Download Current ramping an ignition coil primary not only opens up

EyeOnElectronics
Current ramping an ignition coil primary not only opens
up a whole new world of diagnostics, it can also help
you better understand how ignition coils really work.
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Basic understanding of waveforms begins with
an understanding of what you’re looking at on
the scope’s screen. Properly set up, a scope provides a visual picture of how voltage in a circuit
changes in relation to time. The key is to understand why the voltage behaves as it does, and
what it ultimately means regarding the performance of the vehicle.
For almost all automotive applications, what
we’ll be looking at are pulse waveforms. A pulse
waveform can be a single event such as a glitch,
or a repetitive sequence, or pulse train, such as
you’d see on a computer data bus or at the ignition coil primary.
Unlike sine waves, pulse waveforms contain
many frequencies within the pulse. A good example is the common square wave. Even though
it looks like the DC voltage is simply switched off
and on, a square wave is actually a complex
waveform made up of many different sine wave
frequencies that, when combined together, give
it its square shape.
The waveforms you see on a scope
screen are a combination of the energy
pulse that’s applied and the effect the
circuit, and its impedance, has on it.
Impedance consists of inductive and capacitive elements, as well as DC resistance. Since both inductance and capacitance are frequency-sensitive,
there’s an interaction with the frequency content of the waveform.
An excellent example of this is a tank
circuit. Imagine a capacitor in parallel
with an inductor. If we start with a
charged capacitor, then connect the inductance, the capacitor will discharge
its current through the inductor, which
builds a magnetic field. When the current from the capacitor stops, the magnetic field collapses, sending its energy
Battery voltage=14.2 volts. Module and coil at room temperature.
back to the capacitor. If you were to
put a scope probe on this circuit, you’d
One of the key bits of diagnostic information in this current ramp is the oscillasee a sine wave.
tions in coil current that occur after the transistor switches off. This indicates
Waveform interpretation becomes
that the secondary isn’t able to accept the energy from the primary coil.
Waveform: Mike Dale
Mike Dale
rabbing a waveform is as easy as
turning on a scope, adjusting the
settings and connecting the
probes. Understanding what that
waveform is trying to tell you,
however, is much more difficult.
And without that understanding, it’s impossible to
use a scope’s power to help you fix cars.
Being able to analyze waveforms correctly is
not a new problem. Since the 1940s, TV repair
guys and other scope users have struggled to figure out what the waveforms they were looking
at meant. The variations in scopes, TV design
and probes meant wide variations in the patterns displayed. Years back, Sun Electric, among
others, actually had plastic templates with waveforms printed on them for use as screen overlays for their automotive scope testers. But
while comparison is a good way to determine if
a waveform is “wrong” or “different,” it doesn’t
necessarily lead to a successful diagnosis.
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July 2001
Eye On Electronics
useful when we can connect changes in
vehicle performance with changes in a
waveform that result from changes in a
particular circuit. As an example, let’s
take a look at the ramp of the ignition
coil primary shown on the opposite
page. This waveform was grabbed using
a current probe to convert changes in
current into a voltage that can be displayed. At the beginning of this waveform, we see a turn-on of the current
and the initial damped sinusoidal oscillations, which are normal and associated
with the charging of the secondary inductance. Note that the change in current here is enough in some coils to induce a feed-forward voltage in the secondary coil of a couple of thousand
volts. Some coils contain a diode to
block this voltage from accidentally firing the spark plugs out of time.
If these sinusoidal oscillations are not
present, it indicates that something is
wrong in the coil secondary. For instance, if the line goes straight up (no oscillations) before turning into a ramp, it’s
an indication of a full or partial short in
the secondary coil. In this case, it’s the
change in the shape of the waveform
that has the diagnostic information in it.
In an ignition coil, the resistance and
the inductance of the primary determine the time the ramp takes to get
from its starting point to its highest
point. A coil with a shorted primary will
have lower inductance and resistance,
thus its current will rise faster. In this
case, it’s the scope’s ability to measure
time that’s useful in diagnosis.
At the top of the ramp, the coil current may or may not show a flat spot.
Older systems traditionally have this
flat spot, which represents coil currentlimiting. Most coils have primary currents in the range of 6 amps. With primary resistances in the vicinity of .5
ohm, battery voltage could drive nearly
30 amps through the primary if the current was not somehow limited. In
waveforms with a flat top, a power transistor in the module is being turned
partially off to limit the flow of current.
For the period of time that it’s limiting
current to the coil, the transistor driver
may have to dissipate as much heat as a
light bulb (66 watts).
More popular these days, and for
good reason, is the ramp-and-fire system. In this system, battery voltage and
ambient temperature information are
used to calculate how soon before the
spark is needed it is necessary to turn on
the coil. If the accuracy of the prediction
is good enough, the dwell current will
just be passing through 6 amps when
the coil needs to fire. This type of system
avoids the heat in the power transistor
caused by it being used as a current-limiter. The diagnostic information is shown
by the shape of the waveform and your
ability to measure the amount of primary current at the firing point.
The falling edge of the coil primary
current ramp represents the turn-off of
the ignition coil by its driver. Although
it looks like a completely vertical line,
there really is some slope to it. Usually
the fall time is in the 3- to 4-microseccontinued on page 22
July 2001
21
Eye On Electronics
ond range. If the time is very much
longer than that, the output of the coil
will be affected. A slow falling edge typically indicates that something is wrong
with the transistorized coil driver.
The falling edge of the coil current is
also quite interesting in another way. If
you’ve ever wondered how an ignition
coil really works, this is the part of the
waveform that tells the story. By Faraday’s law, the voltage induced into a coil
depends on the number of turns of wire
multiplied by the change in flux divided
by the change in time. The faster the flux
moves, the greater the induced voltage.
It’s the speed at which the coil turns
off that’s ultimately responsible for the
voltage output of the coil primary. As
an example, 14.8 volts (battery voltage)
applied to the primary is converted to
350+ volts by the sudden collapse of
the magnetic field. It’s this 350-volt
spike that’s actually sent across the
transformer into the secondary. With
the typical 100:1 step-up turns ratio in
the coil, the approximate secondary
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July 2001
output voltage can be as high as 35kV.
The falling edge of the primary current waveform can also give you information about the condition of the secondary circuit. Once the magnetic field
collapses, its energy has to go somewhere. If the secondary coil can’t accept
it, you’ll see current oscillations as the
energy fluctuates between the inductance of the primary and the distributed
capacitance of the coil windings. The
waveform on page 20 shows this exact
circumstance. The oscillations in the primary current can be seen in the area
right after the initial switching.
As you can see, there’s a lot of diagnostic information in a coil primary current waveform. That doesn’t mean that
everything you might want to know can
be seen there. Still, in these days of
coil-on-plug ignitions where there’s no
secondary wire around which to put an
inductive pickup, a great deal can be
learned from the primary circuit.
Waveform analysis is something that
has to be studied and learned. The hard
part has always been knowing what a
good waveform is supposed to look like.
It would be terrific if manufacturers
would print appropriate waveforms as a
part of their service literature, or at
least post them on their websites. In
the meantime, there are variety of other resources available to you. For example, the International Automotive
Technicians Network (iATN) has some
4400 stored waveforms that members
can download. Some scope makers include a library of sample waveforms
with their scopes. And scope seminars
often include a book of waveforms as a
part of the course material. Finally, if
you have a spare $500, there are books
of waveforms for sale on the Internet.
Probably the best way to learn about
waveforms is to collect your own set of
known-good and known-bad patterns.
By building your own library, you can
limit the number of waveforms to the
kind of work you do, on the kind of vehicles you most often work on. Not a
bad deal, is it?