A couple of questions
Subject: A couple of questions
From: richard.quick-at-slug-dot-org (Richard Quick)
Date: Sat, 6 Jan 1996 03:52:00 GMT
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Quoting Walt Donovan <walt-at-rendition-dot-com>:
> The problem was, the neon-sign transformers kept burning out,
> and I finally ran out of the ones I had salvaged from torn-down
> buildings. Does anyone have any ideas why they burned out?
Neon sign transformers are simply not built for use as Tesla coil
power supplies. The tank circuit dishes out enough abuse in the
form of RF leakage and high voltage kickback that an unprotected
neon has a very finite life span.
The neon transformer requires protective circuitry for long life
as a Tesla coil power supply. The first line of defense is a safety
gap with an RF grounded center post that is placed across the high-
voltage bushings of the neon power supply. This provides a low
impedance path to ground for over-voltages that are typical of kick-
back from the Tesla tank circuit. The activity at the safety gap also
gives the coil builder an indication of the tune.
The next line of defense is a bypass capacitance between each HV
bushing and ground. The bypass capacitance need not be large, but
it must be rated with plenty of voltage safety margin. Typically a
capacitance of .0008 microfarads, or less, between each HV bushing
and the RF ground is more than sufficient. A few dozen picofarads is
a good starting point. Too large a value of bypass capacitance can
cause problems. The best dielectrics for this bypass capacitance are
the ones with the highest RF dissipation factors; glass, PVC, or DC
rated barium titanate dielectrics are ideal in this application. The
higher the RF dissipation factor of the bypass capacitor the more RF
bleedover is converted to harmless heat. The voltage rating on these
capacitors should start around 50-60 KVDC.
Between the bypass capacitors and the tank circuit you need a couple
of fairly heavy RF chokes. My experience is that the best chokes for
this application are large ferrite core toroids, say about two inches
in diameter weighing around 1/4 - 1/3 of a pound each, wound with
around 15-20 turns of some fairly thin PVC jacketed stranded wire.
Here is a wiring diagram of a protective circuit for a neon power
||O | | RF1
||O ------- BC1 |
---O||O ------- |
O||O | *
O||O | *
---O||O ------- |
||O ------- BC2 |
||O | | RF2
X1 = Is the step up xfmr (neon or neons) with grounded center tap
GRND = Dedicated RF Ground for the HV/RF wiring and secondary coil
BC1 = Bypass Capacitance
BC2 = Bypass Capacitance
SG = Safety Gap w/grounded center post
RF1 = Ferrite Toroid Radio Frequency Choke
RF2 = Ferrite Toroid Radio Frequency Choke
This circuit should be set up such that the values of BC1 & BC2 are
matched or at least close in value. The same would also go for the
values of the inductance RF1 & RF2.
How it works is like this: the RF chokes reduce the level of RF "hash"
or bleedover present in the 60 cycle feed lines. This RF will gradually
deteriorate the delicate HV windings on the neon sign transformer. I
have made relative measurements of these types of RF chokes, the simple
instructions I have provided will produce a a choke that will work very
well in this application. I sometimes purchase these chokes surplus and
strip the old windings off of them, then wrap the cores in a few layers
of friction tape for a little insulation, and finally wind the core with
The bypass capacitors are placed between the chokes and the HV bushings
on the transformer as shown in the diagram above. The level of RF in
this section of the line will already be lower due to the presence of
the chokes, but the remaining RF is bypassed to ground or converted to
heat by the capacitor dielectric. The result is an acceptable level of
RF on the HV bushing itself.
None of this circuitry is going to stop a HV kickback originating from
the tank circuit. Such kickbacks are quite powerful, megawatt peak
powers being possible in a high performance "desk top" coil these days.
Kickbacks are seeking low potientals. Without a safety gap across the
HV bushings of the neon, the kickback will seek the transformer core
through the winding itself. The safety gap places a lower impedance
pathway to ground which effectively shunts powerful kickbacks that
would otherwise destroy a neon sign transformer.
... If all else fails... Throw another megavolt across it!
___ Blue Wave/QWK v2.12