Phat Bytestard wrote...
What are some HV coax types (p/n) you like to use?
We used a military rooted, PTFE dielectric (IIRC), teflon jacketed
SPC multistranded core with a full, tightly bound SPC shield. It was
a little over an 8th inch in outside diameter. I think it was 30kV
form factor, but we used it for ultra low noise supplies. You be
amazed (or not) how much noise the supply injects into itself from the
multiplier array. I developed arrays that were shielded all around,
and the output node was a big round blob of solder. It helps when you
are trying to get a 4kV supply down to 4mV ripple. :-] The whole
thing was in a can, but the multiplier array was itself shielded from
the feedback network and output lead and RC filtering. It does make a
difference at those levels. I could more than double the output
capacity by not incorporating the RC filter, and the supply would push
it, no problem. We needed noiselessness, not brute power, however.
I can't recall exactly how many pF per foot we got from the coax,
But we used them as HV loads, and various other uses, like for
characterizing a feed forward needed in an HV feedback loop. Most
were between 5 and 20pF across several hundred MegOhms or more of
feedback resistor. Some supplies worked well with a little feed
forward on the feedback... some needed none.
BTW, it should be pointed out that the coax in the home-made
probe above does not see HV, only the 2pF capacitor does. It
should also be pointed out that stray capacitance conducting
any signal current to the coax center wire, besides the 2pF,
will reduce the desired 100:1 division ratio. This means
the 2pF and its LV output wire must be shielded.
Yes... noise is easy to inject in small signal electronics in an
unshielded environ.
Our stuff was DC, and the ripple was so small as to really be
insignificant. We ran at frequencies anywhere from 17kHz to a couple
hundred on our HV switcher driven supplies. We also made supplies for
transmitters. The England facility made up to 400kV supplies. We still
shielded things.
The issues and techniques involving this shield and the method
of creating the 2pF and the heat shrink to hold it all together
and the nice but safe finger-hold and the sharp probe tip and
the various schemes to adjust or calibrate the probe (take a
breath) presents considerable opportunities to a clever mind.
We used the left overs from potting sessions placed in a dixie cup.
It makes great stand offs for HV. One could have a one inch or a six
inch plug (drinking glass shaped) holding a circuit element... several
actually. One can even cut V shaped grooves in it to help "hold" the
wire or object in question,
One more comment. That 2pF had better never break down.
One should always have a coax rated several times greater than the
voltage being tested. Corona can poke holes right through teflon.
Good thing coax doesn't have air gaps. Another reason why the HV
shrink is needed over the tail.
The coax you suggested has a 2kV breakdown strength on the core
insulator, IIRC.
Have to watch out for poke throughs on that stuff too. More even
than my "Made for the purpose" coax.
It's like RG-174 on steroids. :-]
