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Thread: ignition condensor

  1. #21
    Bushytails Guest

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    Quote Originally Posted by randyscycle View Post
    I believe if I remember correctly, that if you look at the pattern of the Secondary Ignition on an oscilliscope, at the end of the pattern you'll see a small "bump" which we always called the "extinguishing spike." I think (not certain!) that there is where the condenser discharges......

    Help me if I am wrong!
    Sorry to dig up an old thread, but figured I'd add some more information... the condensor doesn't suddenly discharge, except in the case that engine rpms are high enough that the points re-close before the spark is completed. The combination of the condensor and the coil forms a resonant circuit, which rapidly oscillates, with the condensor going from fully charged with a positive voltage to no charge to fully charged with a negative voltage and back and forth several times slowly decaying to nothing.

    As an example, here's a 'scope display from when I showed someone what a misfire looks like:


    On the first one, there's a sharp spike when the points open, a level area during the spark itself, a small peak once the spark ends, then lots of quick oscillations. (while not relevant to this discussion, the second one shows a no-spark misfire - the sharp peak happens, but rather than showing a level area as it sparks, it just decays without ever sparking)

    During these oscillations (the squiggles to the right of the spark), the condensor is charged and discharged thousands of times a second with alternating polarity, slowly decaying due to resistance in the circuit.

    --Bushytails

  2. #22

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    Quote Originally Posted by Bushytails View Post
    Sorry to dig up an old thread, but figured I'd add some more information...
    No problem brining up this old thread. Your input adds more correct information.

    I now see where the discharge of the condenser (capacitor) is not linear but is in an oscillating discharge.

    The below charts are from an amplified ignition but the values would be similar to a points ignition. They show a primary voltage spike of 326.8 volts and an oscillating decay near the 40 volt mark and eventually going lower. These also show the resonance you described above.












    This link has a very good description of the above charts that show the high voltages across the points and condenser.

    http://www.mavericktechnology.co.uk/...ed_primary.htm

    Thank you for your input.

  3. #23
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    Man, you guys are way over my head.

  4. #24
    Bushytails Guest

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    Quote Originally Posted by fng View Post
    Man, you guys are way over my head.
    Well, how about a mechanical analogy?

    Consider a mass (weight)... it has momentum. If it's not moving, you need to apply force to it to make it move. And when it's moving, it'll keep moving unless you do something to stop it.

    If you apply a small amount of force over a long time, you build up stored energy. If you stop it suddenly, you get back a very large force over a very small period of time. The classic example is a hammer - you apply a small, constant force over the entire swing of the hammer, then the hammer applies a very strong force to the nail/stuck bolt/bent wheel arch/brake drum/etc for a short period of time.

    Now, if you intentionally want this sudden large force, you sometimes also don't want whatever you're beating to break. To do this, you add a small spring to it (like putting a piece of rubber on the hammer, to make a rubber-faced mallet), so while the hammer is trying to smash with a very strong force, the spring absorbs some of that force, storing energy itself, preventing too much force from the hammer blow.

    When the swing is over, the spring will push the hammer back slightly, then the hammer and spring will sit there jiggling up and down or vibrating, until either friction with the air slows it to a stop, or you pull back the hammer for another blow.


    An ignition system works much the same way... While the points are closed, a constant voltage is applied to the ignition coil. A coil, like a mass, builds up stored energy with time, and also like a mass, doesn't like suddenly stopping. When the points open, the coil wants to keep power flowing through it, but with the circuit disconnected, it has to suddenly stop. Like the hammer hitting the nail, this sudden stop produces a very large, short burst of energy - visible as the big spike in the oscilloscope pictures, this is what triggers the spark.

    However, just like the hammer, unless there's something to absorb just a little bit of the energy, things will break. A condensor (capacitor) acts like a spring - you can stick energy into it, and the more energy it's storing, the harder it is to push more in. Some of the energy released from the coil is stored in the condensor, instead of doing harmful things, which in the case of a points ignition, is burning the points.

    Like the hammer+spring example, now that the condensor has stored some energy, it wants to spring back... it discharges the energy it just got back into the coil (with power flowing the opposite direction). Then the coil puts some energy back into the condensor. Etc, etc, etc. Like a weight and a spring jiggling up and down, this process resonantes until imperfections (resistance/friction) reduces it to nothing - these are the jiggles you see on the graphs.

    Now, all this only describes the primary side of the circuit. The coil also has a secondary side, which for this example, can be thought of as a long lever - the big impulse when the points open is translated to an even bigger one on the other end of the lever, which is why you have a 35,000 volt spark from the 350 volt primary spike...


    Did that help, or was it such a horrible analogy no one has any idea what I'm talking about?

    --Bushytails

  5. #25

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    I remember my dad using a snap-on scope to diagnose problems back in the 70's. Always thought it was cool to look at all the wavy lines but didn't have a clue what they meant. From looking at the lines a good mechanic could quickly diagnose the source of the ignition problem.

  6. #26
    Bushytails Guest

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    Yep. Hence why I was showing someone what ignition problems look like on a 'scope.

    --Bushytails

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