Roll Your Own CDI

Discussion in 'Lighting and Electrical' started by rohmell, Apr 3, 2011.

?

Are You Going to Roll Your Own CDI?

  1. No, too complicated and difficult.

    24.3%
  2. Yes, but only one, for my own MB.

    33.3%
  3. Yes, a few for my own use.

    34.2%
  4. Yes, many, I'm gonna sell 'em!

    8.1%
  1. Venice Motor Bikes

    Venice Motor Bikes Custom Builder / Dealer/Los Angeles

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    #621 Venice Motor Bikes, Dec 3, 2015
    Last edited: Dec 3, 2015
  2. Ryoskate

    Ryoskate New Member

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    I'm sorry but your method of experiment doesn't really say a lot.... Please, I'm appreciative of your effort and I'm not personally attacking you, but the timing you posted are nothing without context...

    In order to make out anything useful from timing degrees, you'll have to record both RPM and timing at multiple points and plot it in a graph to see the actual CURVE, if any at all.

    Does the timing retard after it reaches max advance? How steep is the curve? etc etc. You can't get any of that from just a single measurement.
     
  3. WECSOG

    WECSOG Member

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    He is comparing various CDI units at a given (3000) rpm, starting from the same fixed mechanical timing. While that doesn't plot a curve, it does give us useful information. None of these units actually advance the timing; they retard it at low rpm and the retard becomes less as rpm increases.
     
  4. Venice Motor Bikes

    Venice Motor Bikes Custom Builder / Dealer/Los Angeles

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    I was simply trying to give this community some kind of info to help them figure out what part will work best for their needs.

    If you've been around here for any length of time, you'd know that none of the makers of these CDIs ever provided a single bit of of test info about what their parts actually do.
    (All they ever did was talk trash & say theirs works the best).
     
  5. LS614

    LS614 New Member

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    Made my own

    Hi guys,
    I finally built one, I saw this thread years ago and finally got around to it. My only concern is that I used a pair of 250V caps. Am I doomed to a CDI that fails? What I don't understand is I thought the caps were the place where the voltage is really multiplying from whatever the magneto puts out to what goes into the ignition coil. If that's the case then why is it bad to use a lower voltage cap? Someone please explain what's going on and if I need to swap the caps out before I test it.

    [​IMG]

    [​IMG]
     
  6. WECSOG

    WECSOG Member

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    250 volt caps will probably be fine. It is doubtful that the stator on these engines will produce much more than 200, anyway.
    There is no voltage increase inside the CDI. The stator outputs around 200 volts AC which is rectified to DC and stored in the cap until the right moment when the SCR triggers and dumps the charged cap into the coil primary. Cap voltage is whatever the stator outputs.
     
  7. LS614

    LS614 New Member

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    Thanks for the reply! Is there any measure I could/should take to protect the caps or should I just try it out and see what happens? The explanation of what specifically is going on helps a bunch. :)
     
  8. LS614

    LS614 New Member

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    Hey guys, so an update. I took the magneto cover off and put a 14mm socket on my drill, and hooked test wires up to my multimeter. My drill spins the engine around 750 to 1000 rpm, approximately idle speed. I found that my magneto generates 32.8 VAC consistently at idle speeds. I find it hard to believe that in normal operation I would be getting anywhere close to 300 VAC from the magneto coil. So I tested the circuit using more test leads and my drill and here is the result. A nice fat blue spark. Thank you to everyone who helped fine tune this circuit.

    https://www.youtube.com/watch?v=FQcmAN0XSJI
     
    #628 LS614, Jan 22, 2016
    Last edited: Jan 22, 2016
  9. WECSOG

    WECSOG Member

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    I would just try it and see what happens. The only way you could really protact the caps would be to add resistance to the circuit, and that would introduce several additional problems.
    Even if the caps blow (which I think is unlikely), it shouldn't take anything else out. You should be able to just replace the caps and be good to go again.
     
  10. thecoolsundar

    thecoolsundar New Member

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    I'm not able to see any of the image shack files. is anyone else having the same problem?
     
  11. thecoolsundar

    thecoolsundar New Member

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    Has anyone disassembled the rotor? I'm curious as to what kind of magnets they use and how the laminated stack looks on the inside
     
  12. Ahmedy

    Ahmedy New Member

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    Hi guys. I've got a problem. I've built Rohmell's cdi twice now and I tried building the jag one also. I'm not getting spark from either. Then when I hook up the original Cdi it works with no problem. Could it be the Chinese coil I'm using or could it be that the original stator coil isn't producing enough volts.
     
  13. nyemi

    nyemi New Member

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    Welcome everyone.
    Circuit your opinion?
    Resistance(P1) to change the ignition timing.
    Your test circuit.zpt

    Regards nyemi


    Postscript:I optimize circuit.Sorry.
     

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    #633 nyemi, Sep 8, 2016
    Last edited: Sep 13, 2016
  14. humanj

    humanj New Member

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    Hi Rohmell

    Please help me out, I noticed in the first circuit you posted a 33 Ohm resitor was present. The later circuits it is not present. At first I tried the circuit with the 33 Ohm resitor but no spark. Eventually I came right after removing it. Is the purpose of this resistor the limiting of current on the Gate Resitors?

    I must say this circuit performs very well givven its simplicity. I used a Yamaha Generator Ignition Coil.

    Thanks for your effort!!
     

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  15. humanj

    humanj New Member

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    Hi Toad

    You can use a Yamaha Generator coil -- Yamaha High Voltage Set ET950 -- Works well.

    In the orginal Circuit you added another capacitor but you did not include the capacitance value. I would also like to experiment, please supply the vallue.

    The orginal circuit works well I must say.
     

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  16. rohmell

    rohmell Active Member

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    Another CDI schematic.

    Single coil CDI..jpg
     
  17. nyemi

    nyemi New Member

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