U.S. patent number 3,732,433 [Application Number 05/256,981] was granted by the patent office on 1973-05-08 for combustion control circuit for a fuel burner.
This patent grant is currently assigned to Webster Electric Company, Inc.. Invention is credited to Ronald F. Lourigan.
United States Patent |
3,732,433 |
Lourigan |
May 8, 1973 |
COMBUSTION CONTROL CIRCUIT FOR A FUEL BURNER
Abstract
A combustion control circuit for an oil burner installation
includes a triac for controlling the energization of both a burner
motor and an ignition circuit. A control circuit carries out a safe
start component check and then operates the triac when a thermostat
closes and when no flame is detected at the burner. If ignition
fails to take place in a trial-for-ignition period of predetermined
time, a thermal switch is heated by a heater in the control circuit
and opens to shut off the burner motor. However, if ignition takes
place, flame is detected within the predetermined period by an
improved, positively acting, reliable resistance bridge flame
detector circuit. The flame detector circuit serves to render
nonconductive a silicon controlled rectifier in the control circuit
in order to disconnect the thermal switch heater while maintaining
operation of the burner motor. Simultaneously, the ignition circuit
is deenergized by a novel ignition disabling circuit connected
between the silicon controlled rectifier and the ignition circuit.
Failure of the triac in a short circuit condition prevents an
unsafe condition due to connection of the thermal switch to the
motor and due to the use of a second heater in circuit with the
ignition circuit.
Inventors: |
Lourigan; Ronald F. (Kenosha,
WI) |
Assignee: |
Webster Electric Company, Inc.
(Racine, WI)
|
Family
ID: |
22974398 |
Appl.
No.: |
05/256,981 |
Filed: |
May 25, 1972 |
Current U.S.
Class: |
307/117; 431/69;
431/74 |
Current CPC
Class: |
F23N
5/203 (20130101); F23N 5/082 (20130101); F23N
5/08 (20130101); F23N 2227/16 (20200101); F23N
2231/12 (20200101); F23N 2231/10 (20200101); F23N
2229/00 (20200101); F23N 2227/36 (20200101); F23N
2223/16 (20200101); F23N 2239/06 (20200101) |
Current International
Class: |
F23N
5/20 (20060101); F23N 5/08 (20060101); H01h
035/00 () |
Field of
Search: |
;307/116,117,118,119,112
;431/24,25,26,66,71,74,69,27 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Schaefer; Robert K.
Assistant Examiner: Ginsburg; M.
Claims
What is claimed and desired to be protected by Letters Patent of
the United States is:
1. A combustion control circuit for controlling the operation of a
fuel burner having a burner motor and an ignition device and
comprising:
a triac in circuit with both the burner motor and the ignition
device;
a thermostat;
a control circuit for placing the triac in a conductive condition
to energize both the burner motor and the ignition device in
response to operation of the thermostat;
a flame detector coupled to the control circuit for providing a
flame indication in response to operation of the burner;
means coupled to said control circuit for disabling the ignition
device in response to a flame indication;
a thermal delay switch for disabling the burner motor in response
to being heated to a given extent;
a first heater connected in said control circuit for heating said
thermal delay switch during the time period beginning with
operation of the thermostat and ending with production of a flame
indication;
and a second heater connected in circuit with the ignition device
for heating said thermal delay switch during operation of the
ignition device.
2. The combustion control circuit of claim 1, said thermal delay
switch being connected in series with said triac.
3. A combustion control circuit for a fuel burner used to heat a
space and including a burner motor and an ignition device, said
combustion control circuit comprising:
a switching means coupled to both the burner motor and the ignition
device and operable to energize the burner motor and ignition
device simultaneously;
a flame detector disposed adjacent the fuel burner to provide a
flame indication;
a thermostatically controlled switch in the heated space movable to
an operating position in response to a demand for heat;
a first control means coupled to said switching means and to said
thermostatically controlled switch for operating said switching
means in response to movement of said thermostatically controlled
switch to the operating position;
a second control means coupled to said ignition device and to said
flame detector for discontinuing operation of said ignition device
in response to a flame indication;
a thermal delay switch connected to the burner motor for preventing
operation of the burner motor in response to being heated to a
predetermined extent;
a first heater adjacent said thermal delay switch and connected to
said first and second control means for energization during
operation of said first control means prior to operation of said
second control means; and
a second heater adjacent said thermal delay switch and connected to
said ignition device for energization during operation of said
ignition device.
4. The combustion control circuit of claim 3, said switching means
comprising a triac.
5. The combustion control circuit of claim 4, said thermal delay
switch being connected in series with said triac.
6. The combustion control circuit of claim 5, said first control
means including relay means having a winding means and having a
contact means connected in controlling relation to said triac, and
an SCR having output electrodes and a gate electrode, said
thermostatically controlled switch being coupled to said output
electrodes, and said flame detector being connected to said gate
electrode.
7. The combustion control circuit of claim 6, said flame detector
comprising a light sensitive variable resistor, a bridge circuit
including said detector in one leg, means for driving said bridge
with a regulated driving signal, and a capacitor coupled to the
bridge output and to the gate electrode of the SCR.
8. The combustion control circuit of claim 5, said second control
means comprising a coupling transformer connected between said
flame detector and said ignition device.
9. The combustion control circuit of claim 5, said second control
means comprising a relay connected between said flame detector and
said ignition device.
10. The combustion control circuit of claim 5, said second control
means comprising a lamp and light sensitive resistor connected
between said flame detector and said ignition device.
11. A combustion control circuit for a fuel burner comprising:
control means for operating the burner in response to energization
of the control means;
a controlled conduction device including electrodes forming an
output electrode pair and a control electrode pair;
said output electrode pair being coupled to said control means for
controlling the energization thereof;
a variable resistance flame detection device disposed in flame
sensing relation to the burner;
a resistance bridge having opposed input and output terminals and
having one leg including said flame detection device and having
three legs including only fixed resistance; and
driving signal means coupled between a source of operating
potential and said bridge input terminals;
said bridge output terminals being coupled to said control
electrode pair of said controlled conduction device.
12. The combustion control circuit of claim 11, said controlled
conductive device comprising an SCR.
13. The combustion control circuit of claim 11, said detection
device comprising a photoelectric cell.
14. The combustion control circuit of claim 11, said driving signal
means including a zener diode.
15. The combustion control circuit of claim 11, said flame
detection device having dark and light resistance conditions, said
fixed resistances being chosen so that the bridge is unbalanced in
response to the absence of flame and oppositely unbalanced in
response to the presence of flame.
16. A combustion control circuit for controlling a fuel burner
installation including a burner motor and an ignition circuit of
the type including an ignition circuit gated device periodically
rendered conductive by a gating signal to create a burner ignition
condition, said combustion control circuit including:
motor start means and motor hold means coupled to the burner motor
for sequentially initiating and then maintaining operation of the
burner motor;
a normally conductive controlled conduction device including
electrodes forming an output electrode pair and a control electrode
pair;
said output electrode pair being coupled to said motor start means
for controlling the energization thereof;
a variable resistance flame detection device disposed in flame
sensing relation to the burner;
a resistance bridge having opposed input and output terminals and
having one leg including said flame detection device; and
driving signal means coupled between a source of operating
potential and said bridge input terminals;
said bridge output terminals being coupled to said control
electrode pair of said controlled conduction device for rendering
said controlled conduction device nonconductive in response to
detection of flame to disable said motor start means;
and an ignition disabling circuit coupled between said output
electrode pair and the ignition circuit gated device for disabling
the gated device in response to detection of flame.
17. A combustion control circuit for a fuel burner installation of
the type including a burner motor and an ignition circuit having an
active element, said combustion control circuit comprising in
combination:
a triac having a gate electrode and having output electrodes
connected in current supplying relation between a power source and
both said motor and said ignition circuit;
a thermal switch connected in current supplying relation with the
motor;
a flame sensitive variable resistance device adjacent the
burner;
a bridge circuit including said device in one leg;
means for driving said bridge with a regulated voltage;
a capacitor connected to said bridge for storing a potential
corresponding to the balance condition of the bridge;
a controlled conduction device having a control electrode coupled
to said capacitor for conduction in response to the absence of
flame and for nonconduction in response to the absence of
flame;
a relay having pull-in winding means and hold winding means and
having a normally open set of contacts coupled to the gate
electrode of said triac;
means including a thermostat for energizing said hold winding means
and said controlled conduction device in response to a demand for
heat;
said pull-in winding means being connected for energization by said
controlling conduction device;
an ignition disabling means coupled between said controlled
conduction device and the ignition circuit active element for
disabling the active element in response to nonconduction of said
controlled conduction device;
a first heater for said thermal switch connected in circuit with
said pull-in winding means; and
a second heater for said thermal switch connected in circuit with
said ignition circuit.
18. The circuit of claim 17, said thermal switch being in series
with said triac.
19. The circuit of claim 17, said controlled conduction device
comprising an SCR.
20. The circuit of claim 17, said thermostat being in series with
said triac.
21. The circuit of claim 17, said thermostat being connected in
current supplying relation to said pull-in winding means and said
hold winding means.
22. The circuit of claim 17, said disabling means comprising a
transformer.
23. The circuit of claim 17, said disabling means comprising a
relay.
24. The circuit of claim 17, said disabling means comprising a lamp
and a light sensitive resistance.
25. A combustion control circuit for a fuel burner and for an
ignition circuit including an ignition circuit SCR and including a
gating circuit for periodically applying a gating signal to the
cathode and gate electrodes of the ignition circuit SCR for
producing an ignition condition, said combustion control circuit
comprising:
control means for operating the burner in response to energization
of the control means;
a solid state controlled conduction device having a control
electrode and having output electrodes coupled to said control
means for controlling the energization thereof;
a flame detection circuit coupled to the control electrode of said
solid state controlled conductive device for rendering said device
nonconductive in response to detection of burner flame; and
an ignition disabling circuit coupled between said solid state
controlled conduction device and said ignition circuit;
said disabling circuit including a transformer having one winding
connected across the output electrodes of the solid state
controlled conduction device and having another winding connected
across the cathode and gate electrodes of the ignition circuit SCR
to reverse bias the ignition circuit SCR in response to
nonconduction of said solid state controlled conductive device. A
combustion control circuit for a fuel burner and for an ignition
circuit including an ignition circuit SCR and including a gating
circuit for periodically applying a gating signal to the cathode
and gate electrodes of the ignition circuit SCR for producing an
ignition condition, said combustion control circuit comprising;
control means for operating the burner in response to energization
of the control means;
a solid state controlled conduction device having output electrodes
coupled to said control means for controlling the energization
thereof;
a flame detection circuit coupled to the control electrode of said
solid state controlled conduction device for rendering said device
nonconductive in response to detection of burner flame; and
an ignition disabling circuit between said solid state controlled
conduction device and said ignition circuit;
said disabling circuit including a lamp connected across the output
electrodes of the solid state controlled conduction device and a
light sensitive resistance connected across the cathode and gate
electrodes of the ignition circuit SCR to prevent gating of the
ignition circuit SCR in response to nonconduction of said solid
state controlled conduction device.
Description
The present invention relates to an improved combustion control
circuit for reliably and safely controlling the operation of a fuel
burner.
A conventional fuel oil burner heating installation of a type
widely used in the past includes an ignition device such as a
transformer or other ignitor for igniting fuel at the burner,
together with a primary control using two or more relays for
operating the burner in response to a thermostat. Upon closing of
the thermostat in normal operation, the burner motor and the
ignition device are energized during a trial-for-ignition period.
If burner flame is detected, the burner motor is continued in
operation and, if the installation is of the intermittent ignition
type, the ignition device is deenergized. On the other hand, if
flame is not detected during the initial period, a thermal delay
safety switch shuts down the installation.
The complexity and expense of conventional control systems has made
it desirable to provide a safe, economical and reliable combustion
control circuit making use of inexpensive solid state components in
place of expensive relays, large transformer windings, and the
like. Devices of this general class have been proposed, and one
example may be found in U. S. Pat. No. 3,624,407 -- Bauer. However,
solid state devices developed to date have suffered from several
disadvantages such as lack of complete safety in operation,
unreliability under some circumstances, the need for expensive
components, and excessive cost and/or complexity.
Objects of the present invention are to provide an improved
combustion control circuit for controlling the operation of a fuel
burner installation including a burner motor and an ignition
circuit; to provide an improved combustion control circuit having
desirable safety features including a novel arrangement for
protecting against component failure; to provide an improved
combustion control circuit in which the necessity for expensive
components is reduced; to provide an improved, highly reliable
flame detection circuit for use in a combustion control circuit;
and to provide a combustion control circuit having an improved
arrangement for discontinuing the operation of the ignition circuit
when burner ignition is accomplished.
In brief, the above and other objects and advantages of the
invention are realized by the provision of an improved combustion
control circuit for an oil burner installation of the type
including a burner motor and an ignition circuit. A triac switching
device is connected for simultaneously energizing both the burner
motor and the ignition circuit. A thermostat is located in the area
to be heated by the oil burner installation, and a light sensitive
variable resistance flame detector is disposed adjacent the fuel
burner. A control circuit operates to place the triac in a
conductive condition to energize both the burner motor and the
ignition device when the thermostat closes upon demand for heat.
When ignition is accomplished, the control circuit operates in
response to the flame detector for discontinuing operation of the
ignition circuit. On the other hand, if ignition is not
accomplished during a predetermined period of time, a thermal delay
switch connected to the burner motor prevents further operation of
the burner motor upon being heated to a predetermined extent. A
first heater in the control circuit operates from the time that the
thermostat closes until such time as a flame is detected. A second
heater for the thermal switch protects against failure of the triac
and heats the thermal switch at all times that the ignition circuit
is in operation.
In accordance with another feature of the invention, there is
provided a novel flame detection circuit comprising a resistance
bridge provided with a regulated operating potential and including
a light sensitive variable resistance flame detector in one leg
thereof. In the absence of flame the resistance bridge is
maintained in an unbalanced condition. Upon the detection of flame,
the resistance bridge is moved through the null point to an
oppositely unbalanced condition, and the control circuit is
operated positively and effectively.
An aspect of the invention resides in a novel circuit for disabling
the ignition circuit in response to the detection of flame. In one
embodiment, a transformer coupled between a switching device in the
control circuit and a controlled conduction device in the ignition
circuit enables or prevents operation of the ignition circuit in
response to presence or absence of flame. In other embodiments, a
relay and a light and light sensitive device are used.
The invention and its objects and advantages may be best understood
from consideration of the following detailed description of
embodiments of the invention shown in the accompanying drawing,
wherein:
FIG. 1 is a schematic and diagrammatic illustration of a combustion
control circuit embodying the principles of the present invention;
and
FIGS. 2 and 3 are fragmentary schematic illustrations of
alternative embodiments of the invention.
With reference now to FIG. 1 of the drawing, there is illustrated a
combustion control circuit designated generally by the reference
numeral 10 and constructed in accordance with the principles of the
present invention. The circuit 10 controls the operation of a
typical oil burner installation including a burner 12, a burner
motor 14 and an ignition circuit generally designated by the
reference numeral 16. In normal operation, the burner motor 14 and
the ignition circuit 16 are operated upon a demand for heat by a
burner circuit generally designated as 18 under the control of a
control circuit generally designated as 20. Demand for heat is
signalled in conventional manner by a line voltage thermostat
switch 22 or by a low voltage thermostat switch 24 located to sense
temperature in a region heated by operation of the burner 12. In
any given installation only one of the switches 22 or 24 is used,
the other being replaced by a permanent closed circuit.
Upon closing of the controlling thermostat switch 22 or 24, fuel at
the burner 12 is ignited by the ignition circuit 16 and flame is
detected by a flame detection circuit generally designated as 26.
Upon detection of flame, the control circuit 20 serves to continue
operation of the burner motor 14 while an ignition disabling
circuit generally designated as 28 discontinues further operation
of the ignition circuit 16. Should ignition fail to take place
within a predetermined time, a thermal delay device designated as a
whole by the reference numeral 30 discontinues operation of the
fuel burner installation. In accordance with a feature of the
invention, the device 30 also discontinues operation of the
installation in the event of certain undesirable malfunctions of
components in the circuit 10.
Referring now in more detail to the construction and operation of
the combustion control circuit 10, the circuit includes a pair of
power supply terminals 32 and 34 adapted to be connected to a
standard nominal 115 volt, 60 hertz power supply. The control
circuit 20 and the flame detection circuit 26 are supplied with
operating potential by means of a pair of secondary windings 36 and
38 of a transformer 40 having a primary winding 42 adapted to be
coupled to the power supply terminals 32 and 34. In addition to the
line voltage thermostat switch 22 (when used), energization of the
burner circuit 18 and the control circuit 20 can be interrupted by
a conventional over-temperature limit switch and fan switch 44 and
by operation of the thermal switch structure 30.
In operation of the circuit 10, control of the energization of the
burner circuit 18 is provided by a gated, bidirectional solid state
controlled conduction device 46 in the form of a triac. The output
electrodes of triac 46 are in current supplying relation with both
the burner motor 14 and with the ignition circuit 16. Thus triac 46
must be rendered conductive by coupling of its gate electrode to
one of its output electrodes in order to bring about operation of
the burner circuit 18.
In FIG. 1 the combustion control circuit 10 is illustrated in its
normal, temperature satisfied condition wherein the thermostat
switch 22 or 24 is open and wherein the triac 46 is in its high
resistance or open circuit condition so that neither the burner
motor 14 nor the ignition circuit 16 is energized. When a demand
for heat is sensed by closing of the controlling thermostat switch
22 or 24, a heating cycle is initiated under the control of the
control circuit 20. Upon initiation of this heating cycle, a safe
start component check takes place, and assuming the circuit
components to be operating properly, the control circuit 20 serves
to render the triac 46 conductive thereby simultaneously energizing
the burner motor 14 and the ignition circuit 16 throughout a
trial-for-ignition period having a duration determined by the
operation of the thermal device 30.
In order to operate the triac 46, control circuit 20 includes a
pair of relay windings 48 and 50, which windings control a normally
open set of relay contacts 52 connected in series with a resistor
54 and with the gate electrode of the triac 46. Windings 48 and 50
are arranged in known manner relative to contacts 52 through
adjustment of variable resistances 49 and 51 respectively so that
energization of both windings is necessary to move or pull in the
set of contacts from the open to the closed position, while
energization of only the winding 50 suffices to hold the contacts
in the closed position. Energization of the windings 48 and 50 is
controlled by operation of switch 22 or 24, while energization of
winding 48 is further controlled by a gated solid state controlled
conduction device 56 in the form of an SCR having its gate
electrode controlled by the flame detection circuit 26.
With reference now to the flame detection circuit 26, the function
of this circuit is to control the condition of the SCR 56 in
accordance with the presence or absence of flame at the burner 12.
In accordance with important features of the invention, the circuit
26 carries out a stable and reliable control operation despite such
unfavorable factors as supply voltage fluctuations, intermittent or
unstable ignition, intermittent extraneous light, or the like.
More specifically, circuit 26 includes a zener diode 58 serving
together with a resistor 60 to supply a voltage regulated driving
signal to a resistance bridge 62. Due to the inclusion of diode 58,
the bridge 62 operates substantially independently of supply
voltage variations. Bridge 62 includes a pair of input terminals 64
and 66 connected across the zener diode 58 and a pair of output
terminals 68 and 70 connected to the gate and cathode electrodes of
the SCR 56 and shunted by a capacitor 72.
One leg of bridge 62 includes a variable resistance photoelectric
device 74 disposed adjacent the burner 12. In the absence of flame
the device 74 is in a high resistance condition, and when
illuminated by the presence of flame the device 74 assumes a low
resistance condition. The remaining three legs of bridge 62 include
resistances 76, 78 and 80 having values such that the bridge 62 is
unbalanced in opposite directions when the device 74 is
alternatively in its high and low resistance conditions.
The novel flame detection circuit 26 has important advantages in
operation because it reliably and positively controls the condition
of the SCR 56. When no flame is present at burner 12, device 74 is
in its high resistance condition and as a result the bridge 62 is
unbalanced and bridge terminal 70 is maintained at a positive
potential relative to bridge terminal 68. This potential appears
across capacitor 72 in such a way as to maintain the gate electrode
of the SCR 56 positive relative to its cathode. Accordingly, the
SCR is maintained in condition for conduction of current through
its output circuit. When flame is detected the resistance of device
74 drops to a low level and the bridge 62 moves through its null
point or balance condition to an oppositely unbalanced condition
wherein terminal 70 is negative relative to terminal 68, thus
biasing the gate electrode of the SCR 56 negative relative to its
cathode to assure that SCR 56 ceases conduction and remains
nonconducting.
Capacitor 72 provides a smoothing or filtering effect which
prevents unwanted response of the flame detection circuit to
various possible transient effects. Moreover, since the incident
light level at device 74 must change substantially enough to change
the condition of the bridge from one unbalanced condition to the
oppositely unbalanced condition, positive and reliable operation is
achieved. Furthermore, stability of operation is aided by the use
of the zener diode 58 which provides a regulated driving voltage
for the bridge 62 substantially independent of variations in the
supply voltage.
As indicated above, upon a demand for heat the operation of the
combustion control circuit 10 commences with a safe start component
check. When switch 22 or 24 closes, a circuit is completed for
energization of the control circuit 20 from the transformer 40. At
this time the triac 46 is rendered conductive only if the
components of the flame detection circuit 26 and control circuit 20
are operational. If the photoelectric device 74 provides a false
indication of flame, the SCR 56 is nonconductive and winding 48
cannot be energized to close normally open relay contacts 52.
Similarly if SCR 56 is inoperative and cannot conduct current,
winding 48 cannot be energized. The same result follows if because
of any other fault the control circuit 20 or flame detector circuit
26 fails to operate.
If all components are operational, closing of the thermostat switch
causes both windings 48 and 50 to be energized. More specifically,
winding 50 is energized at a continuous DC level by a diode 82 and
capacitor 84 connected across secondary winding 38 in parallel with
a resistor 86. Winding 48 is energized simultaneously by current
flowing through the conductive SCR 56 and through a resistor 87
associated with the thermal delay circuit 30.
Simultaneous energization of windings 48 and 50 causes contacts 52
to move to their closed condition thereby gating the triac 46 to
its conductive condition. Voltage fluctuations due to the inductive
nature of the load and switching transients and the like are
shunted by a circuit including a capacitor 88 and resistor 90
providing reliable turn-off of the triac 46. A further aspect of
the safe start component check is that the triac 46 cannot be
rendered conductive following a failure of the relay which prevents
contacts 52 from closing, or if the triac itself fails in the open
circuit condition.
Operation of both the burner motor 14 and the ignition circuit 16
takes place when the triac 46 is rendered conductive. When the
burner motor operates, a combustible mixture of fuel and air is
emitted at the burner, and is ignited by the ignition circuit 16 to
produce a stable burner flame.
Referring more specifically to the structure of the ignition
circuit 16, this circuit may take any desired form, and as
illustrated includes a pair of spark electrodes 92 and 94 located
in ignition relation to the combustible air - fuel mixture at the
burner 12. Although other types could be used, the ignition circuit
16 is similar to the ignition circuit illustrated and described in
detail in U. S. Pat. No. 3,556,706 -- Campbell, to which reference
may be had for a more complete disclosure. Briefly, the circuit 16
includes a capacitor 96 which charges repetitively during alternate
half-cycles of the power supply waveform through a diode rectifier
98, a resistance 100 associated with the thermal delay circuit 30,
and an inductance 102. Repeatedly during the half-cycle, as the
voltage across the capacitor 96 reaches a predetermined threshold
level, a trigger circuit including a pair of voltage divider
resistors 104 and 106 and a phase shift capacitor 108 applies an
operating voltage to the gate electrode of an ignition circuit SCR
110. At this point the SCR 110 is placed in a conductive condition
and rapidly discharges the capacitor 96 in a pulse or surge through
the primary winding 112 of a transformer 114 having a secondary
winding 116 connected to the spark gap electrodes 92 and 94. As a
result, during alternate half-cycles of the power supply waveform
there are produced adjacent the burner 12 a high frequency series
of discrete, high energy ignition sparks.
Following initial energization of the burner motor 14 and ignition
circuit 16, the combustion control circuit carries out a
trial-for-ignition operation during a predetermined time period the
duration of which is established by operation of the thermal delay
device 30. In normal operation, ignition takes place during this
period. If ignition fails to occur, then continued emission of fuel
from the burner 12 could create an undesirable and potentially
unsafe condition. For this reason, the thermal delay device
functions to shut down the burner motor 14 and the combustion
control circuit 10 at the end of the trail-for-ignition period if
ignition is not detected by the flame detecting circuit 26.
Proceeding now to a description of the thermal delay device 30,
this device includes a normally closed set of switch contacts 118
controlling current flowing via the triac 46 to the burner motor 14
and ignition circuit 16. Contacts 118 are controlled by a
bimetallic switch actuator 120 which opens the contacts 118 when
the actuator is heated to a predetermined extent. Preferably, the
contacts are arranged so that they are latched open and cannot
reclose until released by a service man or operator. Heating of the
actuator 120 is carried out at a relatively high rate of thermal
transfer by the resistor 87 in series with the relay winding 48 and
the control circuit SCR 56. Heating at a slower rate is carried out
by the resistor 100 in series with the triac 46. The heat
transferred to the actuator 120 by the resistor 87 is such that
contacts 118 open in about 30 seconds.
Although the thermal delay device 30 may take various forms, one
device highly suitable for the purpose is disclosed in U. S. Pat.
application Ser. No. 204,491, filed Dec. 3, 1971, of Donald F.
Dalziel and Charles H. Heide. Reference may be had to that
application for a further description of the structure and
operation of the device 30.
Throughout the trial-for-ignition period, the heater resistor 87 is
energized because it is in series with the relay winding 48. If
ignition fails to occur within the predetermined time, contacts 118
open and deenergize the burner motor 14 and the ignition circuit
16. The burner installation remains shut down until such time as
the cause for ignition failure is corrected and the contacts 118
are reclosed.
In normal operation, ignition occurs before opening of the contacts
118 and the control circuit 20 places the combustion control
circuit 10 in an operating condition wherein the burner motor 14 is
continuously energized and the ignition circuit 16 is disabled. The
latter function is carried out by the novel ignition disabling
circuit 28.
More specifically, upon ignition a flame at the burner 12 is
detected by the photoelectric flame detecting device 74. In the
manner described above, this results in the SCR 56 being placed in
a nonconductive condition. Accordingly, the heater resistor 87 of
the thermal delay device is deenergized and actuator 120 is no
longer heated by this resistor. Although relay winding 48 is
deenergized at this time, contacts 52 remain closed due to the
continuing energization of the relay winding 50. The burner motor
14 continues to operate because the triac 46 is maintained in its
conductive condition.
When the control circuit SCR 56 becomes nonconductive, the ignition
circuit SCR is disabled by the circuit 28. Ignition disabling
circuit 28 includes a coupling transformer 122 having a primary
winding 124 connected to the anode and cathode of the SCR 56 and a
secondary winding 126 connected to the gate electrode and cathode
of the SCR 110. When SCR 56 is nonconductive, current flows through
the primary winding 124 in half cycles when the ignition circuit
116 would otherwise operate, and the gate electrode and cathode of
the SCR 110 are reverse biased. Due to this reverse bias the SCR
110 cannot be rendered conductive and no capacitor discharge takes
place to produce ignition sparks. Conversely, when SCR 56 is
conductive, it acts as a low resistance shunt of primary 124 to
prevent reverse biasing of the ignition circuit SCR 110.
FIG. 2 illustrates an alternative ignition disabling circuit
generally designated as 28A. In this arrangement a relay winding
128 is energized by a DC potential produced by a diode rectifier
130 and capacitor 132 to close a normally open set of relay
contacts 134. Closing of contacts 134 effectively connects the gate
electrode of SCR 110 to its cathode and prevents firing of the SCR
110. When SCR 56 conducts, it provides a low resistance bypass of
the winding 128 and contacts 134 remain open, permitting SCR 110 to
operate in its normal fashion in ignition circuit 16.
FIG. 3 illustrates yet another alternative ignition disabling
circuit generally designated as 28B. In this arrangement a light
sensitive resistance 140 is coupled between the gate electrode and
cathode of the ignition circuit SCR 110. A lamp 142 is disposed to
illuminate the resistance 140 and is connected across the output
electrodes of the SCR 56. When SCR 56 conducts, it shunts the lamp
142 so that resistance 140 is in its high resistance condition and
the ignition circuit operates. When SCR 56 is biased to its
nonconductive condition, the lamp 142 is energized to reduce the
resistance of the device 142 and disable the ignition circuit.
If at any time during normal operation the burner flame should be
extinguished, another trial-for-ignition period is commenced. If
flame detecting device 74 does not detect flame, the SCR 56 is
operated once again. The ignition circuit 16 operates and the
heater resistor 87 is energized until such time as ignition is
reestablished, or, alternatively, until the contacts 118 open.
Returning to a description of a heating cycle carried out by the
combustion control circuit 10, after ignition the burner motor 14
continues to operate and the flame at the burner 12 heats the
region including the thermostat 22 or 24. When the demand for heat
is satisfied, the triac 46 is rendered nonconductive and the
installation returns to its initial temperature satisfied
condition.
One important feature of the present invention is that the circuit
10 includes novel structure to prevent against unsafe conditions
arising from failure of the triac 46 in a short circuit condition
wherein it remains conductive regardless of the control signal
applied to its gate electrode. This is a common failure condition
of such devices. In prior art units if a controlling triac shorts
out, the burner motor is energized without any monitoring by the
flame detector, and without control of the thermostat in some
cases, giving rise to a highly undesirable and potentially unsafe
condition. Thus, in prior circuits it has been necessary to
purchase expensive triac devices of a high quality in order to
reduce the probability of failure as much as possible.
In the circuit 10, failure of the triac 46 does not lead to an
unsafe condition. Should such a failure occur with a control
circuit thermostat 24 in the open position, the burner motor 14 and
ignition circuit 16 are energized, and the control circuit 20
exercises no monitoring function. Moreover, the ignition circuit 16
is not disabled by the SCR 56 and the disabling circuit 28 or 28A.
However, the continuously operating ignition circuit 16 draws
current through the heater resistor 100 of the thermal delay device
30. After a period of several minutes or so, the resistor 100 heats
the actuator 120 and the contacts 118 open to shut down the
installation in a safe condition.
Should the triac 46 fail with a low voltage thermostat 24 in the
closed circuit condition, the circuit 10 operates normally until
the thermostat opens. At this time the burner motor 14 continues to
operate and the ignition circuit 16 begins to operate. After a
delay period, heater resistor 100 heats the actuator 120 and the
circuit 10 goes out on safety.
When a line voltage thermostat is used, a potentially unsafe
condition exists in prior circuits wherein the thermal switch
contacts are located in a low voltage circuit and not in series
with the triac. In such an arrangement, should ignition fail to
occur or should the flame be extinguished, the burner motor is
maintained energized even if no flame is present because the
thermal switch contacts are not in circuit with the burner motor.
The present invention avoids this unsafe condition because if flame
is not present and the thermal switch contacts open, current to the
burner motor is positively discontinued without reliance upon the
operability of the triac 46.
Although the present invention has been described with reference to
the details of the illustrated embodiments, it should be understood
that other modifications and embodiments will be apparent to those
skilled in the art. The details of the illustrated embodiments are
not intended to limit the scope of the present invention as set
forth in the following claims.
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