Ignition system for fuel burning apparatus

An ignition system is disclosed which controls an electrically operated fuel valve and a spark generating apparatus to cause the fuel to be ignited at a fuel burning apparatus. The system also includes a flame sensing means which inhibits operation of an oscillator a predetermined time interval after operation is initiated in the event a flame at the fuel burning apparatus is not detected by the flame sensing means. The output of the oscillator is applied to a voltage converter which, in turn, controls valve actuation and spark generation. The invention can be used to light either a main burner directly, or indirectly through a pilot burner.

BACKGROUND OF THE INVENTION 
There are many different types of fuel ignition systems known in the prior 
art. One type of system which has become popular employs a pair of spark 
electrodes which create a spark to ignite fuel issuing from a fuel burner. 
Fuel flow to the burner is controlled by an electrically operated valve, 
and generally, the rectifying property of a flame is used to detect the 
presence of a flame at a burner. 
As shown in U.S. Pat. No. 4,019,854, it is known to employ a multivibrator 
in such systems and, in particular, to apply the multivibrator output to a 
voltage converter, the output of which is applied to the spark generator 
and gas valve. In the arrangement shown in this patent, a gate controls 
the multivibrator. A timing capacitor is charged to actuate the gate so as 
to initiate operation of the multivibrator. If a signal from a flame 
sensing circuit is not received within a predetermined amount of time, the 
gate is deenergized to deenergize the multivibrator. 
Another device which employs a multivibrator to control a voltage converter 
which, in turn, controls the spark generator and the gas valve is shown in 
U.S. Pat. No. 3,853,455. It will be seen that a charge built upon a 
capacitor is used to initiate operation of a multivibrator. In the event a 
flame sensing circuit supplies power to the multivibrator before the 
charge on the timing capacitor dissipates, the multivibrator continues 
operating. If, however, no flame appears before the charge on the 
capacitor dissipates, the multivibrator is prevented from operating 
further. 
Another patent disclosing the idea of an oscillator controlling the spark 
and fuel valve is U.S. Pat. No. 3,514,240. The device disclosed in this 
patent also utilizes a safety timer lock-out circuit comprising a timing 
network and a transistor to deenergize the oscillator circuit if a flame 
has not been detected at the main burner. 
One of the problems associated with prior art devices has been sensing 
small current which flows through the flame so as to "prove" ignition. Due 
to the small magnitude of the flame sensing current in the prior art, it 
has generally been necessary to provide some sort of amplification in 
order to properly sense the current. The additional amplification adds to 
the complexity of the control circuitry and increases the chance of a 
failure. 
SUMMARY OF THE INVENTION 
It is thus an object of this invention to provide an ignition system for a 
fuel burning apparatus which is simple, troublefree, reliable in 
operation, and which obviates the problems associated with prior art 
devices. 
This object as well as others which will become apparent as the description 
proceeds are accomplished by utilizing an astable multivibrator in an 
ignition control circuit which is comprised of at least one logic gate 
having a high impedance inhibit input. By using a gate with a high 
impedance input, the need for additional amplifier(s) is eliminated, thus 
inherently simplifying the design of the ignition control circuit. The 
inhibit input to the oscillator is normally held at a positive voltage 
level which is sufficient to prevent oscillation. However, when 
oscillation is to be initiated, the inhibit input is connected to ground 
through a discharged capacitor which forms a lock-out time control 
function. A flame sensing circuit is also connected to the inhibit input 
in such a manner that the oscillator input is held close to ground 
potential if a flame is sensed so as to maintain oscillation. In the event 
a flame is not sensed, the lock-out time control capacitor charges after a 
predetermined time interval to a voltage level sufficient to inhibit 
oscillation. The oscillator output is applied to a voltage converter which 
controls valve actuation and spark generation in a conventional manner.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to FIG. 1, the ignition system is adapted to be connected to 
a suitable power source by conventional methods at lines L1 and L2. The 
power source may be a 24 volt AC source such as is commonly employed in 
furnace control circuits and the like, or it may be a 12 volt DC source 
such as is encountered in recreational vehicles, campers and the like. A 
normally open, single pole single throw thermostat 10 is connected to L1 
such that it controls current flow to a valve actuation circuit 12 and a 
voltage regulator and surge protection circuit 14 which supplies regulated 
power between lines L3 and L2. Connected between lines L3 and L2 is a 
purge timer 16 which serves to provide a time delay between thermostat 
closure and spark actuation during which fuel is purged from the burner 
area. As it will hereinafter be seen, the purge timer is old in the art. 
An astable oscillator circuit 18 providing low frequency output pulses is 
connected at two points to line L3 and at two points to a pair of lock-out 
time control circuits 20a and 20b and a flame sense circuit 22. The flame 
sense circuit 22 is essentially a bypass circuit which precludes charging 
of lock-out time control circuits when a flame is established. The 
oscillator circuit 18 is comprised of logic gates as will hereinafter be 
described in connection with FIG. 2 and because it is conventional not to 
show the power supply and ground connections for such components, they 
have not been shown in FIGS. 1 and 2. The output of the purge timer on 
line L4 is applied to the lock-out time control circuits 20a, 20b and to a 
voltage converter driver 24 along with the oscillator output on line L5. 
The voltage converter driver 24 is essentially a switching circuit, 
enabled by the purge timer and supplying an output switching signal on 
line L6 to cause a voltage converter circuit 26 to produce AC voltages at 
its output. A first AC voltage on line L8 at the voltage converter output 
is applied to the valve actuation circuit 12 which serves to connect the 
electrical operator of an electrically operated fuel valve 28 across lines 
L1 and L2 via line L7 and thermostat 10. The second AC output voltage from 
voltage converter 26 is applied on line L9 to a trigger and spark 
generating circuit 30 having a pair of output electrodes arranged in close 
proximity to a fuel burner 32 arranged to burn fuel supplied to it from 
valve 28. In addition, the second AC output voltage from voltage converter 
26 is applied on line L10 to the flame sensing circuit 22 which senses a 
flame at burner 32 as a result of the flame rectified current flowing from 
the flame sensing circuit 22 to flame rod 34 which is situated in the 
flame, through the flame, and to ground. It will be seen that the flame 
sensing circuit essentially holds the inhibit input to the oscillator near 
ground level if a flame is sensed to maintain the oscillating condition of 
the oscillator. However, if a flame does not occur the lock-out time 
control brings the inhibit input to a voltage level which is sufficient to 
prevent oscillator oscillation so as to deenergize the voltage converter. 
Reference will now be made to FIG. 2 for a more thorough discussion of the 
various components of the system described in FIG. 1. More specifically, 
the voltage regulation and surge protection circuit 14 may include a solid 
state diode 36 having its anode connected to line L7 and located in series 
with a filter capacitor 38. A voltage divider network consisting of 
resistors 40 and 42 is connected across capacitor 38 with line L3 
connected to the junction of resistors 40 and 42. An additional filter 
capacitor 44 and voltage stabilizing zener diode 46 are connected in 
parallel with resistor 42 to insure that line L3 is held at a 
substantially constant voltage level. 
The purge timer 16 preferably is controlled by a timing network comprised 
of a resistor 48 and a capacitor 50 connected in series across lines L3 
and L2. A NOR gate 52 having both inputs connected to the junction of 
resistor 48 and capacitor 50 and its output connected to the junction of 
lock-out time control circuits 20a and 20b responds to the timing network 
such that its output on L4 is normally high and switches to a low 
condition when capacitor 50 accumulates a sufficient charge. A solid state 
diode 53 is connected across resistor 48 to provide a discharge path for 
capacitor 50 whenever power is removed from the system. 
Preferably, the astable oscillator circuit 18 includes a pair of input 
resistors 54a and 54b each connected to L3 and leading to an input of a 
pair of NOR gates 56 and 58 respectively. The output of NOR gate 56 is 
coupled to the other input of gate 58, the output of which is applied to 
line L5. A timing circuit which is effective to cause the oscillating 
condition is comprised of a capacitor 60 responsive to the output of NOR 
gate 58, a resistor 62 connected between the output of gate 56 and the 
capacitor 60, and a resistor 64 connected between the second input to gate 
56 and the junction of resistor 62 and capacitor 60. The values of 
resistors 54a and 54b are chosen so that the voltage at the inputs to 
gates 56 and 58 is substantially equal to the output voltage of gate 52 
for reasons which will hereinafter become apparent. It will be noted that 
whenever the inputs to gates 56 and 58 are switched to near ground 
potential, the oscillator circuit 18 will begin to oscillate at a 
frequency which is determined by the relative values of capacitor 60 and 
resistor 62. 
Each of the lock-out time control circuits is comprised of a single 
capacitor (66a and 66b respectively) which is connected between the output 
of gate 52 and the input of gate 56 and 58, respectively. It will thus be 
seen that whenever the output of gate 52 is high, capacitors 66a and 66b 
will essentially be discharged because there will be very little voltage 
difference across them. However, when the output of gate 52 is switched to 
near ground potential, the inputs to gates 56 and 58 will be near ground 
potential also due to the discharged state of capacitors 66a and 66b, so 
as to cause the oscillator circuit 18 to oscillate. The lock-out time 
control capacitors 66a and 66b will immediately begin to accumulate a 
charge, however, and unless the flame sensing circuit 22 acts to hold the 
input to gates 56 and 58 at near ground level, the oscillator will cease 
to oscillate when a sufficient charge is built up on lock-out time control 
capacitors. 
In accordance with the present invention, the flame sensing circuit 22 
includes a pair of solid state diodes 68a and 68b each having its anode 
connected to the input of gate 56 or 58 respectively and their cathodes 
connected together. A resistor 70 is connected between the junction of 
diodes 68a and 68b and a flame sensing rod 72 which is situated to be 
enveloped by the flame at burner 74. As is well known in the art, the 
flame acts as an electrical conductor so that the junction of diodes 68a 
and 68b is brought to near ground potential when a flame is present at 
burner 74. Thus, whenever a flame is present at burner 74 the inhibit 
inputs to gates 56 and 58 will be held to near ground potential to insure 
that the oscillator continues to oscillate after a flame is detected. A 
capacitor 76, connected between L10 and the flame sensing rod 72 acts as a 
filter for the flame sensing circuit. 
As will be seen in the drawing, the output of the purge timer on line L4 
and the output of the oscillator on line L5 are applied to the voltage 
converter driver 24 which comprises a NOR gate 78, which provides a high 
output on line L6 when lines L4 and L5 are near ground potential. Thus, 
the output on line L6 consists of a pulse train whenever the oscillator is 
oscillating and is near ground potential when it is not. 
The voltage converter circuit 26 consists of a conventional transformer T1 
of the type normally employed in such control circuits and having a 
primary winding 80 connected in series with a resistor 82 and a gated 
solid state switching device 84 situated to be gated by the signal on line 
L6. In addition, the transformer T1 has a pair of secondary windings 86 
and 88 which are situated to each provide a different output voltage, one 
for the valve actuation circuit 12 and the other for a spark generation 
and trigger circuit 30. The valve actuation circuit 12 is situated in 
circuit with secondary winding 86 and has a solid state diode 90 having 
its anode connected to line L8, a relay coil 92 connected to the cathode 
of diode 90 and to line L2, and a capacitor 94 connected between the 
cathode of diode 90 and L2. The valve actuation circuit further includes a 
normally open single pole single throw electrical contact 96 controlled by 
relay coil 92 connected to line L7, and an electrically operated valve 
actuator 98 which is then connected to line L2. Capacitor 100 in parallel 
with electrically operated valve actuator 98 acts as a smoothing 
capacitor. 
Spark generating and trigger circuit 30 is connected in circuit with 
secondary winding 88 and is substantially conventional in design. It 
should therefore suffice to say that it includes a solid state diode 102, 
a timing network comprising capacitor 104, resistor 106 and capacitor 108. 
In addition, the spark generating and trigger circuit includes a voltage 
breakdown device 110 such as a neon tube in series with a diode 112 and a 
resistor 114 connected in parallel with trigger capacitor 108. The gate of 
an SCR 116 is connected intermediate diode 112 and resistor 114 such that 
it is rendered conductive in response to breakdown of neon tube 110. 
Located in series with SCR 116 is the primary winding 118 of high voltage 
transformer T2 which has a secondary winding connected in circuit with a 
pair of conventional spark electrodes 122 arranged to ignite fuel issuing 
from burner 74. 
Now that the circuit of FIG. 2 has been described in detail, its operation 
will be briefly described. First, it will be assumed the circuit is in the 
off condition. Under such conditions, thermostat 10 will be open and the 
output gate 52 will be high so as to maintain the oscillator in the 
non-oscillating condition. Thus, voltage converter 26 will be deenergized 
to prevent energization of electrical valve operator 98 and spark 
discharge electrodes 122. When the thermostat closes, however, the purge 
timer 16 will cause line L4 to go to ground so as to initiate operation of 
oscillator 18. The oscillator's output will drive the voltage conversion 
circuit which will actuate valve actuation circuit 12 and spark generation 
and trigger circuit 30 to open the electrically operated valve and cause 
sparks to occur at the spark generating electrodes 122. After a flame has 
been generated at the burner 74, the inhibit input to gates 56 and 58 will 
each be held near ground potential so as to maintain the oscillating 
condition of the oscillator as a result of the conductive path to ground 
through the flame electrode 72 and the flame. In addition, the spark 
electrodes will be shorted by the flame to discharge trigger capacitor 108 
through the secondary winding 120 of high voltage transformer T2 so as to 
deenergize the sparking circuit. In the event, however, a spark is not 
generated or a flame is not ignited, the lock-out time control capacitors 
66a and 66b will accumulate a sufficient charge and cause the inhibit 
inputs to gates 56 and 58 to be at a sufficiently high enough level to 
prevent oscillation so as to deenergize the valve actuation and spark 
generation and trigger circuits. 
It will be appreciated that the circuit of FIG. 2 utilizes a number of 
conventional components, but that the use of an oscillator circuit having 
at least one gate with a high input impedance is one of the novel aspects 
of this invention. It will further be appreciated by those skilled in the 
art that the high input impedance NOR gates can be implemented with CMOS 
technology. Another novel aspect of the circuit of FIG. 2 lies in the 
interaction between the oscillator, lock-out time control circuits and the 
flame sensing circuit. It will also be noted by those skilled in the art 
that in the circuit of FIG. 2 resistors 54a and 54b are redundant as well 
as capacitors 66a and 66b and diodes 68a and 68b. These components have 
been redundantly designed for the degree of safety necessary for this type 
of system. 
The inventive concepts embodied in the system shown in FIGS. 1 and 2 are 
applied to a system in which direct ignition of the main burner takes 
place. If desired, the inventive concepts may be applied to a pilot 
relight type system as well. Such a system is shown in FIG. 3 and is, in 
general, the same as the direct light system of FIGS. 1 and 2 with the 
additional provision of an additional electrically operated valve and 
associated valve actuation circuitry operated by a second voltage 
converter which responds to the oscillator output and a signal from the 
flame sensing circuit. 
More specifically, in FIG. 3 the circuits which are essentially the same as 
in FIG. 2, have been enclosed in dotted lines and have been given the same 
reference numerals. In addition, in FIG. 3, the main burner is identified 
as reference numeral 126, the pilot burner as reference numeral 128, the 
electrically operated main burner valve is identified as reference numeral 
130 and the electrically operated pilot valve is identified by reference 
numeral 132. The pilot relight system disclosed in FIG. 3 additionally 
includes a NOR gate 134 having one input connected to the oscillator 
output on line L5 and another input connected to the junction of a 
resistor 136 and a capacitor 138 serially connected across lines L3 and 
L2. Also connected to the other input of NOR gate 134 is the anode of a 
solid state diode 140 which is connected to the flame sensing rod 72 
through a current limiting resistor 142. Thus, the output of gate 134 on 
line L10 is normally low, except when a flame is sensed it oscillates in 
the same manner as the oscillator. The output of gate 134 is applied to a 
second voltage converter including a solid state switch 144 and a third 
transformer T3. The primary winding 146 of transformer T3 is connected in 
series with a voltage reducing resistor 147 and solid state switch 144 
across lines L3 and L2. Secondary winding 148 of transformer T3 is 
connected in series with a diode 150 and a relay coil 152 which actuates 
normally open single pole single throw switch contacts 154 which act to 
connect the electrical operator of the electrically operated main valve 
130 in series with the switch contacts of the operator for pilot valve 
132. Thus, the electrically operated main valve 130 is actuated when a 
high signal on line L10 is present and electrically operated pilot valve 
132 has been previously actuated. 
The invention has been disclosed in two different embodiments which have 
been used for exemplary purposes only. It is intended that the scope of 
the invention be determined by the claims.