Re: A TC Grounding and Filtering Scheme

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Original poster: "Gerry Reynolds" <gerryreynolds-at-earthlink-dot-net> 

Hi Dave,

I have 4 comments

1. I would remove the "RF block" components (ie the 1K//2mh).  Other words,
don't make any connection between the RF gound and the line ground.  The NST
should be connected to the RF ground (like you show).  Any secondary arc
will want to return its current to the base of the coil.  If it strikes
something earth grounded (other than thru the power cord), the RF ground
pipe will provide this path for the return current.  With the RF block
components, some of the return current will also return via the line safety
ground.

Yes there will be common mode stuff on the NST and the common mode choke in
the line filter will turn that into differential stuff that can more easily
be filtered.  The line filter should be grounded to line ground (like you
show).  Just remove the RF block components.  I can't  comment on reversing
the direction of the filter since I don't know if it was designed to prevent
stuff from getting from the line to the load or stuff getting from the load
to the line.  You might try it both ways if you have an interference
problem.

2.  The number of MOVs you have add up to a greater voltage then the ones in
theTerry Filter.  For a 15KV NST, I believe Terry has 8 MOVs (at 1800V each)
between each bushing and NST ground.  Reduce your number to13 - 1KV MOVs to
get close.  BTW digikey has the 1.8KV parts

3.  You are paralleling three 2.5 KV caps to get .0047 uf at 2.5KV and then
using 8 groups of these in series.  Thats a lot of parts.  That should work
but may be more expensive than needed.  I'm using eight .0047 uf at 1600V in
series for each bushing (One third your number of parts.  Terry indicates
that breakdown voltage on the caps he recommends is not a problem.  However
I did increase his 7 series string to 8 in series and adjusted the value
(like you did) to get the same effective capacitance.

4. I'll let others comment on the two safety gaps.  I thinking that the one
center grounded safety gap will do the entire job (protect both common mode
and differential mode overvoltage) if set properly.

 >
 > I've put together a schematic of my TC grounding and filtering scheme
 > that I'm planning on using for my next iteration on my coil.   The
 > schematic shows the whole system which, for me, makes it easier to
 > visualize where currents are flowing.   I'm interested in anyone's
 > comments.
 >
 > http://hot-streamer-dot-com/hvdave/temp/Daves_TC_Grounding_n_Filtering.pdf
 >
 >
 > The goal I was trying to achieve...
 > (1) Protect my NST's
 > (2) Keep as much RF crap as possible from feeding back into my 60Hz line
 > and ground.
 >
 > I turned the Corcom EMI filter around backwards so the common mode choke
 > was facing the NST primary.  That should better block common mode
 > currents from the NST primary to my line ground.
 >
 > I'm also planning on putting in an RF block (1k // 2mH) between my line
 > ground and NST chassis for the same reason.   The NST's will have some
 > common mode RF voltages on them but that shouldn't be a problem since
 > differentially they will be well protected.
 >
 > The NST output filter is pretty much a straight up Terry Filter.  I'm
 > using some different components (values) than what Terry's originally
 > selected, but its pretty much the same thing.
 >
 > The first safety gap I was planning on building with two 1/2 brass
 > spheres set off a small flat metal plate (for ground electrode)
 > equidistance.   I'll use the metal plate as my connection point between
 > the RF ground and NST output ground.
 >
 > Safety gap #2 is located very close to the main gap.
 >
 > The RF ground is about 10 feet of 3/4 inch copper pipe sunk into the
 > ground with about 12 feet of 2 AWG wire to the coil secondary.
 >
 >
 > Thanks
 > Dave Lewis
 >
 >