Steve,
I am glad you asked whether the correct way to read coil voltage is with
the coil grounded. I would rather rely on a reply by Theo Smit, as the
electronics expert, but I will stick my neck out and expound a little on
this issue.
Your considerations are all based on DC (direct current) electrical
relationships. The coil is NOT a DC device at all. In fact, it is a
high voltage transformer. It consists of a heavy gauge primary coil
winding around a central core that is probably made of non-magnetic soft
iron. The FlameThrower II is oil filled / cooled, as well. This
primary coil has few windings around the core, and is what you connect
your DC input to. There is another coil wrapped around the core as
well. This has a great many more turns of a finer gauge wire, and is
the "secondary" of this transformer.
If this were an AC device you would view it as a voltage multiplier (or
reducer) as used in small equipment. Typically from 110/220 VAC to 6 to
12 volts AC depending on battery to be charged or replaced. A full wave
rectifier (4 diodes) are usually contained in such devices if intended
for DC battery charging or replacement.
The coil on a car is the reverse of this. It has few primary turns, and
many secondary turns, intended to significantly increase the output
voltage. In the car case a typical coil would convert a 9-12 volt input
into 9 - 10,000 volts. This voltage multiplication is done by the turns
ratio of the two concentric coils. In an ignition coil, the operation
is intermittent, rather than continuous. Input voltage application
forms an electromagnetic field that "charges" the core material to
saturation. At this point, it's job is done, and the input voltage can
be disconnected. The central core remains magnetically charged, and
slowly will leak down. Before that happens, however, the secondary
windings are connected to ground. In the car, the "ground" is the spark
plug. The high magnetic charge creates an electrical current in the
secondary wires which seek that ground, even arcing across the small
spark plug gap. Wallah! Ignition. At this point the core is
demagnetized, and no current flows in the secondary windings.
How is all this controlled, one might ask. By the points, or their
electronic replacement. The "points" connect the 12 volt DC input
voltage to the primary winding during a charging cycle. There is no
need to continue this application of voltage past the time required to
saturate the core magnetic field. The primary voltage is disconnected.
The secondary circuit is disconnected during this entire episode, and is
only completed when the rotor gets close enough to the distributor cap
terminal to allow arcing across the small gap in the distributor, and
the spark plug. At this balance point the core collapses, magnetically,
and the secondary voltage is running to the spark plugs.
In the Pertronix system, this timing and dwell are controlled by the
original rotor shape and the internal electronics to control the "dwell"
(a later subject). The FlameThrower II coil is a MUCH higher voltage
device than a stock coil, and the collapse of it's magnetic field can
generate peak spark plug voltages near 50,000 volts. Yes, it does take
more primary current, but not that much.
The message you should be getting is this is a dynamic function, not a
static one, and DC electrical relationships do not apply. AC theory,
magnetic, and field collapse do. Your questions as to the voltages seen
when the coil is dead grounded have no relationship to how this really
operates, and could also damage your equipment.
OK, that's the limit of my "quickie" electromagnetic device lecture.
I'll rely on Theo to correct my errors, and give you more advice yet.
You need turns ratios, coil core magnetic reluctance figures, dwell and
distributor spark duration / coil charging cycles.
Steve
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