6 volt versus 12 volt coils

Hi all --
My 1975 Norton Mk III uses 6 volt ignition coils. To accommodate the 12 volt battery they use a "ballast resistor" of about 2 ohms in series with the power supply.

My question is, what's really the difference between a 6 volt coil and a 12 volt coil? Why can't you put 12 volts through a 6 volt coil? I doubt anything will "fry", and it seems the spark created won't be effected much because that comes primarily from the rate of change of magnetic field in the primary coil, not so much the input voltage.

Anyway, do I really need that ballast resistor? What would happen if I left it out?

Thanks -- Scott
1975 Norton Commando Mk III Roadster

If the auto-transformer ratio is higher for the 6V coil, you indeed may 'fry' something. The 68/70 models did indeed use 12V coils, but they were ran as separate circuits, not the wasted spark method and used no ballast. I suppose you could hook up the system without the ballast, but you might think about getting 12V coils and reverting to the 6/70 circuit.

If you go EI, they will recommend 6V coils with the wasted spark method.

Dave
69S

scott12180 said:

Hi all --
My question is, what's really the difference between a 6 volt coil and a 12 volt coil? Why can't you put 12 volts through a 6 volt coil? I doubt anything will "fry",

Click to expand...

You will overheat the coils and yes they can fry.

and it seems the spark created won't be effected much because that comes primarily from the rate of change of magnetic field in the primary coil, not so much the input voltage.

Click to expand...

The battery voltage causes current (Ohm's Law), which in an inductor is stored in the form of a magnetic field. The energy stored is the current squared times the inductance. The induced voltage is the inductance times rate of change of current. Twice the input voltage, twice the current, 4 times the stored energy (Joules) and twice the rate of change of current when the field collapses. Yes, you get more spark.

Anyway, do I really need that ballast resistor? What would happen if I left it out?

Click to expand...

Overheated coils, much shorter life of breaker points, or if using electronic ignition likely catastrophic failure of switching transistor.

drc

DogT said:

The 68/70 models did indeed use 12V coils, but they were ran as separate circuits, not the wasted spark method and used no ballast.

Click to expand...

The ballasted 6V coil arrangement used from 1971 also runs as two circuits (with two sets of points and condensers). EI systems generally use the wasted spark system with the two 6V coils connected in series and no ballast resistor.

The 6V coils and ballast resistor ignition system was apparently fitted in readiness for the electric starter that the factory intended to fit in 1971 but didn't get around to doing until the 1975 Mk3 model.

I was never sure why they went to ballast and 6V coils. Now I know.

Dave
69S

In the low dwell point fired 6v coil its the heat of 12 v current a ballast is needed to avoid. 6v coil can spark better on lower voltage than 12 v coil, until ballast heats up then about the same. In shorter dwell EI wasted spark kits its the total ohm resistance through the coils wired together that must be in range the electronics can handle. Analog sparkers tend to handle higher resistance Lucas coils while the digitals use lower ohm composite coil packs. Only takes a few times of losing fingerprints on a coil to believe in the concept of a fried coil.

You could get the 6 volt coils & leave them on the railway track & video the improvement .

Lucas coils were generally regarded as a bit Naff . A BLUE / WHITE spark JUMPING a Half Inch . ( THIRTEEN m.m. to you )
is minimum requirement for trouble free blowing off of whizz bangs and the like .

If it wont spark 10 mm , follow initial proceedure .

6V running on 12 will get 9 if you leave it out in the snow overnight . This is for James Bond & his cohorts , skiers , etc .Inadviseable in Arizona nowadays .