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Re: [TCML] Help the neophyte

Hi Doug,

I would agree with what David said - that you are probably looking at a
solid state inverter-type of transformer - useless in our application.  You
want to look for a "Core and Coil" type of NST.  I recommend checking every
sign shop in your driving radius for a used NST - sometimes you get lucky!

To your other question about two capacitors - it's complicated.

Let's say you have a 9kV NST.  The peak secondary voltage with nothing
connected to the secondary would be about 1.414 x 9000 or about 12.7KV.
Now let's say you were to connect your capacitor across that secondary,
with nothing else, no spark gaps.  The voltage across the cap might charge
to (WAG) +5KV, then down to -8KV, then up to +12KV, then down to -15KV,
etc, on alternate 60Hz half-cycles.  How far in the etc's this continues
depends on several things - how close to 60 Hz resonance your capacitor is
with the NST secondary inductance, and what the insulation strength is on
your NST.  If it's a 9kV NST, they don't spend any money on insulation
stronger that what is needed, so when the voltage rings up high enough,
something in the NST will break down and arc and burn, and that's the end
of your NST.

Now that situation should not occur, because you have a spark gap across
the NST that you've carefully set to break down at just above the
open-circuit voltage of the NST.  So you have two capacitors - one rated at
10KV, the other at 30KV.  If you have a 9KV NST, you will almost certainly
exceed the voltage rating of the 10KV capacitor, so that cap may burn out,
whereas the 30KV unit will not.  The behavior of both capacitors will be
identical, up to the point where the lower-voltage one fails.

The stuff about the voltage building higher and higher (resonant-rise) and
exceeding the NST rating is not intuitive and comes as a shock to most
beginners, but it's critical to believe in and understand this, because
setting your spark gap width incorrectly WILL result in the death of the

Regards, Gary Lau

On Tue, May 26, 2015 at 9:00 AM, Doug <doug11642@xxxxxxxxxxx> wrote:

> Hi Guys, I am in need of some advice. I wish to acquire a NST in the range
> of 10KV-30MA, but as I shop I see [current limited] or [adjusts from 9KV to
> 12KV automatically] ETC.ETC.ETC! I do not know how all these parameters
> apply to what I should purchase! Can someone give me some guidance, please?
> OR, if anyone has one they wish to sell let me know.
>   Another question, lets say I have 2 capacitors, Cap#1 is 10nf-10KV, Cap
> #2 is 10nf-30KV, and my NST is rather small, lets say 6KV-30MA, is there a
> difference in the ability of my NST to charge them?
>    I realize all this must be BASIC knowledge to most of you, but I am
> nowhere near the [basic knowledge] stage yet!
>    This TC thing is very addictive! There is this big grey thing on a pole
> in my yard I have my eye on.:-D [That':-D s a joke]
> Doug J
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