Re: Protection Circuit

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

Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>

Hi Don,

See:

http://hot-streamer-dot-com/TeslaCoils/Misc/Filter.jpg

The filter you are talking of is on my small coil.  The "real" "Terry
filter" is at:

http://hot-streamer-dot-com/TeslaCoils/Misc/NSTFilt.jpg

That is 0.66nF with a 1K resistor so Fo = 241kHz.  That is about the
frequency of many TCs.  The filter really is not meant to filter the
fundamental of the coil but rather the very high frequency 10, 20 ,60...
MHz spikes.  One could filter the fundamental well but it tends to hurt the
coil performance too much.  NSTs can handle TC frequencies but the very
high frequencies do not distribute well in the secondary windings and tend
to cause local high voltages that are "bad".  So the filter takes out the
high MHz stuff.

NST windings appear to distribute frequencies of several hundred kHz well.
The windings and layer dimensions just seem to work out.  The manufacturers
probably just happened on this accidently over time.  The present NST
designs were probably found to stand up well to arcing and such.  However,
when the gap first fires, there are terrible high frequency voltage spikes
that can rip up the secondary windings.  Those are what the filter is
designed to stop.

The safety gaps and the MOVs are especially effective at stopping high
voltages from blowing the windings which is a very common killer of NSTs
too especially in resonant systems.

Cheers,

	Terry



At 06:31 PM 4/22/2002 -0400, you wrote:
>Hello,
>
>I am studying various circuits from several people and it seems the 
>protection circuit is 
>a low pas filter, like Terry's small coil specs, where he uses 360pF and 5K 
>resistor 
>which gives a 3dB point of 88.4kHz. Is it that straight forward or do you 
>really need to 
>factor in the secondary inductance of the NST?
>
>My daughter is doing a High School project and I am trying to explain each 
>section of 
>the circuit based on what I use to know and have long since forgotten.
>
>I have also seen RF chokes used in the protection circuit. Is it really 
>necessary?
>
>I have a OBT 10Kv 23mA and so we are on the low end of the power spectrum. 
>I've 
>ordered some 400pf 30KV doorknobs and so I guess a 5K resistor seems just
fine.
>
>The more we include, the more has to be explained and I want to keep it as 
>simple as 
>possible so it doesn't tax my brain too much.
>
>Thanks.
>
>Don
>