Power protection toriods
Subject: Power protection toriods
From: richard.quick-at-slug-dot-org (Richard Quick)
Date: Wed, 19 Jul 1995 23:31:00 GMT
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Quoting mconway-at-deepthnk.kiwi.gen.nz (Mark Conway)
RE: Power protection toriods any good?
MC> I was reading Duane Byland's book the other day and he said
MC> in it that he often wonders if protective power supply
MC> toroids on neon sign transformers do any good at all. He
MC> said that for them to do any good they would have to have an
MC> impedance as big as or bigger than the impedance of the
MC> neon. Since neons have such a huge impedance (they have
MC> thousands of turns around an iron core) surely a piddly
MC> little toroid with a few turns of wire wrapped around it
MC> will do almost nothing? Won't the huge impedance of the neon
MC> (compared to the toroid) mean that any voltage kickback will
MC> be dropped almost entirely over the neon and hardly any at
MC> all over the toroid?
Good Question! First let's reference a bit before I attempt to
render a decent answer.
Mark is referring to page 91 of Duane A. Bylund's book: MODERN
TESLA COIL THEORY, 1990, published by the Tesla Book Company,
P.O. Box 121873, Chula Vista, CA 91912 USA, Tel: 1-800-398-2056.
BTW, Duane just published a 20 page supplement to this book and
it is available to TCBA members for a nominal $3.00 (others
$4.00). The supplement is available from Duane (140 S. 700 E.,
Spanish Fork, UT 84660 USA) and has several interesting and
worthwhile experiments performed with CW coils. Duane has
indicated that sections from his publications may be reproduced
for non-profit educational purposes only:
RADIO FREQUENCY CHOKE
"The main purpose of the choke is to pass 60 cycle current from
the transformer to charge the discharge capacitor and prevent the
high frequency capacitive discharge from getting back into the
transformer and damaging it. To accomplish this task the choke
must have a low impedance to 60 cycles and a high impedance to
the oscillation frequency. If you were to measure the impedance
looking into the secondary of the high voltage transformer you
would find that it has a high impedance. To be effective the
choke should have a higher impedance than the secondary of the
transformer. To make the choke have a higher impedance than the
transformer, the choke may become large and may also affect the
60 cycle charging current. I sometimes wonder if chokes do any
good at all. I have made several small Tesla coils without chokes
and have not run into any problems. My advice is to use a choke
if that is what makes you happy. As to calculating the value of a
choke, I have no idea."
First, the choke is not designed to prevent "the high frequency
capacitive discharge from getting back into the transformer", nor
is it intended to attenuate kickback. The RF choke is simply the
first line of defense to reduce the level of RF "hash" or "line
noise" in what we wish to be a clean 60 cycle line. The effect-
iveness of the RF choke in this function can be measured with a
remote lead running to a resonate pickup coil placed outside of
the inductive field of the coil system. The goal of the choke(s)
is that RF line noise on the transformer side of the choke be
somewhat lower than the line noise on the coil side.
Second, no mention is made in the quoted passage of bypass
capacitors used in combination with RF chokes. Bypass caps
provide the second line of defense, and aid to further reduce the
levels of RF noise in the 60 cycle lines connecting the trans-
former to the oscillator tank circuit. Bypass capacitors should
be placed across the line on the transformer side of the choke.
Nothing but a properly set safety gap is going to attenuate
kickback. Kickbacks can neatly pass thru RF chokes and over
bypass caps as if they were nonexistent; they might help some,
but not enough that I have ever noticed a drastic reduction in
the force of a kickback. Leave the kickback to the safety gap.
What then, you may ask, does the RF choke and bypass capacitors
do to protect the transformer?
Kickback is responsible for only 50% of the on-line transformer
failures that I have encountered. Continuous high levels of RF
leakage from the tank circuit accounts for the rest of the failed
transformers that I have pulled out of service. Neons often have
a tendency to weaken and fail slowly when used as Tesla power
supplies. This is due to a gradual breakdown of insulation in the
transformer secondary as high levels of RF "crawl" and jump be-
tween turns. This RF in the windings is seeking a lower impedance
path to the RF ground at the core. Eventually this RF leakage
breaks down the insulation and damages the winding to the point
that the voltage output is reduced. Finally the winding fails
completely. This slow damage also appears to make the transformer
even more susceptible to sudden failure from a moderate kickback.
I have seen this RF leakage with the naked eye crawling all over
unpotted, dry fired, neons. In one case the secondary winding of
a rebuilt neon had the outside turn of paper removed, exposing
the last layer of the winding. RF corona and tiny sparks jumping
turns and passing through the paper to the winding layers below
were quite apparent. Even in potted neon transformers it must be
understood that RF leakage does not really recognize tar as an
insulator, but rather as another impedance; and there is no
potting at all inside the HV secondary windings.
Proper use of RF chokes and bypass capacitors greatly extend the
life of neon transformers pressed into service as Tesla coil
power supplies. The transformers will not only last longer, but
they maintain a higher output, and tend to be more resistant to
kickback over their useful life spans.
... If all else fails... Throw another megavolt across it!
___ Blue Wave/QWK v2.12