Telling the difference between DC and AC sparking coil

[mrkprsn]

Sorry if this has been discussed somewhere. I have a 200gt battery ignition and I have a couple of questions.

1. Is there any way to tell the difference between a DC and AC sparking coil?

2. I have two coils. When you mount them with the lettering not upside down. On the left terminal is a # 1 and the right a # 15. Can I assume the # 1 is for the white positive wire from the ignition switch and the # 15 is for the black grounding wire to the points.

Thanks, Mark

[mrkprsn]

I have another question. I have a four spare coils with no labels.

Two show 1.8 ohms between the outside terminals and 6.5 k ohms between the outside and central terminal and two show 4 between the outside terminals and 11 k ohms between the outside and central terminal.

Can anyone test a know good coil of each type and let me know what you get? Can I assume that the coils with the lower numbers are DC?

Thanks -Mark

[Harvey]

The 4 ohm primary coils are for the battery DC system.

[Ventodue]

To build on my reply posted at The Other Place , Mark, this is how I understand it :

In a DC system, the battery provides relatively low voltage but relatively high current (amps). And it does so constantly.

In an AC system, the source coil provides relatively high voltage but relatively low current. And it does so only briefly.

So:
The windings of an AC coil must have relatively lower resistances (which means fewer turns or thicker wire), so that when volts from the source coil are applied to it the ignition coil builds its magnetic field quickly.

What this means in practice is that if you use an AC coil in a battery system, you risk burning the coil out. And if you use a DC coil in an AC system, it probably won't fire the plug.

(Why? Because the low power coming from the source coil is unlikely to allow the magnetic field to grow enough to provide the step-up in voltage in the secondary winding that is needed to jump the gap at the plug).

[mrkprsn]

Ventodue, I like this place better . I have a contradictory opinion.

JimF mentions that the AC spark coil has a resistance of 4 ohms between the + and - terminals. My green label AC coils measure that way also. My DC coils measure 1.8 ohms. I think the wire is thicker for the DC coil because the voltage is lower, there is more current and the current is constant.

I think using an AC coil in a DC system would burn it up but using the opposite should work unless the lower resistance of the DC coil messes up the ignition system of an AC bike somehow. -Mark

[Ventodue]

Ventodue, I like this place better

Me too. You meet a better class of person here ...

I have a contradictory opinion.

Oh, good. I like those ...

JimF mentions that the AC spark coil has a resistance of 4 ohms between the + and - terminals.

Well, Jim, now he know his stuff. So I ain't gonna gainsay him . Shoot him a mail if he doesn't pop up on the thread. In the meanwhile, allow me to ponder the matter further ...

(Btw, "between the + and - terminals" = the primary winding).

[Ventodue]

I said I'd ponder, so here goes ... It's very probably wrong. You have been warned.

An ignition coil is a transformer whose purpose is to provide more volts at the output than arrives at the input.

To achieve that, it would be best to start with as many volts as possible at the input, aka the primary winding. So, since V= I/R (from Ohms law, I = V/R), to get maximum volts in the primary winding we need to either increase the current or decrease the resistance.

With input juice coming from a magneto/alternator coil, the current is pretty low, especially when the thing is only turning over at kick start speed. So to my mind, that means you have no choice but to keep the resistance in the winding as low as possible.

In a battery-fed system, however, the current is relatively quite high, viz. modern motorcycle batteries capable of delivering over 400+ ‘cold cranking’ amps. That suggest to me the opposite: you can put more turns of wire on the primary winding, so increasing its resistance but protecting it from burning out and yet without reducing its voltage.

But what do I know?

[bw_nh]

Good evening-

I have wrestled with this same problem- in October of 2015, I wrote to this list, with this information-

o remind you all... I have two different ignition coils that could be used. The coil that was originally on the bike has a green label, with some numbers... 11.06 printed on it. As far as I know, that is the original correct coil. I also have a "motoplat" coil that also has "barcelona" on the top. Same dimensions, but different resistances...

original green label coil... 4 ohms resistance across #1 and #15 . 10k ohms across #1 and center spark.


motoplat coil... 2 ohms resistance across #1 and #15. 6.4k ohms across #1 and center spark.

Which to use? But neither actually seems to produce a good spark...

I may try bringing up a 6volt battery from the Mach I and try getting it running by temporarily re-wiring the coil and points... and maybe start saving up for an electronic ignition conversion kit... Gah.

This summer, I tried again, giving up the struggle to get the original 6v AC ignition system to work- I used a 6v lithium battery (inside of a gutted 6v AFA battery) , and re-wired most of the bike to use that battery, and a small 6v regulator/rectifier bought on ebay. With this, the bike starts and runs well. I would have to check, but I am pretty sure I am using the motoplat/barcelona coil. I am also still using the AC ignition advance unit, that I probably should change out to the DC ignition advance bit...

I hope this helps!

Sincerely,

Bob Woolner
Hillsboro, New Hampshire USA

[mrkprsn]

I guess until one cuts open a known coil we'll never know for sure how it is constructed. My original interest was in trying to determine coil type without it having a label or other markings. It seems to me from the evidence presented, the AC coil has a 4 ohm resistance of the "primary winding" and the DC coil has around 2. I sent an email to Jim F to get his take. I'll let you know his response. In the meantime if anyone has a known coil would they test it and let us know what the reading is? Thanks, Mark

[Ventodue]

<snip> On the left terminal is a # 1 and the right a # 15. Can I assume the # 1 is for the white positive wire from the ignition switch and the # 15 is for the black grounding wire to the points.

Oops, missed this. Apologies.

The DIN numbering convention is:
15 = switched battery feed
1 = connection at the coil (in this case, to ground).