Re: Why the disagreement on placement of resistor???

Posted by Rick K... my two cents worth (10 cents with inflation) on September 02, 1999 at 12:33:08 from (205.181.231.254):

In Reply to: Why the disagreement on placement of resistor??? posted by john d on September 01, 1999 at 18:27:44:

First off, there is a lot more going on in the ignition circuit than just the series combination of the coil primary and the ballast resistor.

However, from a practical point of view, my best guess is that it shouldn't matter a whole lot. The high voltage current going to the spark plug will have to travel through the ballast resistor if the resistor is placed between the coil and the battery, since this is generally the return path for the spark current in most coils. More resistance means less spark, however, the resistance is so small, and the current is so small, that the voltage drop will amount to virtually nothing.

There is a "circulating" current that flows between the coil primary and the condensor, but that would flow through the resistor reguardless of which side the resistor is put.

When car manufacturers switched from 6 to 12 volts, they typically put the resistor between the battery and the coil. They did this for a reason. They usually put an extra contact on the starter relay, which jumped out the resistor when cranking, making for more voltage applied to the coil during the cranking cycle. It is much easier to jump out the resistor when it is located in between the battery and the coil, rather than between the coil and the points. On a Farmall, which doesn't even use a starter relay, installing this feature would be a little bit of a pain in the butt, so I doubt that most people are doing it.

So, from a practical point of view, I don't think that it matters a whole lot where you put it. You might get a 10th of a percent more spark if you put it between the coil and the points, but a tenth of a percent is roughly zero in my book.

On a related subject, if you switch from positive to negative gorund, I don't think that you should swap the wires on the coil primary. The polarity markings on the coil will not agree with the polarity applied, but the return connection for the high voltage secondary should remain connected to the battery, not the points. If you swap them, you will at the very least reduce your output voltage by an amount equal to the "flyback voltage" on the primary, which might be a few hundred volts. You may tend to arc the points out as well, although I am not sure. The condensor may deal with that problem. The polarity of the output spark will be reversed, but spark polarity should not matter when trying to ignite gasoline.

Note that the above logic applies only to a coil that was designed to be used with positive ground originally, as it was marked with that in mind. If the coil came out of a 66 Chevy, or something like that, which was negative ground by design, then it should be hooked up with the positive terminal going to the battery.

The above is just my opinion, although I have been designing high voltage stuff for a living for 15 ears or so, so it isn't just a totally wild guess. This thread will probably go on a while... kewl.

Rick K

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