Note: Descriptions are shown in the official language in which they were submitted.
- 1078915
BACKGROUND OF THE INVENTION
The present invention is directed to a capacitor discharge
ignition circuit for an internal combustion engine and more
particularly to a capacitor discharge ignition circuit in which
all of the operative coils are wound on the same magnetic core.
Capacitor discharge ignition circuits are well known.
Such circuits generally include a charging coil in which is
generated the current utilized to charge a storage capacitor
and a trigger coil utilized to generate the current necessary
to effect operation of an electronic switch in the discharge
circuit of the capacitor. The discharge circuit of the capa-
citor includes the primary winding of a high voltage transformer
so that the operation of the electronic switch to discharge the
capacitor through the primary winding provides ionization
potential across the air gap of an ignition device such as a
spark plug for an internal combustion engine.
SU~-~RY OF THE INVENTION
The present invention provides a capacitor discharge
ignition circuit comprising:-
an ignition capacitor;
a charge coil for charging said capacitor;
a high voltage transformer;
an SCR for discharging said capacitor through said high
voltage transformer;
a trigger coil for providing a trigger coil waveform;
a selectively operable shut-off coil positively induc-
tively coupled to said trigger coil to inhibit
generation of the trigger coil waveform; and,
circuit means including means for gating said SCR in
response to a discharge inducing compon nt of
the trigger coil waveform and for force commuta-
ting said SCR by applving the trigger coil
1(~78915
waveform to the SCR to induce a reverse anode-
to-cathode bias.
The "shut-off" coil is selectively switched into the
ignition circuit when it is desired to terminate the operation
of the engine. Such a shut-off coil is described in U.S. Patent
No. 3,894,524 issued July 15, 1975 assigned to the assignee o~
this invention. ;
It is known in such capacitor discharge ignition circuits
that the charging coil and trigger coil may be wound on the
same magnetic core whereby the timing of the charging and dis-
charging of the ignition capacitor may be controlled. In
accordance with another feature of the invention r the charge,
trigger and shut-off coils are wound in the same direction on
a common core. Three separate coils may be wound on the core
or one or two separate coils may be wound intermediate taps to
separate the charging and control functions to effect a substan-
tial reduction in the size and expense of the circuit as well
as a minimization of the assemblying process.
These and further features of the present invention
will become apparent to one skilled in the art to which the
invention pertains from a perusal of the following detailed
description when read in con~unction with the appended drawings.
THE DRAWINGS
Figure 1 is a schematic circuit diagram of the circuit
of the present invention illustrating certain current paths;
Figure 2(a) and 2(b) are illustrations of the waveforms
generated by the charge coil and trigger coil of the present
invention;
Figure 3(a) through (f) are illustrations of waveforms
generated at various points in the circuit of the present
invention; and,
--2--
.
Figure 4 is a schematic circuit diagram of a second
embodiment of a triggering and shut-off subcircuit which may
be employed in the circuit illustrated in Figure 1.
Figure 5 is a schematic circuit diagram illustrating a
third embodiment of the present invention; and,
Figure 6 is a timing diagram for the waveforms generated
in the circuit of Figure 5.
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DETAILED DESCRIPTION
To facilitate an understanding of the circuit of the
present invention reference may be had to the following detailed --
description of the circuit of a first embodiment of the present
invention shown in figures 1 and 4 and the circuit of a second r
embodiment of the present invention shown in figure 5. The
operation of each of the circuits in performing its various
functions may be found following the description of the respec-
tive circuits.
10The Circuit of the First Embodiment of the Present Invention
Referring first to Figure 1 ~ere a capacitor discharge
ignition circuit is illustrated as including a charge coil 10
formed, for example, of 2,500 turns of No. 36 wire and connected
in a circuit including a diode 12, an ignition capacitor 14,
and the primary winding 18 of a high voltage transformer 20.
Rotation of a flywheel magnet of the engine (not shown) into
and out of flux cutting proximity to the charge coil 10 operates
in the conventional manner to induce a voltage in the coil 10
having a waveform as generally depicted in Figure 2(a).
As illustrated in Figure 2(a), the charge coil waveform
may comprise a relatively small positive portion 50, a larger
negative portion 52, a large positive portion 54, and a rela-
tively small negative portion 56. The current generated
responsively to the voltage of the positive portions of the wave-
form illustrated in Figure 2(a) will be passed through the diode
12 to effect charging of the capacitor 14 to ignition potential.
The path of conventional current associated with the above-
mentioned charging of the capacitor 14 is denoted by the letter
"A" in Figure 1. The diode 12 will block the passing of the
current induced by the voltage of the negative portions of the
charge coil waveform illustrated in Figure 2(a~.
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With continued reference to Figure 1, the ignition capa-
citor 14 is connected in a discharge circuit including the
primary winding 18 of the transformer 20 and an electronic
switch such as the illustrated SCR 16. It should be understood,
however, that any switch capable of being electronically trig-
gered may be substituted therefor with appropriate changes in
the polarities of the several biasing diodes associated with
the switch and appropriate changes in the polarities of wave-
forms applied to the switch. In the case of the SCR 16, it
may be triggered into conduction by the application of a
positive gate-to-cathode potential at any time that the SCR has
a positive anode-to-cathode bias.
When triggered into conduction, the SCR 16 will effect the
discharge of the capacitor 14 through the primary winding 18 of
the transformer 20 as illustrated in Figure 1 by a current path
denoted by the letter "B". The primary winding 18 of transformer
20 may comprise 100 turns of No. 26 wire and the secondary win- --
ding 22 of the transformer 20 may comprise 7,000 turns of No. 44
wire. It will be apparent that the discharge of capacitor 14
through primary winding 18 will be inductively coupled by the
secondary winding 22 of transformer 20 to a conventional ioniz-
ation discharge device such as spark plug 24. The potential -
developed across the secondary winding 22 may serve as a gap
ionizing potential applied to spark plug 24 for engine ignition.
Current resulting from firing the spark plug 24 may flow
as indicated by current path "C" with the particular direction
of flow depending on the orientation of secondary winding 22
with respect to primary winding 18. The discharge of capacitor
14 may serve to temporarily store energy in the magnetic field
established in transformer 20 by current passage through the
primary winding 18. As that magnetic field collapses upon
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1()7891S
. .,
the cessation of current, a current path denoted by the letter
"D" in Figure 1 may be established through a diode 26 and the
capacitor 14 to effect a partial recharging of the capacitor
14 and, at the same time, causing the spark plug 24 to arc in
the reverse direction. With the trigger potential remaining on
the SCR gate electrode, the SCR 16 will again be triggered into
conduction to once more effect the discharge of the capacitor
14 through the primary winding 18 of transformer 20 to again
reverse the polarity of the arc of the spark plug 24. This
process will continue in the presence of a triggering potential
until the charge on capacitor 14 has been dissipated and has
been found to produce about four or five closely spaced arcs.
The charging sequence of the circuit of the present inven-
tion is illustrated graphically in the timing diagram of Figure
3. Figures 3(a) and 4(b) depict the charging coil and trigger
coil waveforms discussed below in connection with Figure 2.
Figures 3(c) through 3(f) depict voltage waveforms measured at ~ -
various points in the circuit with Figure 3(c) illustrating
the voltage appearing across the capacitor 14. At the point on
the waveform designated "80", a initial discharging of the
capacitor has been induced by the negative swing in the trigger
coil waveform. Voltage oscillations 82 represent the "ringing"
induced in the circuit by the repeated sequential chargings and
dischargings of the capacitor 14.
Referring once again to Figure 1, a control coil 30 m~y be
coaxially wound with the charge coil 10 may be tapped to provide
a trigger coil 31 and shut-off coil 32 alternatively the coils
may be wound separately on the core and the start end of the
shut off coils connected to the finish end of the trigger coil.
The trigger coil 31 may be formed of 100 turns of No. 36 wire
and the shut-off coil formed of 200 turns of No. 36 wire.
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1078915
Voltages may be induced in coils 31 and 32 by the passage of the
flywheel magnet (not shown) into and out of flux cutting prox-
imity with the control coil 30. The windings of the coils 10
and 30 are so oriented that the open eircuit output waveform of
trigger coil 31 and the waveform of charge coil 10 are substan-
tially in phase with one another. The phase relationship of
the trigger coil and charge coil waveforms is depicted in
Figure 2, wherein the output waveform of the trigger coil 31
measured from the center tap to ground appears as is illustrated -
in Figure 2(b). From a comparison of Figure 2(a) with Figure
2(b) it may be observed that the voltage waveforms are substan-
tially identical in shape and differ only in amplitude as a
funetion of the number of turns and impedanee of the coils.
The trigger coil 31 is connected to the gate electrode of `
SCR 16 by the novel circuit of the present invention. The
finish end of the trigger coil 31 may be connected to the
cathode of the SCR 16 by way of a resistor 34 in series with
the parallel combination of a resistor 36 and a diode 38. The
cathode of the SCR 16 may be connected to the gate of the SCR
16 through a resistor 40 and to ground through the parallel
combination of a resistor 42 and a diode 44. The gate of the
SCR 16 may be grounded by a diode 46. The finish end of the
trigger coil 31 is eonneeted at the tap 33 to the start end of
the shut-off coil 32 and the finish end of the trigger coil 31
may be grounded by the grounding of the finish end of the shut-
off coil 32 through a conventional manually operable switch 48.
The circuit illustrated schematieally in Figure 1 employs
a triggering and shut-off subeircuit comprised of the control
coil 30, the resistor 34 and the conventional switch 48. For
purposes of illustration, the triggering and shut-off subcircuit
is depicted as being connected to the remaining circuitry by
junctions 47 and 49.
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Referring now to Figure 4, a schematic circuit diagram of
a second embodiment of the triggering and shut-off subcircuit
of the present invention is illustrated. The subcircuit of
Figure 4 may be attached to Figure 1 by electrically connecting
junctions 60 and 62 of the circuit of Figure 4 with junctions
47 and 49, respectively, of the circuit of Figure 1.
Again referring to Figure 4, the junction 60 may be con-
nected to a resistor 64 and the cathode of a diode 66. The
junction 60 may be selectively grounded by a conventional,
manually operable switch 68. The anode of the diode 66 may be
connected to the finish end of a shut-off coil 70. Advanta-
geously, the shut-off coil may be formed of 100 turns of ~30
wire. Resistor 64 may be connected to the finish end of a
trigger coil 72. Advantageously, the trigger coil may be formed
of 100 turns of #30 wire. The start ends of the coils 70 and
72 may be grounded by connection to junction 62. Voltages may
be induced in the coils 70 and 72 by the passage of the flywheel
magnet (not shown) into and out of flux cutting proximity with
the coils. Coils 70 and 72 may be so oriented with respect to
one another and to charging coil 10 of Figure 1, that the out-
put waveforms with respect to ground of all three coils are
substantially in phase. This effect may be achieved by winding
the coils 70, 72 and 10 on the same core so that the coils are
positively inductively coupled.
With reference again to Figure 1, a gating current path
denoted by the letter "E" is established for the negative por-
tion of the trigger coil waveform induced from ground potential
(i.e., the start end of the coil 31 or coil 72 of Figure 4)
through the diode 46, the resistor 40, the diode 38, the
junction 47 and the triggering and shut-off subcircuit. It
will be apparent that when a negative voltage with respect to
1~)7~915
ground is induced at the junction 47, the cathode of the SCR 16
will be held negative with respect to the gate due to conduction
of the diode 46 whereby the gating of the SCR 16 may be effected.
Figure 3(d) illustrates the cathode-to-gate voltage which
reaches the minimum value necessary to trigger the SCR 16 into
conduction at the point 84 on the waveform. The positive going
spikes 86 in the waveform of Figure 3(d) reflect the repeated
swamping of the cathode potential imposed by the trigger coil
waveform due to a positive forward voltage developed across the
diode 44 when current flows along path "B".
With reference once more to Figure 1, the negative compon-
ent of the trigger coil waveform may tend to establish a flow
of conventional current along the path designated by the letter
"F". As will hereinafter appear, this current flow tends to
force commutate the SCR 16. -
When a positive voltage with respect to ground is induced
at the junction 47 during the positive portion of the induced
waveform of Figure 2(b) it will be apparent that the resistors
36 and 42 act as a voltage divider which will impose a positive
voltage on the cathode of the SCR 16 to prevent the conduction
thereof. The current path so established is denoted by the
letter "G" in Figure 1.
Force Commutation
It is important that conduction of the SCR 16 be inter-
rupted after firing since the current normally utilized to
charge the capacitor 14 will otherwise be shunted to ground
through the SCR 16 and the diode 44. A negative cathode-to-
gate voltage on the SCR applied at the junction 47 by the
trigger coil will serve to gate the SCR 16 into conduction and
to develop a voltage drop across the diode 44 due to the forward
resistance thereof. The voltage drop developed across diode 44
107891S
raises the voltage at the cathode of the SCR 16 and swamps the
negative gating voltage provided by trigger coil 31. As a
result, the conduction of the SCR 16 immediately serves to
back bias the diode 46 out of conduction and to thereby remove
the negative cathode-to-gate bias until the capacitor is dis-
charged. The relationship between the capacitor voltage and
the cathode-to-gate voltage is illustrated in Figures 3(c) and
3(d).
When the charge on the capacitor 14 is reduced below that
value which effects back biasing of the diode 46, a negative
cathode-to-gate potential may again be imposed by the negative
component of the trigger coil waveform. The SCR may remain
conductive permitting current flow along the conventional -
current path denoted by the letter "F". This latter current -
path pulls the SCR anode negative and reverses the charge on
the ignition capacitor. This removes the positive anode-to-
cathode SCR bias and force commutates the SCR out of conduction.
The effects on the anode potential due to the current flow
along path "F" are illustrated in Figure 3(e) by the small
negative voltage 88 appearing at the anode after discharge of
the capacitor. This small negative voltage may operate to
place a slight reverse charge on the capacitor.
Gate Bias Protection
The continued presence of the negative voltage component
of the trigger coil on the SCR cathode after discharge of the
capacitor may not produce an excessive gate-to-cathode current
in the circuit of the present invention. The SCR gate voltage
is the voltage drop across the diode 46 and the SCR cathode
voltage is the drop across the diode 26 and the SCR anode-to-
cathode junction. The maximum forward voltage which may appear
,.,, --10--
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107891S
.
across the diodes 46 and 26 will be nearly the same and will inany event, be limited to approximately 1 volt depending on the
composition of the semi-conductor material out of which the
diodes are constructed. Thus the maximum gate-to-cathode
potential is limited, approximately to the forward voltage
across the SCR from anode-to-cathode. This voltage drop will
be insufficient to damage the SCR when applied gate-to-cathode.
Circuit Temperature Stability
It is known in the art that both the triggering requirements
of an SCR and the anode-to-cathode holding current are dependent
on the temperature of the device junctions. It is desirable to
compensate for these temperature variations to provide ignition ;~
sparks of uniform timing and duration through a broad range of
ambient temperatures.
The biasing and interconnection of SCR 16 and diode 44 in
the present invention may provide improved circuit stability
over wide variation in ambient temperature. The forward resis-
tance across diode 44 remains relatively stable as temperature
incraases when compared to the forward resistance of the SCR 16.
The amount of current flowing through the series combination of
the SCR 16 and the diode 44 will depend on the sum of the for-
ward resistances presented by the devices. The thermal stability
of the forward resistance of diode 44 renders the series
resistance of the combination more stable. As a result the ~-
voltage required to maintain the holding current through the
SCR tends to remain constant.
Temperature compensation of the gate signal to SCR 16
may be obtained by the selection of trigger circuit parameters
to cause the impedance of the trigger signal source, i.e.,
trigger coil 31, resistor 34 and resistor 42, to increase with
1078g1S
increasing temperature and thereby reduce the gate current
in accordance with the reduced gate current requir~ment of
SCR 16 with increasing temperature. Hence, the conduction of
the SCR can be made quite stable over a wide range of temper-
ature changes.
The temperature stability of the circuit is also greatly
enhanced by the gate-to-cathode resistor 40. The selection of
the value of the resistance of the external resistor 40 several
orders of magnitude below that of the internal resistance of
the SCR insures that most of the current will flow through the
resistor 40. The impedance of the external resistor 40 is
relatively constant with respect to changes in temperature
while the internal impedance of the SCR changes nonlinearly
with variations in temperature.
Gate Sensitivity
With continued reference to Figure 1, the trigger coil
waveform and the circuit connecting the coil to the gate and
cathode of SCR 16 are operative to inhibit triggering of the
SCR 16 during the charging of the capacitor 14. This is a
desirable result, since triggering of the SCR 16 during capa- ;
citor charging could prevent proper charging of the capacitor ;~
14 and could discharge the gap ionization device 24 at an
incorrect time in a combustion cycle of the engine for which
the circuit provides an ignition spark.
With reference also to Figure 2 as previously noted, the
positive voltage component 54 of the charge coil waveform de-
picted in Figure 2(a) is applied to the capacitor 14 through
the diode 12. A positive waveform component 60 of the trigger
coil waveform depicted in Figure 2(b) appears at the tap 33
substantially in phase with positive charging component 54 of
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:1~)7891S
the charge coil waveform. This positive waveform component 54
of the trigger coil waveform results in a current along the
current path denoted by the letter "G" in Figure 1. Due to
the forward resistance of the diode 44, this current flow tends
to hold the cathode of the SCR 16 at a positive voltage with
respect to ground. Since no current flows through the back
biased diode 46, the gate and cathode of the SCR are held at
essentially the same positive voltage by the absence of current
through the resistor 40. The value of the current along path
"G" and the degree of back bias of the diode 46 is controll-
able by the selection of the value of the resistor 36. The
resistor 36 also suppresses the end of charging current tran-
sient and substantially diminishes the undesired triggering of
the SCR.
Referring to Figure 3, the cathode-to-gate voltage wave-
form is illustrated for the circuit of the present invention in
Figure 3(d). Figure 3(f) illustrates the cathode-to-gate
waveform for the circuit of the present invention where the
resistor 36 has been eliminated. A transient spike, denoted
by the numeral 90, is caused by back emf occurring at the end
of charging of the capacitor 14. The transient spike 90 may ~-
be sufficient to cause extraneous triggering at an incorrect
time in the combustion cycle of the engine. As may be noted
with reference to Figure 3(d), the back emf transient is
completely damped out in the circuit of the present invention
incorporating the resistor 36.
Referring once more to Figure 2(b), it may be noted that
the positive waveform component 60 of the trigger coil depicted
in Figure 2(b) is preceded by the negative waveform component
58 of the trigger coil waveform. As discussed above, triggering
` 1()7891S
of the SCR 16 induced by the appearance of negative voltage
component 58 at tap 33 of the trigger coil.
Engine Shut-Off
With reference to Figure 1, the operation of the circuit
heretofore described has assumed a condition in which the con-
tacts of the manually operable switch 48 have remained in an
open condition. The closure of the contacts of switch 48 by the
operator will insure engine shut-off.
As discussed above, the circuit of the present invention
may include a shut-off coil 32 connected in series with the
trigger coil 31 and disposed in flux cutting proximity to a fly-
wheel magnet of the engine. The shut-off coil 32 may be oper-
able to reduce the magnetic flux in proximity of the trigger
coil 31 when the series combination of the two coils is shorted
by means of the switch 48. Where the shut-off coil and trigger
coil are positively inductively coupled, i.e., where the flux
in the coils tend to induce current flow through both coils in
the same direction the shorting of the series combination of
the two coils loads the cores of the coils and inhibits
generation of the trigger coil waveform. The shut-off coil
and trigger coil may be positively inductively coupled by being
wound in the same direction about a common core and may be
separate coils or portions of the same coil as earlier described.
The shorting of the shut-off coil or the series combina-
tion of the shut-off coil and trigger coil also loads the core
for the charge coil and reduces the amplitude of the capacitor
charging waveform. Thus, closing of the switch 48 inhibits
the gating of the electronic switch 16 and, at the same time,
prevent overcharging of capacitor 14 and overloading of
electronic switch 16 by the loading of the core shared with
the charge coil.
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1~)7891s
Note that embodiments of the present invention may be operative
to facilitate engine shut off by the selective loading of the
flux confining core of either or both the charging coil and
the trigger coil. For the purposes of this application,
the term "laoding" when used in conjunction with the word
"core" indicates a reduction in the value of the flux induced
by the rotating magnetic me~ber and confined within said core.
The reduction of flux may be of sufficient magnitude in rela- -
tion to the core parameters and winding parameters of the coils
to accomplish any or all of the following: prevent overcharging
of the capacitor while the engine coasts to a stop; prevent
sufficient charging current to be induced in the charging coil
to permit firing of the gap ionization device; and prevent a
sufficient triggering signal to be induced in the trigger coil
to trigger the electronic switch. In the embodiment of the
present invention above described, engine shut off may be
achieved if the circuit selectably connecting the shut-off
coil 32 to ground has a resistance in a range from zero to ten
ohms. Sufficient loading may also be achieved by directly
shorting trigger coil 31 to ground.
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~()789~5
~``
With reference to Figure 4, the functioning of an alternate
triggering and shut-off subcircuit will be described. Where
the subcircuit of Figure 4 is connected to the gating circuitry
of Figure 1 in place of the triggering and shut-off subcircuit
of Figure 1, the manually operable switch 68 is in an open
position during engine operation. The closure of the switch
68 by the operator will insure engine shut-off.
As discussed above, the triggering and shut-off subcircuit
of Figure 4 may include the trigger coil 72 and the shut-off
coil 70, connected in series and positively inductively coupled
with each other and with the chargir.g coil 10. During engine
operation, the negative component of the trigger coil waveform
is delivered to the gating circuitry through the resistor 64.
The negative component of the shut-off coil waveform is blocked
by the diode 66. The positive components of both the triggering
coil and shut-off coil reinforce one another and tend to hold
the cathode of the SCR 16 at a positive voltage with respect to
ground, thus inhibiting undesired triggering of the SCR during
the charging of the capacitor 14.
Closure of the manually operable switch 68 grounds the out-
puts of both the trigger coil and the shut-off coil. Thus no
negative gating pulse is applied to the cathode of the SCR and
consequently, engine ignition will cease. However, subsequent
charging waveforms continue to be generated by the charging coil
10 as the engine coasts to a stop. Because the trigger and
shut-off coils are inductively coupled to the charging coil, the
output of the charging coil is inhibited by the loading of the
core shared by the coils. In this way, overcharging of capa-
citor 14 is prevented.
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1()78915
Independently Variable Controls
A significant advantage of the present circuit is
the independence of the gate sensitivity (as provided by the
resistor 40), gate current (as provided by the resistor 34 or
resistor 64 of the embodiment of Figure 4) and the degree of
back bias controls (as provided by the resistor 36). In addition,
the value of the resistor 42 independently determines the volt-
age picked off the voltage divider network and thus the point
in the magnetic cycle at which the threshhold voltage of the SCR
lQ is reached. Gate noise is also minimized by the resistor 42
and the stability of the SCR is improved by the ground connection
established therethrough. The value of resistor 42 may be sel-
ected to fix the minimum engine speed at Whichtriggering of the
SCR will occur. The values of each of these components may be
selectively varied to vary one circuit operating parameter with-
out affecting the other operating parameters of the circuit.
Circuit Values
In the exemplary circuit of Figure 1, the values
of the various circuit components may be as follows:-
SCR 16 G.E. No. C1106D
Resistor 34 10 ohms, 0.5 watt
Resistor 36 100 ohms, 0.25 watt
Resistors 40 &
42 18 ohms, 0.25 watt
Capacitor 14 0.68 microfarads
~iodes 38,44 *
& 46 GI No. GlB*
Diode 26 GI No. GlH*
Diode 12 GI No. HG4 , 6v.
In the exemplary circuit of Figure 4, the values
of the various circuit components may be as follows:-
Resistor 64 18 ohms, 0*5 watt
Diode 66 GI No. GlB
Trade Mar~s
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107891S
ADVANTAGES OF THE CIRCUIT
. _
As has been explained in connection with the exemplary
circuit of Figure 1, the present invention provides force com-
mutation subsequent to the complete discharge of the ignition
capacitor. The commutation is induced by the control signal
provided by the triggering coil.
In addition, the circuit provides gate bias protection by
limiting the voltage drop between the control and output
electrodes of the electronic switch during the application of
the triggering wave component of the control signal to the
electronic switch.
Further, the circuit provides operating stability through
variations in ambient temperature by the following mechanisms:
(1) connecting a thermally stable resistive element in series
with the electronic switch in the output current path of the
electronic switch; (2) matching thermal variations of the
impedance of the control signal source with thermal variations
in the triggering requirements of the electronic switch; and,
(3) clamping the control electrode of the electronic switch to
an output electrode of the switch by means of a resistive
element with a value several orders of magnitude smaller than
the control-to-output resistance of the electronic switch.
Yet a further advantage of the present invention is that it
provides a parallel combination of a unidirectional impedance
element and a bidirectional impedance element, which combination
is operative to transmit the triggering component of the
control signal to the control electrode of the electronic switch
while being operative to damp triggering inducing transients
in the absence of the triggering component of the control signal.
An additional advantage of the present invention is that it
provides a capacitor discharge ignition circuit which may be shut
down by shorting the trigger coil to ground by means of a low
impedance, manually operable
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1071391S
switch. A further advantage of this shut down function is that
the shorting of the trigger coil loads a common core on which the
charging coil is wound thus preventing overcharging of the ignition
capacitor as the engine coasts to a stop.
Yet a further advantage of the present invention is that it
provides for independent variation of gate sensitivity, gate cur-
rent, engine speed and SCR back bias through a design choice of
values of three discrete resistors.
THE CIRCUIT OF A SECOND EMBODIMENT
With reference to Figure 5 where a two-legged magnetic core
110 is illustrated, two coils 112 and 113 are wound about one leg
thereof. The coil 113 may be easily converted into two separate
coils 116 and 118 during the manufacturing process by the tapping
thereof at a point intermediate the ends thereof. In Figure 5,
for example, the coil 112 may comprise 2,500 turns which are
utilized as a charging coil 114 for the ignition circuit subsequently
to be described. One hundred turns of the coil 113 may be utilized
as the trigger coil 116 for the ignition circuit, and an additional
100 turns utilized as the shut-off coil 118 for the ignition
circuit.
One end of the shut-off coil 118 may be connected through a
suitable conventional manually operable switch 120 to ground and
the tap between the trigger coil 116 and the shut-off coil 118
connected through a diode 122 to the gate electrode of a grounded
cathode of an SCR 126. The gate electrode of the SCP 126 may also
be grounded through the parallel combination of a capaci~or 128
and a resistor 130.
The ungrounded end of the charging coil 114 may be connected
through a diode 132 to the anode of the SCR 126 and to the series
combination of the ignition capacitor 134 and the primary winding
3~
`~ 136 of the ignition transformer. A di~u-a~ is connected across
the SCR 126 for commutating purposes.
--19--
` 1078915
The secondary winding 140 of the ignition transformer may be
connected to the gap ignition device 142 such as a conventional
spark plug of an intern~l combustion engine.
In operation, the flywheel responsive movement of a magnetic
element into and out of proximity to the free ends of the core 110
will generate positive, negative and then positive impulses. The
first positive impulse will be passed through the diode 132 but
effects little charging of the storage capacitor 134 at speeds
below about 8,000 r.p.m. The negative impulse will be blocked by
the diode 132 and the second positive impulse, far larger in
ampl'tude as shown in Figure 6, will effect charging of the capacitor
134.
During this same time interval as shown in Figure 6, negative,
positive and then negative impulses will be generated within the
trigger coil 116 followed by a smaller positive impulse effectively
filtered by the capacitor 128 and resistor 130 to so effect. The
negative impulses will be blocked by the diode 122 during the
charging of the ignition capacitor 134 by the current generated
within the charging coil 114 and the large positive impulse
effects operation of the SCR 126.
As shown in Figure 6, the impulses in the trigger coil 116 -
are 180 degrees out of phase with the impulses in the charging
coil 114 and the next subsequent generation of a positive pulse
in the trigger coil 116 after the capacitor 134 has been charged
by the major positive pulse in the charging coil 114 will be pas-
sed through the diode 122 to the gate electrode of the SCR 126
thereby insuring the conduction thereof. The conduction of the
SCR 126 provides a discharge path for the potential of the storage
capacitor 134 and this discharge current is inductively coupled
through the primary winding 136 and secondary winding 140 of the
high voltage transformer to supply ignition potential to the
ignition device 142.
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1078915
OPERATION OF THE CIRCUIT
. . . _
During the normal operation of the circuit as above
described, the switch 120 will remain in an open position thereby
removing the shut-off coil 118 from the ignition circuit. In the
event that engine shut-off is desired, the contacts of the switch
120 may be closed so that negative and then positive going impulses
will be generated in the shut-off coil 118 in synchronism with the
impulses generated in the charging coil 112 as illustrated in
Figure 6. The positive going impulses are larger in magnitude
than the corresponding netative pulses of the trigger coil due to
the resistance 124 in the trigger coil circuit. These positive
impulses will be passed through the diode 122 to the gate electrode
of the SCR 126 to insure the conduction thereof during the time
interval in which the charging coil 114 is seeking to charge the
ignition capacitor 134. The conduction of the SCR during this time -
interval shunts current away from the capacitor 134 and prevents
the accumulation thereon of sufficient charge to provide gas
ionization potential to the ignition device 142.
Because all of the coils 114, 116 and 118 are wound on the
same leg of the core 110, and because the trigger coil 116 and
shut-off coil 118 induced currents are opposed in polarity, either --
the number of turns in the shut-off coil 118 must be at least as
great as the number of turns in the trigger coil 116 to insure the
conduction of the SCR 126 during the normal capacitor 134 charging -
cycle when engine shut-off is desired, or the impedance in the
trigger coil circuit must be greater.
ADVANTAGES OF THE CIRCUIT
~ _ .
A significant advantage of both of the circuits as above
described includes the removal of the engine shut-off means from
the charging circuit. AS is frequently the case where ignition
circuits such as those herein disclosed are utilized in hostile
environments such as portable chain saws, sawdust and/or other
107891S
debris together with moisture may provide a shunt between the
leads for the charging coil, particularly where these leads are
exposed for connection to a mechanical shut-off switch. As the
resistance of this shunt decreases, more of the current from the
charging coil will be shunted away from the ignition capacitor.
In the circuits of the present invention, the mechanical switch
has been eliminated from the high voltage charging circuit and
only the relatively low voltage of the relatively few turns of the
shut-off coil will be subject to this shunt. Since the trigger
current can be greatly reduced without inhibiting operation, and
since the more critical high voltage charging coil is protected,
operation of the circuit in a hostile environment is greatly enhanced.
An additional and very significant advantage is the simplicity
of manufacture achieved by the present invention. A significantly
less expensive circuit results as a result of both manufacturing
~; and assemblying techniques.
The present invention may be embodied in other specific forms
without departing from the spirit or essential characteristics
thereof. The presently disclosed embodiments are therefore to be
considered in all respects as illustrative and not restrictive, the
scope of the invention being indicated by the appended claims
rather than by the foregoing description, and all changes which
come within the meaning and range of equivalency of the claims
are therefore intended to be embraced therein.
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