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Neon transformers and HV



TE>dewzenol-at-inlink-dot-com, on or about Mon May  1 10:16 MDT 1995,
shorted-out, spluttered, and wrote as follows:

        I could be off-base by a couple of spark-gap lengths, but there
        are several things in your description which jump across to me:

TE>I've been running a very small tesla coil off of a 6kv, 1.2ma supply.
TE>I want to replace the supply with a 9 or 15kv neon xfmr (30ma)

        Althought any number of Tesla Coils have been built by
        experimenters using neon sign transformers - they are =not= a
        good choice for this.  Many reasons.

TE>I do realize that the neon xfmrs are center ground, and have wired
TE>them as such... Am I missing something obvious???

        Yike!  You're siphoning off all your rf energy (or most of it).
        There is nothing in the =primary= circuit of a Tesla Coil which
        requires grounding.

        The center tap of a neon transformer is grounded to permit the
        suppression of interference when they are used in sign service -
        get it???

        The interference is caused by unwanted =rf energy= incidental to
        the operation of a neon tube.  In a Tesla Coil we =want= rf
        energy.  Lots and lots of it.  Lots of good, wonderful,
        free-flowing, warm and juicy rf energy. We do not want to
        suppress any part of it. None. Nix. Nil. Nyent. Nada. Okay?

TE>#2: I purchased a couple of G.E. caps  which say:
TE>"DC Filter Service Only (cat14F1354) 0.39mf  25000vdc(50deg celcius)"
TE>Apparently these are not polar caps, but they say they are for use as dc
TE>filters only??

        Yes!!  What G.E. is telling you is "Rf-wise we didn't take the
        proper precautions, so these capacitors are leaky as a Chinese
        junk after a typhoon - better not use them on anything rougher
        than damn-near pure dc."

        Listen to the man, gee.

        The capacitors of choice for Tesla Coils should be mica
        transmitting capacitors, ceramic, or glass.  Note that the
        voltage, capacitance, =and rf current handling= capabilities
        must be appropriate.

        The 25000-volt rating is a little shaky, too.  Traditionally,
        capacitor makers have always played fast and loose with voltage
        specs.  Most working engineers would not think of using a
        capacitor on anything greater than half it rated working
        voltage.
                That 25000-v. is the =working= voltage, isn't it?  I
                mean it's not the peak voltage, hey?

         So, in practical terms, you have a 12,500-volt capacitor.
         Unfortunately you are likely putting several times that
         voltage into it.

        Your neon transformer is rated at 9kv.  That's 9kv, r.m.s.  The
        =peak= voltage (which is what counts in high-voltage work
        - always) is 9kv. x 1.414 = 12,726v.  Mighty doubtful, I'd say
        on a 12,500-v. capacitor not intended for rf work in the first
        place.

        Unfortunately, you've likely got voltage peaks up to several
        times 12,726v.   It's in the nature of resonant circuits.
        Voltage peaks can exceed input voltages by a factor of several.
        So - you've got maybe 2 - 3 times 12,726v., maybe more (12,726 x
        2.5 = 25,452-v.).   My, but you do like to play it close to
        line.

                              - - - - - - - -

        To extract yourself from this sea of miseries, unground
        everything in the primary of your Tesla Coil (and don't go doing
        anything like that again).   Then connect your capacitors in
        series - this will effectively double their voltage rating.
        Tune your Tesla Coil (primary and secondary) to the resonant
        frequency this series-connected capacitor bank provides.  You
        may get sufficient results to satisfy.  Else - go buy some glass
        plates, aluminum foil and shellac at the hardware store and
        stop messing with store-bought capacitors.  What was good
        enough for Dr. Tesla should be good enough for you.

                                Megavoltly,
                                Robert Michaels - Detroit, U.S.A.