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How to connect the oscilloscope a primary ignition circuit
Plug the x20 adapter into channel A on
the ADC-212 and plug a BNC test lead into the adapter.
Placing a large black crocodile clip on the test lead with
the black moulding (negative) and an insulation piercing probe
onto the test lead with the red moulding (positive). Place the black
crocodile clip onto the battery negative terminal and probe the
coil's negative (or number 1) terminal with the insulation piercing
probe, as illustrated in figure 44.1.
Fig. 44.1
The example waveform shows that the voltage seen during
this test is relatively high and the scaling of the oscilloscope is
therefore adjusted to suit. It is important that the x20 adapter
is used in all situations when a voltage exceeding 20 volts is to
be measured.
With the example waveform displayed on the screen you
can now hit the space bar to start looking at live readings.
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Example primary (single cylinder) waveform
Ignition primary waveform notes
The ignition primary waveform is looking at and measuring the readings seen on the negative side of the ignition coil. The earth path of the coil can produce over 350 volts.
Within the primary picture there are several sections that need closer examination. In the waveform shown, the horizontal voltage line at the centre of the oscilloscope is at fairly constant voltage of approximately 40 volts, which then drops sharply into what is referred to as the coil oscillation, these can also be seen in Figure 44.2.
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Fig. 44.2
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Fig. 44.3
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The length of the aforementioned horizontal voltage
line is the 'spark duration' or 'burn time', which in this particular
case is 1.036 ms, this can again be seen in figure 44.3. The
coil oscillation period should display a minimum number of 4 to 5
peaks (both upper and lower). A loss of peaks would indicate that
the coil needs substituting for another of comparable values.
There is no current in the coil's primary circuit until the dwell period (figure 44.4), this is when the coil is earthed
and the voltage seen drops to zero. The dwell period is controlled
by the ignition amplifier and the length of the dwell is determined
by the time it takes to build up approximately 8 amps. When this predetermined
current has been reached, the amplifier stops increasing the primary
current and it is maintained until the earth is removed from the coil,
at the precise moment of ignition.
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Fig. 44.4
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Fig. 44.5
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The vertical line at the centre of the trace is in excess
of 200 volts, this is called the 'induced voltage'. The induced voltage
is produced by a process called magnetic inductance. At the point
of ignition, the coil's earth circuit is removed and the magnetic
field or flux collapses across the coil's windings, this in turn induces
an average voltage between 150 to 350 volts (Figure 44.5).
The coil's High Tension (HT) output will be proportional to the induced
voltage. The height of the induced voltage is sometimes referred to
as the primary peak volts.
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Technical information - primary ignition circuits
The primary ignition is so called as it forms the first
part of the ignition circuit. This circuit is used to provide the
initial stage towards the secondary High Tension (HT) output. The
primary circuit has, over the past few years, evolved from the basic
contact breaker points and condenser to the distributorless and coil
per cylinder systems in common use today. The basic origin of all
of these ignition systems evolves around the magnetic inductive principle.
Magnetic Inductance
This principle is based around a magnetic field (or
flux) being produced, as the coil's earth circuit is completed by
either the contacts or the amplifier providing the coil negative terminal
with a path to earth. When this circuit is complete, a magnetic field
is produced and builds until the coil's magnetic field becomes maximised
or saturated. At the predetermined point of ignition, the coil's earth
is removed and the magnetic field or flux collapses across the coils
250 to 350 primary windings, which in turn induces a voltage of 150
to 350 volts.
This induced voltage will be determined by :-
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The number of turns in the primary winding.
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The strength of the magnetic flux, which is proportionate to
the current in the primary circuit.
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The rate of collapse, which is determined by the speed of the
switching of the earth path.
Dwell period
Dwell is measured as an angle: with contact ignition,
the points gap determines the dwell angle. The definition of contact
ignition dwell is: 'the number of degrees of distributor rotation
with the contacts closed'.
As an example, a 4 cylinder engine will have a dwell of approximately
45 degrees, which is 50% of one cylinders complete primary cycle.
The dwell period on an engine with electronic ignition is controlled
by the current limiting circuit within the amplifier or Electronic
Control Module (ECM).
The dwell on a constant energy system will be seen to expand as the
engine speed increases, to compensate for a shorter time period. The
term 'constant energy' refers to the available voltage produced by
the coil. This, regardless of engine speed, will remain constant,
as opposed to contact ignition where an increase in engine speed means
the contacts are closed for a shorter time period. This reduces the
effective time that the coil has to fully saturate and maximise the
strength of the magnetic flux.
The induced voltage on a variable dwell system will
remain constant regardless of engine speed, while this voltage will
reduce on contact systems. This induced voltage can be seen on a primary
waveform.
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