
amplitude are provided by said pulse generating means to cause
deenergization of said valve means to prevent the flow of said
gaseous fuel to the burner apparatus whenever the pilot flame is
extinguished.
2. An electronic control circuit as set forth in claim 1 wherein
said pulse generating means includes a normally non-conducting
controlled switching device, first circuit means including
capacitor means connected to an input electrode of said controlled
switching device and means connected to a source of potential to
provide a charging path for said capacitor means, discharge circuit
means including said controlled switching device for discharging
said capacitor means whenever said controlled switching device is
rendered conductive, said pilot flame sensing means including
second circuit means connected to a control electrode of said
controlled switching device and operable to periodically render
said controlled switching device conductive permitting said
capacitor means to discharge over said discharge path to provide
said pulse outputs.
3. An electronic control circuit as set forth in claim 2 wherein
said second circuit means includes further capacitor means
chargeable at a first rate whenever the pilot flame is established
to provide a potential at said control electrode to render said
controlled switching device conductive at a time when said
capacitor means of said pulse generating means has charged to a
first voltage and chargeable at a second rate whenever the pilot
flame is extinguished to provide a potential at said control
electrode to render said controlled switching device conductive at
a time when said capacitor means of said pulse generating means has
charged to a second voltage.
4. An electronic control circuit as set forth in claim 2, wherein
said pilot source includes a pilot valve means operable when
energized to supply pilot gas to an outlet, and pilot ignition
means operable when energized to generate ignition sparks for
igniting pilot gas emanating from said outlet to establish a pilot
flame, said normally non-conducting controlled switching device
being responsive to pulses at said first amplitude to effect the
deenergization of said pilot ignition means.
5. An electronic control circuit as set forth in claim 4, wherein
said switching means includes a further controlled switching device
which is normally disabled and enabled by pulses at said first
amplitude to effect energization of said valve means.
6. An electronic control circuit as set forth in claim 11, wherein
said valve means includes a solenoid operated valve having a main
gas valve coil and a secondary coil, said secondary coil being
energized to cause the pilot ignition means to be disabled whenever
said main gas valve coil is energized.
7. An electronic control circuit as set forth in claim 5, wherein
said switching means further includes normally deenergized relay
means, said relay means being energized by said further controlled
switching device to control the energization of said gas valve
means whenever pulses of said first amplitude are provided by said
pulse generating means.
8. An electronic control circuit as set forth in claim 7, wherein
said relay means is operable when energized to effect
deenergization of said pilot ignition means.
9. An electronic control circuit as set forth in claim 1, further
including power means adapted to be coupled to a direct current
source of power for supplying a DC power signal to said control
circuit, and inverter means responsive to said DC power signal to
provide alternating current signals for said control circuit.
10. In an automatic fuel ignition system, pilot ignition means
operable when energized to generate ignition sparks for igniting
pilot gas emanating from a pilot source to establish a pilot flame,
pilot flame monitoring means including flame sensing means having
sensing electrode means located adjacent said pilot source for
sensing said pilot flame and pulse generating means including a
controlled switching device having first and second control
electrodes, first capacitor means connected to said first control
electrode, second capacitor means connected to said second control
electrode, means for permitting said first capacitor means to
charge to a predetermined value to provide a first potential at
said first control electrode, means permitting said second
capacitor means to charge to a predetermined value at a first rate
whenever said pilot flame is extinguished and at a second rate
whenever said pilot flame is established to provide a second
potential at said second control electrode, said controlled
switching device being enabled whenever the potential difference
between said first and second control electrodes exceeds a
predetermined value to permit said first capacitor means to
discharge over said controlled switching device to effect the
generation of pulse outputs of a first amplitude whenever the pilot
flame is established and pulse outputs of a second amplitude
whenever the pilot flame is extinguished, switching means
responsive to pulses of said first amplitude to de-energize said
pilot ignition means and to prepare an energizing path for gas
valve means which supplies gas to gas burner apparatus, said
switching means being disabled whenever pulses of said second
amplitude are provided by said pulse generating means to interrupt
said energizing path for said gas valve means and effect
energization of said pilot ignition means whenever said pilot flame
is extinguished.
11. An electronic control circuit as set forth in claim 10 wherein
said switching means includes normally de-energized relay means
having contact means connected in an energizing path for said valve
means and a normally disabled controlled switching device connected
in series with a coil of said relay means across a source of
potential, said controlled switching device of said switching means
being enabled responsive to pulses at said first amplitude to
effect energization of said relay means to operate said contact
means to permit energization of said gas valve means.
12. An electronic circuit as set forth in claim 11 wherein said
relay means includes further contact means connected in an
energizing circuit for said pilot ignition means and operated
whenever said relay means is de-energized to permit energization of
said pilot ignition means.
13. In an automatic fuel ignition system including pilot source
means for establishing a pilot flame and valve means operable when
energized to supply a gaseous fuel to burner apparatus, an
electronic control circuit for monitoring the pilot flame and
controlling the energization of said valve means said control
circuit comprising pilot flame monitoring means having a controlled
switching device, first circuit means including capacitor means
connected to a source of cyclical AC voltage and to a first control
electrode of said controlled switching device to permit said
capacitor means to charge during each first half cycle of the AC
voltage thereby providing a potential at said first control
electrode, second circuit means connected to said AC voltage source
and to a second control electrode of said controlled switching
device including sensing electrode means located in the proximity
of the pilot source means for sensing the pilot flame such that
said second circuit means provides a potential at said second
control electrode effective to render said controlled switching
device conductive at a first time during each first half cycle of
the AC voltage whenever the pilot flame is extinguished and to
render said controlled switching device conductive at a later time
during each first half cycle of the AC voltage whenever the pilot
flame is established, output means for providing a discharge path
over said controlled switching device for said capacitor means
whenever said controlled switching device is rendered conductive to
provide a pulse output, and switching means responsive to the pulse
output provided by said flame monitoring means whenever the pilot
flame is established to prepare an energizing path for said valve
means, said switching means being disabled to interrupt said
energizing path for said valve means whenever the pilot flame is
extinguished, and said controlled switching device being maintained
non-conductive in the event of a component failure in said pilot
flame monitoring circuit means.
14. An electronic control circuit in a fuel ignition system as set
forth in claim 13 wherein said pilot source means includes a pilot
source and pilot ignition means operable when energized to generate
ignition sparks for igniting pilot gas emanating from said pilot
source to establish a pilot flame, said switching means including
normally de-energized relay means having contact means for normally
connecting said pilot ignition means to said AC voltage source to
energize said pilot ignition means whenever the pilot flame is
extinguished, said relay means being energized to operate said
contact means to thereby disconnect said pilot ignition means from
said AC voltage source whenever the pilot flame is established.
15. An electronic control circuit in a fuel ignition system as set
forth in claim 14 wherein said switching means further includes a
further controlled switching device connected in series with a coil
of said relay means to said AC voltage source, said further
controlled switching device being normally non-conducting and being
rendered conductive responsive to the pulse output of said flame
monitoring means to effect energization of said relay means during
each first half cycle of said AC voltage whenever the pilot flame
is established, said relay means having further contact means
connected in an energizing circuit for said valve means and
operated whenever said relay means is energized to permit
energization of said valve means.
16. An electronic control circuit is a fuel ignition system as set
forth in claim 15 wherein said switching means includes means for
maintaining said relay means energized during each second half
cycle of the AC voltage.
17. An electronic control circuit in a fuel ignition system as set
forth in claim 16 wherein said output means of said pilot flame
monitoring means includes a pair of resistors connected in parallel
between an output electrode of said controlled switching device and
a point of reference potential for said AC voltage source.
18. In an automatic fuel ignition system including a pilot source
for establishing a pilot flame and valve means operable when
energized to supply a gaseous fuel to burner apparatus, an
electronic control circuit for monitoring the pilot flame and
controlling the energization of said valve means comprising input
means, said control circuit including first and second conductor
means for supplying a cyclical AC voltage to said control circuit,
a normally non-conducting controlled switching device, first
circuit means connected between said first and second conductor
means, said first circuit means including first capacitor means
connected to a first control electrode of said controlled switching
device and chargeable during each first half cycle of said AC
voltage to provide a potential at said first control electrode,
second circuit means including second capacitor means connected
between a point of reference potential and said second conductor
means, means for connecting a second control electrode of said
controlled switching device to said point of reference potential,
sensing electrode means, and means for connecting said sensing
electrode means to said first conductor means, said sensing
electrode means being located in the proximity of the pilot source
and positioned in a spaced relationship with said point of
reference potential to provide a gap therebetween which is bridged
by the pilot flame whenever the pilot flame is established such
that a charging path for said second capacitor means is provided
between said first and second conductor means over said gap to
permit said second capacitor means to charge at a first rate
whenever the pilot flame is extinguished to thereby provide a
potential at said second control electrode and to permit said
second capacitor means to charge at a faster rate whenever the
pilot flame is established and bridges said gap to thereby provide
a potential at said second control electrode, said controlled
switching device being rendered conductive whenever the potential
difference between said first and second control electrodes exceeds
a predetermined amount such that said controlled switching device
is rendered conductive at a first time during each first half cycle
of the AC voltage whenever the pilot flame is extinguished and at a
later time during each first half cycle of the AC voltage whenever
the pilot flame is established, output means for permitting said
first capacitor means to discharge over said controlled switching
device whenever said controlled switching device is rendered
conductive to effect the generation of a pulse output, and
switching means responsive to the pulse output provided over said
output means whenever the pilot flame is established to prepare an
energizing path for said valve means, said switching means being
disabled to interrupt said energizing path for said valve means
whenever the pilot flame is extinguished.
19. An electronic control circuit as set forth in claim 18 wherein
said first circuit means includes means for limiting the potential
at said first control electrode of said controlled switching device
to a predetermined value.
20. An electronic control circuit as set forth in claim 18 wherein
said second circuit means includes means connected between said
second controlled switching device and said second conductor means
for limiting the potential at said second control electrode.
21. An electronic control circuit as set forth in claim 18 wherein
said switching means comprises normally de-energized relay means
having an energizing coil and contact means connected in the
energizing path for said valve means and a further normally
non-conductive controlled switching device connected in series with
said coil between said first and second conductor means, said
further controlled switching device being rendered conductive by
the pulse output provided over said output means during each first
half cycle of the AC voltage whenever the pilot flame is
established to effect energization of said relay means to operate
said contact means.
22. An electronic control circuit as set forth in claim 19 wherein
said further controlled switching device is rendered non-conductive
during each second half cycle of the AC voltage and said switching
means further includes means for maintaining said relay means
energized during the second half cycle of the AC voltage.
23. An electronic control circuit as set forth in claim 3 wherein
said controlled switching device is maintained non-conductive in
the event of a component failure in said pilot flame monitoring
means.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to fuel ignition systems, and more
particularly to an electronic pilot ignition and flame detection
circuit for use in such systems to monitor a pilot flame and effect
the de-energization of a gas valve in response to failure of the
pilot flame or a component failure in the pilot ignition and flame
detection circuit.
2. Description of the Prior Art
Many known fuel ignition systems employ a thermocouple device to
monitor a standing pilot flame and a relay controlled by the
thermocouple device to effect de-energization of a gas valve, for
example, to shut down the system in the event of a pilot flame
failure. The thermocouple used to sense the pilot flame in such
systems has a response time of approximately 20-45 seconds before
the control relay will be de-energized to effect closing of the
main gas valve. It is desirable to eliminate a standing pilot flame
to conserve gas and at the same time provide a fast response time
of the control arrangement therefor.
A fast response time makes possible the elimination of standing
pilots, a very desirable function considering the impending gas
shortage and the vast amount of gas consumed by standing
pilots.
In order to accomplish this and still retain the inherent safety of
a pilot ignition of a main burner, it becomes necessary to turn the
pilot off when the thermostat is not calling for heat. This means
that any time the thermostat calls for heat the pilot burner must
be ignited first, second the presence of the pilot flame verified
and then the main burner turned on.
The response of this system is so fast that less than 1 second is
required for the complete sequence. This means that the circuit can
replace a standing pilot system without any detectable difference
in performance to the heating system.
SUMMARY OF THE INVENTION
The present invention has provided an electronic pilot ignition and
flame detection circuit including an electronic flame sensing
circuit which has a much faster response time than prior art
systems employing a thermocouple-relay combination. The response
time of the electronic flame sensing circuit may, for example, be
one second or less thereby eliminating the need for a gas-wasting
standing pilot flame without otherwise effecting the heating
system.
Moreover, the flame detection circuit of the present invention may
include an electronic pilot ignitor controlled by the electronic
flame sensing circuit such that the generation of ignition sparks
for igniting the pilot is provided automatically whenever loss of
pilot flame is detected. Also, the electronic flame detection
circuit of the present invention is extremely fail-safe in that any
given component failure for an open-circuit condition to a
short-circuit condition will not result in an unsafe condition in
which the main gas valve may be energized when the pilot flame is
not present.
In accordance with an exemplary embodiment of the invention, the
electronic pilot ignition and flame sensing circuit includes pilot
ignition means operable when energized to generate ignition sparks
for igniting pilot gas emanating from a pilot source to establish a
pilot flame.
A switching means is normally de-energized when the pilot flame is
extinguished to extend a cyclical AC signal supplied to the circuit
over input means and a pair of input conductors to the ignition
means for energizing the ignition means. The switching means is
operable to effect the de-energization of the ignition means and to
prepare an energizing path for gas valve means which supplies gas
to gas burner apparatus whenever the pilot flame is lit.
A flame sensing means operable as a pulse generating circuit
provides pulses at first and second levels for controlling the
de-energization and the energization of the switching means as a
function of the presence or absence of the pilot flame. The flame
sensing means includes a controlled switching device having a pair
of control electrodes, a first circuit means including first
capacitor means connected between a first one of the control
electrodes and one of the input conductors, and second circuit
means including second capacitor means connected between the second
control electrode and the one input conductor.
The flame sensing means further includes means connected between
the input conductors for providing a charging path for the first
capacitor means to permit the first capacitor means to charge to a
predetermined value during each first half cycle of the AC signal
to provide a first potential at said first control electrode, and
means for providing a second capacitor means to charge to a
predetermined value at the first rate whenever the pilot flame is
extinguished and to charge to said predetermined voltage at a
faster rate whenever the pilot flame is established to provide a
second potential at said second control electrode.
The controlled switching device is rendered conductive at a time
during each first half cycle of the AC signal when the potential
difference between the first and second control electrodes exceeds
a predetermined value, permitting the first capacitor means to
discharge over the controlled switching device to effect generation
of pulses at the first level for energizing the switching means
whenever a pilot flame is established and to effect the generation
of pulses at the second level causing de-energization of the
switching means whenever the pilot flame is extinguished.
In addition, in the event of a component failure in the pilot
ignition and flame detection circuit, such as in the flame sensing
means, for example, the controlled switching device will be
maintained non-conductive so as to disable switching means and
interrupt the energizing path for the main gas valve solenoid.
Thus, the electronic pilot ignition and flame detection circuit of
the present invention has three levels of operation. For the
condition of a pilot flame failure, the system is operable at a
first voltage level in which the pulses provided by the flame
sensing circuit are ineffective to enable the switching means and
accordingly the energizing path for the main gas valve solenoid is
interrupted. Moreover, the pilot ignition means will be energized
to effect re-ignition of the pilot flame.
The presence of a pilot flame places the system at a second
operating level in which the flame sensing circuit is operable to
provide pulses for maintaining the switching means enabled
permitting the main gas valve solenoid to be energized. In the case
of a component failure, the system is operable at a third level in
which the flame sensing circuit is disabled such that the switching
means will remain de-energized whether or not the pilot flame is
present.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic circuit diagram of a preferred embodiment of
the invention operable from an alternating current source and using
a relay energized by a silicon controlled rectifier for controlling
main burner valve operation;
FIG. 2 is a schematic circuit diagram of another embodiment of the
invention, but substituting a solenoid of the main valve for the
relay coil of FIG. 1, thereby controlling the main burner valve
directly by the silicon controlled rectifier and utilizing an
ignitor circuit which is self-extinguishing;
FIG. 3 is a schematic circuit diagram of still another embodiment
of the invention similar to FIG. 2 in that the main valve solenoid
is controlled directly by the silicon controlled rectifier but
operable from a D.C. source and using an ignitor which is
controlled by means of a second coil wound on the solenoid of the
main valve; and
FIG. 4 is a schematic circuit diagram of yet another embodiment of
the invention, similar to FIG. 1 in that a relay is responsive to a
silicon controlled rectifier to control the main valve, but is
energized from a D.C. source.
DESCRIPTION OF A PREFERRED EMBODIMENT
Referring to FIG. 1, there is shown an exemplary embodiment for an
electronic pilot ignition and flame detection circuit 10 provided
by the present invention. The electronic flame detection circuit
10, which may be employed in a fuel ignition system, includes an
electronic pilot ignition circuit 11 which supplies voltage pulses
derived from an AC source to a pair of ignition electrodes 12 and
13 to ignite a gaseous fuel emanating from a pilot source 14 of a
gas burner apparatus (not shown).
Energizing power for the flame detection circuit 10, including the
pilot ignition circuit portion 11, is supplied over an input
transformer T2 which has a primary winding 15 connectible to an AC
voltage source which, for example, may be a standard 60 Hertz, 120
volt A.C. line voltage source, and a secondary winding 15A. The
input transformer T2 is a step-down transformer which provides
approximately 24 volts A.C. between terminals 40 and 41 of winding
15A when the primary winding 15 of the transformer T2 is connected
to a 120 volt A.C. source.
Terminal 40 of the secondary winding 15A of the input transformer
T2 is connected through normally open THS thermostatic contacts
over a conductor L3 and normally closed contacts 21A of a relay 21
to a first terminal 22 of the ignition circuit 11. Terminal 41 of
the secondary winding 15 of the input transformer T2 is connected
over a conductor L4 to a second terminal 24 of the ignition circuit
11.
The ignition circuit 11 is more fully described in United States
patent application Ser. No. 307,077 filed Nov. 16, 1972, now U.S.
Pat. No. 3,806,305 and has an output winding 25 having a first
terminal 26 connected to one of the ignition electrodes 12 and a
second terminal 27 connected to the other ignition electrode
13.
Accordingly, when the thermostat calls for heat THS contacts close
and power is applied to the flame detecting circuit 10 over the
input transformer T2, AC current flows through the ignition circuit
11 producing voltage pulses across the output winding 25 of the
ignition transformer T1, which pulses are applied to the electrodes
12 and 13, producing sparks between the ignition electrodes 12 and
13 for igniting gas emanating from the pilot source 14. When power
is applied to the ignitor circuit 11 from transformer T2 on the
first half wave of applied voltage and with terminal 40 assumed
positive with respect to 41, current flows through thermostat
contacts THS over line L3 through normally closed relay contacts
21A through capacitor C10, resistor R20, capacitor C11, diode D10
to terminal 24, charging capacitors C10 and C11. On the next half
cycle or reverse polarity, current flows from terminal 41, terminal
24, diode D11, capacitor C10, normally closed contacts 21A and
thermostat contacts THS. This circuit acts as a voltage doubler,
substantially doubling the charge of capacitor C11, which charge is
applied across the anode-cathode circuit SCR T1. As this voltage
doubling effect occurs, the bias across diode D10 of SCR T1 is
sufficient to cause SCR to fire across its anode-cathode circuit
causing capacitor C11 to discharge through the anode-cathode
circuit and through the primary winding 23 of ignition transformer
T1. This pulses the secondary 25 of the transformer causing a high
voltage spark across electrodes 13 - 12 to attempt ignition of the
pilot gas coming from pilot burner 14. This occurs once every cycle
of the applied voltage until the pilot gas is ignited.
The flame detection circuit 10 also includes a flame sensing
circuit, indicated generally at 30, which is supplied with
approximately 80 volts A.C. through isolating step-up transformer
T3. Circuit 30 is operable to provide a pulse output indicative of
the presence or absence of the pilot flame. The flame sensing
circuit 30 includes a flame sensing electrode 31 and a controlled
switching device 32, embodied as a programmable unijunction
transistor (PUT), such as the type 2N6028 commercially available
from Motorola. The flame sensing circuit 30 also includes an anode,
a control network 33 and a gate control network 34 for the PUT
device 32.
The flame sensing electrode 31 is connected over a resistor R1 to
conductor L1 and is positioned in a spaced-relationship with a
ground reference point 35 for the electronic flame detection
circuit 10, normally providing a high resistance path between
conductor L1 and the reference point 35. The ground reference point
35 may, for example, be a metallic ground provided by a gas burner
apparatus or the pilot source 14. The flame sensing electrode 31 is
located in the region in which the pilot flame is to be produced
such that the pilot flame will bridge the gap 36 between the
electrode 31 and the reference point 35 thereby lowering the
resistance of the current path over the electrode 31 between
conductor L1 and the reference point 35 whenever the pilot flame is
present.
The gate control network 34 determines the gate potential for the
normally non-conducting PUT device 32. The gate control network
includes a capacitor C1 which is connected between the reference
point 35 and conductor L2. Capacitor C1 will charge at a first rate
whenever the pilot flame is unlit. However, whenever the pilot
flame bridges the gap 36 between the sensing electrode 31 and the
reference point 35, the resistance of the charging path for
capacitor C1 will be lower and capacitor C1 will charge at a faster
rate.
The gate control network 34 further includes a resistor R2 which is
connected between the reference point 35 and the gate electrode of
the PUT device 32, and a resistor R3 which is connected between the
gate electrode of the PUT device and conductor L2. Resistors R2 and
R3 form a bleeder path for capacitor C1. A second capacitor C1' is
connected redundantly in parallel with the capacitor C1 for safety
purposes.
In addition, a resistor R3 and a transistor 50 are connected
between conductor L2 and the gate electrode of the PUT device 32
forming a clamping circuit to limit the voltage swing at the gate
to a predetermined amount.
The potential at the anode electrode of the PUT device 32 is
determined by the anode control network 33. The anode control
network 33 includes a capacitor C2 connected between the anode
electrode of the PUT device 32 and conductor L2. The anode control
network 33 further includes a voltage divider network comprised of
resistors R5, R6 and R7 which are serially connected between the
conductors L1 and L2 with the junction of the resistors R5 and R6
at point 37 being connected to the anode electrode of the PUT
device 32 and thus to one side of capacitor C2. Accordingly, a
charging path is provided for capacitor C2 from conductor L1 over
resistor R5 and capacitor C2 to conductor L2.
The PUT device 32 is normally non-conducting and is rendered
conductive whenever the potential at the anode electrode exceeds
the potential at the gate electrode by approximately 0.6 volts as
determined by the action of the anode control network 33 and the
gate control network 34.
Whenever the PUT device 32 is rendered conductive, a discharge path
is provided for capacitor C2 over the anode-cathode circuit of the
PUT device 32 which supplies pulses provided by the flame sensing
circuit 30 to a control electrode of a second controlled switching
device 39, embodied as a silicon controlled rectifier, which may be
the type C106A manufactured by General Electric Co.
The normally non-conducting silicon controlled rectifier (SCR) 39
has an anode-cathode circuit connected in series with a coil of
relay 21 between conductors L1 and L2. The control electrode or
gate of the SCR 39 is connected over the resistor 38 to the
conductor L2, a redundant resistor 38' being connected in parallel
with the resistor 38 for safety purposes.
Relay 21 may comprise a DC relay having a coil resistance of
approximately 2.5K ohms so that in the case of a short circuit
condition for the SCR 39, AC current flowing through the coil 21
will generate a high impedance and thereby precluding energization
of the relay.
Alternatively, relay coil 21 may have a low resistance of
approximately 450 ohms and a fuse (not shown) may be connected in
the branch of the circuit including the relay. In such case a short
circuit condition for the SCR device 39 will cause the current
flowing over such branch to change from half wave to full wave,
thereby blowing the fuse and preventing operation of the relay
21.
Relay 21, which is normally de-energized, has normally closed
contacts 21A which are connected in series with the normally-open
THS contacts and conductor L3 between terminal 22 of the ignition
circuit 11 and terminal 40 of the secondary winding 15A of the
input transformer T2. Relay 21 has a pair of normally open contacts
21B which are also connected in series with normally open
thermostat switch contacts THS and a gas valve solenoid 45 which
controls the flow of gas to a main gas burner (not shown). A pilot
valve 47 is connected in parallel with the contacts 21B and the
valve 45 to cause gas to flow from the pilot source 14 when the
contacts THS close.
Thus, whenever the flame detection circuit 10 is energized, relay
21 will be de-energized in the absence of a pilot flame, permitting
current flow over the normally closed contacts 21A from the input
transformer T2 to the ignition circuit 11 to effect the generation
of ignition sparks between the ignition electrodes 12 and 13.
When the pilot gas is lit, the SCR 39 will be rendered conductive
by the pulse output of the flame sensing circuit 30 effecting
energization of relay 21. As relay 21 operates, contacts 21A will
be opened thereby terminating the generation of ignition sparks at
the pilot ignitor 11, and contacts 21B will be closed energizing
the main burner gas valve solenoid.
OPERATION OF THE FLAME DETECTION CIRCUIT
For purposes of illustration of operation of the pilot ignition and
flame detection circuit 10, it is assumed that the circuit 10 is
initially unenergized and that the SCR 39 and the PUT device 32 are
cut-off and relay 21 is de-energized. When power is applied to the
primary winding 15 of the input transformer T2, 24 volts AC will be
produced across conductors L3 and L4, if the THS contacts are
closed causing AC current to flow through the ignition circuit 11
and the pilot valve 47 to cause gas to flow from the pilot source
14. Accordingly, voltage pulses will be induced in the output
winding 25 of the ignition circuit 11, as was previously described,
and applied to the ignition electrodes 12 and 13, generating sparks
for igniting the gas emanating from the pilot source 14. When the
pilot gas has been ignited, a pilot flame will bridge the gap 36
between the sensing electrode 31 and the reference point 35.
The proper phase relationship between the pilot ignition circuit 11
and flame sensing circuit 30 is obtained by connecting transformer
T3 windings such that terminals 19 and 24 are both positive at the
same time. This phases the circuit 10 so that the spark is at the
ignition electrodes 12 and 13 when the potential at conductor 22
and conductor L1 are both negative and therefore the flame sensing
circuit 30 is not sensing.
Accordingly, during a first half cycle of the AC line voltage
applied between conductors L1 and L2 when conductor L1 swings
positive relative to conductor L2, current will flow from conductor
L1 through resistor R1 sensing electrode 31 and the pilot flame to
the reference point 35, and over capacitor C1 to conductor L2,
permitting capacitor C1 to charge. The voltage across capacitor C1,
which is connected over resistor R2 to the gate electrode of the
PUT device 32, establishes a gate potential for the PUT device
32.
During the same half cycle, capacitor C2 is charged over a path
extending from conductor L1 over resistor R5 and capacitor C2 to
conductor L2, establishing a potential at the anode of the PUT
device 32.
The values of capacitors C1 and C2 are selected such that some time
before the peak of the AC line voltage during the first half cycle
of the AC line signal the anode to gate potential of the PUT device
32 will exceed +0.6 volts so that the PUT device will conduct,
permitting capacitor C2 to discharge. Also capacitor C2 will have
charged to a voltage sufficient to effect the generation of a
voltage pulse across the redundant resistors 38 capable of
rendering the SCR 39 conductive. The speed of response of the flame
sensing circuit 30 is a function of the value of capacitor C1 and
resistors R2 and R3 which form the bleeder path for capacitor
C1.
When the SCR 39 is rendered conductive, an energizing path is
completed from conductors L1 and L2 for relay 21 which then
operates opening contacts 21a to inhibit further sparking between
the ignition electrodes 12 and 13 of the ignition circuit 11, and
closing contacts 21b to energize the main gas valve solenoid
45.
Accordingly, once the pilot flame has been established and bridges
the gap 36 between the sensing electrode 31 and the reference point
35, the action of the flame sensing circuit 30 will be effective to
provide enabling pulses to the gate of the SCR 39 during alternate
half cycles of the applied AC line signal. During the next half
cycle of the AC line signal, when conductor L2 swings positive
relative to conductor L1, the SCR device 39 will be cut off.
However, relay 21, once energized, will be maintained energized by
capacitor C3 during the portion of the half cycle of the line
voltage in which the SCR 39 is non-conductive. The above conditions
will occur every cycle when a pilot flame is present at the sensing
electrode 31.
It should be understood that the only time pulses will be passed to
the PUT device 32 and the gate of the SCR 39 is when the voltage at
the anode of the PUT device 32 exceeds that of the gate by plus 0.6
volts and the SCR 39 will be enabled only when the capacitor C2 has
charged sufficiently to provide the pulse energy required to render
the SCR 39 conductive.
For the condition when the pilot flame is extinguished, a high
impedance path will be provided over the sensing electrode 31 to
reference point 35 such that capacitor C1 will have a longer
charging time. Accordingly, as capacitors C1 and C2 are charged,
the voltage at the gate of the PUT device 32 will be lower relative
to the voltage at the anode since capacitor C2 will be charged at a
faster rate over the voltage divider path provided by resistors R5,
R6 and R7. Consequently, the anode voltage will exceed the gate
voltage early in the half cycle of the AC line signal before
capacitor C2 is fully charged. Charge on capacitor C2 is limited by
voltage on capacitor C1 which is very low when pilot flame is
extinguished and less than that required to trigger the SCR 39 into
conduction. Accordingly, whenever the pilot flame is not present,
pulses provided by the flame sensing circuit 30 will be ineffective
to enable SCR 39 to cause relay 21 to be energized.
When the pilot gas is ignited and relay 21 is operated, the gas
valve solenoid 45 will be energized permitting gas to flow to the
main gas burner for ignition by the pilot flame. When the main
burner flame is lit, a current path is provided through the pilot
flame and the main burner flame to the ground reference point 35.
Consequently, the resistance between sensing electrode 31 and
reference point 35 will decrease effecting a further increase in
the charging current for capacitor C1.
When the main gas burner is lit, the clamping circuit, including
the transistor 50 and resistor R3 limits the amplitude of the
voltage provided at the gate electrode of the PUT device 32 to a
desired operating range which may, for example, be 1 to 4 volts.
Accordingly, with the gate electrode being clamped at a
predetermined voltage level, the potential at the anode electrode
as provided by the charging of capacitor C2 will be capable of
exceeding the gate potential by an amount sufficient to trigger the
PUT device 32 into conduction and provide pulses for maintaining
the relay 21 energized. If desired, the clamping circuit may
comprise alternatively a Zener diode in series with a
resistance.
The electronic pilot ignition and flame detection circuit 10 is
also characterized by a fail safe feature by maintaining the proper
magnitude and phase relationship between the voltages that are
applied to the gate and the anode of the PUT device 32 in the
normal operating mode. The normal operating voltage range is 1 to 4
volts for voltage levels at the anode or gate electrodes of the PUT
device 32. For values above this, as may be caused by a component
failure, for example, the anode voltage will not exceed the gate
voltage and accordingly the PUT device 32 will not conduct. On the
other hand, for voltage values below the operating range, the anode
voltage will exceed the gate voltage before the charge on capacitor
C2 is sufficient to pulse the gate of the SCR 39.
Thus the electronic pilot ignitor and flame sensing circuit 10 may
be considered as a pulsing system wherein the flame sensing circuit
30 is a pulse generator that stops generating pulses for any
component failure or flame-out condition.
The pilot flame which bridges the gap 36 between the sensing
electrode 31 and the reference point 35 serves as both a resistance
and a rectifier, and the flame sensing circuit 30 utilizes the
rectification properties of the flame to maintain the charge on
capacitor C1 within a desired operating range. Therefore, any value
of resistance between the sensing electrode 31 to the reference
point 35 will not result in a condition where the main gas valve is
energized when pilot flame is not present. Also, the rectification
property of the flame enables the flame sensing circuit to detect
the difference between a flame and leakage resistance between the
sensing electrode 31 and the reference point 35.
In one exemplary embodiment, the components of the electronic flame
sensing circuit 30 may have the values listed in Table I.
______________________________________ Resistors 38 and 38'
2-220.DELTA.ohms in parallel Resistor R1 470K ohms Resistor R2 2.2
Megohms Resistor R3 4.7 Megohms Resistor R7 5.6K ohms Resistor R5
270K ohms Resistor R6 18K ohms Capacitors C1 and C1' 2-0.047
Microfarad in parallel Capacitor C2 0.47 microfarad Capacitor C3 22
microfarads Transistor 50 2N3394
______________________________________
It is to be understood that the operating voltage range which
establishes the relationship between the gate and anode voltages,
both phase and magnitude, for the PUT device 32 is a matter of
choice and that there are a large number of combinations of values
for the resistors R5 and R6, capacitors C1 and C2, resistor R3 and
the transistor 50 that can be adjusted to provide satisfactory
operation of the circuit.
With reference to FIG. 2, the electronic pilot ignition and flame
detection circuit 10B is similar in structure and operation to that
of the circuit of FIG. 1 with the exception that the solenoid coil
21 of the main burner valve 45 is substituted in the anode circuit
of SCR 39 in place of the relay 21 of FIG. 1. In addition, a
different pilot relighter circuit is utilized which is
self-extinguishing and which is designated as 11A. This pilot
relighter circuit is more fully described and claimed in copending
application of the same assignee and of which Matthews is
co-inventor, filed Nov. 16, 1972, as Ser. No. 307,077, now U.S.
Pat. No. 3,806,305.
When thermostat contacts THS are closed to energize the pilot valve
to provide gas for ignition, igniter 11A provides a spark across
electrodes 12 and 13 to ignite the pilot gas. As is described in
the previously aforementioned, U.S. Pat. No. 3,806,305 a spark is
produced once every cycle of applied voltage when the SCR 15
conducts discharging capacitor C11 through the primary 25 of
ignition transformer T1. The ignitor circuit is self-extinguishing.
When its flame sensing portion detects flame, current flows across
electrode 13 to pilot 14 and thence to ground, as is described in
the copending application. This shorts the gate to cathode
electrodes of SCR 15 of the ignitor circuit causing pulsing of the
ignition transformer T1 to cease. Should the pilot flame be
extinguished, the ignitor circuit automatically reapplies a pulse
across electrodes 12, 13.
When SCR 39 conducts through its anode-cathode circuit as was
previously described for FIG. 1, main valve 45 is energized
directly through its solenoid coil 21 in the anode circuit of SCR
39.
With reference to FIG. 3, the electronic pilot ignition flame
detection circuit 10C shown is energized from a 12-volt D.C. source
(not shown) instead of the 120-volt A.C. source (not shown) of the
previous FIGS. 1 and 2. The circuit 10C, however, operates
substantially the same as that described for FIG. 2 where the
solenoid of the main valve 45 is actuated directly by SCR 39. The
circuit operation differs in the following respects: the pilot
valve 110 instead of being operated directly through thermostat
contacts THS as described for FIG. 2, operates through normally
closed contacts 112 of a warp switch connected in series with the
ignitor circuit 11B. As the ignitor circuit attempts to ignite the
pilot gas as was previously described for FIG. 2, current flows
through warp switch heater 111 heating the warp switch, which is of
a conventional type. Should the ignitor circuit continue to draw
current for more than a predetermined selected time, the warp
switch heater 111 is heated sufficiently to energize and open its
normally closed contacts 112, terminating energization of pilot
valve 110 and the ignitor 11B. Pilot valve 110 then closes,
stopping the flow of gas to the pilot burner. In this case, the
ignitor and pilot valve are in "lockout" position and must be
manually reset. This is done by manual actuation of the warp switch
into its normal position in which contacts 112 are reclosed,
placing ignitor 11B and pilot valve 110 subject again to
energization through thermostat contacts THS.
The circuit shown in FIG. 3 is powered from a 12 volt D.C. source
and utilizes a conventional inverter 123 to change the 12 volt D.C.
to 110 volt A.C. at secondary 16B to supply power to the flame
detection circuit 30 previously described.
Considering now the operation of the circuit shown in FIG. 3, a 12
volt D.C. signal applied to terminals 118 and 119 causes current to
flow through warp switch contacts 112, warp switch heater 111 to
spark generator 11B and pilot valve 110 to ignite the pilot burner.
Current also flows to inverter circuit 123 to generate 110 volt
A.C. at secondary 16B to supply power to flame detector circuit
30.
The presence of pilot flame on electrode 31 causes SCR 39 to
conduct as previously described, thereby causing the energization
of the valve coil 21 on main gas solenoid and causing main burner
gas to flow and to become ignited by the pilot. Coil 107, also
located on the main gas valve solenoid, acts like the secondary
winding of a transformer, coil 21 being the primary, and supplies a
voltage to disable the spark generator 11B whenever the main gas
valve is energized.
Considering the operation of the 12 volt D.C. spark generator, the
high voltage transformer T1 has three windings, high voltage
secondary 101, primary winding 100 and feedback winding 102.
Current flows into emitter of transistor 109 to collector 121 and a
voltage divider consisting of resistors 103 and 104. The voltage at
the junction of resistors 103 and 104 is sufficient to cause
current to flow through feedback winding 102, diode 113, base 116
to emitter 115 of transistor 117 to 12 volt terminal 119. As a
result, transistor 117 conducts and allows current to flow from 12
volt terminal 118 through contacts 112 and heater 111 through high
voltage primary winding 100 to induce a high voltage in the
secondary winding 101 to produce sparks for ignition at the
electrodes.
As the current in the primary winding increases, feedback winding
102, located on the same magnetic core as primary winding 100,
causes transistor 117 to conduct further until it becomes
saturated. At that time, the voltage induced into the feedback
winding begins to decrease, thereby decreasing the conductivity of
transistor 117 and the current in primary winding 100 decreasing to
induce a negative voltage in the feedback winding 102 to cut off
the conduction of transistor 117 to complete one cycle of
oscillation. The frequency of oscillation utilized is approximately
1000 cycles per second.
After the spark ignites the pilot, sensing probe 31 causes SCR 39
to conduct to energize main gas valve solenoid coil 21 to turn on
the main gas.
Energizing coil 21 induces a voltage in secondary winding 107 to
increase the voltage on base 122 of transistor 109 sufficiently to
cut it off. This removes the bias on SCR 117 and disables the spark
generator. If the pilot becomes extinguished for any reason, coil
21 becomes de-energized removing the voltage generated by coil 107
causing transistor 109 to conduct to cause the spark generator to
produce sparks for re-ignition.
If for any reason the pilot gas did not ignite, warp switch heater
actuates manual resettable contacts 112 to the open condition in
approximately two minutes to de-energize the system. Warp switch
heater 111 normally does not actuate because one amp of current
drawn by the spark circuit is eliminated during a normal operation.
So, at start-up, warp switch heater current is approximately 1.3
amps and warp switch heater current during a normal cycle is
approximately 0.3 amp.
With reference to FIG. 4, the operation of the circuit shown is the
same as for the circuit of FIG. 3 except main valve coil 21 has
been replaced by a relay coil 21 with contacts 125 and 124 for
controlling main valve operation and ignitor circuit 11C.
Contacts 124 energize the spark generating ignitor circuit 11C
which generates sparks in a manner previously described with
respect to FIG. 3, except that when pilot gas is ignited, probe 31
causes SCR 39 to conduct to energize relay 21 to open contacts 124
to disable the ignitor circuit 11C and closes contacts 125 to
energize the main gas valve 126.
If the pilot flame is extinguished for any reason, probe 31 causes
relay coil 21 to de-energize to drop out the gas valve to shut off
the main burner gas and to activate the spark generator 11C. If
after approximately two minutes the spark generator has not turned
off, the warp switch 111 opens its contacts 112 to de-energize the
system, as previously described for FIG. 3.
* * * * *