12 volt front coil

Short version: It makes absolutely no difference whatsoever whether the resistance in a 12 volt points and condensor ignition is external or internal to the coil. And if the resistance is internal to the coil, it makes no difference whether the resistance comes from the coil winding or some resistance element. So, there are no "true 12 volt" coils. There are coils that require external resistors and there are coils that do not require external resistors. Call them what you will.

Now, the long version:

First, make the assumption that all ignition coils have an identical inductance. Now, there are coils that have non-standard inductance, but these are intended for hotrodders using high performance distributors; we are not talking about these coils.

OK, so the spark energy released by a coil is whatever energy was stored in the coil at the instance the points open. That energy is equal to one-half the inductance times the square of the current through the coil. So, if the inductance of several different coils is identical, then whichever coil has the greatest current will have the greatest energy.

But what controls the current through the coil? Well, in steady state it is ohm's law: current equals voltage divided by resistance. We know the voltage applied by the battery is 12 volts. (Actually, it's not; a "12 volt" system is really around 14 volts, but never mind that.) The resistance is the TOTAL resistance from the positive battery terminal to the negative battery terminal. That includes the primary wiring, any external resistor, the resistance of the coil windings, and other resistance in the coil, and a whole bunch of other things such as the points, key switch, splices, etc. So IT DOESN"T MATTER where the resistance comes from, it all gets added up in the end. If the TOTAL resistance is the same, the coil current is the same, and the coil energy is the same.

OK, so someone is likely to say "yeah, but what about AFTER the points open". Well at that instant you have an RLC (resistor/inductor/capacitor) circuit. and yes its behavior is going to vary depending on the resistance in the circuit. But once again it is the TOTAL resistance in the circuit, NOT just the internal or external resistance alone. And that total resistance is going to be the same as it was BEFORE the points opened, with the exception that the points are no longer in the circuit (the current that was passing through the points now passes through the condensor).

OK, flame away.

In the IDEAL situation the ballast resistance (around 1.5 ohms) would be near the same as the coils primary (also around 1.5 ohms) such that a true 50/50 voltage divider existed and the coil saw a full 6 volts as designed for. HOWEVER coils varied from maybe 1.25 ohms to under 2 ohms and likewise Ive seen those ballast from 1.5 to 1.85 ohms SOOOOO OOOOO OO when you bought an of the shelf ballast it may have been on one end of the spectrum and if the coil was on the opposite end you may have ended up with a voltage divider that only applied 4.5 to 5 volts on the coil i.e low current and a weak spark. HOWEVER that problems not there on that full true 12 volt coil.

Or else maybe (say the voltage divider left sufficient voltage on the coil) the old coil was simply showing its age and that brand new just yielded more spark gap energy as there were less losses to heat or other inefficiencies.

Fun chat, theres other possibilities but I gotta run, will check back later

John T another olddddd dd lol retired EE

Bob stated: "Can you PROVE you've EVER seen EVEN ONE coil with an INTERNAL RESISTOR, as opposed to simply more windings? I think the idea of the almost "non-existant" "INTERNAL" resistor is REALLY a persistant MYTH "

It's not a myth at all. It used to be common in many systems including Remy, Delco, Atwater Kent, Connecticut,Willys-Knight, etc. Many had a removeble caps that pried off the top of the coil so the resistor could be repaired or replaced without changing the whole coil. The resistors were not there just to preserve point and coil life, they were also there to prevent dead batteries when igntion switches were left on. Here's a description from a 1930s Dykes Auto and Tractor Repair manual:

"The purpose of the ingition resistance unit is twofold, one being to protect the battery from discharging back through the primary winding and thus overheating the winding and discharging the battery if the switch is left on and the engine is not running and the breaker points are closed. The igntion resistance is made of iron wire, usually mounted inside the top of the coil mounted on a porcelain spool. It turns cherry red when the primary current flows through it for any length of time. If current is left on for a long period of time, the resistance may burn out. This sort of thing happens frequently. "

I AM well aware of those.

What WAS being talked about in my post was 12-VOLT COILS from the relatively MODERN ERA.

Are you aware of ANY 12-Volt coils from the relatively modern era with an actual resistor INSIDE the "can"?????

You posed the question "Can you PROVE you've EVER seen EVEN ONE coil with an INTERNAL RESISTOR . . ."

You did NOT say one "newer" coil, or one "12 volt" coil.

Seeing how this forum has a primary focus on antique equipment, I didn't think 1930s - 1940s electrical systems were not included in the discussion. Also - about your most recent comment about the images I posted - and the resistors NOT being in the coils? I guess it depends on what you call "in." They certainly were considered part of the coil, but the coil top could be pried off for access to the internals. Go pick up an Oxford English Dictionary and look up the word "in", and I think you'll find it applies here. In regard to 12 volt coils, I have worked on many specifically referred to as internal resistor coils. I have some specs on `1960s - 1970s outboard motors and some British and German cars that go as far as give resistance specs just for the resistor inside the coil - separate from the primary and secondary windings. But, I can't say I've ever bothered to saw one in half just to look inside. Once the tops no longer popped off, I tended to just throw coils out when they stopped working.

How many have you cut in half?

AND, I'm beginning to see why you got kicked off the other forum, as you are being needlessly argumentative here.

I am not going out and seeking arguments for intellectual practice. I am also limited by expressing myself with a keyboard and it seems I come across differently to different people. You are the first person that has ever overtly accused me of such on this forum.

If more people other then you, want me off the forum - well, seems for that reason alone - I wouldn't want to be here anymore.

Again, GREAT PICTURE n thanks for more info, we (or us sparky types) got a lot of mileage out of such a simple but interesting lil coil question.....

Happyyyyy y Newwwww w Yearrrrr rr Yalllll lll

John T

I never gave this much thought before this - since I like having an external resistor so I know what I've got, and I know I can bypass it when needed.

and all 12V coils I have seen have an internal resistor, not more windings. Seems if more windings they would be twice as big ?

Hope this helps yall

God Bless n Happy new Year

John T Retired Electrical Engineer

Can you PROVE you've EVER seen EVEN ONE coil with an INTERNAL RESISTOR, as opposed to simply more windings?

I think the idea of the almost "non-existant" "INTERNAL" resistor is REALLY a persistant MYTH.

Just because the coil is marked "NO external resistor required" does not mean there's a DISCRETE resistor inside, just more primary turns.

By FAR the most space inside a coil is taken up by the SECONDARY windings. The PRIMARY winding, which is what we are concerned about here, takes up a small portion of the space inside an ignition coil, and there is PLENTY of room for increasing the number of primary turns.

I see val blt shows a 12 volt coil for the 8N in their 2006 catalog. Might be YT has such a thing.

A 6 volt coil with a resistor the same resistance as the coil primary will work well on 12 volts.

There's another reason for a resistor and fewer primary turns. Not important for a tractor, but for high speed engines, the inductance slows the current rise and the low inductance coil with the current limiting resistor allows a faster current rise and so higher speed engine operation. Some hot 6 volt Chrysler products used a 3 volt coil and a resistor for that reason.

Gerald J.

6 VOLT COIL

Has around 1.5 ohms of low voltage primary winding resistance as measured between its lil + and - terminals.

It is engineered and designed for ONLY 6 volts use and 12 volts would cause it to overheat plus the points would burn up quickly from switching excess current!!!!

12 VOLT COIL

Has around 3 ohms of primary coil/winding resistance as measured between its lil + and - terminals and is designed to have 12 volts (NOT 6) applied across its terminals.

If you were to only use 6 volts it will have a weak spark !!!!!

Although some refer to them as being "internally ballasted" most (slight exceptions) achieve their higher 3 ohms of primary resistance by use of more turns or length of primary coil windings OR ELSE they could use higher resistance wire.

One reason for 6 volt coils to have only 1.5 ohms of low voltage primary winding resistance while a 12 volt coil has twice that (around 3 ohms) is to limit the amount of current necessary for the points to switch to around 4 amps to avoid premature burning.

If you wish to use a 6 volt coil on a 12 volt tractor you can add an external series voltage dropping (12 to 6) ballast resistor so the coil only sees 6 volts plus the current the poinst have to switch is again reduced/limited to around 4 amps to avoid premature burning. One advantage of such a system is so the externally ballasted system (NOT so if theres no external resistor as on a 12 volt coil) can by by passed during start up to improve cold weather starting.

Hope this helps yall understand the 6 volt and 12 volt coil thing.

God Bless n Happy New Year

John T

There's all sorts of 12-Volt coils, which, contrary to popular belief, are simply wound with more turns in the primary winding, and, with very few exceptions do NOT have an "internal resistor", just MORE windings.

Hapy New year

John T

Keith

He's looking for the NAPA # for a 12-Volt "square-can" coil for the "front-mount" distributor on a Ford 9N/2N/Early 8N tractor.