Posted by JMOR on April 09, 2009 at 05:15:12 from (72.181.158.237):
In Reply to: Farmall 460 coil posted by IOWA FARM on April 08, 2009 at 06:53:45:
John T said: (quoted from post at 14:49:44 04/08/09) Yo Jim, Im not aware of any Thermistor (a resistor that increases in ohms over time as it heats up) use in our old tractors, but believe Ford or someone used it in some automotive applications?????????? It would save a lot of the start by pass and normal ballast problems fer sure.
While on the subject, many here refer to 12 volt coils as being "internally ballasted" sort of like in part of the can theres this stand alone discrete actual RESISTOR. Although a few such did exist, some of which had a lil ring around the can separating the resistor portion, and jdemaris or SoundGuy or someone once even posted pics of old coils with actual internal resistors (some of which were used in circa thirties cars I believe), MANY OF THE TYPICAL GARDEN VARIETY 12 VOLT COILS ACTUALLY HAVE NO INTERNAL BALLAST RESISTORS, but their increased resistance is simply due to more coil wire length or higher resistance wire ANYONE WANNA TAKE ISSUE WITH THAT feel free, its the best info I have available.
Maybe you or Bob M or jd or someone has details or specs or data on the subject?? But far as I know most typical 40's n 50's tractor 12 volt tractor coils DO NOT have an internal pure stand alone discrete resistor hid somewhere inside there
Yall take care now
John T
Copper wire winding.
You don't need any external current control (thermistor, temperature dependent resistance or whatever) to obtain a higher start current than the all-day-run-current.
Example:
1.7 ohm primary, 200 turns of 26GA wire or about 42 feet
current @ 68F is 7.4v/1.7 ohms=4.35Amps
then pull hard all day & say coil is 200F (heat from current + engine it is bolted to). It won't take you long to feel of it!
copper wire resistance increases as temp rises from 68 to 200=33% or is now at 2.26 ohms and current is down to 7.4/2.26=3.27Amps
a decrease of 1.08Amps or 25% or stated another way, running hot at 75% of cold start current.
Related is the fact that Energy (the heat of the spark ) is proportional to current squared, and thus the hot all-day- run Energy is only 50% of your cold start Energy.
All that with only the copper winding of the coil.
NOTE: Nothing above considers the loss in battery voltage while starter sucks large current.
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