coil voltage?

[northvale]

question: I converted my TO20 to 12V and have always used a 12V ignition coil. I also have a JD 2010 gas which is a factory 12V system. I went to my JD dealer to get a new coil for the JD, and they gave me a 6V coil and explained they used a 6V so the points wouldn't burn. The tractor ran fine on either type, so what is the difference?

 

[Grits]

In general I'd say you got bogus advice, but it is hard to say specifically without real electrical data on the specific coils (coil resistance, coil inductance, and primary to secondary turns ratio).

In general, with a lower voltage system the coil has a slightly lower inductance in combination with a resistance (total internal + possibly some external) suited for 6V operation - the resistance keeps the stalled or cranking coil current from going higher than some limit, typ about 4 or 5 amps. Without getting into the whole story about spark energy, let me just say that applying 12V to this coil designed for 6V will give twice the current stalled/low rpm (where the points are closed for a relatively long time and it is only the coil circuit resistance limiting the peak current) because you just put twice the voltage on the same part. This is harder on the points since it is twice what they were intended to carry. The energy stored is proportional to the square of the current and the higher current will store MUCH more energy - about 4x as much as when driven by 6V. The coil will also tend to get hot since you are running more current - may even fail after a bit.

Even running, when the current through the coil is limited by the inductance rather than just the resistance, the current will be higher since a typical coil for 6V will have a lower inductance value. The lower inductance is needed to allow the lower voltage to build enough current to store enough spark energy in the time the points are closed. So there will be a higher rate of rise and a higher final (and average) current, and so the coil will run hotter here also.

Some of the excess energy that would get stored would inevitably get released at the points and will quickly wear on the points and condenser.

So you have to look at a whole system and know the specifics of the coil, but in general it is best to assume that they label it 6V if it was designed by professionals to be put in a 6V system; ditto for 12V parts. They aren't just directly interchangeable, and doing so is not usually good in the long run.

 

[Bob]

The 2010 has a ballast resistor ahead of the coil, and a starting bypass.

A 12 Volt coil marked "use WITH external resistor" is more or less the same as a 6 Volt coil, so, rather than stock a 6 Volt coil and a (more or less identical) 12 Volt coil marked "use WITH external resistor" Deere simplifies inventory and stocks only the one marked 6 Volt.

So that explains why they sold you a 6 Volt coil!

Had they sold you a true 12 Volt "no external resistor required" coil, it would produce weak spark in your 12 Volt-WITH-resistor system!

Clear as MUD?

 

[Bruce(OR)]

Well. . .Then again,
I am in the process of repairing a no start Ford 8N 12V conversion.
Come to find out the 12V coil has 2.5 ohm resistance against the .5 (?) ohm on the 6V coil.
I am in need to convert the ignition system to a starter bypass using a Ford twin pole solenoid so that when the engine is cranking the coil will get full voltage with the ballast resitor, I plan to add, being bypassed.
My goal is to get the coil voltage down to about 7 to 8 volts while running at about 1200 RPM with full alternator voltage output.
Really clear as mud now?

 

[JMOR]

(quoted from post at 01:44:16 09/02/11) Well. . .Then again,
I am in the process of repairing a no start Ford 8N 12V conversion.
Come to find out the 12V coil has 2.5 ohm resistance against the .5 (?) ohm on the 6V coil.
I am in need to convert the ignition system to a starter bypass using a Ford twin pole solenoid so that when the engine is cranking the coil will get full voltage with the ballast resitor, I plan to add, being bypassed.
My goal is to get the coil voltage down to about 7 to 8 volts while running at about 1200 RPM with full alternator voltage output.
Really clear as mud now?

"My goal is to get the coil voltage down to about 7 to 8 volts while running at about 1200 RPM with full alternator voltage output. "

If you are speaking of voltage across the coil primary, as measured with an averaging (standard) DC volt meter, then you will never see 7 to 8 volts & if by some freak of nature, you actually did, the coil would burn very quickly, as the power would be (8X8)/ 2.5 = 25.6 Watts! Whereas the normal running power in those coils is just over 1 Watt, that's correct, one Watt.

 

[Grits]

Maybe you can check my math. On a 4-cyl there are 2 ignitions per revolution. 1200rpm = 20revs/sec. Times 2 = 40 ignitions/sec @ 1200rpm. Now if a typical ignition delivers about 80mJ of energy, then 40sparks * 80e-3mJ/spark =3.2J/s and that is 3.2W. This neglects the coil resistance. Does this sound right?

I've seen values of 45mJ to 150mJ for standard coil ignition systems, the latter value would be a very hot spark. Since most coils sold for tractor service don't come with inductance values, you'd have to measure a coil to get the exact value so I picked a value kind of in the middle for the calculation. If you picked the 45mJ value and dropped the rpm a bit it would get closer to the 1W level. Still, 3.2W is only going to make the coil feel slightly warm, barely more than sitting next to the hot block.

 

[Grits]

But I guess the 3.2 Watts is stored in the magnetic field and delivered to the load and only the resistive losses are heating the coil, right? So maybe that 1 Watt value is closer to reality.