RE: Static Spark Gap question

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Lau, Gary" <gary.lau-at-hp-dot-com> 

The thing that kills NST's is insulation breakdown on the secondary, due
to too high a secondary voltage.  When using a static gap, the peak
charging voltage on the cap and gap is governed strictly by the gap
setting.  Increasing the NST primary voltage will cause the cap to be
charged faster and the gap will fire more frequently, but the peak
voltage will be the same.  The NST primary current will be higher and
the NST will run hotter, but if run times are kept reasonable, this
should not be a problem.

Any NST in Tesla coil service is always run much harder than in a neon
sign application.  And MMC caps are typically designed to be used far
above their AC voltage ratings.  But since we don't expect our
components to run for 20 years 24-7, we can get away with a certain
amount of overstress.  If we wanted to be really conservative, our
components would cost many times what we're accustomed to paying.

On the multi-gap breakdown voltage issue, I have seen experiments
performed where the sum of breakdown voltage of individual gaps does not
remotely add up to the breakdown voltage of a series chain.  I no longer
recall which was higher, and I have heard suggestions that high voltage
biasing resistors across individual gaps may correct the situation
(similar to what is done with MMC caps), but I've not heard of anyone
actually trying this.  But since it's probable that single gaps have
lower losses, it may be academic.

Gary Lau
MA, USA
------------------------------------------------------------------------
-------------------------

Original poster: "WIZZARD ." <pbobyk-at-hotmail-dot-com>


GARY

I have been led to believe that overstressing a component beyond what
has
been designed to do is just asking for trouble. ( I know an NST was not
made for a Tesla coil ) I guess I'm being too cautious? I have not
actually
tried this on my system because of the high cost of replacing my
transformers.

I stand corrected on the gap issue too.
What seems logical doesn't always make it true :-)
Web of a million lies.... I hate to be contributer to that.

Thank You for the corrections.

I would like to understand why. If you have the time.

Wizzard
Dwight, Ontario
Canada

 >It is commonly assumed that a multi-segment N-segment gap of X inches
per
 >segment will have the same breakdown voltage as a single (N*X) inch
gap.
 >This is not at all true.  The only way to determine the proper number
of
 >gaps in a multi-gap is to do so experimentally.
 >
 >What's wrong with applying 140V to an NST through a Variac?
 >
 >Gary Lau
 >MA, USA
 >
 >
 >
 >Original poster: "WIZZARD ." <pbobyk-at-hotmail-dot-com>
 >
 >Apply the 15KV accross a safety gap and adjust it so it "just" stops
arcing.
 >Remove power, then measure the opening of the safety gap.
 >Divide this number by the number of gaps between your pipes.
 >Pipes should be installed as parallel to each other as possible. Each
gap
 >doesn't have to be perfectly spaced unless you really like that
appearance.
 >
 >More pipes means less space in between each pipe.
 >Small gap spaces get clogged and/or shorted by surface irregularities
on
 >the pipe usually in a short period of operation time. Overall coil
 >performance suffers. ( Fine sandpaper time )
 >
 >My NST 15KV/120mA NST powered coil uses a RQ gap with 10, 3/4" Cu Pipes
 >about 4.5" long.
 >A good circulation of air through the gaps is needed for this power
level.
 >
 >Tips to avoid dissapointment :-)
 >         1) Don't make the main gap larger than the safety gap.
 >         2) Don't use a mains resonant capacitor.
 >         3) Don't apply 140 VAC to NST through a variac.
 >         4) Use a Terry Filter!
 >
 >Members correct me if I'm wrong.
 >
 >
 >
 >
 >
 >
 >
 >
 >
 >
 >
 >
 > >Original poster: "Gerry Reynolds" <gerryreynolds-at-earthlink-dot-net>
 > >
 > >Hi All,
 > >
 > >I'm building a 15kv 120ma coil system and want to start with a
 > >multi-segmented spark gap.  I know that Dr R doesn't like total spark
gaps
 > >in excess of 240 mils but I do plan on setting my safety gap
appropriately
 > >and leaving it alone.  I'm planning on a 16 pipe string with each
copper
 > >pipe being 3 inch long by 0.625 diameter.  Three gap spacing
arrangements
 > >are being considered that will allow me to vary the total gap in 5
mil
 > >increments.  This will be done by having the center gaps be of
smaller
 > >spacing than what the string is based on.  The smaller gaps will be
 > >shorted out (as necessary) in various combinations to get the 5 mil
 > >increment.  The following shows the smaller gaps in the center of the
 > >string based on 20, 25, and 30 mil spacings:
 > >
 > >a. 20 mil spacing   .... 20 10 15 20 ....            maximum total
gap 285
 > >mils
 > >b. 25 mil spacing   .... 25 10 15 20 25 ....       maximum total gap
345
 > mils
 > >c. 30 mil spacing   .... 30 15 20 25 30 ....       maximum total gap
420
 > mils
 > >
 > >Questions:
 > >
 > >1. For this power level, what is the current thinking on maximum
number of
 > >segments?
 > >
 > >2. Is there a rule of thumb on the maximum gap between any two
segments?
 > >
 > >3. What is the "expected" optimum total gap required for this power
source
 > >and how much would this need to be increased for my 5000 foot
elevation?
 > >
 > >Answers to these question will help me identify a design center and
choose
 > >an appropriate spacing range.
 > >
 > >Many thanks for any help,
 > >
 > >Gerry R
 > >Ft Collins, CO
 > >