Re: NST protection

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

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
>