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Yes, I stuffed up with the resistor being in the secondary... May I write that off as due to being ill and at work?
The resistor is in the primary to limit the maximum current through the coil. But it increases the time to build up the field. The time constant for current flow is RL (resistance x inductance). It takes "RL" seconds to reach approximately 2/3 of the maximum current (and thus maximum field strength). But having only 1.5 Ohms means that you will drive 8A of current through those fine coil winding wires... You will generate about the same heat as a 100W light bulb. Raising the resistance to 3 Ohms means you will drive only 4A through the winding which generates the same heat as a 25W light bulb. So less heat, but also double the time to build up the field. From these figues, you can see that 3 Ohm is a good compromise.
There are configurations that will actually bypass the external ballast resistor while cranking the engine to give it the extra power for starting, but I can't remember where (which cars) that is done.
The resistance (or rather impediance) of the secondary winding is what helps protect the coil from the ignition event. When the event occurs, the condenser (capacitor) conducts the current through it (remember that a capacitor conducts AC current, blocks DC current). It protects the points, but also (and more importantly) provides a low impediance path for the ignition pulse so the current does not have to try and run through the battery or arc across the contacts.
Good comment about the primary common joined to the secondary. With CD ignition you do not need this junction, proof again that the coil is used differently for kettering and for CD ignition systems. With kettering the coil is used as an inductor, and inductors have the characteristic that it will do it's best to prevent a change in current flow. When the points open and break the current flow, it increases the terminal voltage as much as it can to attempt to keep the current flowing. The equivalent "common man's" explanation for a capacitor is that a capacitor will try it's best to keep the voltage across it the same by allowing current to flow through it. That's how the capacitor provides a low impediance path for the ignition current pulse, attempting to keep the voltage across the capacitor 0V. Eventually (milliseconds later) the capacitor has charged up to 12V, but then the points close on it. Now the spark on the points is probably (not exactly sure at this moment) because the capacitor also has built-in resistance which is higher than the short circuit of the points, but now we are getting too deep into things.
OK, time to give my fingers a rest. And I still have not heared what Clayton's background in electronics is... (Not trying to put him down, there is a lot he can teach me about engines or something else that I know nothing about.) Wow, a rear thought provoker, KLR142!
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