[SEL] Coil Question

[Peter Scales]

In message <20041231.095955.1208.0.jlb94 at juno.com>, jlb94 at juno.com
writes
>
>I had an engine running with a coil & battery.
>The coil was from a Techumsa ???  The battery - a small gell filled 12V.
>
> Once I got the engine running, I decided to "Streamline" by taking the
>iron frame from around & through the coil.
>
>After hooking it back up - I didn't seem to be getting much spark -  When
>all of a sudden
>a loud SNAP !!!  and SMOKE - (The coil blew up.)
>
>Question :::  Does a coil NEED the iron core running throught the middle
>to work properly ?
>

Apparently so...

Paraphrased slightly from

http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/igcoil.html

 and

http://www.tortech.com.au/toranalysis.html

"The primary winding of an ignition coil is wound with a small number of
turns and has a small resistance. Applying the battery to a coil causes
a sizeable DC current to flow (Ohm's law) even though it has a
significant inductance which slows the increase of that current to its
maximum value. The secondary coil has a much larger number of turns and
therefore acts as a step-up transformer. But instead of operating on AC
voltages, a coil is designed to produce a large voltage spike when the
current in the primary winding is interrupted. Since the induced
secondary voltage is proportional to the rate of change of the magnetic
field through it, opening a switch quickly in the primary circuit to
drop the current to zero will generate a large voltage in the secondary
coil according to Faraday's Law. The large voltage causes a spark across
the gap of the sparking plug to ignite the fuel mixture.

The current in the primary winding is caused to drop to zero quickly by
the opening of a switch. This causes the magnetic field in the primary
winding to drop rapidly, generating a large voltage in the secondary
winding. The generated voltage is proportional to the rate of change of
the magnetic field according to Faraday's Law, and the amount of change
is multiplied by the effect of the iron core in the coil."

"The difference between air and iron for magnetic permeability is about
4,000", meaning that it requires 4,000 times more magnetizing force to
create the same magnetic field strength in a given volume of air than in
the same volume of iron.  The magnetizing force is a direct linear
function of the current that flows in the primary winding, i.e., twice
the current, twice the magnetizing force.  However the heating effect of
the current rises as the square of the current - twice the current, four
times the heating effect, three times the current, nine times the heat,
and so on.

Your coil might have blown up just as easily with the iron in it as
without, though.  Did the Tecumseh engine it came from have a battery or
was the coil energised from a flywheel dynamo or magnet?  I'm betting
that the coil was not designed to see a 12V lead-acid cell connected
across it for any length of time and was originally designed to operate
with either a flywheel dynamo/magnet or points-based or Hall effect
ignition, any of which would have interrupted the current and reduced
the effective heating (low duty cycle).  The original circuit may also
have included a ballast resistor to limit the current seen by the coil.

Happy New Year to one and all!

     Pete
-- 
Peter Scales