Mon, 31 Mar 1997, Derek Evans wrote:
<< i don't believe that the connections to the ignition coil should be
<< reversed when changing from positive earth to negative earth. ignition
<< coils are not polarity sensitive devices.
True, but the polarity of the secondary output is dependendant on the
polarity of the primary, and spark plugs are somewhat polarity sensitive, as
stated by M. Porter in his post. The secondary circuit is electricaly
isolated from the primary circuit, and from the battery.
<< the ignition system in a car works by interrupting the flow of current
through the coil by opening
<< the points, setting up a resonant circuit with the capacitor (condenser).
It is true that the coil/condenser combination forms a resonant circuit, but
I believe the time constant is large with respect to the open time of the
points. i.e., there is not enough time for the circuit to complete one whole
oscillation before the points close again. Plus, the internal resistance of
the coil adds a damper to the resonance.
<< it is this oscillating current transfering energy between
<< the coil low tension (12volt) winding and the capacitor that induces a
<< voltage in the high tension winding of the coil ( the bit that connects
<< to the spark plugs via the distributor).
When the points open, the coil discharges through the capacitor, allowing the
field to collapse. This collapse is very rapid. A very rapidly moving field
from the primary coil of a transformer cutting through the secondary windings
will produce a voltage spike from the secondary. An oscillating voltage in
the primary would produce a high voltage output, depending on the turns
ratio, but the energy content would be low.
<< the resulting voltage that fires the plugs is not D.C. as shown in some
car manuals but is in fact
<< a short burst of radio frequency energy.
A pure sine wave contains only one frequency. A TRUE square wave contains ALL
frequencies. The output of the secondary is neither, but more closely
approximates a square wave. That is why there is RF interference from a coil
discharge. That, plus the fact that a spark discharge, whatever the source,
is rich in harmonics, including frequencies in the RF range.
<< the point of all this is that the coil doesn't care which way the
<< current flows through it, only that the points interupt the current and
<< that there is a healthy capacitor ( condenser) with which to resonate.
Again, the coil doesn't care, but the plugs do.
<< changing the coil connections radically alters the circuit. it will
<< still run but with less spark.
I believe the converse is true. If you don't change the coil connections, the
spark direction at the plug points will be reversed, which is inefficient.
<< the fact that some coils are marked with
<< '+' and '-' terminals is misleading the + connection always goes to the
<< ignition switch ( or 'hot wire') and the - minus always goes to the
<< points regardles of battery polarity.
I don't believe this is true. I think the coil indications are just what they
say.
Now, having said all the above, I must confess to a great deal of ignorance
on the subject, and I stand by, ready and willing - eager even - to be
corrected.
Dan Masters,
Alcoa, TN
PS: I learned a long time ago, that when theory conflicts with reality, a
wise man changes his theory. I am not a wise man yet, but I am working on it!
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