Slash five tuning help

[guy]

The reason you cannot use 2 12 volt coils in parallel to replace 2 6 volt coils series is, resistance in series is added while resistance in parallel is sum of resistance divided by the product of the resistance.
6 volt R1+ R2 = RT (Additive)
12 volt R1 X R2 / R1 + R2 = RT (Product over the Sum)
Guy

[Ken in Oklahoma]

The reason you cannot use 2 12 volt coils in parallel to replace 2 6 volt coils series is, resistance in series is added while resistance in parallel is sum of resistance divided by the product of the resistance.
6 volt R1+ R2 = RT (Additive)
12 volt R1 X R2 / R1 + R2 = RT (Product over the Sum)
Guy

Sorry Guy, I can't agree with that. Sure enough you know how to calculate total resistances in both series and parallel configurations, but a 6V coil's primary winding will have a different resistance than that of a 12V coil. The energy from the secondaries of both a 6V coil and a 12V coil will be approximately the same. The turns ratio between the primaries and secondaries in both coils will have been designed to yield the same energy from the secondary windings.

Now some people might argue that with two 12V coils in parallel, the primary resistances will be different due to manufacturing tolerances. Thus the coil with the lesser resistance will hog the current away from the coil with the higher resistance. But for the purpose of this discussion we are talking about two good 12V coils operating within the designed parameters. Connect a voltmeter to the positive and negative of one coil and it's going to read exactly the same voltage as the voltage measured across the other coil. They have to be the same because in effect the positive terminals of both coils are strapped together just as are the negative terminals. The points will open the ground path on both coils at exactly the same time. Since the coils are both operating within their designed parameters they will both spark both plugs nicely.

Another way to look at it is that energy from the primary winding on a coil will equal the energy to the secondary winding, less resistive losses and mutual induction losses. So two 6V coils will be equivalent to two 12V coils 'energy-to-the-plug' wise. It matters not whether the two coils are in series, as with our airhead engines, or in parallel as with my hypothetical configuration. The energy transfers remain the same.

My thoughts,
Ken

[Duane Ausherman]

Guy, that would depend upon the resistance of the primary. Were it 4 times higher, then Ohm's law would work out OK. Suppose that each 6 volt coil was 2 Ohms and each 12 volt coil was 8 Ohms.

Two coils in series with 2 Ohms = 4 Ohms. Two coils in parallel with 8 Ohms = 4 Ohms.

Then the two circuits would end up with the same resistance.

I don't know the resistance of those coils, just saying about the math.

[guy]

The reason posted was that when you figure the parallel resistance of the two 12 volt coils it will be lower than the resistance for one 12 volt coil. Not trying to start an oil thread just additional information so when you try the experiment you have some idea what all is involved.
Guy

[Garnet]

The reason posted was that when you figure the parallel resistance of the two 12 volt coils it will be lower than the resistance for one 12 volt coil. Not trying to start an oil thread just additional information so when you try the experiment you have some idea what all is involved.
Guy

That's what we need to do.... some sperimentin.

Let's find all our old 12v coils and send them to Ken.

[Frog]

This thread is closed for business.

Just kidding...haha.

[Duane Ausherman]

Ken, I think that you missed something in DC 101. If the two coils have a disparate primary resistance in either the 6 V or 12V configuration, the result is exactly opposite. I would be surprised if the coils have a closer manufacturing tolerance than 10%.

In that case, in the 6 V config. the voltage measured across the lower resistance coil will be lower and the energy transfer to the secondary would be lower. The spark would be less.

In the 12 V parallel config the voltage measured across the coils would be identical, because if you are measuring one, you are measuring both. However, the current flow through the lower resistance coil would be greater and the energy transfer to the secondary would be greater. The spark would be greater.

That is one basic difference in the two configurations.

In the real world, it matters very little, as the design allows for a large fluctuation in voltage and all operates OK. I would expect the ignition system to run the engine anywhere from 9-10 Volts on the lower end up to well over 14 volts. At the high end they would eventually overheat. As long as the coils are still within parameters, little or no difference will be seen in such a lowly tuned engine such as we are discussing.

From something that I read a hundred years ago, I remember that the Q of these coils is about 2. That affects the rpm range over which the coil will operate well enough, but I am thinking more about low rpm.

What am I missing?

Disclaimer, I am not an electrical engineer.

[Ken in Oklahoma]

Ken, I think that you missed something in DC 101. If the two coils have a disparate primary resistance in either the 6 V or 12V configuration, the result is exactly opposite. I would be surprised if the coils have a closer manufacturing tolerance than 10%.

In that case, in the 6 V config. the voltage measured across the lower resistance coil will be lower and the energy transfer to the secondary would be lower. The spark would be less.

In the 12 V parallel config the voltage measured across the coils would be identical, because if you are measuring one, you are measuring both. However, the current flow through the lower resistance coil would be greater and the energy transfer to the secondary would be greater. The spark would be greater.

That is one basic difference in the two configurations.

In the real world, it matters very little, as the design allows for a large fluctuation in voltage and all operates OK. I would expect the ignition system to run the engine anywhere from 9-10 Volts on the lower end up to well over 14 volts. At the high end they would eventually overheat. As long as the coils are still within parameters, little or no difference will be seen in such a lowly tuned engine such as we are discussing.

From something that I read a hundred years ago, I remember that the Q of these coils is about 2. That affects the rpm range over which the coil will operate well enough, but I am thinking more about low rpm.

What am I missing?

Disclaimer, I am not an electrical engineer.

Duane, I think we may be talking about two different things. In my above musing I did not mean to imply that the performances of the twin coils would be equal in either the 6v series or 12V parallel configuration. You are quite right (if I understand your point correctly) that if the coils are not matched one of the coils in each pairs will supply greater energy to it's spark plug than the other one will.

I made a big deal out of stipulating coils operating within their manufacturing tolerances. Those tolerances might yield a set of coils where one of them is indeed 10% off compared to the other. In the case of two 6V coils in series, it apparently doesn't matter. We have a long history of that configuration in our airheads. In practice we don't seem to worry about whether the coils are well matched. We open the boxes of two new coils and expect them to perform properly. Perhaps with your experience you have seen in practice that new coils sometimes need to be matched to each other. And if so I'll yield the point. With my limited experience I haven't seen it and I haven't heard of it, yet anyhow.

I'm a little more confident when I talk about two 12V coils in parallel. You correctly said that with the two coils strapped together we're measuring the voltage across both coils. Can't be any other way. Interestingly, that's my point. An in-tolerance coil, with in tolerance 12volts fed to it's primary winding is going to create a satisfactory spark on its spark plug. In a sense it would be like two independent 12V ignition systems fed by a common battery, that is if we don't count the shared capacitor across the shared set of points.

What I haven't talked about is the dynamic or "ac" performance of coils and the capacitor across the points. The reason is simple. I don't know enough about those transients to speak with any real confidence. And yet a battery/coil ignition system in a sense depends on those transient characteristics to work at all. To wit, the spark across the plug being initiated by the opening of the points and now all of a sudden we are talking about mutual inductance between the primary and secondary windings, the inductive voltage kick, and the reflected kick to the secondary, and that other energy storer and releaser, the capacitor across the points.

Though I'm writing with somewhat confident tones, I'm very aware that I may be blundering, ignoring or simply not getting a key point. Though I would like to be right, I'd be a lot smarter if I'm made aware of my blunder.

Thanks for trying

By the way, I have very much enjoyed this thread and this discussion.


Ken

[Duane Ausherman]

"By the way, I have very much enjoyed this thread and this discussion."

Me too. I tried to make it clear that I was musing, and not too serious about it being important, only interesting.

My comments were only about DC and that is useless in an ignition system. It must be analyzed as an AC circuit. Sort of a very low frequency transmitter. My experience with radio makes me convert everything to radio concepts.

Besides, some inganeer will blow us out of the water with our simple understandings.

[guy]

Duane is right about analyzing as an A.C. circuit. The coil is just a transformer changing the input voltage to a higher output voltage with an inverse current value. Transformers work from the collapsing field in the primary windings inducing an elevated voltage in the secondary windings. The change in voltage is calculated using the turns ratio, primary to secondary. The points opening stop charging the primary windings causing the primary field to collapse inducing the secondary voltage through the wire to the spark plug. A word of warning when checking the primary winding resistance make sure there is a safe path for the secondary winding to discharge if not it will reflect back through the primary when the test probes are removed.
Guy