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Transformer for CDI ignition power supply

  1. Jan 5, 2016 #1
    I have an old motorcycle which I would like to fit with a modern CDI ignition.

    The ignition would be of the type typically fitted to dirt bikes and which require a high voltage AC source. They have a permanent magnet alternator with a separate high voltage stator for the ignition in addition to a 12V stator.
    My bike has a permanent magnet alternator which contains two 6V stators wired in parallel. I am wondering if it is possible to use a transformer to provide a ~300V source from the stock alternator to power the CDI. I could dedicate one of the stators to the ignition if necessary.
    Is providing the high voltage supply as simple as this or am I missing something? Is there any specific type of transformer or characteristics/features I would need?
  2. jcsd
  3. Jan 10, 2016 #2
    Thanks for the post! This is an automated courtesy bump. Sorry you aren't generating responses at the moment. Do you have any further information, come to any new conclusions or is it possible to reword the post?
  4. Jan 10, 2016 #3

    jim hardy

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    It'd be easy to say "Sure a 6 to 300 transformer is all you need"
    but we dont really know if that'd work.

    The CDI's i have encountered indeed make a few hundred volts DC from a coil
    but it's not continuous wave like your housepower from a wall socket
    it's a single pulse every time the magnet passes by the CDI's supply coil
    which they capture with a rectifier and store with a capacitor
    then deliver to the coil using a thyristor at the proper time to fire the sparkplug.
    Rectifier, capacitor and thyristor are inside the "Power Pack" usually a sealed and potted plastic box that you can't open.
    You probably knew that already....
    In outboards it's a winding on the stator , often but not always one winding for each cylinder.
    The coil is just like a charging coil but with more turns
    the coil is physically located so as to charge the capacitor slightly before the CDI needs to make use of that charge,
    In other words so magnet passes coil before TDC + timing advance .

    Do you have any electrical knowledge ?
    Would it be within your ability to experiment -
    connect a small transformer, rectifier and capacitor to one of your 6 volt coils,
    and see if it'll capture a couple hundred volts when you kick over the motor?

    A small transformer from a wall-wart might work..
    Maybe you could rewind one of those charging coils for high voltage.

    That's the closest i can come to an answer.

    Can you post a photo? And DC resistance of those charge windings? And open circuit voltage at around 3000 RPM ?

    PS it's not a '60's British bike is it? Old BSA's positioned the magnets and charge coils so you can use a huge capacitor instead of a battery . Racers liked that.
    My '56 Enfield had a magneto....

    old jim
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