12 volt coil without resistor

The main thing is to understand why to have a six volt coil for a 12 volt job.
It is simply for better STARTING spark. End of story.
The way it works is at starting, a 12 volt coil may not get enough voltage (due to heavy current drain of starter) to produce a strong enough spark. So the designers fit a 6 volt coil which will give a good spark on a reduced voltage, 12 volt system for starting. But normal running would very quickly burn out contacts and/or coil, so they added a resistor to drop the current through the coil during normal running. It was called a ballast resistor in this application.
So your wiring will feed the coil directly from the starter solenoid (bypassing ballast resistor) only when the starter motor (solenoid) is energised (starting engine) and through the ignition switch via the ballast resistor for normal running.
Now what you actually need is determined by how or what your application has been designed or modified for.
Regards, RAB

HOWEVER if its labeled "12 volts for use with an external ballast resistor" then its in reality a 6 volt coil and does indeed need the external series voltage dropping (12 to 6) Ballast Resistor

The more typical use of the external series voltage dropping (12 to 6) Ballast Resistor is when youre using a 6 volt coil on a 12 volt tractor. It drops 6 volts across it leaving the remaining 6 across the 6 volt coil so it runs at its designed current and voltage and resultant heat.

A typical 6 volt coil will have around 1.25 to 2 ohms primary resistance measured between its lil + and - terminal while a true full 12 volt coil will have around 2.5 to 3 and over.

NOTE; If you use an external series ballast, it should be wired after the ignition switch and before the coil NOT between the coil and distributor. After the points open the condensor is in series with the coil and some ringing occurs.

Hope this helps, post back any questions, good luck and God Bless

John T

Keith

They are simply wound with more turns of wire in the primary, to accomodate 12-Volt power without excessive primary current draw that would overheat the coil and/or burn the points.

And, KEB, I fully agree with you about the efficacy of the external resistor with a starting bypass.

That's also why the case of an ignition coil does not have to be grounded for it to work - the high voltage current does not return through the case of the coil. If you put the ballast resistor between the coil and the points, you've added series inductance to the high voltage point path (wire wound resistors like a typical automotive ballast resistor have significant inductance and are intended to be used only in DC circuits).

Put the resistor between the power source and the coil, not between the coil and the points.

Keith

Thanks for posting that!

You are absolutely correct. I have posted that info so often here, and it doesn't seem like anyone evers sees the correct information, or remembers it.

There is more to a Kettering ignition system than many people realize. The coil and condensor form a resonant circuit, and the coil "rings" at each spark event. The better the "Q" of the primary circuit, the better the coil "rings" and the "hotter" the spark. You DON'T want the resistor between the coil and the points/condensor to add resistance OR inductance there.

Anyone who has ever viewed the operation of an ignition system on a 'scope is familiar with this.