ignition coil

[VictorVictor]

Does any one know what the letters SW and CB stand for on the ignition coil diagram.

 

[Billm]

switch (for ignition switch) and contact break (for "points)

 

[10musketeer]

hmm. I wouldn't have guessed that in a thousand years! Makes sense though!

 

[jlaird]

Gesh, I learned something as well. Good stuff. Thanks.

 

[dklawson]

Use caution when installing coils with the SW and CB labeled terminals. They were used most commonly on the older positive ground installations. You're supposed to reverse these coil connections if you convert the car to negative ground. According to others on this forum, failure to swap the coil terminals when switching to negative ground will result in a significant reduction of the available spark voltage. Ping Dave Russell for details.

 

[jlaird]

Naaa. Can not be. Works fine either way. Electric will go through a coil either direction with the same result.

 

[Sarastro]

Use caution when installing coils with the SW and CB labeled terminals. They were used most commonly on the older positive ground installations. You're supposed to reverse these coil connections if you convert the car to negative ground. According to others on this forum, failure to swap the coil terminals when switching to negative ground will result in a significant reduction of the available spark voltage. Ping Dave Russell for details.

This is another one of those things in the conventional wisdom that puzzles me. I have also read, in one place or another, that ionization in the plug gap occurs more easily with one polarization, so the voltage needed to fire the plug is about 15% lower. The fact that the coil's primary terminals are marked (the CB and SW labels) lends credence to this idea, but I can't imagine why it would be true. Indeed, in Astley's book, MG Electrical Systems, the section on positive-to-negative ground conversion doesn't mention it.

The only other reason I can see for marking the coil primary terminals is that the bottom end of the secondary coil is connected to the CB terminal, the one that goes to the points. So, in normal operation, the secondary current goes to ground through the capacitor. If you reverse the terminals, the ground return is now through the electrical system. The current is pretty small, so it probably doesn't matter; but, if anything, it implies that you should NOT switch coil terminals.

If you do switch the terminals, you might get more electrical noise in the system, which might affect radios or other modern electrical components. To avoid this, add a 0.25 uF, 500V capacitor to the CB terminal (which is now connected to the switched 12V power, not the points). It won't affect performance, but might reduce electrical noise.

 

[Bayless]

There is a reeason the virtually every coil manufacturer labels the connections for polarity. I figure the guys who design and build these thing probably know more than I do about them. Although electricity will go through the coil either way, your spark plugs will deliver a hotter spark if the charge is negative. Electrons are more easily ejected from a hotter surface than a cooler one. Also, they will last longer and maintain gap longer if fed negative charge. The center electrode is hardened so that electron ejection is less damaging. The ground electrode is not.

Be safe and reverse the coil connections if you change polarity of the system.

 

[Sarastro]

This makes a lot of sense. It's still a bit of a puzzle to me, because the spark current is AC, not DC. But the connection with electrode temperature and direction of the initial current, when the gap breaks down, seems persuasive to me.

I do remember something else I've read, now that I read this: the polarity is arranged so that the erosion of the plug electrodes involves metal transfer from the outer electrode to the center one. This results in longer plug life, as the metal loss from the outer electrode is distributed over a wider area than it would be if the center pin eroded. If you don't switch the coil terminals, the erosion is in the wrong direction.

 

[jlaird]

Now that I can beleive.

 

[dklawson]

Jack, I got my rear end chewed off on the Healey forum by Dave Russell and Keoke who took two approaches to explaining the coil polarity issue to me. Keoke slapped me down and simply said in essence "go look it up, it's well documented". Dave took more time to explain it to me. I agree that it doesn't make a lot of sense, but there are as Keoke said, many years of test results out there that show incorrect polarity connections on the coil result in lower output voltage. I don't pretend to understand why. However, as Bayless said above, the coil makers mark the coils for polarity which indicates that there is some reason to hook one side high and the other side low. I don't disagree with you, the coils will work either way, I have just had it pointed out to me several times that improper connections result in reduced performance.

EDIT:
Just for grins, please visit the following links:
https://www.mgexperience.net/article/neg-convert.html
First paragraph talks about coil wire swapping.

https://www.chicagolandmgclub.com/techtips/general/574.html
This article discusses coil polarity as part of ground conversion.

Also see:
https://www.xvo73.dial.pipex.com/ht/coil.htm

And see step #8 of these technical instructions on ground conversion:
https://www.minimania.com/web/SCatagory/E...81/ArticleV.cfm

 

[Bayless]

Steve, the spark current is DC. It flows in one direction through the coil primary while the points are closed then, when they suddenly open, the breakdown has the effect of a reverse flow through the secondary. So although the induced current in the secondary does go both ways, the flow with the points closed is building the field in the coil and is not nearly strong enough to induce a spark. Thus the spark is always one way.

 

[Sarastro]

I'm not sure I understand this. The initial coil current builds up relatively slowly while the points are closed. Then, when they open, you have an inductor in series with a capacitor. This creates a sine wave of current, about 10 KHz in Sprites, whose magnitude decreases with time. I have measured this (actually, the coil voltage, but the current must have the same basic waveform); a plot can be seen in my treatise on tachometers, https://www.nonlintec.com/sprite/Sprite_Electronic_Tach.pdf.

It's true, though, that the spark forms in the first half cycle, so the breakdown occurs with a specific polarity. However, it does continue for about a millisecond, which is about 100 cycles of the 10 KHz AC.

What's interesting is that the sine wave is somewhat asymmetrical; the initial positive voltage excursion is much greater than the negative-going. (12V of this difference is simply the battery voltage, but the offset is more like 40V total.) So, there is a polarity, of sorts, and that should be selected to make breakdown easiest. Incidentally, a positive voltage at the point where I measured it corresponds, as you suggest, to a negative voltage at the plug.

Good discussion, though--

 

[jlaird]

Wow, wonder if I understood all that. Must go look at Miss Agatha. I am sure I have the coil set up for Positive ground when in fact I did convert to Neg ground. Did not reverse coil wires though.

Makes sense I think. What we are doing is providing a bit more power and life to the plugs by running the current the proper direction.

 

[piman]

Hello all,

I think that this discussion is getting too deep.
Basically the spark is able to arc across the plug electrodes better from the hot central electrode to earth (ground) than the other way around so the plug body needs to be the same polarity as battery earth. Or to put it another way, more H.T. voltage is required if the central electrode is the same polarity as the battery earth; unless you have a very high output engine or do not drive very hard it's likely that you will not notice the difference.

Alec

 

[Sarastro]

OK, I'll back off. I'm a technogeek--what more can I say?

 

[racingenglishcars]

That stuff is all very interesting

 

[jlaird]

Technogeek, that's a good thing.