Jim,
The spark coil works by the effects of a collapsing magnetic field. All
transformers operate by changing magnetic fields. The basic theory is that
a magnetic field crossing a wire will induce a voltage in the wire. A good
example of how well this works are stop signal switches (that coils of wire
in the ground). Your car has a weak magnetic field around it as you drive
over the coil you induce a voltage in the wire and the controller turns the
light green for you. The output voltage is calculated by taking the input
voltage and multiplying by the turns ratio of the primary to secondary.
Transformers are generally used to change the value of AC voltage. Spark
coils are one of the exceptions to the rule. When the points are closed
current flows through the coil to ground through the points. This builds a
magnetic field around the primary winding of the coil. When the points open
current flow stops and the field collapses. The magnetic field which was
static (not moving think of an electro magnet) now collapses. The magnetic
lines of force are now moving, this creates a voltage in the secondary of
the spark coil. Since there are many more turns of wire (several thousand)
the voltage sent out to the spark coil is very high.
As to the capacitor this is really an integral part of the system and is to
a large extent "tuned" to the spark coil. When you build the field in the
primary, and then collapse it to build the field in the secondary, the
secondary side of the system stores a bit of energy (like a capacitor),
which then has to go somewhere. The somewhere is to build a field in the
secondary, which in turn collapses, causes a voltage to be induced in the
primary, which charges the capacitor that's across the coil. This repeats
until resistive losses bleed off the current. The effect of the capacitor
is to stretch out the high voltage spike sent to the plugs and to keep the
points from arcing while open.
Hope this helps
Steve Hanselman
Manager, Defense Systems
Datagate Incorporated
+1.775.882.1313 x253
-----Original Message-----
From: owner-6pack@autox.team.net [mailto:owner-6pack@autox.team.net]On
Behalf Of Don Malling
Sent: Monday, June 30, 2003 6:45 PM
To: Jim Swarthout; 6-Pack
Subject: Re: Ballast resistor question
I had always thought the capacitor was charged as the points opened,
thus prevented arcing across at the points when they opened. Arcing
transfers point metal from one side to the other. Pit on one side,
"point" on the other.
Don Malling
Jim Swarthout wrote:
> Hmmmm,
>
> I had always assumed that the ,(capacitor), condenser prevented a
> current overload at the points? Current causing the pitting of the
> points?
>
> Occasionally I'm wrong!
>
> Anyone care to describe how an ignition coil works? After-all we are
> converting 12V to 25,000V
>
> Jim Swarthout
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