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Re: New Coil - Barney was (half) right!

To:, mgs@Autox.Team.Net
Subject: Re: New Coil - Barney was (half) right!
Date: Sat, 15 Aug 1998 12:32:34 EDT
In a message dated 98-08-15 02:08:20 EDT, writes:

> But alas,
>  only a few people have caught the fact that the single spark ignition coil
>  does not have the high voltage side grounded to the case.  


Actually, I did catch that, but didn't comment on it. As a matter of fact, I
and a few others from the Triumph list are invovled in a very lengthy
discussion of this, and the points you raised below, as we speak. Off the list
of course. I have drawn up a few diagrams of the ignition circuit to try to
make some sense of this, but as yet, I don't fully understand it. If you have
any interest, I would be happy to send them to you.

>  Now it's time for the brain tickler and quiz.
>  1.) Why does the high energy spark not fry the ignition switch and all the
>  other devices that draw power from it?

Just guesses, but:

a) short duration.
b) most of the energy has been dissipated in the spark.
c) In your description, you showed how the return path from the secondary gets
from the hot terminal of the battery, through the circuit, and then to ground.
You left out the path from ground to the hot terminal of the battery. The
internal resistance of a good battery is near zero, so all of the other
components are bypassed by the battery.
d) Damage is typically done by two actions: heat and arcing. a) answered the
heat concern. Arcing is not a problem as long as all the connections in the
circuit are good. It's only when there is a gap in the condution path that
arcing can occur.

>  2.) What is the function of the capacitor in this circuit?

To decrease the primary field collapse time, maximizing the spark output. The
faster the field decays, the higher the output voltage.
>  3.) Will the system generate a spark without the capacitor being connected?

Yes, but a very weak one. The engine will not run without it. Actually, I
should rephrase that. The coil will produce a high voltage, but no where near
as high as with the condenser. It may or may not produce a spark, depending on
conditions in the engine, but it certainly won't be sufficient for the engine
to operate.
>  4.) Will the system generate a spark with the capacitor being shorted out
>  internally?

No, this is the same situation as the points sticking closed.  

>  And now two questions for extra credit:
>  5.) Exactly how and why does the coil generate such a high voltage in the
>  secondary winding when the magnetic field collapses?

Before I answer this question, let me ask one of my own. Will a transformer
work on DC? The answer is yes!  The output voltage from the secondary is the
result of the rapid (the emphasis is on rapid) decay of the magnetic field in
the primary. The faster the decay, the higher the output voltage. As the
magnetic field collapses, the lines of magnetic force cut through the
secondary windings. The movement of magnetic fields through a wire is the
basis of transformer action. As these fields cut through the windings, current
is produced. Current through resistance produces voltage. It is only necessary
that the lines of force move; there is no requirement that the polarity
changes. Thus, an ON-OFF DC voltage will work in a transformer just as well as
an AC voltage. In fact, a coil is nothing more than a simple transformer, no
different in principle that a power supply transformer in a radio or TV.
Naturally, the parameters are quite different - turns ratio, wire size, etc.

Bonus point? If the secondary windings are short circuited, current will flow,
but no voltage will be produced. Because there is no current path, a high
voltage is generated in the coil. Not an instantaneous voltage, but a rapidly
rising voltage. When the voltage gets high enough, it will produce an arc
across the points. At this point, the voltage across the plug gap drops to
zero. That's why you have to increase your plug gap to take advantage of the
higher voltage output of a high performance coil.  If you don't, the plugs
will fire at the same voltage as before, and the coil output will not be
allowed to reach the high voltage you paid for.

>  6.) Approximately how much current is flowing in the wire from the ignition
>  switch to the coil when the spark is happening (within one order of
>  magnitude)? 

Zero amps are flowing steady state. How much is flowing as a result of the
spark return to the secondary, I haven't a clue. If you do, please tell me.
I spent my entire electrical engineering career designing instrumentation and
control circuits. I deliberately avoided anything to do with transformers,
motors, generators, etc. As a result, not surprisingly, I am rather ignorant
of these things. If you can shed some light on this, I would be greatly

If you know the answers to your questions, please tell us.

Dan Masters,
Alcoa, TN

'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
'74 MGBGT---3000mile/year driver, original condition - slated for a V8 soon
'68 MGBGT---organ donor for the '74

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