Re: NST protection

["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>

Hi Terry,

Is there a simple rule of thumb to determine the value/type of fuses needed
for NSTs?
I'm running 15/30s but thought a general rule would be useful for all.


Rick williams
Salt Lake City

----- Original Message -----
From: Tesla list <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Thursday, August 16, 2001 12:55 PM
Subject: Re: NST protection


> Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>
>
> Hi Justin,
>
> There are three ways to kill and NST aside from it just randomly dying
from
> old age.
>
> 1.  The RF voltage gets back into the secondary winding.  The high
> inductance of the winding tends to concentrate the voltage on the outer
> layer of turns.  The RF may also tend to cut through the tar insulation
> much more easily than 60Hz.  My filter uses resistors and capacitors to
> remove much of the RF.  Also, placing the NST across the main gap instead
> of the primary cap vastly reduces the RF going back to the NST.
>
> 2.  Resonant rise occures with the NST's impedance and the primary
> capacitor ring at 60Hz.  If left unstopped, the voltage can jump to around
> 80kV which will kill just about any NST.  Resonant size primary caps are
> very prone to this failure and probably account for 90% of NSTs blowing.
> LTR coils have primary caps large enough that this cannont happen.  I
> usually recommend LTR caps now just for this reason.  My filter has saftey
> gaps that will fire and stop an over voltage situation.  It also has solid
> state MOVs which will also stop the over voltage if the saftey gap does
not
> (like it is miss adjusted).
>
> 3.  If an NST is used with a large primary cap.  There is an odd condition
> where the current can become very high due to the magnetic shunts in the
> NST saturating.  This vary's depending on the NST's contsruction.  A
simple
> fuse on the NST input will blow if this happens and protect the NST.
>
> If you use a good filter and input fuses, I don't think you can break a
> good NST without a hammer ;-)  I tend to like "my" filter at:)
>
> http://hot-streamer-dot-com/TeslaCoils/Misc/NSTFilt.jpg
> http://hot-streamer-dot-com/TeslaCoils/Misc/Filter.jpg
> http://hot-streamer-dot-com/TeslaCoils/MyCoils/BigCoil/protection.jpg
> http://hot-streamer-dot-com/TeslaCoils/MyCoils/SmallCoil/small_protection.jpg
> http://hot-streamer-dot-com/TeslaCoils/MyCoils/MyCoils.htm
> http://hot-streamer-dot-com/TeslaCoils/MyPapers/MyPapers.htm
>
> Cheers,
>
> Terry
>
>
> At 02:18 AM 8/16/2001 -0400, you wrote:
> >Well, I burned out another one of my 15kV 60mA NST's today. That's the
second
> >one, and I've only got one left. After the cursing stopped, I got to
> >thinking. After I while, I came across an idea for protecting NST's. I
don't
> >know if it will work, but I'll ask you guys and see what you think. A
couple
> >of days ago, I was asking about using microwave oven caps as a tesla tank
> >capacitor. Everyone that responded told me that they won't work because
they
> >are lossy at RF frequencies. They are intended for use at about 60 Hz. So
> >here's the idea: Why not place lossy type capacitors between the tesla
tank
> >circuit and the NST. I'm not an electronics expert, so someone would have
to
> >come up with a proper setup. The 60Hz coming off the NST would pass
through
> >the caps without any problem, but any RFI coming back through the circuit
> >would be turned into heat before it could damage the NST. Of course some
> >other type of capacitor could be used beside microwave oven caps, which
would
> >be quite bulky. I have no idea if this will work, but I thought I might
as
> >well throw it out there. I'd like to know what some of you guys out there
> >think.
> >                                Justin
> >
>
>