[AT] AT Digest, Vol 42, Issue 31

[Ken Knierim]

>
> I don't see how grounding the plastic case or the isolated metal case that a
> standard tractor ignition coil sets in is doing much good.. except perhaps
> emi/rfi interference deadoning, in the situation where the coil outter shell
> material is metal.
>

You're right. Not much help other than EMI reduction.

> In standard kettering ignition systems, and using common ignition coils on a
> points based distribuitor / battery ignition system, I don't see any
> examples yet where anyone shows any thing to the contrary other than the
> ponts are the ground for the primary.. and the inuslated / isolated case
> that houses the coil is nothing more than a container on the standard
> tractor application.

The "ground" path for the high voltage section comes through the
battery power terminal. Think about it for a minute... the high
voltage section develops somewhere on the high side of 10KV so the
fact that the low voltage section has up to 12 volts applied makes
little difference on 10KV (10000 volts, plus or minus 12 volts... not
much difference). The points turn on the energy to the primary by
grounding the primary AND secondary (on many systems. they vary, I'm
sure).

When the points open and the field collapses in the primary it induces
voltage in the secondary. Since they are connected together the
"ground" connection is the 12 volt (or whatever) electrical system.
Current passes through the secondary, reaches the disconnected
"ground" at the points and proceeds back through the low impedance
primary to the battery terminal.
       There isn't a lot of current (very low milliamps or high
microamps) backfed into the power/battery terminal so you don't notice
it, but the battery connection has to be strong enough for at least an
ampere or more to feed the primary. This give you the "ground"
connection for the high voltage. It's rather simple and eliminates a
separate connection to the coil. There IS a ground as far as the high
voltage is concerned; it's just done through the battery terminal when
the points are open and disconnected from the circuit.

My 2 cents worth anyway, and it's not intended as a blanket statement
for all ignition systems. There are too many ways to get from point A
to point B for that.

Ken in AZ