Potting compund question

[innkeeper]

I am looking for what to use for potting compound.

I've built myself some solid state rectifier replacements for tube rectifiers into tube bases, mostly for testing purposes.  Which means i will tend to be plugging them in and unplugging them a lot (not live of course).

I want to pot them so i don't mess them up.  When I looked at some potting compound from GC it was like 50 bucks.  I' looking for a suitable CHEAP potting compound that is solid somewhat thermally conductive, and won't break down over time and become conductive.  voltages involved will be under 500v .. actually for my initial uses probably under 200v

[floobydust]

I've put silicon diode strings into 866 rectifier 4-pin sockets, you don't have to pot them.
Just need a lid to keep it safe.
I've blown up 3kV solid-state diode strings and you would not be able to fix that once it is potted... so your design has to be good, not just a bunch of diodes in series.

But if you want to pot them, I would use silicone encapsulants which cure like a hard rubber.
The two-part ones perform better but more expensive and messy to mix, like Dow Corning SYLGARD 170. You might try for free samples. Potting compound can leak out the tube socket pins which makes a mess.

RTV silicone with a dielectric rating would be okay. GE RTV6700 is at the hardware store and 16kV/mm. I don't like GE's latest environmetally friendly products, I've had some that never cure.

Heat is not so much an issue because little is generated in solid-state diodes, at typical plate currents. Unless you are putting in series resistors to try match the tube's internal resistance. Most potting compounds help transfer heat.

https://www.mgchemicals.com/products/rtv-silicones/potting-compounds] [url]https://www.mgchemicals.com/products/rtv-silicones/potting-compounds[/url]

[coppercone2]

Hmm what you might wanna do with a cheap compound is embed some thermocouples into your PSU (use thermal epoxy to attach them to the relevant places before potting) and measure them once they are running. I am planning on doing this, so I can scan  the assembly later, if I ever get around to potting anything. If you make them yourself from wire and give them a quick test it should be very cheap, then you can leave them as test points you can hook into for trouble shooting or integrity test to see if something is deciding to get hot like 6 months or 6 years later. It's also generally interesting.

Keep in mind too you can route some wires out of a potted prototyped you soldered to relevant places to act as test points, so you won't be completely in the dark if something is wrong and you can get an idea of what to do maybe on v2 without difficult de-potting.

[innkeeper]

floobydust, just capping it might be the option i have to take. seems the potting options are outrageously priced.

[coppercone2]

Can you cut a tube with a glass cutter, do a even cleave and then glue it back together? I wonder if that glass can take it. Possibly put a thread in it so it can be opened up. Using a piece of threaded pipe would probobly be safer though since its not a glue seal. It seems like you can use a bit of copper pipe thats soldered shut. But their conductive, I don't know if you can trust a big heat shrink insulator in a high temperature area surrounded by vacuum devices.

I am not sure what a good solution would be to enclosure. What are you using? I imagined a copper pipe that has a bunch of threaded feedthroughs soldered to it, with the stuff inside, and then the feedthroughs soldered to the tube socket by heavy gauge stiff wire and some kind of reinforcement glue, then ground the whole thing to be safe. Using star washers etc.

[floobydust]

For a 5U4 plug-in, I have used octal plugs with snap on cover, and also gutted octal relays. There is room to add some parts.
Amphenol made them originally and QQQ Japan does today.

[innkeeper]

enclosure wise..I was actually able to get my hands on unused tube bases, so the circuit fits nicely inside the tube base, no problems.

thing is, i'll be using these for testing, so the will get plugged in and unplugged repeatedly and end up knocking about in a drawer when not used, so, you can see why i would want to protect the guts so to speak.

[Ian.M]

Cylinders of expanded brass mesh with disks of FR4 PCB soldered in top and bottom would make a neat job of it.  Cut (or etch) away any copper you don't need to support parts.  There's space on the top to mark them with an identifier, and plenty of air circulation for cooling resistors  for approximating the valve's characteristic or maintaining the heater chain in series heater sets.   Ceramic loaded epoxy putty or electrical grade silicone could bond them into or onto your bases.

[David Hess]

I would use a tube made from fishpaper and fill it with a suitable wax.  If necessary, the wax can be removed later just by heating and reused.

[Conrad Hoffman]

I think you'll find commercial thermally conductive solutions are all expensive. I've used Eccobond 285 in the past, but same problem. It also stinks. I've added alumina powder to cheap epoxy with so-so results. I like the above if you can find a wax with a high enough melting temperature to suit your needs.

[coppercone2]

I wonder what they do to make thermal epoxy. Do they use different formulations for the base?

I thought it might just rely on screened additive to basic epoxy mixtures. Like between a particular mesh range (not a random powder). It would be nice if someone figured out some ratios so we can just make custom epoxy mixtures for various jobs instead of paying ridiclous fees for safe/reliable electronics. I bet you they are making 10x markup on the stuff just because no one knows wtf the process is. And it expires. Screw that.

I still wanna know why epoxy goes bad as soon as you break the seal and if you can just vacuum seal it to maintain its tube life.

Yea ima complain about this. Spend 150$ a year in handy epoxy tax or get random work delays. Shitty deal. Just keeping JB weld, peramtex black plastic weld and a conductive epoxy. Let alone loctite prices  . Talk about bourgeoisie. Freaking edible items keep better then glue! It's crazy.

[Zero999]

What's the current?

I don't see the need for many rectifiers in series, at these low voltages. The 1N4007 is rated to 1000V and is cheap.

I agree, there should be no need to pot, for electrical insulation, but there might be a reason for mechanical purposes i.e. vibration and moisture ingress.

[Wolfram]

I've used the RS 552-668 thermally conductive epoxy potting compound before. The thermal performance is pretty good at 1.26 w/mk. It's also available from Allied in the US, but at twice the price. There are alternatives from Electrolube, like ER2220. From your application, it sounds like you might get away with lower thermal conductivity, in which case this is a cheaper option: https://www.alliedelec.com/rs-pro-71006383/71006383/

[innkeeper]

i was looking on one of the epoxy sites. they were selling "marble dust" or calcium carbonate as a filler for epoxy that also increased its thermal conductivity.  i also noticed that glass filled epoxies have a higher thermal conductivity, probably for the same reasons the filler increases it