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Detecting points opening on a magneto question

Discussion in 'Electronic Basics' started by eatmorepies, Dec 4, 2011.

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  1. eatmorepies

    eatmorepies Guest


    I've been asked to help set the ignition points on a 1950 Triumph
    motorcycle - it has a magneto. I want to detect when the points open. My
    first two circuits have attempted to detect the small (around 2.4R)
    resistance change. Both circuits were based on potential dividers, one with
    a NAND gate and the second with a bridge circuit and an op-amp. To keep the
    current drawn from the PP3 low I had to use reistors in the order of low k
    values - the tiny resistance change is thus hard to detect.

    Now for my question. The magneto is basically a low inductance coil that is
    or is not shorted by the contact breaker - so how do I set about detecting
    the change from shorted to small inductance?

    There's a capacitor across the points, I did think of detecting a current
    spike as the PP3 charged C when the points open. I'd like to try detecting
    the coil first. Any help would be appreciated.

  2. Jamie

    Jamie Guest

    The coil is very inductive. Inject a high freq signal into the coil side
    of the points and use a AC to DC detect circuit from that signal at the
    point where it is connected.

    With the induction there, you'll see the signal until the points close.

    you'll need to factor in the cap that is on the points too, so don't get
    to high in freq..

    Most likely something in the range of 1khz from a 555 timer will do it.

    Make sure you put a back to back Zener for protection so not to damage
    your input sensing circuit.

  3. eatmorepies

    eatmorepies Guest

    This is a 1950 motorcycle using a magneto.

    1. The manual suggests testing for the opening of the points using a small
    piece of cigarette paper. This method gives +/- 10 degrees of uncertainty in
    the timing. A visual method would be even worse.

    2. It's a magneto - no pigtail. These things are fully enclosed with just
    the high tension lead(s) emerging. You get at the points by removing a small
    cover. You can't even see the capacitor, coils or any wiring without
    disassembling it - which requires special tools.

    Try Googling magneto and have a read.

  4. eatmorepies

    eatmorepies Guest

    This seems to be the way to go. Thanks.

  5. Rich Grise

    Rich Grise Guest

    In the olden days, we used to set the points by just setting the gap using
    a feeler gauge.

    Can a dwell meter handle a magneto?

    Good Luck!
  6. Jamie

    Jamie Guest

    Looking back over the years, I find it amazing that the point system
    worked as well as it did.

    Taking into account for worn cam shafts, loose cam bearings, cam
    pads, spring tension etc.. And let us not forget the crappiest
    condensers ever made along with the normally stripped out screws holding
    it all in.

  7. Jasen Betts

    Jasen Betts Guest

    battery life is less of a problem is you use a low voltage op-amp (LM324)
    or comparitor (LM319) and a 3V supply
    You could use a higher frequency than DC, say 10KHZ, the magneto will
    have a higher impedance at that frequency.

    10Khz | |\ /|
    --------[100R]---+------------| >========| | speaker
    ------------------------------|/ \|
    --- audio amp
    if the flywheel magnet is in place there will be other effects tat
    depend on the speed of rotation etc.
    Disconnect the magneto from the points and just measure resistance?

    put a coil (old degausing coil? 100 turns of telephone wire? a spool
    of hook-up wire?) near the magnetto supplied with stepped-down AC or an audio signal and connect an amplifier
    and loudspeaker (pc speakers) to the plug terminal and chassis?

    when the points oen the sound should get louder.
  8. Rich Grise

    Rich Grise Guest

    (slapping forehead) Of course! Duh! We'd set the timing by turning
    the distributor itself!

    Sorry for the brain fart.
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