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Neon Sign transformer / Tesla Coil question

Discussion in 'General Electronics' started by J Shrum, Mar 12, 2005.

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  1. J Shrum

    J Shrum Guest

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

    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

    Jim
    KC9FFX
     
  2. Robert Baer

    Robert Baer Guest

    Neon transformers are built somewhat like a Sola (regulating)
    transformer.
    The neon transformers are basically constant current sources.
    A capacitor across the transformer is a "low" impedance.
    Take 8nF at 60Hz and one gets Xc~~350K; a relative short circuit for
    10KV.
     
  3. J Shrum

    J Shrum Guest

    One additional comment:

    Whats very strange about this, is that I don't even have the primary
    connected. Its just the NST, cap, and gap in parallel.

    I only connected one side of the capacitor to the gap, so the current isn't
    even going through the cap, and still the sparks are very light and easy to
    blow out. W/ just spark gap, bright sparks that can't blow out. But when I
    hook even one side of the capacitor, it goes out. Does it appear that there
    is something wrong w/ the NST?

    If it helps, I can draw a diagram...

    Thanks again for all the help.
     
  4. I suspect the several picofarads of stray capacitance are loading down
    the voltage of the neon sign transformer - which I would not expect unless
    the transformer is one of those high frequency electronic ones - in which
    case putting just one nanofarad across the secondary will probably load
    down the voltage to less than 1 kilovolt.

    If the transformer is a conventional one, I suspect the spark gap is
    marginal (too wide) and only intermittently sparking through, although I
    thought that if it sparks through without the capacitor the spark would
    become a somewhat flame-like (but not really bright) arc. Also check for
    bad connections.

    CAUTION with the more conventional transformers and capacitors - do not
    connect the capacitor without a spark gap. If the capacitance is that
    which results in resonance with the leakage inductance (around a few
    nnofarads) and no spark gap is present to limit the voltage, the voltage
    may become excessive for the transformer's insulation.

    - Don Klipstein ()
     
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