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Neon Sign transformer / Tesla Coil question

Discussion in 'Electronic Basics' started by J Shrum, Mar 12, 2005.

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  1. J Shrum

    J Shrum Guest

    I have been working on a small tesla coil based around a 10KV 30ma neon

    I've built the spark gap, tested, works.
    I've built a 12 pack of bottle capacitors, measured 8nF.
    All seemed well until I tried to hook up the spark gap and the capacitor. W/
    the spark gap alone, it throws sparks just fine... but as soon as I
    introduce the capacitor in either series, or parallel... it stops the spark
    gap from firing until I turn off the supply and turn it back on. But if the
    capacitor is hooked to it... nada.

    Could this be a protection in the neon transformer that shuts off w/
    overload? Shunt? I'm not real certain what to do at this point, as I've
    spent weeks searching google for everything I could find prior to building
    this thing, and now I'm beyond frustrated.

    Any help is much appreciated. Thanks


    (cross posted to sci.electronics)
  2. I suspect 8 nF is rather high for many to maybe most neon sign
    transformers and will load down their secondary voltage.

    I suggest:

    1. Reducing the capacitance. I think 8 nF is quite high to charge within
    half a cycle of AC line frequency from a neon sign transformer. 8 nF at
    30 mA only gains 3.75 KV per millisecond, and at 60 Hz zero-to-peak time
    is about 4.17 milliseconds, and also consider whatever may not be ideal...
    I have experience of only a couple nF doing well with a neon sign
    transformer for this!

    2. Blowing forced air through the spark gap if the sparking through the
    spark gap with the lower capacitance results in the "spark" conducting in
    an excessively continuous (quiter) manner. The sparking should be a very
    bright bluish white bright enough to put spots in your eyes and noisy
    enough to interfere with normal conversation at close range unless noise
    abatement measures are taken.
    Also make the spark gap electrodes bigger, bulkier, beefier, and better
    able to stay cool!

    CAUTION - such sparks generate nitrogen dioxide - a gas that is to some
    extent corrosive to lung tissue and maybe some rubbers, has an
    orangish-brownish color when accumulated in a confined area (I saw this
    happen in a jar enclosing a Tesla coil primary spark gap), and has an odor
    somewhat like somewhere between chlorine and ozone.

    3. Increasing the number of turns of the primary, in a proportion that is
    inverse square root to capacitance as capacitance is changed. Reduce
    accordingly (as in proportional to square root of capacitance) the
    secondary voltage.

    4. ALTERNATIVE - fullwave rectify the output of the neon sign transformer
    before feeding this to the capacitor. Expect the sparking frequency to be
    on the low side, less than once per half cycle of the AC line frequency if
    the problem turned out to be requiring tghis fix or the above fix due to 8
    nF being too much capacitance to charge to an adequate voltage within a

    5. Make sure the spark gap is not too wide. Depending on spark gap
    geometry, the voltage required to spark through can somewhat easily be as
    much as about 3 KV per millimeter plus 300V! More extreme - 3.3 KV per
    millimeter plus 300V!

    - Don Klipstein ()
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