Hi all,
I took my Pertronix Ignitor II and the Flame Thrower II to the test
dungeon and connected up the coil through a 0.05 ohm resistor, so that I
could measure the current (as well as the dwell time). The ignitor
module is installed in a derelict distributor body, and I'm using a
cordless drill to spin up the shaft. In the near future I'll either
build a high-speed motor setup or else switch to a computer controlled
coil driver to test at higher RPM (similar to what I'm doing for tach
calibration), but for now, this setup answers lots of questions and
gives some good insights into what the Pertronix is doing.
The picture at
http://members.shaw.ca/tsmit/tachmod/pertronix/DSC00214.JPG shows a coil
current plot I captured, with the distributor being driven with the
drill. Because of the way the coil and series resistor are connected,
the flat part of the line represents zero current, and the down-going
slope represents increasing current to the coil. The upwards step is
where the ignitor shuts off the current, and this is when the secondary
in the coil generates a big voltage step in response and fires the spark
plug. The time (horizontal) scale is 5 milliseconds per division (50 ms
from one side of the screen to the other), and the vertical scale
represents the current, with the scale being equivalent to 10 amps per
division (I was using 0.05 ohms resistance, and a voltage scale of 0.5
volts per division).
So what's there to see? The ignitor is being fired approximately every
13 milliseconds, which corresponds to about 1150 RPM. The coil is being
energized for about 5 ms prior to being fired - this is constant at
least down to about 400 RPM, by the way, and it's controlled by the
Pertronix internals. At very low speeds (flicking the distributor shaft
by hand) the coil gets turned on for longer, and the coil current
reaches a steady state current of about 10 to 12 amps. This means that
there is something inside the Pertronix that limits the coil current,
because a transistor could easily be turned on hard enough to pull 15
amps or more through the coil. I was not able to turn the coil on
continuously regardless of how I fiddled with the distributor shaft.
This is a good thing.
Unanswered questions: You can see that the coil current doesn't
instantly jump to the final value - it smoothly ramps up to about 10
amps over a period of 5 ms. This is as expected because a coil presents
a big inductive load and it resists instantaneous changes in current.
However, the coil is being energized for 5 ms in this example, and it's
idle for 8 ms. What happens when we reach 3000 RPM and the coil has to
fire every 5 ms? There's no more idle time if the coil drive circuit
doesn't change its mode of operation, and operation at higher RPM would
be compromised severely because the coil wouldn't have enough time to
reach saturation current. Soooo... I expect something to happen as the
RPM is increased to near 3000 RPM and higher - I just can't make it
happen right now.
Stay tuned!
Theo
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