RE: weird beginner's-thoughts on RF-chokes

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

Original poster: "Lau, Gary by way of Terry Fritz <teslalist-at-qwest-dot-net>" <Gary.Lau-at-hp-dot-com>

Hi Christoph:

The short answer to the question about chokes is "don't use any chokes, use 
an R-C filter instead".  Until recently, for years, protection chokes were 
widely used and recommended.  Then folks started using circuit simulators 
and noticed that bad things happen when chokes were used on spark gap 
coils.  L-C filters work very well when a continuous signal is applied, but 
in a spark gap coil, unwanted resonances in the choke and bypass caps are 
excited, and these resonances are no less in magnitude than the RF that 
you're trying to filter out.  Series damping resistors can reduce the 
duration of these resonances, but not completely.

R-C filters are easier to build, have no unwanted resonances, and have a 
very good track record.  See my page at 
http://www.laushaus-dot-com/tesla/protection.htm, or Terry Fritz's filter 
schematic at http://hot-streamer-dot-com/TeslaCoils/Misc/NSTFilt.jpg.

How was your spark gap width set?  If it's too wide, it will damage the 
NST's regardless of filters.  With only the gap across the NST, the gap 
width should be set so that it just barely fires.  Any wider is asking for 
trouble.

Regards, Gary Lau
MA, USA

  -----Original Message-----
From: 	Tesla list [mailto:tesla-at-pupman-dot-com]
Sent:	Tuesday, January 28, 2003 2:20 PM
To:	tesla-at-pupman-dot-com
Subject:	weird beginner's-thoughts on RF-chokes

Original poster: "Christoph Bohr by way of Terry Fritz 
<teslalist-at-qwest-dot-net>" <cb-at-luebke-lands.de>

    Hi Folks!

I spent some more time optimizing my coil-design with wich I had some
initial problems last week that were discussed on the list.
I changed the thin wire for 1/4" copper tubing and got a bunch of OBIT's to
power the thing up to 1,2KVA
although I only replaced the xfrmers the results were really astonishing.
The streamers were actually 4 to 5 feet or even longer, much more than I
expected.
In fact I was a little bit frightning as the arcing was much more violent
than expected, I should have known that something was about to go wrong. at
this point.
Nevertheless after some rather short runs I ended up with one xformer less
than the 6 ones I stared with ;-)
It just burned out in some way. I can't measure anything wrong but it won't
deliver any voltage anymore...at least it was the oldest one of the
xformers.

This might have happened because of RF-kickbacks that should be surpressed
with RF-chokes if my information is right.
I had two air-chokes ( 40turns of insulated HV-capable wire on an 2"
PVC-pipe ) that caused me a lot of trouble as they got the primary circuit
ringing, causing breakouts even at the most unlikely spots.
After I removed these chokes everything worked fine at 400VA. During the
short runs at 1,2KVA I observed another interesting phenomenon:
sometimes, after switching th coil of ( sorry, no variac at this time, full
power or no power...) It wouldn't re-ignite ( sorry for the term, I didn't
find a better one ).
After some 30 seconds I was able to fire the coil as if nothing had
happened. This was repeated 2 times and then the xformer was gone. :-(
Furthermore the sound of my static sparc-gap was really annoing the
neighbours as this time it wouldn't make cracking sounds at about 200Hz, but
almost somthing sounding like a whistle and really loud.

Now I tried to build some RF-chokes. Therefore I got 4 ATX-PSU's from our
dumpster an disambled them to get some ferrite-cores, and thats what I got:

4 torus-shaped ferrite.cores, ca 1 1/8" in diameter

and

2  pole-shaped cores qinch in length and ca 1/4" thick.

On all of the cores I applied 2 layers of very-high quality electrical tape
and put about 40 turns on each of the toroid cores and about 30 turns on the
pole-shaped cores.
The idea was to make up two chains, each consiting of 2 toroid and one
pole.shaped cores in it with the pole-shaped core in the middle like in this
asci-drawing: O-O.
But while holding th single parts in my hand some questions came in my
mind..:

  > I'm not quite the mathematician and I don't own a meter for inductances,
so can the inductance of this combined chokes be high enough for my
purposes? What should it be?

( just to get an impression on what I use: 6 or 6OBITS, 10KVrms/15KVpeak /
20mA each, making up a 1,2KVA power-source, for an 11nF tank-cap. )

  >  is it bad if some of the cores are wound clockwise while the other ones
are wound counterclockwise ind one row... or will it cause any kind of
erasement....?

I'd really like to power the thing back up, but I'm very afraid that I will
loose all my OBITS which were really hard to find. ( Almost only gas-heaters
in my region )

So I'd be glad for any suggestions.

Sincerely


Christoph Bohr