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RE: Advice needed on capacitor tests: update: BIG resonance!

>Original Poster: Alex Crow <user-at-alexcrow.clara-dot-net> 
>
>Dear all,
>
>I will add to this after another test....
>
>I hooked up my cap across my NST again, but this time put my multimeter on
>the output of the NST. I turned up the voltage, and at only 1 or 2 percent
>of line voltage I was getting about 600-700 volts on the meter!!  I tried
>it without the cap and all I was getting was around 80-100 volts on the
>same setting... All I can think of is that the NST windings are resonating
>with my cap at 60Hz and causing this voltage rise of about 6 to 8 times!!
>Instead of shunts, these things have a small gap in the core which provides
>enough leakeage inductance to limit the current.
>
>In fact, I'm just doing the test again right now - with the cap connected I
>set the variac for an NST output of 800V. I removed the cap and I'm only
>getting about 85V. This must mean that I was actually getting about 10kV
>across my cap, no wonder I got the big sparks! this is a rise of nearly
>10x!  Help!!! does this mean my xfrmr is useless for Tesla work? I can't
>see that my RC filter (desingned for the Tesla frequency) is going to stop
>this!
>
>Please respond, I am at my wit's end as to what to do - can I fill the gap
>in my core and add limiting on the input to my NSTs? maybe this would
>reduce the inductance and stop the ringing. Help!



You are clearly seeing your NST secondary resonating with your tank cap.
The voltage rise you see will easily take out your NST and cap if you let
it.  BUT - if you understand it and treat it right, mains resonance is a
GOOD thing, assuming that you use a static, not rotary spark gap.

If you just run your NST into your cap with no spark gap, the voltage
across your cap/NST will build on each successive 60 Hz cycle until
something breaks down.  This is bad because the peak voltages are far in
excess of what your components are designed for.

BUT - if you add the static spark gap, SET PROPERLY, the maximum voltage
will be absolutely limited to what the static gap permits.  This means
that your gap may be firing more than once per half-cycle, but this is a
GOOD thing.  The more times your cap charges and discharges per time
interval, the more power you are pulling through your system.  Running
at mains-resonance can allow you to pull double or more times the
faceplate current out of your NST.

Note the all-caps used in the "set properly" phrase.  The importance of
this cannot be over emphasized.  If the gap is set too wide, the charging
voltage will rise correspondingly too high.  If you accidentally bump
your gap setting too far, well, too bad for your cap.

Just what gap setting is "proper" for a given NST or cap charging voltage
is not well documented.  Ideally we would all have one of Terry's fiber
optic VI probes and be able to monitor just what voltage our caps are
seeing.  The breakdown voltage of a gap is determined mostly by its total
length, but also by altitude and atmospheric pressure and gap electrode
geometry, and possibly airflow.  As a single point of reference, I use a
0.36" gap on my 15KV NST (at near sea level).  I have no idea to what
voltage this breaks down at...

Regards, Gary Lau
Waltham, MA USA