Which Coil for CDI Ignition?

[Stratos23]

Hans,
Thanks for your detailed explanation.
By quickly charging the capacitor (from charging winding or the battery) this takes care of the dwell problem with inductive ignitions (not enough time to fully charge coil at high rpms). One source states cdi systems have higher energy and shorter duration spark. I'm thinking it might help if Greg increased his plug gap (.045"?). Maybe the 3 ohm coil is better because it increases spark duration. Increasing gap with the cdi coil may have the same effect. Anyway Electrex put this system together and should provide the info. I guess their customer service is limited.

Regarding other cdi coils. Ignitech offers three different cdi coils. One is a dual output. They are described on their website.

Perhaps other electrex users can describe their experiences.

Dave

Hi Dave, thanks for some interesting insights!

Your suggestion to increase plug gap makes sense in theory - wow, that's a mighty large gap (1.15mm!!). It's an easy experiment to try (I'm assuming you mean increased gap with 3ohm coil). My concern is whether the larger resistance (3ohm) coil might damage the CDI long term? And I agree that Electrex have no doubt paired the supplied 0.6ohm coil with their system for a reason (although its quality appears dubious). I tried to get some info on the subject from them, but they were frustratingly un-conversant (their response was something to the effect of "this is the coil we supply - anything else is at your own risk"). They didn't seem interested in WHY their system wasn't performing very well on my bike - nor interested in discussing possible solves. Their product is certainly well-made and I'm very happy in this regard - I just want to get it running as it should.

I've had a look at the coils you suggested on the Ignitech site (and in fact I use an Ignitech CDI ignition on another Ducati twin paired up with 3ohm Dyna coils - but that's another whole scenario). It seems that the coils they recommend for CAPACITOR discharge ignitions are all low resistance (0.2 & 0.3ohm). The coils recommended for INDUCTIVE ignitions are around 0.8 - 3.7ohm) - although some of these are recommended for their own CDI ignitions (including the one I run on my other twin cyl Ducati). Perhaps I ask Ignitech what they might suggest. They've been very helpful in the past.

And as you also suggested - DOES ANYONE ELSE USING ELECTREX CDI's ON THEIR SINGLES HAVE ANYTHING TO SHARE ON THIS TOPIC?

Thanks! Greg

[Stratos23]

Sorry,

you are right with the dwell problem - but a CDI charges it's storage capacitor to 300 - 400V! No battery can do that directly. In the early days of CDI development (by a Mr. Winterburn, especially for car aftermarket), an electronic HV converter circuit provided the charge voltage. It was powered by the battery, indeed. See https://en.wikipedia.org/wiki/Capacitor_discharge_ignition and https://en.wikipedia.org/wiki/Ignition_system#Battery_and_coil-operated_ignition for more insight.
Btw, I still run my '73 350M3D with the original Ducati Elettronica CDI. As yours, it's spark looks somehow weak, quite different when compared to that from a points/battery system. But the engine starts perfectly, I never had to complain.

I have no direct experience with Electrex, but if their system is equipped with a 0.6 Ohms coil, this was surely done intentionally. A replacement coil should be of same quality.

Energy storage or transformation in an ignition system is subject to a physics law: E = 0.5 * L * I^2
where E is the spark energy, L is the primary coil inductance, and I is the primary current. This current is constant in a point/bat ignition, as long as the points are closed and the dwell is sufficien, but it is allways a function of time (exponentially decaying) in a CDI. The secondary voltage across the plug is dependant on the coil's turns ratio and the switching speed of the primary current. So, not only the primary resistance but also the turns ratio is an important parameter, which has to be defines the coil's characteristics, so must not be neglected.


Hans

Thanks again Hans - more learned wisdom! Now I remember you mentioning you worked in an electronics research lab! So forgive me if I struggle to keep up - electronics is my least favourite area of engineering (unless of course it's playing with synthesisers in the studio!!).

In previous projects where a CDI was employed, I've never had to incorporate a battery in the circuit. If you get a moment, here is a link to the set-up from their website. From what I can see (in my limited understanding), the CDI/ignition circuit seems to be independent of the battery - would you agree?

https://www.dropbox.com/s/6skgdxuul6gkz ... 4.pdf?dl=0

And yes, I've heard of people talking about the coil's TURNS RATIO as another factor to consider. A friend in NZ here who sells Nology coils was stressing this point. But it seems that manufacturers seldom give this specification.

Interesting that you're still getting good performance from the original Ducati Electronica CDI (my 450 is the older points model so I don't have that luxury). Do you know the specifications of that original coil?

At the end of the day, I just want to improve its willingness to start without damaging the CDI. Any more recommendations out there?

Thanks for all your considered thoughts so far BTW - I love this forum - where would we be without it!

[graeme]

Greg, do you have a resistor plug cap?
Do Electrix recommend one or not?

Like Hans I have 2 450 singles with the original '74 red box ignitions and both start easily even though it has to be a dark place to see the spark.

Graeme

[Ventodue]

In previous projects where a CDI was employed, I've never had to incorporate a battery in the circuit.

Hi Greg,

Yup, to be clear about this: a CDI ignition system can be designed to get its initial power from either a battery or from a starter coil. In operational terms, it makes no difference which.

I've heard of people talking about the coil's TURNS RATIO as another factor to consider. A friend in NZ here who sells Nology coils was stressing this point. But it seems that manufacturers seldom give this specification.

The ratio of the number of turns between the primary and secondary windings of a coil is what determines the voltage step-up of the coil. The greater the ratio, the higher the step-up. Explanation:

Coils are, in electronic terms, transformers. if you have a transformer with 50 turns of wire on the primary and 100 turns on the secondary, i.e. a ratio of 1:2, and you put 120 volts into the primary you will get 240 volts out of the secondary. Works the other way round too, of course: if you put the same 120 volts through a transformer with a turns ratio of 10:1, then you will get 12 volts out of the secondary.

Remember, several thousand volts are needed to make the charge jump the gap in the spark plug and, in so doing, make the spark! And the further the spark has to jump, the more voltage is required. (And normally, this also produces a spark of shorter duation - but that's another story).

Effectively, the number of turns on the windings in a coil is represented by their resistance. A 3 Ohm coil will have more turns on its primary winding than a 1 Ohm coil.

How this works for you is that with a CDI system, you should be able to run an ignition coil with fewer turns, and therefore with lower resistance (and which will also be physically smaller, to boot) because the voltage that is supplied to it is already higher than with a inductive (points) system. What this means is that the ignition coil has to step up the voltage less - often, much less.

And that's why Electrex have specified a low resistance coil. Now, as you'll understand from what I've said above, what your substitute high(er) resistance coil is doing is stepping up the volage to a level that is high enough for it to jump the gap and give you a spark. The unanswered question is:
WHY is it having to do this?

At the end of the day, I just want to improve its willingness to start without damaging the CDI. Any more recommendations out there?

Sorry, but I've gotta ask: are you SURE that the rest of the bike is set up right?

P.s I'm not sure that running a high resistance coil will necessarily damage the CDI - but best to check with Hans about that!

Interesting that you're still getting good performance from the original Ducati Electronica CDI (my 450 is the older points model so I don't have that luxury). Do you know the specifications of that original coil?

Me too, still running the original battery-less DUCATI Elettrotecnica CDI system . But sorry, I don't know what the coil's resistance might be. And as you may be able to infer from the above, it's not so important, really, because each system will have its own particular characteristics. For example, my little system starts off with 70 volts AC delivered straight from the ignition coil - vroom!

[Jordan]

How this works for you is that with a CDI system, you should be able to run an ignition coil with fewer turns, and therefore with lower resistance (and which will also be physically smaller, to boot) because the voltage that is supplied to it is already higher than with a inductive (points) system. What this means is that the ignition coil has to step up the voltage less - often, much less.

As I understand it, with points on an inductive system, the primary voltage can go to hundreds of volts when they open. That is what gets multiplied at the HT coil.
The primary coil circuit itself is enough to cause sparking at the contact breaker points.

[Jordan]

At the end of the day, I just want to improve its willingness to start without damaging the CDI. Any more recommendations out there?

If a part is cheap enough that a failure won't hurt the bank balance too much, you could try? :

Ebay item 171613262872

I have had one on my widecase for several months of occasional use so far, hasn't given trouble yet.

[Ventodue]

As I understand it, with points on an inductive system, the primary voltage can go to hundreds of volts when they open.

True, Jordan. When the magnetic field in the coil collapses, a voltage is induced in the primary winding in the region of 250-350 volts - which, as you say, is enough to arc across the points.

It's one of the reasons a condenser - properly called a capacitor - is included in the circuit: the charge in the primary takes the path of least resistance to ground, i.e. via the condenser. Result: no (or much reduced) arcing across the points.

Coil.jpg

That is what gets multiplied at the HT coil.

This is something I am less clear about. Please do say more if you know!

I have certainly read that, once it is fully charged, the condenser re-releases its charge back into the primary winding. This causes the magnetic field in the coil to reform. Which in turn then collapses again, so inducing new charges in the primary and secondary windings. And so on until all the energy has been dispersed by the secondary winding.

However, I find physicists do not always deliver the clearest of explanations .This is about the best I could find, and to get to this took quite some editing. It also implies that when the field first collapses, no current is induced in the secondary winding. Which doesn't seem logical. Me, I dunno. Maybe I've misunderstood.

"When the points open:
- The ground contact is broken, stopping the flow in the primary winding. This causes the magnetic field in the coil to collapse. The collapsing magnetic field induces a current to flow momentarily in the same direction, charging up the capacitor.

This current raises the voltage at the capacitor to about 300 volts. This high voltage stops the current flow. The current then reverses direction, being driven backward into the primary winding by the high voltage at the capacitor. Once the charge in the capacitor is depleted, the current in the coil re-charges the capacitor in opposite polarity. The cycle then reverses. The result is an electrical oscillation which reduces rapidly with time.

The initial 300 volt spike in the primary winding will drive an output in the secondary winding of up to 30,000 volts. The current going to the spark plug will oscillate in harmony with the input current for as long as the output voltage is high enough to jump the spark gap under compression."

[ecurbruce]

Ventodue says;
When the magnetic field in the coil collapses, a voltage is induced in the primary winding in the region of 250-350 volts - which, as you say, is enough to arc across the points.

Hi Craig,
Just as a (related) anecdotal,
When I am charging/jumping a car battery at 40 amps with my industrial 120 volt battery charger, and if I grab the power cord and snatch it out of the 120 wall outlet while the charger is still running and connected to the car battery at 40 amps, it will burn the 120 volt power wire right off the primary side of the transformer. Same reason as the surge on the primary side of the ignition coil...the secondary sends that high power surge back into the primary at the time of collapse.

Bruce

[Jordan]

Reply to Ventodue:

You seem to have as much info and knowledge as I do on this subject. Sorry, but I can't add to that.
I read that the primary voltage when points open briefly is quite high - as you have also researched.
If you web search for oscilloscope waveforms of primary voltage, there are many images to see.
For example:
https://www.picoauto.com/library/automo ... y-voltage/

[Stratos23]

Greg, do you have a resistor plug cap?
Do Electrix recommend one or not?

Like Hans I have 2 450 singles with the original '74 red box ignitions and both start easily even though it has to be a dark place to see the spark.

Graeme

Hi Graeme,

yes, I've considered this - Electrex didn't advise either way, but I'm currently running a non-resistor NGK cap. Very easy to try one as I have heaps on the shelf. These are typically 5 Kohms - so 5000 Ohms, which seems like a LOT of added resistance. How does this correlate with the nominal resistance ratings of the HT coils themselves?

Cheers, Greg