Roll Your Own CDI

[rohmell]

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]

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]

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.

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]

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!)

 

[DuctTapedGoat]

LOL! Lemme know if it works!

 

[rohmell]

Even an old computer monitor should provide useful parts for this project.
Think green and recycle!

 

[rohmell]

Quick and dirty video of spark length using carbon-core suppressor wire:

http://img845.imageshack.us/img845/9383/dh7.mp4

 

[GearNut]

Wow! That's a very strong ignition system! When you pulled the wire away far enough to break the spark gap, I hope you didn't get zapped.

 

[DuctTapedGoat]

I totally scored at a thrift shop today - capacitance and resistance substitution boxes for 4 bucks!

 

[rohmell]

Sorry for the poor quality video, I was holding the video cam in one hand and the spark plug wire in the other hand, and I wanted a third hand to rev the engine.
I will try to make another, nice clear video soon.

Quick and dirty video of spark length using carbon-core suppressor wire:
http://img845.imageshack.us/img845/9383/dh7.mp4

 

[rohmell]

In a bind for parts?

For those of you who are having a hard time finding parts for this project, here is an sampling of what is available:

Capacitor:
ARCOTRONICS MKP C.4G Capacitors 1.0uf / 850v / 450vac | eBay

SCR:
NEW S8040R SCR NON-ISOLATE 800V 40A TO220AB TECCOR - eBay (item 350314480438 end time Jul-02-11 16:59:02 PDT)

Coil:
IGNITION COIL CHINESE ATV DIRT BIKE 50 70 90 110 125 cc | eBay
or
IGNITION COIL 70CC 90CC 110CC 125CC MOPED DIRT BIKE | eBay

 

[skrew37]

I wish I would have seen that. I did a direct search for the cr80 coil and got this:

CR80 CR85 CR125 CR250 CR480 CR500 XR600 IGNITION COIL | eBay

...that could have saved me $10.

 

[rustycase]

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]

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.

 

[DuctTapedGoat]

Now, I've tried looking this up online and came short. What about making our own coils? What's that process like?

 

[rustycase]

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]

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]

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]

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]

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

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.