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Transistor Ignition shematic

Discussion in 'General Electronics Chat' started by easy.rahil, Jul 17, 2011.

  1. easy.rahil

    easy.rahil Member

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  2. vintagekarter

    vintagekarter New Member

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    Hate to bring up an old thread. But what size capacitor is used on the circuit simmilar to the ATOM module. I know it is .47, but .47 uF, pF, F?
    Thanks a bunch.
     
  3. jpanhalt

    jpanhalt Well-Known Member Most Helpful Member

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    0.47 µF, probably 400V to 600V or so. Need to see the actual design to be sure about the voltage rating.

    John
     
    Last edited: Nov 21, 2012
  4. dave

    Dave New Member

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  5. debe

    debe Active Member

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    This is the Atom ign circuit. I used a .47uf 100v cap in the circuit.
     
  6. vintagekarter

    vintagekarter New Member

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    Thanks. That's what I needed to know. I am intending on using this on an old McCulloch go-kart engine because the atoms are no longer available.
     
  7. bigmrk

    bigmrk New Member

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    need help also

    dear debe did you change the value of R2 or is it the same value as the original atom im needing to build one of these for a '63 johnson outboard in witch the points and coils sit under the flywheel and i was wondering if you had ore pics of the layout of the ciruit because im not that good at reading schematics lol il admit im a newbie lol
     
  8. bigmrk

    bigmrk New Member

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    dear debe did you change the value of R2 or is it the same value as the original atom im needing to build one of these for a '63 johnson outboard in witch the points and coils sit under the flywheel and i was wondering if you had ore pics of the layout of the ciruit because im not that good at reading schematics lol il admit im a newbie and thanks in advance
     
  9. easy.rahil

    easy.rahil Member

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  10. MisterJames

    MisterJames New Member

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    Bringing up the old thread, but....

    When contact points are closed Transistor Q1 is closed i.e. not conducting. When contact points open Q1 is conducting and the coil is energized. Why the operation is inverted?, normal CDI ignition system energizes the coil when points are closed and when points open the spark is created.

    James
     
  11. spec

    spec Well-Known Member Most Helpful Member

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    Hi MJ,

    With traditional Kettering ignition systems, the points are closed and current ramps up in the primary winding of the ignition coil, to typically 4A maximum.

    At the ignition point, the points open and the coil generates a back emf, of typically 400V, which is transformed to between 15KV to 33KV across the secondary winding of the coil.

    On the other hand, with a CDI (capacitor discharge ignition) system an inverter converts the normal battery voltage (12V) to 100V to 400V to charge a capacitor, typically 1uF to 4.7uF, to 100V to 400V.

    At the ignition point the charged capacitor is connected to the primary winding of the coil, by a bipolar junction transistor (BJT), insulated gate bipolar transistor (IGBT), MOSFET, or SCR. This generates a fast, high energy, pulse across the primary of the ignition coil, which is transformed to 15KV to 33KV across the secondary winding of the coil.

    So the functioning of a Kettering ignition system and a CD ignition system are quite different and a CDI system can be arranged to trigger from a positive or negative pulse.

    spec
     
    Last edited: Dec 7, 2016
  12. dr pepper

    dr pepper Well-Known Member Most Helpful Member

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    Welcome.
    Thinking about it, if you inverted the point operation on a engine if you then adjusted the timing the igntion system would still work, however the dwell time would be all messed up.
    The op mentions a hall sensor so maybe his 'cam', the device that triggers the hall sensor is custom made, or the system charges the coil at a variable dwell time.
     
  13. spec

    spec Well-Known Member Most Helpful Member

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    With CDI there is no dwell as such.

    With a Kettering ignition the the dwell is crucially important, because during the dwell period the coil current is ramping up and if the dwell is not sufficient the current build-up will not be enough to generate a healthy spark. One of the shortcomings of the Kettering system is that as the engine revs increase, by definition, the dwell reduces, thus the spark energy also reduces, just when you need a good spark (at high revs).

    On the other hand, with CDI, the high voltage generator can run all the time, and the high voltage generator can be made as powerful as you like, so it can be made to charge up the CDI capacitor as fast as you like.

    spec
     

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