[AT] question on use of resistor on coil

[Dudley Rupert]

Cecil,

In the last year I've bought a coil from NAPA and one from my nearest Case
IH dealer.  The instructions that came with both of these coils said to use
an external ballast resistor when connecting to 12 volt systems but for 6
volt systems the external resistor was not needed.

FWIW - A typical automotive ignition coil has an internal resistance of
approximately one ohm.  When starting an engine with a 12 volt system the
output of the battery is normally routed directly to the coil thus resulting
in a current through the coil of approximately 12 amps.
Note: The current through the coil is determined by dividing the battery
output (volts) by the coil resistance (ohms).  Once the engine has started
(that is, once the key is released from Start to Run) the output from the
battery is usually "re-routed" through an external resistor (often called a
ballast resistor) of a few ohms and then to the coil.

If this external resistor were not in 12 volt systems the continual high
current (i.e., the 12 amps) through the coil would likely burn it up after a
relatively short period of time.
Note: The heat generated in (and dissipated by) the coil is the product of
the current squared times the resistance (that is, 12 times 12 divided by 1)
or approximately 144 watts.

In 6 volt systems the external ballast resistor is normally not used.  The
current though the coil, which is approximately 6 amps, is the same both
when starting the engine as well as when running the engine.  A current of 6
amps through a one ohm coil results in approximately 36 watts of generated
heat which the coil should be able to dissipate.

Hope this doesn't cause any confusion -
Dudley
Snohomish, Washington



In typical use the coil is connected in series with a ballast resistor of a
few ohms to reduce the voltage, which in turn reduces the current through
the coil so it doesn't burn up. The ballast resistor isn't used during
cranking (when the battery voltage is usually reduced to around 6-8 volts).
A typical DC coil resistance would be around an ohm, and at 12 Volts, the DC
current would be 12 Amps, dissipating more than a 100 watts.


-----Original Message-----
From: at-bounces at lists.antique-tractor.com
[mailto:at-bounces at lists.antique-tractor.com]On Behalf Of Cecil E Monson
Sent: Saturday, October 23, 2004 4:19 AM
To: Antique tractor email discussion group
Subject: [AT] question on use of resistor on coil


	I've been trying to get my JD 420 crawler to start after sitting
all summer. At first I had no spark. With the loader down and the hood
on the tractor, it is almost impossible to get to the coil and to where
you can see to work on the distributor. I took the hydraulic connections
apart yesterday and got the hood off. Substituted a new coil for the old
one without success. Took the cap and rotor off the distributor and used
a "point file" I got at Sears to clean up the points. Now I have a sort
of yellow spark when I lay a spark plug where it is grounded and crank
the engine over. The yellow spark indicates to me it is a weak spark
or still bad contacts at the points.

	The question is - the coil came out of the box with a note saying
it must be used with a resistor but no resistor is there. I figured that
being as it is marked 12 volts on the coil and I'm using it on a 6 volt
system, I should not need a resistor. Is this true or do both 6 volt and
12 volt systems use the same coil with a resistor?

	Another question is that I can feel pitting on the points on one
side when I run the file over them. Would you just change out the points on
general principals?  I have already changed out the condenser as I initially
thought it might be the problem.

	In case someone wonders about fuel, I smell gas in the exhaust when
cranking so it is getting gas.

Cecil
--
The nicest thing about telling the truth is you never have to wonder
what you said.

Cecil E Monson
Lucille Hand-Monson
Mountainville, New York   Just a little east of the North Pole

Allis Chalmers tractors and equipment

Free advice

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