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Restoration & Repair Tips Board

Re: ohms for 6v verses 12v coil?

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Author  [Modern View]

10-19-2012 11:54:35

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I haven"t checked every coil made but the ones I have checked showed a primary reistance of ~1.6 ohms for 6v systems and ~ 3.2 ohms for use on 12v systems. Both of these yield ~4amps max primary current which appears to be a design point for long point and coil life.

The ballast reisitors I have seen have a positive temperature coeficient which means they have a low resisitance at cold temperatures and the resistance increases as the resistor heats up. This gives max current and a higher spark potential for cold starting and then causes the current to reduce for running to increase point and coil life.

Your 2.9 ohm coil appears to be for use on a 12V system.

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John T

10-20-2012 05:52:01

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 Re: ohms for 6v verses 12v coil? in reply to Jerry/MT, 10-19-2012 11:54:35  
You raised a good point that I forgot to mention, in the Kettering ignition points and condensor coil ignitions systems used on "many" typical OLD TRACTORS (NOT talkin elec ignition or high energy or after market or more later cars mind you) A MAJOR LIMITING FACTOR WAS TO LIMIT THE CURRENT THE POINTS HAD TO SWITCH TO 4 AMPS AND UNDER SO THE POINTS DIDNT BURN UP PREMATURELY.

THEREFORE 12 volts/3 ohms (coil) = 4 amps 6 volts/1.5 ohms (coil) = 4 amps

Of course, the system can be designed to have coils of different primary resistance values different from the above, Im ONLY talkin typical and general values here (for many tractors not all) for stock old tractor coils in the 40's to 60's vintage.

Good post and fun chat, God Bless yall

John T

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