U.S. patent number 3,770,365 [Application Number 05/292,142] was granted by the patent office on 1973-11-06 for burner control.
This patent grant is currently assigned to Sundstrand Corporation. Invention is credited to Robert J. Lenski.
United States Patent |
3,770,365 |
Lenski |
November 6, 1973 |
BURNER CONTROL
Abstract
A control system for an oil burner including a motor for pumping
fuel to the burner, an igniter for lighting the fuel, and an
electronic switch for controlling energization of the motor and the
igniter, all in a line voltage circuit, together with a low voltage
circuit for controlling operation of the motor and igniter switch
including a relay controlling the gate of the switch, a silicon
controlled rectifier and a thermostat in circuit with the relay for
energizing the same, a second silicon controlled rectifier for
triggering the first mentioned rectifier, and a gate circuit for
triggering the second mentioned rectifier including a light
sensitive flame detector cell.
Inventors: |
Lenski; Robert J. (Rockford,
IL) |
Assignee: |
Sundstrand Corporation
(Rockford, IL)
|
Family
ID: |
23123408 |
Appl.
No.: |
05/292,142 |
Filed: |
September 25, 1972 |
Current U.S.
Class: |
431/79 |
Current CPC
Class: |
F23N
5/082 (20130101); F23N 5/203 (20130101); F23N
2231/10 (20200101); F23N 2227/28 (20200101); F23N
5/08 (20130101); F23N 2223/20 (20200101); F23N
2229/00 (20200101); F23N 2239/06 (20200101) |
Current International
Class: |
F23N
5/20 (20060101); F23N 5/08 (20060101); F23n
005/08 () |
Field of
Search: |
;431/79,24,78,75 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Favors; Edward G.
Claims
I claim:
1. A burner control system comprising:
a. means providing a line voltage circuit,
b. burner means in the line voltage circuit adapted to be
selectively energized,
c. a first electronic switch in circuit with the burner means,
d. a circuit for triggering said switch including a controlled
relay element,
e. means providing a source of low voltage including a normally
closed safety switch,
f. a controlling relay element in circuit with the low voltage
source for controlling said controlled relay element,
g. a thermostat in circuit with said controlling relay element for
conditioning the circuit responsive to to a call for heat,
h. a second electronic switch in circuit with said controlling
relay element and thermostat for energizing said controlling relay
element,
i. a circuit for triggering said second electronic switch including
a third electronic switch in circuit with the low voltage
source,
j. a heater for said safety switch in circuit with said third
electronic switch,
k. a gate circuit for triggering said third electronic switch in
circuit with said low voltage source and said thermostat, and
l. a light-sensitive flame detector in the gate circuit for the
third electronic switch normally providing a relatively high
resistance to trigger said third switch in the absence of a burner
flame when the thermostat is closed, and provide a relatively low
resistance in the presence of a burner flame to render the third
switch nonconductive when the burner is lighted.
2. A burner control system as defined in claim 1, wherein the
burner means comprises a motor for supplying fuel and an igniter
for lighting the fuel.
3. A burner control system as defined in claim 1, wherein the flame
detector comprises a cadmium type photosensitive cell.
4. A burner control system as defined in claim 1, wherein the first
electronic switch comprises a Triac including one main terminal
connected to the burner means, another main terminal connected to
the line voltage circuit, and a gate for rendering the switch
conductive across the main terminals when the gate is
energized.
5. A burner control as defined in claim 1, wherein the second
electronic switch comprises a silicon controlled rectifier having
an anode, a cathode, and a gate for triggering the switch, and
remains conductive after the triggering circuit therefor is
deenergized, while the thermostat remains closed calling for
heat.
6. A burner control system as defined in claim 1, wherein the third
electronic switch comprises a silicon controlled rectifier having
an anode, a cathode and a gate for triggering the switch.
7. A burner control system as defined in claim 1, wherein the
controlled relay element comprises a switch and the controlling
relay element comprises an electromagnetic coil.
8. A burner control system as defined in claim 1, wherein the
controlled relay element comprises a light sensitive element and
the controlling relay element comprises a light transmitting
element.
9. A burner control system as defined in claim 8, wherein said
light sensitive element comprises a phototransistor.
10. A burner control system as defined in claim 8, wherein the
light transmitting element comprises a light emitting diode.
11. A burner control system comprising:
a. means providing a line voltage circuit including selectively
energizable burner means,
b. a first electronic switch in circuit with the burner means
including a gate for triggering said switch,
c. a light-sensitive element in circuit with the gate for rendering
the gate conductive,
d. means providing a source of low voltage,
e. a light emitting element for transmitting light to said
light-sensitive element,
f. a thermostat in circuit with said low voltage source and said
light emitting element for conditioning the circuit responsive to a
call for heat,
g. a second electronic switch in circuit with said light emitting
element and thermostat for energizing said light emitting element
including a gate for triggering said second electronic switch,
h. a third normally nonconductive electronic switch in circuit with
the low voltage source for rendering the last recited gate
conductive,
i. a gate for triggering said third electronic switch in circuit
with said low voltage source and said thermostat, and
j. a light-sensitive flame detecting cell in circuit with the gate
for the third electronic switch normally providing a relatively
high resistance to trigger said third switch in the absence of a
burner flame when the thermostat is closed, and provide a
relatively low resistance in the presence of a burner flame to
render the third switch nonconductive when the burner is
lighted.
12. A burner control system comprising:
a. means providing a line voltage circuit,
b. means in the line voltage circuit for supplying fuel and
igniting a burner,
c. a first normally nonconductive electronic switch in circuit with
the fuel supply and ignition means,
d. a gate circuit for triggering said switch including a
light-sensitive element for rendering the gate circuit
conductive,
e. means providing a source of low voltage including a normally
closed safety switch,
f. a light emitting element in circuit with the low voltage source
for transmitting light to said light-sensitive element,
g. a thermostatic switch in circuit with said light emitting
element adapted to close for conditioning the circuit responsive to
a call for heat,
h. a second normally nonconductive electronic switch in circuit
with said light emitting element and thermostatic switch for
energizing said light emitting element,
i. a gate circuit for triggering said second electronic switch
including a third normally nonconductive electronic switch in
circuit with the low voltage source,
j. a heater for said safety switch in circuit with said third
electronic switch,
k. a gate circuit for triggering said third electronic switch in
circuit with said low voltage source and said thermostatic switch,
and
l. a light sensitive flame detector in the gate circuit for the
third electronic switch normally providing a relatively high
resistance to trigger said third switch in the absence of a burner
flame when the thermostatic switch is closed, and provide a
relatively low resistance in the presence of a burner flame to
render the third switch nonconductive when the burner is
lighted.
13. A burner control system comprising:
a. means providing a line voltage circuit,
b. means in the line voltage circuit energizable for supplying fuel
and igniting a burner,
c. a triac switch in circuit with the fuel supply and ignition
means,
d. a circuit for triggering said triac switch including a
light-sensitive phototransistor for rendering the circuit
conductive,
e. means providing a source of low voltage including normally
closed safety switch contacts,
f. a light emitting diode in circuit with the low voltage source
for transmitting light to said phototransistor,
g. a thermostat in circuit with said light emitting element for
conditioning the circuit responsive to a call for heat,
h. a first silicon controlled rectifier in circuit with light
emitting diode and thermostat for energizing said light emitting
diode,
i. a circuit for triggering said first silicon ontrolled rectifier
including a second silicon controlled rectifier in circuit with the
low voltage source,
j. a heater for said safety switch contacts in circuit with said
second silicon controlled rectifier,
k. a circuit for triggering said second silicon controlled
rectifier in circuit with said low voltage source and said
thermostat, and
l. a light-sensitive cadmium type flame detecting cell in the gate
circuit for the second silicon controlled rectifier normally
providing a relatively high resistance to trigger said second
silicon controlled rectifier in the absence of a burner flame when
the thermostat is closed, and provide a relatively low resistance
in the presence of a burner flame to render the second silicon
controlled rectifier nonconductive when the burner is lighted.
14. A burner control system comprising:
a. means providing a line voltage circuit,
b. means in the line voltage circuit for supplying fuel and
igniting a burner,
c. a first normally nonconductive electronic switch in circuit with
the fuel supply and ignition means,
d. a gate circuit for triggering said switch including switch
contacts for rendering the gate circuit conductive,
e. means providing a source of low voltage including a normally
closed safety switch,
f. an electromagnetic coil in circuit with the low voltage source
for controlling said switch contacts,
g. a thermostatic switch in circuit with said coil element adapted
to close for conditioning the circuit responsive to a call for
heat,
h. a second normally nonconductive electronic switch in circuit
with said coil and thermostatic switch for energizing said
coil,
i. a gate circuit for triggering said second electronic switch
including a third normally nonconductive electronic switch in
circuit with the low voltage source,
j. a heater for said safety switch in circuit with said third
electronic switch,
k. a gate circuit for triggering said third electronic switch in
circuit with said low voltage source and said thermostatic switch,
and
l. a light-sensitive flame detector in the gate circuit for the
third electronic switch normally providing a relatively high
resistance to trigger said third switch in the absence of a burner
flame when the thermostatic switch is closed, and provide a
relatively low resistance in the presence of a burner flame to
render the third switch nonconductive when the burner is
lighted.
15. A burner control system comprising:
a. means providing a source of low voltage including a normally
closed safety switch,
b. a controlling relay element in circuit with the low voltage
source,
c. a thermostatic switch in circuit with said controlling relay
element adapted to close for conditioning the circuit responsive to
a call for heat,
d. a first normally nonconductive electronic switch in circuit with
said controlling relay element and thermostatic switch for
energizing said controlling relay element,
e. a gate circuit for triggering said first electronic switch
including a second normally nonconductive electronic switch in
circuit with the low voltage source,
f. a heater for said safety switch in circuit with said second
electronic switch,
g. a gate circuit for triggering said second electronic switch in
circuit with said low voltage source and said thermostatic switch,
and
h. a light-sensitive flame detector in the gate circuit for the
second electronic switch normally providing a relatively high
resistance to bias said second switch to conduct in the absence of
a burner flame when the thermostatic switch is closed, and provide
a relatively low resistance in the presence of a burner flame to
reversely bias the second switch to a nonconductive state when the
burner is lighted.
16. A burner control system as defined in claim 12, wherein the
gate circuit for the first electronic switch is a low voltage
circuit independent of the fuel supply and ignition means so that
triggering is not affected by variation in electrical
characteristics of the fuel supply and ignition means.
Description
BACKGROUND OF THE INVENTION
The present invention relates to controls for a heating system such
as an oil burner of the type utilized in residential buildings, for
example, wherein the burner is responsive to a call for heat by a
thermostat, which is intended to energize a fuel supply means and
an igniter for lighting the fuel.
In systems of the type described, there is usually a relatively
delicate balance between fuel supply and air supply within which
clean, efficient combustion can be expected. If there is a fuel
supply without prompt ignition, or if combustion is initiated and
then discontinued inadvertently or unexpectedly, it is important
that the fuel supply be discontinued promptly in order to avoid
conditions which could be harmful to the equipment or dangerous to
property or personnel. Accordingly, it is important in burner
control systems to provide safe failure in event the flame does not
start when desired, or goes out after starting.
In the past, control systems of the type described have often
utilized electromechanical controls including moving parts which
are subject to wear. As controls have become more complicated,
numerous moving parts have led to relatively short life and
expensive maintenance problems. Also, the controls have often been
incorporated in line voltage circuits which may be dangerous and
destructive of circuit components in event of malfunction.
Recently, there have been some controls utilizing electronic
components in low voltage circuits in an effort to improve safety
and reliability. For example, U.S. Pat. No. 3,624,407 relates to a
furnace control with electronic components in a low voltage
circuit, but the patented control includes a reed switch
controlling the burner which requires separate energizing and
holding coils. U.S. Pat. No. 3,672,811 also relates to a burner
control, but essentially all of the control components are included
in a line voltage circuit, and the fuel supply is controlled by a
light sensitive relay which requires both an activating lamp and a
holding lamp.
It is desirable to provide an improved burner control system with
the control components in a low voltage circuit of simple and
reliable construction with fail-safe capacity assuring that the
fuel supply is discontinued in the event that there is no ignition
or in the event that combustion is unexpectedly discontinued.
SUMMARY OF THE PRESENT INVENTION
The present invention relates to an improved burner control system
in which a thermostat, flame sensor and safety switch are connected
in a low voltage solid state circuit, completely isolated from the
commercial 110 volt AC power supply, in an arrangement which
provides a safe and electric shock-proof circuit for all external
wiring to the control. The system will withstand shorting of any of
the thermostat and flame sensor terminals without serious harm to
the control.
Only a burner motor, for supplying air and fuel, and an igniter,
together with a control switch for the motor and igniter, are
included in a line voltage circuit.
When the thermostatic switch closes, calling for heat, the controls
in the low voltage control circuit function sequentially to verify
operation of a flame detector cell and a safety switch before
energizing the fuel supply and igniter means. If a flame is not
established in a short time, the safety switch opens the control
circuit to prevent energization of the fuel supply and igniter, and
the system cannot be placed in operation again without manually
resetting the safety switch.
In the low voltage control circuit, a relay for energizing the
burner means is connected in circuit with the thermostat, the
safety switch, and an electronic triggering switch, the gate for
the electronic switch is connected in circuit with a second
electronic switch and a heater for the safety switch, and the gate
for the second electronic switch is in circuit with the flame
detector, so that in the absence of a flame when the thermostat is
calling for heat, the electronic switches are conductive, to
energize the relay, and when a flame is detected, the second switch
is nonconductive and the heater is deenergized.
Preferably, the electronic switches in the low voltage control
circuit are silicon controlled rectifiers so that the relay remains
energized after triggering, so long as the thermostat calls for
heat and the power supply is maintained. There is no need for
separate energizing means and holding means for the relay.
When the system is placed in operation, the burner has constant
ignition, because the igniter is wired in parallel with the burner
motor which supplies fuel and air for combustion. In the event of a
power failure, the burner control will remain ready to restart the
burner when power is resumed.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a burner control circuit embodying the principles of the
present invention and utilizing a photo-electric coupling between a
low voltage control circuit and a line voltage circuit for a burner
motor and igniter; and
FIG. 2 is a burner control system embodying the principles of the
present invention and utilizing a magnetic coupling between a low
voltage control circuit and a line voltage circuit for a motor and
igniter.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Referring now to FIG. 1, a line voltage circuit includes a line 20
and line 21 connected across the primary winding of a transformer
22. A burner motor 24 for supplying oil and air to a combustion
chamber, is connected to the line 20 by wire 25 and connected to
the line 21 through a fuse 27 (if desired) and a triac switch T1.
An igniter 28 is connected in parallel with the motor 24 so that
the motor and the igniter are simultaneously energized.
The triac switch T1 includes a main terminal MT2 connected to the
fuse 27, a main terminal MT1 connected to the line 21 and a gate G
connected to a control circuit for triggering the switch when there
is a call for heat by the thermostat. A resistor R2 is connected
between the triac terminals G and MT1 to eliminate false triggering
of the triac due to excessive leakage currents at high temperature
or due to electrically generated stray noise signals. A resistor R1
and a capacitor C1 are connected across the triac terminals MT2 and
MT1 to limit the rate of rise of voltage across the triac T1 to a
safe value when the triac switches from on to off.
The triac T1 is normally in its off or nonconducting state when the
thermostat is not calling for heat. The gate G of the triac T1 is
biased to render the triac conducting by means of a collector
current from a driver transistor T3. The collector of the
transistor T3 is connected to the triac gate G by a conductor 30
including a resistor R3. The resistor combination of R3 and R2
provides proper gate bias voltage and current for the triac T1. The
base of the transistor T3 is connected to the emitter of a
phototransistor T2 by a conductor 31, and the emitter of the
transistor T3 is connected by a conductor 32 to a conductor 33.
The base of the transistor T2 is connected by a conductor 35 and
capacitor C6 to the conductor 33, and the latter is connected by a
conductor 36 and diode D1 to a tap on the primary winding of the
transformer 22. The collector of the transistor T2 is connected to
a resistor R4. The phototransistor T2 is arranged to respond to
light from a light emitting diode D3 in a low voltage control
circuit responsive to the thermostat. Light from the diode D3
striking the transistor T2 initiates a base current flow in the
transistor T2, as a result of which the phototransistor conducts,
amplifying the small light generated base current into considerably
more emitter current. The emitter current from the transistor T2
flows into the base of the driver transistor T3, causing it to
saturate and conduct collector current to trigger the triac T1.
Diode D1 and a capacitor C2 in the conductor 33 function as a
half-wave power supply for the gate trigger circuitry of
transistors T2 and T3. Resistor R4 acts as a current limit for
transistors T3 and T2.
A resistor R5 is connected between the conductors 31 and 33, and
capacitor C6 is provided in the conductor 35. The resistor R5 and
the capacitor C6 insure stabilization of the high gain transistor
pair T3 and T2.
A low voltage control circuit is connected across the secondary of
the transformer 22. The low voltage circuit includes a thermostat
incorporating a normally open thermostatic switch 40 which is
adapted to close responsive to decreasing ambient temperature to
call for heat. The low voltage circuit also includes a light
sensitive cadmium-type photoresistive flame detecting cell 42
located adjacent to a burner 43 so that the cell is responsive to
the presence or absence of a flame at the burner. The cell 42 has a
relatively high resistance, on the order of 50K ohms, in darkness
in the absence of a flame at the burner 43, and a relatively low
resistance, from 300 ohms to 2,000 ohms, depending upon the
distance of the cell from the flame, when the burner is
lighted.
Preferably, the burner control circuitry is incorporated in an
appropriate housing represented at 45, with only the burner motor
24, the igniter 28, the thermostat 40 and the cad cell 42 outside
the housing 45. In order to properly connect the control circuitry
to the external components of the system, the housing 45 includes a
terminal LG for connection to the power supply line 20, a terminal
L for connection to the power supply line 21, and a terminal M for
connection with the motor 24 and igniter 28. Additionally, the
housing 45 includes terminals T for connection of the thermostat,
and terminals F for connection of the flame detector cell 32.
The light emitting diode D3 and the phototransistor T2 may be
appropriately characterized as a light relay generally designated
46, in which the diode D3 is a controlling relay element and the
transistor T2 is a controlled relay element.
In the low voltage control circuit, the light emitting diode D3 is
connected to be energized responsive to closure of the thermostatic
switch 40, provided that certain other conditions exist, as will
appear in the description of the circuitry.
The low voltage circuit includes a safety switch with normally
closed switch contacts 47 and a heater coil 48 arranged to cause
opening of the contacts 47 after a predetermined period on the
order of 15 to 45 seconds. After the switch contacts are opened,
they are latched in open condition and must be manually reset by
means of a button 49 accessible from the outside of the housing 45.
If desired, the physical construction of the safety switch may be
on the order of that shown and described in the aforementioned U.
S. Pat. No. 3,624,407.
In order to provide for energization of the light emitting diode D3
when there is a call for heat, the diode is connected across the
secondary winding of the transformer 22 in a circuit including a
conductor 50 leading from the transformer secondary (+12) to the
safety switch 47, a conductor 51 leading from the safety switch 47
to the thermostat, a conductor 52 leading from the thermostat, a
conductor 54 leading from the conductor 52 to the diode D3, a
conductor 55 leading from the diode D3 to an electronic switch T4,
preferably a silicon controlled rectifier, and a conductor 56
returning from the SCR T4 to the transformer secondary (0). A
resistor R7 is connected across the conductors 54 and 56 and draws
current through the thermostat when it is closed, before the SCR T4
conducts. A diode D2 and a resistor R8 are provided in the
conductor 54. A conductor 58 is connected to the conductor 54 and
includes a power supply capacitor C3. A resistor R9 is connected
across the capacitor between the conducter 54 and the conductor 58.
A current limiting resistor R10 is provided in conductor 54 in
circuit with diode D3.
On the positive half cycle swings of secondary transformer voltage
(+12), diode D2 conducts, charging the power supply capacitor C3 to
about +17 volts through resistor R8. The capacitor will serve to
provide current to the diode D3, when T4 conducts, during negative
half cycle swings of the transformer secondary winding. Resistor R9
serves to completely discharge the capacitor C3 after the
thermostat opens and T4 switches off to its high impedance
state.
In order to provide for triggering the SCR T4, a conductor 60 leads
from the safety switch contacts 47 to the heater 48, and a
conductor 61 leads from the heater to a second electronic switch
T5, also preferably a silicon controlled rectifier. The gate of the
SCR T5 is connected by a conductor 63 to the flame detector cell
42, and a conductor 64 connects the cell 42 to the transformer
secondary winding (-6) through a resistor R16. The cathode of the
SCR T5 is connected by a conductor 66 to the gate of the SCR T4, so
that the SCR T5 may function to trigger the SCR T4. A capacitor C4
connected to conductor 61 dampens stray noise signals due to
contact bounce of the safety switch contacts 47 and the thermostat
contacts 40. Similarly, a capacitor C5 connected to the gate of the
SCR T5 eliminates stray noise signals at the gate of the SCR
T5.
A conductor 68 is connected between the conductor 54 and the
conductor 63 and includes a resistor R11, which together with a
resistor R14 in the conductor 63 and a grounded resistor R15 form a
resistor bias network for controlling the gate of the SCR T5. In
the absence of a flame in the burner, the cad cell 42 has a very
high resistance on the order of 50K ohms, and such high resistance
state causes the gate-cathode junction of the SCR T5 to be forward
biased by the resistor network R11, R14 and R15 during positive
half cycle swings of the transformer winding (-12). The SCR T5
receives enough gate current and gate voltage to remain triggered
as long as the flame detector cell 42 remains in its darkened state
indicating the absence of a flame at the burner 43.
The triggering of the SCR T5, as described above, causes it to
conduct current through the resistive heating element 48 in the
safety switch. The heating element in turn is arranged to heat a
bimetallic strip in the safety switch which ultimately would open
the switch contacts 47 when the bimetallic strip reaches a
predetermined temperature. The time-temperature characteristic of
the safety switch is such that it will open after 15 to 45 seconds
of continuous heating by the element 48. Thus, if no flame is
established at the burner 43 in the 15 to 45 second delay following
closure of the thermostat contacts, the safety contacts 47 will
open to discontinue power to the low voltage control. Once the
safety switch contacts have been opened, they remain open, and the
entire control is disabled, until the safety switch is manually
reset.
During the 15 to 45 second period required to open the safety
switch contacts, SCR T5 conducts current through a resistor R13 and
a redundant resistor R6 connected to the conductor 66, so that
enough voltage appears across them to trigger the SCR T4 through
its gate and a resistor R12 which controls the current to the gate.
When SCR T4 conducts, a circuit is completed through the light
emitting diode D3, so that the latter transmits light to energize
the burner controls. After triggering, the SCR T4 will remain in
its conducting state until the thermostat contacts 40 open, or the
safety switch contacts 47 open, or the power is lost at the line
voltage supply.
If a flame is eatablished at the burner 43 during the 15 to 45
second trial period before the safety switch contacts 47 open, as
will normally occur if the control system functions properly, the
resistance in the flame detector cell 42 will be reduced to a low
value between 300 and 2,000 ohms, depending upon the proximity of
the cell to the flame. The low resistance in the flame detector
cell reverse biases the gate-cathode junction of the SCR T5,
thereby turning off the SCR T5. When the switch T5 does not
conduct, no power is consumed by the heater element 48 in the
safety switch, so that it cools and the contacts 47 remain in the
normally closed condition. As a result, the burner will continue to
operate as long as the thermostat calls for heat and the flame
detector cell reads a flame condition.
The switching devices T1 and T3-T5 may be of the type identified by
various manufacturers as follows:
T1-it210, q185
t3-2n5305
t4-c103y, 2n5060, t1c44
t5-c103y, t1c44
the diodes D1 and D2 may be of the type identified by manufacturers
as follows:
D1-1n4148
d2-dt230f, 1n4148
the capacitors C1-C6 have values generally as follows:
C1-0.1 mfd 400V
C2-220 mfd 25V
C3-100 mfd 25V
C4-0.1 mfd
C5-0.001 mfd Ceramic
C6-390 pfd
Preferably, the light emitting diode D3 and the phototransistor T2
are combined in a single commercially available integrated circuit
such as Monsanto Company's photocoupler MCT 26.
The resistances R1-R16 have values approximately as follows:
R1-100 ohm 1/2W
R2-330 ohm 1/2W
R3-82 ohm 1W
R4-3.3-6.8k ohm 1/3W
R5-82k ohm 1/3W
R6-4.3 ohm 1/3W
R7-47 ohm 5W
R8-10 ohm 1/3W
R9-5.6k ohm TO 10K ohm 1/3W
R10-560 ohm 1/2W
R11-15k ohm 1/3W
R12-330 ohm 1/3W
R13-4.3 ohm 1/3W
R14-2.2k ohm 1/3W
R15-8.2k ohm 1/3W
R16-1.8k ohm 1/3W
To summarize the operation, when there is no call for heat, the
thermostatic switch contacts 40 are normally open, and the
components of the system are at rest. When the thermostatic switch
is closed, a circuit is completed through the conductor 68 to the
gate circuit for the SCR T5. If the flame detector cell is
functioning properly, when there is no flame the high resistance at
the cell causes the SCR T5 to conduct, for purposes of triggering
the SCR T4. A circuit is completed through the safety switch
contacts 47 and the thermostat to energize the light emitting diode
D3 when the SCR T4 is conducting. Energization of the diode D3
results in transmission of light to the phototransistor T2. The
emitter current from the phototransistor T2 causes the transistor
T3 to conduct. The collector current from the transistor T3 biases
the gate G of the triac T1 so that the latter is rendered
conducting, to energize the burner motor 24 and the igniter 28. The
motor drives a pump for supplying fuel oil to the burner 43 and the
igniter causes ignition of such fuel. The existence of a flame
reduces the resistance in the flame detector cell 42, rendering the
SCR T5 nonconducting. The heater coil 48 is deenergized, and the
safety switch contacts 47 remain closed.
If for some reason, the normally expected cycle of operation
described above is not completed, and no flame is established at
the burner 43, the high resistance in the unlighted flame detector
cell 42 will maintain the SCR T5 conducting until the heater
element 48 opens the safety switch contacts 47, thereby shutting
the system down until it receives manual attention.
Among the advantages of the system illustrated, the flame sensor
and the thermostat are connected to low voltage circuits completely
isolated from the 110 volt commercial power supply, by virtue of
the light sensitive relay providing a photoelectric coupling
between the low voltage circuit and the line voltage circuit. This
results in a shock-proof arrangement for external wiring to the
control. The photo-electric relay coupling the low voltage circuit
and the line voltage circuit is composed of highly reliable solid
state devices with extremely long life expectancy. The
photoelectric triac gate triggering circuit provides an improvement
over previous controls, in that the circuit is isolated from the
motor and igniter by virtue of a separate tap on the transformer
winding, so that the circuit is not affected by variations in the
characteristics of the windings in the motor and igniter. The
isolated photoelectric triggering circuit insures good starting of
the motor at low temperatures, when more triac gate current may be
required, by providing full 360.degree. A.C. sine wave conduction.
The circuit also eliminates false starts at high temperatures due
to leakage currents.
The arrangement illustrated provides for safe failure. If a
malfunction condition exists such that the thermostat terminals T
are shorted, and the flame sensor terminals F are open, the burner
control may turn on, but it will not operate longer than the time
required for the heater 48 to open the safety switch contacts 47,
because there will be no reduction of resistance to render the SCR
T5 non-conducting. If the flame sensor terminals F are shorted, the
burner control will never start, because the SCR T5 will be
nonconducting. This sequential safety check insures that the flame
detector is functioning properly before the system is put into
operation when the thermostat calls for heat. In event the flame
sensor terminals are shorted to the thermostat terminals T,
resistor R16 limits the current in the transformer secondary to a
safe value.
The igniter 28 is constantly energized during operation of motor
24. If there is a power failure, the control will restart the
system when power returns.
FIG. 2
The embodiment of FIG. 2 is similar to the embodiment of FIG. 1,
except that the photoelectric relay 46 utilized in the control of
FIG. 1 is omitted, along with the circuit for amplifying the
photoelectric signal, and in place thereof an electromagnetic relay
70 is utilized including a coil 72 and switch contacts 74. The
remaining components of the system in FIG. 2 correspond
substantially to similar components in FIG. 1 and are identified by
similar reference numbers primed. The coil 72 functions as a
controlling relay element in place of light emitting diode D3, and
the switch 74 functions as a controlled relay element connected
directly to the gate terminal G' of triac T1', in lieu of
phototransistor T2 and its amplifying circuit. A conductor 75 leads
from fuse 27' through the relay switch 74 to the gate terminal G'
for triac T1'.
The circuit has the advantage of reducing the control elements
associated with the triac switch controlling the burner motor and
igniter. The switch 74 may be a coil controlled magnetically
sensitive mercury switch of a type manufactured by Fifth Dimension,
Inc., of Princeton, N.J., and called Logcell. In the Logcell
switch, an elongate core for the coil has a mercury coated terminal
at one end in a sealed cap containing a flexible spring disc
contact engageable with the terminal to close a circuit responsive
to energization of the coil. The circuit of FIG. 2 has the
reliability, shock-proof and fail-safe characteristics as described
in connection with FIG. 1.
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