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Jacobs Ladders & Stacking Transformers?

Discussion in 'Electronic Basics' started by [email protected], Mar 9, 2007.

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  1. Guest

    I've build a Jacob's Ladder with an old "Franceformer". The specs
    are:

    Input; 120 volts - 60 Hz. Output;14,000 volts- 30 Ma.

    If I wanted a bigger show and had two identical transformers, could I
    stack them for a bigger arc? Would this even be prudent? If not why
    not?

    If it would be possible, would I wire them in a series or parallel
    configuration? If not why would it not work, and what are the
    pottential hazzards?

    Thanks in advance
     
  2. Tom Biasi

    Tom Biasi Guest

    In theory you could wire them in series aiding and get the sum of the
    voltages out.
    In practice, realize that you are dealing with very high voltages. Your
    wired connections need to be clean and the insulating material needs to be
    rated for the voltages.
    Be careful.
    Tom
     
  3. Since both primaries would be referenced to the same point, I think
    the stress on the transformer insulation would be doubled, and one or
    the other might short out. I.E., suppose you ground one end of one of
    the secondaries; then the most remote point from that ground reaches
    sqrt(2)*28kV = 40kV peak, across insulation designed for half that,
    within the second transformer.
     
  4. Bob Eld

    Bob Eld Guest

    In a word NO! Most likely the 14,000 volt secondary is center taped with the
    center connected to the case. This makes it 7,000 volts to the case from
    either terminal. There is no practical way to series windings from two
    transformers without breaking the center connections. Secondly, if they
    could be series'd there is insufficient insulation in the windings,
    terminals, etc. for the resutant 28kV. The best solution is to get a 30kV
    sign transformer. Keep in mind that these sign transformers are magnetically
    current limited so they can withstand the heavy load of an arcing jacobs
    later. High voltage transformers that are not so protected are very
    dangerous and are not suitable for an arcing application. They can blow
    breakers, burn your house down or worse. Be careful.
     
  5. Rich Grise

    Rich Grise Guest

    Just off the top of my head, I'd say it'd be incredibly stupid. With 14 KV
    at 30 mA, you could make a Jacob's Ladder that could go two stories high -
    it depends on the physical design of the Jacob's Ladder. Once you start
    the arc, as long as it's arcing, it's a negative resistance - the only
    thing that will limit the size of the arc is your ballast. (which, AIUI,
    neon sign transformers have built-in, which might be a snag.)

    But stacking those kind of transformers can be lethal, if you don't
    know _exactly_ what you're doing.

    (I suppose you could put them in a big box and pot them, but from that
    point you're on your own. =:-O )

    Good Luck!
    Rich
     
  6. I think that if the two transformers are the same model, you
    can wire both primaries and secondaries in parallel, and the
    transformers will work okay. You don't get a bit more open
    circuit voltage, but you get twice the arc current that will
    produce a hotter arc that will climb a wider gap. Each
    transformer should protect itself from the effective short
    of the arc.
     
  7. There are some transformers out there that don't stack in series well
    because the secondaries have points somewhere or somewhere else either not
    too well insulated from, or outright connected to, the primary.

    An example is neon sign transformers. Most have secondaries with their
    center taps connected to the case. I doubt the insulation between the
    primary and the case withstands reliably, maybe usually not at all, half
    the secondary voltage.
    If you put two neon sign transformers in series, their cases will have
    one full secondary voltage between them. Even powering the primaries with
    isolation transformers can be a problem if interwinding capacitance is not
    greatly smaller in the neon sign transformers than in the isolation
    transformers - insulation breakdown even with the low current flowing
    through interwinding capacitance will cause a problem sooner or later, so
    in an isolation transformer scheme you need to connect one end of each
    neon sign transformer to its case. You will need two isolation
    transformers that withstand reliably half the secondary voltage, or one
    that reliably withstands the full secondary voltage.

    I think this gets Rube-Goldbergish and is asking for big trouble.

    Two neon sign transformers in parallel will give you an arc of same
    starting length, but that will stretch longer before it breaks. However,
    if you get zapped you are more likely to die.

    - Don Klipstein ()
     
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