RE: Potting compound suggestions

["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>

I've repotted my NST in melted Vaseline petroleum jelly and it's been fine 
since then.  I think it would satisfy your requirements.

Gary Lau
MA, USA

  -----Original Message-----
From: 	Tesla list [mailto:tesla-at-pupman-dot-com]
Sent:	Tuesday, July 22, 2003 2:54 PM
To:	tesla-at-pupman-dot-com
Subject:	Potting compound suggestions

Original poster: "Richter, Rick by way of Terry Fritz 
<teslalist-at-qwest-dot-net>" <rick.richter-at-hp-dot-com>



          Hi all,

          I'm looking for some suggestions for relatively cheap potting
compounds for a couple circuits I'm putting together. I'm looking to
minimize corona loss and sparkover while, at the same time, increasing the
safety factor (insulation) for both circuits. Let me describe each circuit
to give you an idea what I'm dealing with.

          The first circuit is a six-stage Cockcroft-Walton voltage
multiplier circuit that takes 15 kV square wave input at 20 mA with a
frequency of around 20 kHz from a solid-state neon sign transformer. I may
scale this circuit back to have it output around 60 kV instead of around 90
kV.

          I know as the number of stages in a multiplier increases, the
actual output voltage can change significantly according to the load.
Therefore, I'd like to be able to measure the output voltage using a
combination voltage divider and digital multimeter (with about 10 Meg
internal resistance). I've got some 200 Meg and 10 Meg  2% high voltage
resistors I want to stack to form the divider, but again I want to minimize
corona loss and keep the exposed high voltage to a minimum.

          I want to pot each circuit separately and I'm trying to balance
off several factors. Number one is cost, I want to keep things as cheap as
possible. I realize that submersing the circuits in mineral oil is one
relatively cheap way to go, but it's also pretty messy and I'd like to
steer clear of that method unless I have no other choice. Second, from what
I've read, some epoxy-based potting compounds generate significant amounts
of heat during curing. Also, I've heard some types of RTV release corrosive
chemicals while curing. So I'm looking for something that won't melt the
capacitors or digest the diodes in my multiplier. Third is reversibility (
is that a word?). If I blow a diode in the multiplier, it would be nice to
be able to un-pot the circuit to replace the failed component(s). By unpot,
I mean anything from melt under low heat (if potting in paraffin) to gently
chip off pieces of the potting compound. However, being able to unpot the
circuit is the lowest priority.

          Does anyone know of any potting compounds that meet one or more of
these requirements?


          Thanks in advance,

          Rick Richter