Re: Another Coil question

Posted by KEB1 on December 27, 2007 at 11:16:02 from (63.227.47.247):

In Reply to: Another Coil question posted by John T on December 27, 2007 at 07:33:14:

John T,

Spent a while with s similar discussion on another board several years ago. I don't think the issue ever actually got resolved.

I took an automotive coil apart one time to see how they were actually constructured. It's actaully an autotransformer, with the input tapped part way up. Of course, the part of the autotransformer that the primary current flows through is wound with much larger wire than the part that only the secondary current flows through.

What this does is increase the spark voltage by having the back EMF of the primary add to the voltage produced by the secondary. When the points open, the bottom end of the autotransformer is connected to ground via the condenser (for purposes of this dicussion we can ignore whatever part of the secondary pulse flows back through the battery).

Note also that the induced voltage polarity is the reverse of the DC input polarity, so the negative voltage is applied to the center conductor of the spark plug and the positive voltage is applied to the engine block. For the pulse voltage, the engine block would actually be positive with respect to the battery by several hundred volts due to the back EMF from the primary winding. This results in the total voltage across the plug being the back EMF of the primary plus the induced voltage in the secondary, resulting in a hotter spark.

If you leave the "+" terminal connected to the battery with a positive ground connection, the resulting high voltage will be negative at the center conductor of the plug, resulting in a weaker spark.

I can think of two ways to fix this. I'm not sure how positive ground coils are actually wired, as I've never had a chance to take one apart & find out. The first way would be to simply wind the coil so that the primary is wound in the opposite direction of the secondary, so that the magnetic field relationship between the primary and secondary remains the same with a positive ground as a negative ground coil would have.

The second way to do it would be to connect the common point of the two windings to the points/condenser and the lower end of the primary to the battery, such that current flows through the primary in the opposite direction to compensate for the reversed polarity (this is what happens when you connect the "+" terminal to the points and the "-" terminal to the battery in a positive ground system). This connection can be thought of as being a conventional transformer with the bottom ends of the windings tied together rather than an autotransformer. In this case, the spark polarity would be correct (center electrode negative), but one would not gain the advantage of having the back EMF in the primary adding to the spark voltage.

I suspect this has a lot to do with the differing ways the connections are shown in various references. I have never been able to find a definitive answer, nor determine unequivably whether both connections are actually in use.

Keith

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