U.S. patent number 3,832,123 [Application Number 05/306,591] was granted by the patent office on 1974-08-27 for burner control system.
This patent grant is currently assigned to Walter Kidde & Company, Inc.. Invention is credited to Lyman H. Walbridge.
United States Patent |
3,832,123 |
Walbridge |
August 27, 1974 |
BURNER CONTROL SYSTEM
Abstract
Disclosed is a fail safe burner control system with a valve
controller for operating a burner, a flame rectification flame
detector and a spark igniter apparatus. A first electronic switch
opens the valve and starts the igniter. Upon the occurrence of
flame, a second electronic switch, in response to the rectification
detector, disables the first switch thus stopping the spark, but
holds the valve open as long as flame is sensed. If flame is lost,
the first switch is enabled automatically. In the event of failure
to reignite after a loss of flame, the continued operation of the
sparking igniter causes a circuit breaker to lock out the
system.
Inventors: |
Walbridge; Lyman H. (Ashland,
MA) |
Assignee: |
Walter Kidde & Company,
Inc. (Clifton, NJ)
|
Family
ID: |
23185977 |
Appl.
No.: |
05/306,591 |
Filed: |
November 15, 1972 |
Current U.S.
Class: |
431/78 |
Current CPC
Class: |
F23N
5/123 (20130101); F23N 5/203 (20130101); F23N
2231/06 (20200101); F23N 5/20 (20130101); F23N
2231/12 (20200101); F23N 2229/00 (20200101); F23N
2227/36 (20200101); F23N 2229/12 (20200101) |
Current International
Class: |
F23N
5/20 (20060101); F23N 5/12 (20060101); F23h
005/00 () |
Field of
Search: |
;431/66,74,78,67,70,80 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Favors; Edward G.
Attorney, Agent or Firm: Toupal; John E.
Claims
What is claimed is:
1. A burner control system comprising:
valve means for controlling the flow of fuel to a burner;
ignition means energizable to ignite fuel emanating from the
burner;
flame detector means for detecting the presence of flame at the
burner;
first electronic switch means enableable to both activate said
valve means to initiate fuel flow to the burner and energize said
ignition means.
second electronic switch means for maintaining said valve means in
a condition wherein fuel is supplied to the burner in response to
signals from said flame detector means; and
ignition interruption means or deenergizing said ignition
means.
2. A system according to claim 1 wherein said first electronic
switch means and said second electronic switch means comprise
solid-state switching elements.
3. A system according to claim 1 wherein said ignition interruption
means comprises cut-off means responsive to said second electronic
switch means for disenabling said first electronic switch means
when flame is sensed by said flame detector means.
4. A system according to claim 3 wherein said flame detector means
comprises electrode means supplied with alternating electric
current and disposed to be bathed by the flame and direct current
detection means for detecting the flow of rectified current through
the flame.
5. A system according to claim 3 wherein said ignition interruption
means comprises delay timer means for disenabling said first switch
means until said delay timer means has timed out in a predetermined
delay time, and wherein said cut-off means comprises inhibit means
for deactivating said delay timer means and thereby disenabling
said first switch means.
6. A system according to claim 5 wherein said cut-off means
comprises periodic reset means for periodically resetting said
delay timer at intervals shorter than said predetermined delay time
when the presence of a flame is sensed.
7. A system according to claim 6 wherein said delay timer comprises
clamping means responsive to said periodic reset means for clamping
the power supplied to said first switch means.
8. A system according to claim 7 wherein said clamping means
comprises a clamping capacitor that is periodically discharged by
said periodic reset means.
9. A system according to claim 6 comprising ignition timer means
for timing the operation of said ignition means and including power
input means for supplying power to said burner control system and
further comprising switch deactivator means for disenabling said
first electronic switch means in response to timing out of said
ignition timer means in a predetermined period of time.
10. A system according to claim 9 wherein said switch deactivator
means comprises energy accumulation means for normally receiving
energy at a first rate through said first electronic switch means
and comprising threshold means for disconnecting power from said
control system after the accumulation of a predetermined amount of
energy.
11. A system according to claim 10 wherein said switch deactivator
means comprises lock out thermal circuit breaker means.
12. A system according to claim 10 wherein said energy accumulation
means comprises leakage means for dissipating energy stored therein
and wherein said second electronic switch means normally transmits
energy to said energy accumulation means at a second rate lower
than the said first rate, and said leakage means renders said lock
out means non-responsive to said second rate.
13. A system according to claim 12 wherein said first electronic
switch means comprises a first silicon controlled rectifier and
said second electronic switch means comprises a second silicon
controlled rectifier.
14. A system according to claim 13 comprising control circuit means
for causing said first controlled rectifier to conduct for a longer
duty cycle than that of said second silicon controlled
rectifier.
15. A system according to claim 12 comprising a.c. power supply
means for rendering said first electronic switch means conductive
during one half cycle of the a.c. power supplied and wherein said
second electronic switch means is conductive during the alternate
half cycle of the a.c. power supplied.
16. A system according to claim 15 comprising control circuit means
for supplying energy to said energy accumulation means in the event
of conduction by said second electronic switch means during said
half cycle of each cycle and preventing the flow of any substantial
amount of energy through said second electronic switch means to
said energy accumulation means during said alternate half
cycle.
17. A system according to claim 16 wherein said first electronic
switch means comprises a first silicon controlled rectifier and
said second electronic switch means comprises a second silicon
controlled rectifier.
18. A system according to claim 17 wherein said ignition means
comprises spark ignition means.
19. A system according to claim 6 comprising lock out means adapted
to transmit power to said system during normal periodic
energization of said second electronic switch means and to lock out
said system following normal periodic energization of said first
electronic switch for a predetermined period of time.
20. A system according to claim 19 wherein said lock out means
comprises a thermal circuit breaker.
21. A system according to claim 1 wherein said valve means
comprises electromagnetic means for controlling the flow of fuel,
said first electronic switch is connected to transmit electrical
power to said electromagnet means for opening said valve means, and
said second electronic switch means is connected to transmit
electrical power to said electromagnetic means for maintaining said
valve means in open position in response to signals from said flame
detector means.
22. A system according to claim 1 comprising ignition timer means
for timing the operation of said ignition means and including power
input means for supplying power to said burner control system and
further comprising switch deactivator means for disenabling said
first electronic switch means in response to timing out of said
ignition timer means in a predetermined period of time.
23. A system according to claim 22 wherein said switch deactivator
means comprises energy accumulation means for normally receiving
energy at a first rate through said first electronic switch means
and comprising threshold means for disconnecting power from said
control system after the accumulation of a predetermined amount of
energy.
24. A system according to claim 23 wherein said energy accumulation
means comprises leakage means for dissipating energy stored therein
and wherein said second electronic switch means normally transmits
energy to said energy accumulation means at a second rate lower
than the said first rate, and said leakage means renders said lock
out means nonresponsive to said second rate.
25. A system according to claim 1 comprising lock out means adapted
to transmit power to said system during normal periodic
energization of said second electronic switch means and to lock out
said system following normal periodic energization of said
electronic switch for a predetermined period of time.
26. A system according to claim 1 wherein said first electronic
switch means, said ignition means and said valve means are
interconnected such that activation of said valve means by said
first switch requires energization of said ignition means.
Description
BACKGROUND OF THE INVENTION
The invention relates to burner control systems and, more
particularly, to fail safe burner control systems.
Extensive efforts have been directed toward the improvement of
control systems for fuel burners such as gas and oil burners and
the like. Increased system safety and reliability have been primary
objectives of such efforts. These objectives, however, generally
conflict with an obvious desire to limit the cost and physical size
of the systems. Thus system complexity is an important
consideration.
Such systems often ignite the fuel with a spark igniter. Interest
has recently been directed toward systems that extinguish the spark
after ignition to eliminate radio frequency interference. However,
circuits to extinguish the spark have greatly added to the
complexity of the control circuit. This is particularly true since
it is required that if flame is lost for any reason, the system
must respond in one of two ways. Either the valve must be closed to
stop the flow of fuel, or as is preferable if heat is still
required, the ignition apparatus must be activated in an effort to
reestablish flame.
In addition, most burner systems must employ fuel supply valves
that are controlled by flame sensing mechanisms which automatically
interrupt fuel flow in response to a predetermined loss of flame
condition. In accordance with the above requirements, circuits have
been designed wherein the spark apparatus is responsive to the
flame sensor so that when flame is detected the igniter is stopped
and upon loss of flame the igniter is activated to reestablish
flame. A difficulty encountered with these circuits is their
complexity; for example, often a plurality of feedback loops, or
the like, is used. A danger in having such a complex system is that
failure of one or more circuit components can cause an unsafe
condition as, for example, a situation in which the valve remains
open but the ignition apparatus is not activated. An explosive
amount of fuel may thereby enter the atmosphere. Many conventional
circuits provide a capacitor that is charged by the sensor when
flame is present and a valve that opens when the charge on the
capacitor exceeds a predeterminee minimum. To initiate operation,
the capacitor is precharged to open the valve and is kept charged
by the sensor if flame is achived. If no flame is achieved before
the capacitor becomes discharged, the valve closes and the system
shuts down. The unsafe condition can occur in this circuit, for
example, if flame is lost or never established, but a malfunction
in the precharging circuit keeps the capacitor charged and thus the
valve open. This is a result of having to "fool" the system by
precharging.
The object of this invention, therefore, is to provide a burner
control system that automatically activates the igniter to
reestablish flame in the event of a loss thereof. However, it is
desired that if a failure to establish flame occurs, either
initially or after a loss of flame, the valve be closed after a
predetermined time. It is further desired that the system be
rendered fail safe, that is, malfunction of any component or group
of components shall not lead to an unsafe condition.
SUMMARY OF THE INVENTION
The invention is characterized by a burner control system for a
fuel burner comprising a valve apparatus that controls the flow of
fuel to the burner, and a spark ignition apparatus. A flame
detector circuit is also included. A first electronic switching
apparatus is energizable to open the valve and start the ignition
apparatus. A second electronic switching apparatus is coupled to
the flame detector circuit, and, in response to a signal indicating
the presence of a flame, maintains the valve in its open position
and, through an ignition interruptions apparatus, inhibits the
first switch apparatus. As long as power is supplied to the control
system, the first switch is automatically enabled in the absence of
a signal from the ignition interruption apparatus. Consequently, a
control circuit is provided that automatically reestablishes flame
in the event of a loss thereof by immediately enabling the first
switch to start the ignition apparatus and maintain the valve in an
open position. An advantage of providing two switches, each of
which is capable of maintaining the valve in an open position, is
that many of the dangers experienced with precharging circuits,
such as maintaining the valve in an open position without flame,
are eliminated. A separate three terminal solid state element, for
example a silicon controlled rectifier, forms the active element of
each of the electronic switching apparatus. A circuit utilizing
three terminal devices is rendered significantly more fail safe
than conventional relay controlled circuitry. This is so because
conventional circuits utilize multiple pole relays with contacts
controlling a plurality of separate switching circuits. Thus the
valve and the ignition apparatus are generally controlled by
separate circuits. Therefore a component failure in either circuit,
or a pair of relay contacts sticking together can cause the valve
to remain open without activating the ignition apparatus. However,
such an event is less likely to occur in the subject control system
inasmuch as SCRs function as if they were two terminal SPST
devices. therefore, the single SCR in the first switch both
energizes the spark apparatus and opens the valve with the
equivalent of only one pair of "contacts." Thus, for example, it is
unlikely that one of these functions will be performed in response
to the first SCR without the other function being performed
also.
A feature of the invention is the inclusion of a delay timer
apparatus in the first switch in conjunction with a flame detector
system. The timer prevents the enabling of the first electronic
switching apparatus until the timer has timed out in a
predetermined delay time. The flame rectification detector fires
the SCR in the second electronic switch apparatus once during each
cycle of the a.c. voltage supplied to power the burner control
system. Pulses caused by the conduction of the SCR in the second
switch are coupled to the delay timer by a periodic reset apparatus
and each pulse resets the delay timer. Since the delay time is
selected to be longer than the period of the alternating supply
voltage the delay timer can never time out if the flame
rectification detector circuit is sensing flame. However, in the
event that no flame is sensed, the delay timer is no longer reset
and it quickly times out. Thereafter the first electronic switch is
enabled and the ignition apparatus is energized. Thus it is seen
that the ignition apparatus is energized in an effort to
reestablish flame when flame is lost, and furthermore that the
ignition apparatus is deenergized upon establishing flame.
Another feature of the invention is the inclusion of a thermal lock
out circuit breaker that causes the control circuit to lock out
after a predetermined period of ignition unless flame is sensed.
This is important inasmuch as the valve is open while the first
electronic switch is activated so that if there is a malfunction in
the ignition apparatus, the valve is releasing fuel into the
atmosphere with no change of the ignition thereof. Power is
supplied to the burner control system through the circuit breaker.
A wave shaping control circuit within the burner system causes the
silicon controlled rectifier in the first electronic switch to
conduct for a substantially longer duty cycle than is exhibited by
the SCR in the second electronic switch. Furthermore, operation of
the first electronic switch causes power to be drawn directly from
the power supply, but as will be explained below, normal operation
of the second switch does not. The result of this is that the
normal operation of the second electronic switch puts no
significant strain on the thermal circuit breaker, but continued
operation of the first electronic switch, drawing power during its
longer duty cycle, will cause actuation of the circuit breaker
after a preselected period of time. Thus, operation of the first
electronic switch continuously for 15 seconds, for example, will
cause actuation of the thermal circuit breaker.
DESCRIPTION OF THE DRAWINGS
These and other features and objects of the present invention will
become more apparent upon a perusal of the following description
taken in conjunction with the accompanying drawings wherein:
FIG. 1 is an operational diagram of a preferred burner control
system;
FIG. 2 is a schematic diagram of the preferred system; and
FIG. 3 shows various wave forms at different points within the
circuit shown in FIG. 2.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring first to FIG. 1 there is an operational diagram of a
preferred burner control system 21. It should be understood that
the diagram of FIG. 1 is not a conventional block diagram. For
example, not all of the blocks depicted in FIG. 1 correspond to an
easily descernible portion of the circuit shown in FIG. 2, and the
lines coupling the blocks in FIG. 1 may indicate either electrical
or mechanical coupling. It is however, felt that an understanding
of the diagram shown in FIG. 1 will simplify comprehension of the
operation of the circuit shown in FIG. 2. The system 21 is powered
by an a.c. power source (not shown). When power is applied, a delay
time 22 that is part of a first electronic switch 23 begins to time
out in a predetermined delay time that is longer than one cycle of
the a.c. supply current. The delay timer 22 enables a first silicon
controlled rectifier 24 through a line 25. The first silicon
control rectifier 24 fires once during each cycle of the a.c.
current as long as a signal remains on the line 25. In addition,
the signal on the line 25 is carried to a shut down timer 26 that
begins timing out in a preselected shut down time when enabled. If
the shut down timer 26 times out, a signal delivered on a line 27
to a lock out apparatus 28 causes the system 21 to lock out. Firing
of the first SCR 24 produces a signal on a line 29 that performs
three functions. An igniter 31 is energized in response to a signal
on the line 29 and a fuel valve 32 is opened in response thereto.
Thus when the first SCR 24 fires, fuel is supplied to a burner (not
shown) and the igniter 31 seeks to ignite the fuel. Simultaneously,
an ignition timer 33 begins running in response to the signals on
the line 29. If the ignition timer 33 times out indicating that the
first SCR 24 has been firing for a preselected period of time, a
signal on a line 34 is delivered to the lock out apparatus 28 thus
locking out the system 21. As was pointed out above, the first SCR
24 fires whenever there is a signal on the line 25. Thus the
presence of a signal on the line 25 starts operation of both the
shut down timer 26 and the ignition timer 33. When either timer 26
or 33 times out, the lock out apparatus 28 is activated. Thus the
two timers 26 and 33 are both "ignition" timers and the provision
of two separate timers is a safety feature. Disposed near the
burner is a flame sensor 35 that fires a second SCR through a line
37 once each cycle of a.c. power when flame is sensed. When the
signal on the line 37 is delivered to the second SCR 36, it fires
producing pulses on a line 38 that maintain the valve 32 in an open
position and resets the delay timer 22 through a periodic reset
line 39.
During operation of the system 21 power is applied and the delay
timer 22 times out in the delay time of greater than one cycle of
the a.c. supply voltage. When the delay timer 22 has timed out, the
first SCR 24 begins to fire and the shut down timer 26 and the
ignition timer 33 begin to run. Also in response to the firing of
the first SCR 24, the igniter 31 and fuel valve 32 are energized.
Under normal circumstances flame will be established before either
the shut down timer 26 or the ignition timer 33 has timed out. In
that event, the flame sensor 35 begins firing the second SCR 36
which maintains the valve 32 in an open position and, upon firing
once each cycle of the supply voltage, resets the delay timer 22
through the periodic reset line 39. Recalling that the delay time
is greater than the period of the a.c. supply voltage, it is seen
that the delay timer 32 is prevented from timing out while the
second SCR 36 is firing. Thus it is seen that as long as flame is
sensed by the flame sensor 35 the shut down timer 26 and the first
SCR 24 are inoperable. If flame is lost, the second SCR 36 ceases
firing the delay timer 22 soon times out thus causing the first SCR
24 to resume firing. Consequently the effect of a loss of flame is
that the system behaves as it does when initially energized. Thus
if flame is reestablished the second SCR 36 begins to fire again
and the first SCR 24 is inactivated and the shut down timer 26 is
periodically reset.
If flame is not established initially, or following an effort to
reignite, the system 21 is locked out upon the timing out of either
the shut down timer or the ignition timer 33.
Referring now to FIG. 2 there is a schematic diagram of the burner
control system 21. Portions of the circuit corresponding to the
blocks in FIG. 1 have been pointed out with similar reference
numerals where possible. A "hot" line 41 in an a.c. power supply in
connected to a buss 42 by a switch 43 such as, for example, a
thermostat. A grounded line 44 is connected to a lock out thermal
circuit breaker 45, that is part of a power input apparatus, so
that the current flowing through the line 44 passes through an
energy accumulating bimetalic strip member 33. A threshold member
34 in the circuit breaker 45 separates switch deactivator lock out
contacts 28 in the event of a circuit breaker overload as evidenced
by an excessive amount of heat building up in the bimetalic member
33. The heat energy in the bimetalic member 33 is supplied by
heating caused by current flowing therethrough and the surface of
the bimetalic element 33 radiates heat from the strip 33 to the
atmosphere and thus comprises an energy leakage system. Because
energy is radiated by the surface of the bimetalic strip 33, the
circuit breaker 45 will not respond to energy supplied thereto at a
low rate. The circuit breaker 45 connects the grounded line 44 to a
junction 46. The power supplied on the lines 41 and 44 is
alternating current and the term positive half cycle means that
half of the cycle of the alterntating current in which the line 44
is positive with respect to the line 41. It will be appreciated
that the absolute potential on the grounded line 44 does not change
and that changes in voltage refer only to relative values with
respect to power line 45.
Controlled by the system is a fuel burner 47 that is grounded and
is supplied with fuel through a line 48 in response to a valve
control apparatus 32 including a valve control relay coil 49 that
is shunted by a capacitor 51. When the coil 49 is energized the
valve is opened. A diode 52 couples the coil 49 and capacitor 51
combination across a resistor 53. One end of the coil 49 is
connected to the common buss 42 along with one end of the capacitor
51 and the resistor 53. The other end of the resistor 53 is
connected in series with a capacitor 54 and thence another resistor
55. At a junction 56 the resistor 55 is connected to a spark
capacitor 57, the other end which is connected to the buss 42. The
junction is also connected to the anode of the second SCR 36 by a
diode 106 and a resistor 89. During positive half cycles of the
input voltage, the capacitors 54 and 57 charge through the spark
igniter apparatus 31 that includes a resistor 58 and diode 59 in
series with a primary winding 61 of a spark transformer 62. Current
flow in the above described spark circuit is prevented during
negative half cycles of the supply voltage by the diode 59. Also
included within the ignition apparatus 31 is a secondary winding of
the transformer 62 with two spark electrodes 64 and 65 connected
thereto. The magnitude of the charging current is insufficient to
cause sparking between the electrodes. The flame rectification
flame detector aparatus 35 includes a resistor 66 connected between
the electrode 65 and a flame rectification capacitor 67. The other
terminal of the capacitor 67 is connected to the buss 42. Shunting
the capacitor 67 is a resistor 68 and connected to a parallel
combination of a capacitor 69 and complementary silicon controlled
rectifier 71 by another resistor 72. Two capacitors 73 and 74
connected in series and joined at a junction 75 shunt the
complimentary silcon controlled rectifier 71. A resistive voltage
divider including a resistor 76 and a resistor 77 spanning from the
junction 46 to the buss 42 supplies current to the gate 78 of the
complementary silicon controlled rectifier 71.
The first electronic switch apparatus 23 including the first SCR 24
is made to conduct by applying a voltage to a junction 81 that
powers a voltage divider control including two resistors 82 and 83
that supply current to the gate 84 of the SCR 24. The second
electronic switch apparatus 85 including the second SCR 36 receives
power from the junction 46 through a resistor 86, a diode 87, a
inhibit diode 88 and another resistor 89. The preceding circuit is
a cut off control circuit 90. The gate 91 of the SCR 36 is
connected to the junction 75 by the line 37 and to the buss 42 by a
resistor 92.
The delay timer clamping capacitor 22 connects a periodic reset
line 94 to the buss 42. The cut off circuit 90 and the delay timer
clamping capacitor 22 are part of an ignition interruption
apparatus that deenergizes the ignition apparatus 31 upon the
sensing of a flame by the flame sensor 35 as will be described more
fully below.
The shut down timer 26 includes an energy accumulator capacitor 95
and a leakage resistor 96 in series and connected between the line
94 and the buss 42. A junction 97 between capacitor 95 and the
resistor 96 is coupled to the gate 98 of a shut down silicon
controlled rectifier 99 by a neon bulb 101. A capacitor 80 and a
resistor 90 are connected in parallel between the gate 98 and the
cathode of the SCR 99 and the anode is coupled to the line 94 by a
resistor 105. Any energy absorbed by the capacitor 95 is leaked off
through the leakage resistor 96 when the second SCR 36 is firing as
described below. When the SCR 99 fires, it acts as a controlling
apparatus for the first SCR 24 so that the SCR 24 conducts. A
control circuit 102 including a capacitor 103 and a neon bulb 104
supplies current to the gate 84 of the first SCR 24 through the
junction 81. The capacitor is charged through a resistor 105.
Referring now to FIG. 3a there are shown charging curves for the
capacitors 103, 22 and 95. It is to be understood that no specific
time constants are shown because the exact time constants are less
important than the relationship among the three charging time
constants. It should be further understood that the curves shown
are for charging each capacitor disregarding the effect of the
other capacitors. Specifically, the clamping action of the
capacitor 22 on the capacitor 95 is ignored in FIG. 3a. The time t
represents approximately one cycle of the alternating supply
voltage. Thus it is seen by a curve 111 that in this example the
capacitor 103 is nearly fully charged after one cycle. The delay
capacitor 22, as represented by a curve 112, requires several
cycles to obtain a substantial charge and the capacitor 95 requires
many cycles as shown by a curve 113. The capacitor 95 could, for
example, take approximately 10 seconds to charge.
During operation of the system 21 a.c. power is supplied and during
the positive half cycles thereof current flows through the circuit
breaker 45, the diode 59 and the primary winding 61 to charge the
capacitors 54 and 57, which nearly fully charge during one half
cycle. In addition, current flows through the resistor 86 to the
capacitors 103, 22 and 95. During negative half cycles of power,
the capacitor 103 is bypassed by a diode 100 and thus does
discharge. The diode 87 prevents discharge of the capacitors 22 and
95 in the negative half cycles except through the SCR 36. Two paths
of discharge are available for the capacitors 54 and 57. One path
is through the primary winding 61 and then through the SCR 24. The
second is through the resistor 89 and then through the second SCR
36.
To more fully understand the operation of the system 21, reference
should be made to FIGS. 3(b) - (f). A sine wave form 121 shown in
FIG. (3b) represents the alternating current power supplied to the
system 21 and is used to establish a time scale for FIGS. 3(c) -
(f). A curve 122 in FIG. 3(c) shows the delay capacitor 22. A small
amount of charge is gained during each positive half cycle of the
sine wave 121 and the charge on the capacitor 22 remains constant
during negative half cycles. The charge on the capacitor 103 is
shown by a wave form 123 in FIG. 3(d). The capacitor 103 can
substantially charge during one positive half cycle of the sine
wave 121. However, during the positive half cycles the diode 87 is
forward biased and thus is conductive so that the charging of the
capacitor 103 is initially delayed by the clamping of the delay
clamping capacitor 22 as shown in FIGS. 3(c) and (d).
After several cycles the capacitor 22 approaches full charge each
cycle and allows the capacitor 103 to fire the neon bulb 104.
Firing occurs at near the peak of the positive half cycle of the
sine wave 121 as shown at the points 124 in FIG. 3(d). Discharge of
the capacitor then proceeds through the bulb 104 and the wave
shaping resistors 82 and 83 to supply current that causes the first
SCR 24 to conduct. The resistor 82 length the discharge period of
the capacitor 103 so as to prolong the current input to the gate 84
and thereby the conduction period of the SCR 24. After an initial
period of discharge, the bulb 104 stops conducting and the
discharge proceeds as shown by the curved portion 125 of the wave
form 123. Thus the first SCR 24 conducts during half of the
positive half cycle as shown by a wave form 126 in FIG. 3(e).
Inasmuch as the capacitors 54 and 57 absorb substantially a full
charge during each positive half cycle of the wave form 121, they
supply a substantial current to the primary winding 61 as they
discharge through the SCR 24. This current creates sufficient power
in the secondary winding 63 to cause a spark between the electrodes
64 and 65. In addition, the discharge of the capacitor 54 creates a
current through the resistor 53 and a voltage drop thereacross as
indicated in FIG. 2. This voltage drop forward biases the blocking
diode 53 and thus activates the relay coil 49. It should be noted
that even if the diode 52 were to become shorted, the valve would
not open when the capacitor 54 is charging. The current flow then
is to low due to the resistor 58. Generation of a large enough
voltage across the resistor 53 requires storing a charge in the
capacitor 54 and drawing it out in a rapid surge that bypasses the
resistor 58. In addition, the capacitor 51 stores a sufficient
charge to maintain the valve open until the following positive half
cycle. Thus gas is released from the burner 47 and the ignition
apparatus 31 sparks when the SCR 24 fires.
When flame is achieved at the burner 47 current is conducted
between the burner and the electrode 65 in accordance with the
flame rectification phenomena, thereby charging the capacitor 67 to
the polarity indicated in FIG. 2. This charge is filtered and
impressed across the capacitor 73 by the resistors 68 and 72 and
the capacitor 69. The capacitors 73 and 74 are connected in series,
and the combination is in parallel with the capacitor 69 and thus
they are charged with the polarity indicated in FIG. 2. Note that
the capacitor 74 is charged to a lower level than the capacitor 73
due to the drain of the resistor 92. In order for the complementary
SCR 71 to conduct, the gate 78 thereof must receive current from
the anode. This situation occurs during each negative half cycle of
the sine wave 121 due to the resistors 76 and 77. In order for the
second SCR 36 to fire, it must pass current from the gate 91 to the
cathodes. Under normal circumstances, precisely the opposite is
true due to the charge on the capacitor 74. However, when, as a
result of flame rectification, a sufficient charge has built up on
the capacitor 73, the complementary SCR 71 is fired at the negative
going crossovers 127 of the wave form 121. When the complementary
SCR 71 fires, it effectively connects the negatively charged
terminal of the capacitor 73 to the buss 42, thus discharging the
capacitor 73 through the gate 91 of the second SCR 36.
Consequently, when there is sufficient charge on the capacitor 73,
the complementary SCR 71 and the second SCR 36 both fire on the
negative going crossovers 127. The firing cycle of the second SCR
36 is shown by a wave form 128 in FIG. 3(f). Comparing FIGS. 3(e)
and (f) it is noted that the first firing of the second 36 occurs
precisely at the conclusion of a conducting cycle of the first SCR
24. Thus the capacitors 54 and 57 have been previously discharged
by the first SCR 24. However, as shown by FIG. 3(c), the first
firing and each subsequent firing of the second SCR 36 discharges
the delay capacitor 22 through the inhibit diode 88. Inasmuch as
the second SCR 36 fires every cycle if flame is sensed, the delay
capacitor 22 requires several cycles before a sufficient charge can
be built up to permit the first SCR 24 to fire, the first SCR 24
does not fire when the second SCR 36 is firing.
The capacitor 54 and 57 each continue to absorb a full charge
during each positive half cycle of the wave form 121. However,
discharge is now through the second SCR 36. Thus the voltage is
still produced across the resistor 53 to maintain the valve in open
position, however, the primary winding 61 of the transformer 62 is
bypassed and thus the spark ignition apparatus 31 is deenergized
and the spark is extinguished. This mode of operation continues as
long as flame is sensed.
Note the constant time lines T.sub.1 and T.sub.2 in FIGS. 3(b) to
(f). It will be observed that the values represented by the wave
forms are identical at each line. Thus it will be appreciated that
if the firing of the second SCR 36 indicated by a pulse 129 on the
wave form 128 were not to occur, the situation would be precisely
as it was at the time T.sub.1. The pulse 129 will not occur if
flame is lost because the flame rectification capacitor 67 then
becomes discharged. Thus it will be appreciated that if flame is
lost, the system 21 automatically recycles to try for
reignition.
Consider the system operation in the event of a failure to
establish flame. When the delay capacitor 22 becomes charged after
a few cycles, the energy absorbing capacitor 95 begins to charge.
After 10 seconds the capacitor 95 has stored a sufficient charge to
fire the neon bulb 101 which causes the SCR 99 to conduct during
positive half cycles. Conduction of the SCR 99 supplies current to
the resistor 83. Consequently, the SCR 24 conducts during each
positive half cycle as shown by a wave form 131 shown in FIG. 3(g).
Little current limitation is provided by the resistor 58.
Consequently the lock out thermal circuit breaker 45 opens quickly
thereby locking out the system 21.
Shown in FIG. 3(h) is a wave form 132 that represents the current
passing through the lock out circuit breaker 45 when the first SCR
24 is firing normally to establish ignition. The small lobe 133 is
due to the charging of the capacitors 54 and 57. The large
conducting portion 134 corresponds in shape to the firing of the
first SCR 24 as shown in FIG. 3(e) and indeed represents the firing
of the first SCR. The first SCR 24 conducts such a large current
because it fires during the positive half cycles of the supply
voltage and thus a current path is established from the junction 46
through the resistor 58, the diode 59 and the SCR 24 to the buss
42. Consequently, a strain is put on the thermal lock out circuit
breaker 45 during the long duty cycle of the first SCR 24. If the
shut down SCR 99 fails to fire for any reason, the continued firing
of the first SCR 24 in an effort for ignition will cause the
circuit to lock out after approximately 15 seconds. Conversely, if
the control circuit 102 malfunctions such that the first SCR 24
fires late in the positive half cycle, the large lobe 134 will not
occur and continued operation of the SCR will not put a strain on
the circuit breaker 45, the valve may be held in an open position.
In that event, conduction of the SCR 99 is relied on to cause lock
out. The provision of two possible methods for lock out is
beneficial as a safety feature.
FIG. 3(j) shows a wave form indicating current passed by the lock
out circuit breaker 45 when the second SCR is firing. Small lobes
135 correspond to the charging of the capacitors shown by the lobes
133. There is no large current surge when the SCR 36 fires at the
negative going crossovers because no substantial power is being
applied to the lines 41 and 44 and what power is applied during the
negative half cycles reverse biases the second SCR after the
discharge of the capacitors 54 and 57. Thus the only power
conducted by the second SCR 36 is the discharge of the capacitors
22, 54 and 57. Consequently, continued operation of the second SCR
36 will not cause the activation of the lock out circuit breaker
45. However, should the second SCR 36 becomes shorted or leaky,
power will pass therethrough during the positive half cycles,
causing overloading of the circuit breaker 45.
Obviously, many modifications and variations of the present
invention are possible in light of the above teachings. For
example, the preignition timing may be extended to provide
substantial "purge" time and the ignition timer may be adapted to
closing the valve without opening the circuit breaker using
conventional circuitry. It is therefore, to be understood that
within the scope of the appended claims the invention can be
practised otherwise than as specifically described.
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