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The capacitor in a points ignition

  1. Dec 3, 2009 #1
    Hi all

    In a car old points ignition system, I understand the capacitor (condenser) main purpose is that of protecting the points from getting burnt.

    The second purpose for this capacitor, could it be to make a resonant circuit among the ignition coil in order to prevent it from radiation?.

    I mean the coil-condenser would make the system to oscillate at a lower frequency in the way that all the energy would be used to ignite the spark and there would be no loss in the case of a high frecuency radiation.

    Am I wrong?

    Another question. In what way does this capacitor help the primary coil to get those some 200 volts?

    Thanks a lot in advance.
     
  2. jcsd
  3. Dec 3, 2009 #2
    When the points close, the capacitor is shorted and the coil primary charges up to about 2 amps (limited by coil internal resistance of ~6 ohms). When the points open, there is a very large V = L dI/dt voltage, ~300 volts, across the capacitor that resonates back and forth with current in the coil, for ~ 10 cycles. The coil secondary:primary turns ratio is ~100:1 so the coil secondary voltage is ~30,000 volts. See my LTSPICE simulation
    https://www.physicsforums.com/attachment.php?attachmentid=20126&d=1250393723
    The points close at 0 and 4 milliseconds, and they open at 2 and 6 milliseconds.
    Bob S
     
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