Roll Your Own CDI

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Are You Going to Roll Your Own CDI?

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rohmell

rohmell

Active Member
Jun 2, 2010
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To anyone building a CDI:

Please share your build details, what type of ignition coil you are using, pictures, etc.

In the past, we have been able to tune and modify our carburetors to make them work better and deliver a more precise air/fuel mix to the engine.

We have been able to try various exhaust systems so that our engines breathe better.

We have been able to port and polish our engines to make them work better.

I think the CDI deserves equal time!

No longer are we stuck with a mysterious black box, now we can experiment with and change component values and optimize its performance.
 
DuctTapedGoat

DuctTapedGoat

Active Member
Dec 20, 2010
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Nampa Idaho
Nice - what about running it with the SCR and pot, being able to switch between both? I gotta diagram it, and it would add size, but it'd be nice to dial it in when you want, and just go when you don't feel like messing with it.
 
rohmell

rohmell

Active Member
Jun 2, 2010
1,531
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New York
DuctTapedGoat said:
Nice - what about running it with the SCR and pot, being able to switch between both? I gotta diagram it, and it would add size, but it'd be nice to dial it in when you want, and just go when you don't feel like messing with it.
Click to expand...
Absolutely! This CDI circuit is intended to be a bare-bones starting point that can be experimented with, modified and improved.

You could have a multi position switch that selects from some fixed resistors and a potentiometer so that you could have some fixed timing curves plus a continuously variable one as well.
 
rohmell

rohmell

Active Member
Jun 2, 2010
1,531
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Quickie parts list for the CDI:

Diodes: 1000 PIV 3A
Resistors: 18 and 82 Ohm 1/2W
Capacitor: 2uf 600V Film/Foil Polypropylene
SCR: 1000V 3A

Due to the wide variety of ignition coils that can be used it is better to overrate the components to be safe, although for the most part, you can use just about any parts that you may have lying around.

The component values are not set in stone; for example, I used 400 PIV 1A diodes in my prototypes with no problems.
I used an SCR that is just too big for the purpose (600V 63A, inverter grade), but that is what I had lying around, so I threw it in.

An old TV with a CRT should provide all of the parts except maybe for the SCR, although some TVs used an SCR in the power supply circuit. Heck, you might even be able to use the flyback transformer as your ignition coil! (I'm gonna try it!)
 
rohmell

rohmell

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Jun 2, 2010
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rustycase

rustycase

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May 26, 2011
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Thank you for posting those links Rohmell !

...can spend forever searching for good deals on that kind of stuff...

those 1uf caps are the key to the whole thing, aren't they???

also, I might be able to search the nearby scrapyard... what is are apps for that large capacitor?

tnx
rc
 
rohmell

rohmell

Active Member
Jun 2, 2010
1,531
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New York
Due to the wide variety of ignition coils and SCRs that can be used in this project, you might need to play around a little with the value of the capacitor.

I like to use 1uF capacitors, because singularly you have 1uF, put two in parallel and you have 2uF, put two in series you have .5uF. So by stocking up on that value, you can create the value that works best in your particular setup.

A microwave oven from the scrapyard contains a nice big capacitor, but the values vary widely from something like .8 uF to 1.5uF, so you might have to go through several microwave ovens before you find something suitable.
 
rustycase

rustycase

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May 26, 2011
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Tnx rohmell!
Already got a microwave out in the pile I drug home a while back, for parts.
I'll check it out shortly!

Any other sources I might scavenge from that you can think of?

DTG, I went on a search yesterday for coil winding/mod info...
There's not really a whole lot for someone like me who knows nothing.
But here's what I found, so far. Perhaps reading through them you could find valuable info???


The Cushman Permalight Charging System

friese

All-OffRoad - Dirt Bikes - XR600R Magneto Rewind


Good luck
rc
 
DuctTapedGoat

DuctTapedGoat

Active Member
Dec 20, 2010
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Nampa Idaho
I'm actually talking about the ignition coil - I know I don't have the patience and/or equipment to do a magneto/alternator coil, but I'm more curious about the ignition coil and how it's put together, ie: if we can run a series of step up transformers, or whatever it would take to build one from parts.
 
rohmell

rohmell

Active Member
Jun 2, 2010
1,531
6
38
New York
Here's something to consider: If you have the room and clearance, use one of those ignition coils that sit right on top of the spark plug, this type of coil is used in 'distributor less' ignitions, and there's a coil for each plug in the engine.
If you use this type of coil, no more worries about the spark plug wire!
But, I would find a way to give the coil additional support, so there is not too much stress on the spark plug shaft.
 
rustycase

rustycase

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May 26, 2011
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DTG, I spend far too much time custom hacking things to get nearly every task I undertake done. IMO, it is far better to re-purpose something that is already available in some other application, perhaps with some slight modification?

rohmell... THAT is a good idea!

Best
rc
 
DuctTapedGoat

DuctTapedGoat

Active Member
Dec 20, 2010
1,179
10
38
38
Nampa Idaho
rustycase said:
DTG, I spend far too much time custom hacking things to get nearly every task I undertake done. IMO, it is far better to re-purpose something that is already available in some other application, perhaps with some slight modification?

rohmell... THAT is a good idea!

Best
rc
Click to expand...
That's what I'm saying, I rewrote that a few times, trying not to make it sound like "make one from scratch". But, I would like to see how to make one from scratch so that know what we can repurpose.
 
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