Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to improvements
in the opto-electronic ignition system disclosed in our
Canadian Patent No. 1,073,037, issued 4th March 1980.
Our Canadian Patent No. 1,073,037 discloses an
opto-electronic ignition system having a solid state
infra-red radiation source, a photo-transistor and a
plurality of Darlington pairs connected between the out-
put of the photo-transistor and the~primary winding of the
ignition coil, the Darlington pairs switching in inverse
relation with one another in the chain, and the photo-
transistor switching in inverse relation with the first
Darlington pair, the infra-red radiation:from the solid
state lamp being interrupted in timed relation with ::
the engine revolutions by an opaque element in the form
of an apertured or slotted disc, whereby the circuit
is switched between its two stable states in synchronism -.
with the engine. The last Darlington pair is protected :~.
by at least one zener diode connected between the
commoned collector electrodes and the base electrode of :
the first transistor.
The present invention relates to a number of
minor improvements in the design of the opto-
: electronic ignition system of the type disclosed
in our Canadian Patent No. 1,073,037. :.
According to the present invention, there
is provided an opto-electronic ignition system for
an internal combustion engine for use with an
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ignition coil including: a photo-transistor which
will conduct when exposed to radiation; means for
preventing the voltage at the collector electrode
of the photo-transistor from falling below a given
level; a solid state radiation source; a reverse
diode connected across said radiation source; an
opa~ue element having apertures or slots therein
positioned between the radiation source and the
photo-transistor; means for rotating the opaque
element in timed relation to the engine
revolutions; a transistoriæed ignition circuit
having a plurality o~ switching Darlington pairs
connected between the photo-detector and the
i~nition coil, and switching in inverse relation
to one another, the first Darlington pair of the
circuit also switching in inverse relation to the
photo-transistor, transient protection means
connected between the commoned collector eleotrodes
of the last Darlington pair and the base eleotrode
of the first transistor of said last Darlington
pair; and means for slowing down the rate of
switch of~ of said last Darlington pair, said means
being connected in parallel with said transient
protection means.
The means for preventing the voltage at
the collector electrode of the photo-transistor
from falling below a given level may be a Schottky
diode, which is connected in the forward direction
between the base and collector electrodes of
the photo-transistor.
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In an alternative form, the photo-transistor
may be prevented from becoming over-saturated by
designing the chip of the photo-transistor such that
when irradiation exceeds a given level the current
arising from the irradiation of the base area passes
to the collector area and via the collector-emitter
junc*ion so that some of the photo-current is not
amplified.
The present invention wîll now be described
in greater detail, by way of example, with reference
to the accompanying drawings, wherein the sole figure
is a circuit diagram of opto-elec~ronic ignition
system for internal combustion engines disclosed in
our Canadian Patent No. 1,073,037 and including the
improvements referred to above~
Referring to the drawing, the circuit in-
cludes a solid state gallium arsenide lamp 1, a
photo-transistor 2, and two pairs of Darlington
pairs Ql and Q2. The lamp 1 and the photo-transistor
2 are each in series with respective resistors Rl and
R2, and receive a 7.5 volt stabilized supply from a
zener diode Zl. The zener diode is connected across
the 12 volt battery of the vehicle through a resistor
R3. A reverse diode D4 is connected across the lamp
1, in order to protect the lamp against negative
; going transients.
Connected across the emittcr-collector
electrodes of the photo-transistor 2 is a diode
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D1, which serves not only to ensure clean switching
of the photo-transistor 2~ but ensures that a~y
negative transients on the line during the time that
the photo-transistor is non-conductive are conducted
past the photo-transistor, and are thus unable to cause
any damage to its structure. A Schottky diode SD
is connected acrosæ the base and collector :
electrodes of the photo-transistor 2, for the
purpose of preventing the photo-transistor from
becoming over-saturated du~ to excessive irradiation
irom the lamp 1.
It has been found that in the absence of a
Schottky diode, i~ the photo-transistor is
excessively irradiated~ it becomes over-saturated
; 15 with the consequence that, when the irradiation
falls to zero, its switching speed is slow. Under
these conditions of high irradiance the collector-
emitter saturation voltage of the photo-transistor
will fall below a given ~alue which is typically
300 m V.
In an alternative form, instead of providing
the Schottky diode SD, the photo-transistor can
be prevented from becoming over-saturated due to
excessive irradiation by appropriate design of
the chip of the photo-transistor, such that when
irradiation exceeds a given level the current
arising from the irradiation o~ the base area
passes to the::collector area and via the
collector-emitter junction so that some of the
30 photo-current is not amplified.
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The Darlington pairs Q1 and Q2 each consist
of a pair of translstors T1, T2 and T3, T4
respectively, arranged in conventional Darlington
configuration. The base electrode of the transistor
T1 is connected to the collector electrode of the
photo-transistor 2. The commoned collector electrodes
of the transistors T1 and T2 are connected firstly to
the base electrode of the transistor T3 of the
second Darll~gton pair Q2 through a diode D3, and,
secondly, to the 7.5 volt zenered supply through a
reSlstor R4.
A diode D2 is connected between the emitter
electrode of the transistor T4 and the ba~e
electrode of the transistor T3. The diode D1 is
likewise connected between the emitter electrode
of the transistor T2 and the base electrode of
the transistor T1. The diodes D1 and D2 serve to
protect the Darlington pairs against negative going
transients, and also against a careless mechanic
connecting the battery the wrong way around. The
diode D1 also serves to protect the photo-transistor
2 againæt reverse battery connection.
The commoned collector electrodes of the
transistors T3 and T4 are connected to one end of
the primary winding of the ignition coil 4, the
o~her end of which is connected to the positi~e
terminal of the 12 volt battery. Between the
commoned collector electrodes of the transistors
T3 and T4, the base electrode of the transistor
T3, there is connected a circuit comprising a
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pair of serics connected zener diodes Z2 and Z3, a
capacitor C connected in parallel with the zener diodes
Z2 and Z3, and a resistor R5 connected in series with
the parallel circuit comprising the capacitor C and
zener diodes Z2 and Z3. The purpose of this circuit
is, firstly, to control the rate of switching of the
Darlington pair Q2, secondly, to protect the
Darlington pair against positive going transients,
as explained in our Canadian Pate~t No. 932,382 and
thirdly, to prevent undesired electro-magnetic radiation
being radiated from the coil.
The basic operation of the circuit is identi-
cal with that disclosed in our Canadian Patent No.
1,073,037. ~ :
The above described improvements over the
circuit disclosed in our Canadian Patent No. 1,073,037
have the following advantages:-
~a) The provision of the diode D4 across
the gallium arsenide lamp 1 prevents negative going
transients from damaging the lamp.
(b) The provision of either a Schottky
diode SD across the base and collector electrodes of
the photo-transistor 2 or the appropriate alteration to
the design of the chip of the photo-transistor prevents
the photo-transistor 2 from going into the over-
saturated state due to excessive irradiation.
~c) The provision of the capacitor C across
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the zener diodes Z2 and Z3 has the effect of slowing
down the rise time of the first leakage inductance
spike of induced voltage which occurs at the
beginning of the turn off of the Darlington pa~r Q2.
Because the capacitor C slows down the rise time of
the first leakage inductance spike, the rise in
voltage ln the secondary winding is able to follow
the primary rise With complete magnetic interlinkage
between the primary and secondary windlngs. Thus,
the whole of the magnetic energy is kept within the
ignition coil and no energy is radiated externally.
It should be pointed out that in the absence of the
: capacitor C there is some radiation liable to occur
in the radio band of the electro-magnetic spectrum.
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