Potting compound suggestions

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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