Re: MOVs and NSTs

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

Original poster: "Rick W by way of Terry Fritz <twftesla-at-qwest-dot-net>" <rickwilliams404-at-earthlink-dot-net>

Hey all,

I'm in process of building an NST protection circuit according to Terry's
schematic (with SGs). I can't knock the theory and/or the component
selection. 50 bucks to protect my NST bank? Let's see, I'll have 5 NSTs 15K
30ma. All depotted and rebuilt. My time is valuable to me. 2 days each ( 8
hrs/day) to rebuild an NST (OK, I'm careful). My time is worth $10 per hour
(conservative). That's 800 bucks plus the initial cost of 5 NSTs so round up
to a grand.

I don't think a 5% investment for insurance is a bad deal. Besides, I prefer
predictability as opposed to RF going where it shouldn't. RF is funny stuff
and I don't want any surprises. :)

Rick Williams
Salt Lake City




----- Original Message -----
From: Tesla list <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Tuesday, July 24, 2001 7:41 PM
Subject: Re: MOVs and NSTs


> Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>
>
> Hi Peter,
>
> "I" think 90% of NSTs die (used to) because people used resonant cap sizes
> and "somehow" they let the voltage on the NST resonate up far too high.
> The MOVs and safety gaps hit that problem twice over.  The safety gaps
take
> most of the problem (and scare the user to fix what is wrong) but the MOVs
> are there if the user decides to start widening the gaps.  Many NST deaths
> were caused by people trying to get better output by widening gaps :-(
> MOVs don't let them do that :-))
>
> The other problem is what happens on the NST windings if 300KHz at 20kV
> hits them?  The first outer windings are going to see the vast majority of
> the current due to the giant inductance of the windings.  If you put 20kV
> at 300kHz across the first 100 turns of the NSTs secondary, it's gonna
blow
> up.  Thus the RC filtering section to attenuate any RF.  That is a
> "theoretical" effect and not a proven one, but the research there is real
> though :-))  I thought of getting blown NSTs shipped to me for autopsy,
but
> that was not practical...
>
> If one simply had well adjusted safety gaps, that would protect 90% of the
> NSTs out there.  However, as I looked over NST failures and reports of
what
> went wrong, the "user" was a big problem :-))  So I built a lot of stuff
in
> like MOVs to take user error out of the picture.  I also over rated the
> components like the resistors so super hot resistors (300 degrees C is
> normal for power resistors) would not get people "bent out of joint" over
> that :-))
>
> So it has a lot of "human factors" and mistake buffers built into it :-))
> I decided if I was going to design a filter for NSTs, it should not only
> protect against over voltage and RF but might as will protect against user
> error too.  I was thinking far beyond the electrical theory :-))
>
> If someone wants to spend $50 on an NST filter, they deserve to get the
> best known protection out there.  Would less work?  Sure, sometimes...
>
> The input fuse protects against a later discovered effect of NST core
> saturation.  With a large primary cap, the NST current can overcome the
> limits of the NST shunt cores and the current will become unlimited to a
> degree.  Sort of cool that a small NST transformer can put out pig
> currents, but deadly to NSTs...  A simple input fuse stops core saturation
> problems right quick.  Maybe that effect could be controlled to great
> advantage, but too scary to play with for even me :-))
>
> So I did the best I knew how to save NSTs with all the over kill and over
> design I could...  There is a chance that the RC filter part is not
> terribly needed, but I put a lot into "my" NSTs and I was not feeling
lucky
> :-))
>
> NST used to blow like popcorn, but since there has been so much effort
into
> protecting them and LTR stuff, NST death is relatively rare.  So we must
> have done something right :-))  I note that this has become "Terry's"
> filter.  I really didn't want to take a chance of screwing it up :-)))
>
> Cheers,
>
> Terry
>
>
> At 05:18 PM 7/24/2001 -0700, you wrote:
> >Terry,
> >      I've seen your simulation outputs of the difference between
connecting
> >the NST power supply across the tank cap verses across the spark gap, and
> >I am wondering why keep the RC filter in the second case with power
across
> >the SG.
> >
> >It still seems to make sense to me to have safety gaps and/or MOVs, but
if
> >there is no RF ringing to the NST when it is connected across the SG why
> >have the RC filter.
> >
> >-Peter Lawrence.
> >
> >(ps, I have not blown an NST since I started using safety gaps and
started
> >connecting my NST across the SG. I sometimes include an RC filter, but
not
> >always, and not having the RC filter does not seem to have cost me
anything
> >yet..... knock on wood.....)
> >
> >
> >
> >>
> >>Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>
> >>
> >>Hi All,
> >>
> >>Using MOVs with NSTs seems to be catching on :-))
> >>
> >
> >
>
>