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statik

Mon Jul 05 2010, 12:58PM

Registered Member #2967 Joined: Mon Jul 05 2010, 12:49PM
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Posts: 3

Hi All; I've built myself a electrostatic powder coating machine and I'm looking to add a cascade multiplier to increase the output voltage. My current system consists of a Flyback transformer driven by a 555 with frequency tuned to the transformer. I believe I may out around 15KV with this current setup (It is adjustable from around 5KV). I am planning on adding a multiplier circuit on the gun (much as the commercial powder coating systems work). I have no problem assembling the cascade but in order to keep it compact I need to pot it in order to avoid arcing of the final stages of the multiplier. Are there any low cost potting compounds which are accessible to DIYers? Has anyone tried silicone RTV?

doctor electrons

Mon Jul 05 2010, 01:09PM

Registered Member #2390 Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381

I have seen silicone rtv used in some electronics but i never used it myself to pot components.
If the components that you are using aren't going to heat up you can use 2 part epoxy. Make sure
if your components are going to heat up you use a thermally transmissive potting compound!!!!!
Thermally transmissive basically means your potted goodies wont explode when they heat up!
Here is a part number from allied electronics ( 617-7003 ) Its a two part epoxy, about 20 bucks.
You may be able to find the same stuff at a local hardware store cheaper

Good Luck with
the project, hope this helps!

Herr Zapp

Mon Jul 05 2010, 05:07PM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

Statik -

You definitely do not want to use any of the hardware-store grade RTV silicone sealants, as they release acetic acid during their cure cycle (hence the vinegar smell) and will cause severe corrosion of electronic components, and electrical breakdown of anything carrying HV.

There are specialty RTV silicone materials designed specifically for sealing and potting electronic assemblies (like General Electric's RTV 160, 162, 167 series); these materials release alcohol during their cure and are totally non-corrosive. Unfortunately, they are relatively expensive.

How big is the final stage of your multiplier (how much surface area), and how is it constructed? If its just diodes and caps mounted directly on an insulating board, another option is to simply dip-coat the assembly with an insulating varnish, building up thickness through multiple layers. If you've carefully built the multiplier so it doesn't have sharp points sticking out everywhere (corona inception points), this should work well if you build up sufficient coating thickness.

The 2-part epoxy mentioned by dr electrons (Scotch Weld DP270) is fairly viscous, so if you use something like that for potting a HV assembly, you would probably need to use vacuum-impregnation to ensure that there were no voids in the material. Also, the DP-270 is NOT a thermally conductive epoxy, so it would not be a good choice for potting components that dissipate much power. Finally, at more than $10/oz , it's also very expensive; i'd look into conformal coating with varnish first.

Regards,
Herr Zapp

Sulaiman

Mon Jul 05 2010, 06:02PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

Have a look here

I have used 164-321 , it's not viscous and doesn't form air bubbles as you mix it in the pack. It doesn't conduct heat well though.

quicksilver

Mon Jul 05 2010, 06:37PM

Registered Member #1408 Joined: Fri Mar 21 2008, 03:49PM
Location: Oracle, AZ
Posts: 679

It wasn't my imagination that quality sealants cost quite a bit. But I found out the hard way out using hardware store silicone caulking material and it's excretion of acetic acid!
11â‚¤ for 250 grams seems quite expensive but having no "extra-curricula" problems is worth it.

The only time I think using common silicone is worth a shot is if it's not touching any metal surface & simply building up from an existing potted surface to make an extra thick insulation. Otherwise, it appears to "etch" into leads and form oxidation over time.

statik

Tue Jul 06 2010, 06:51PM

Registered Member #2967 Joined: Mon Jul 05 2010, 12:49PM
Location:
Posts: 3

Thanks all for the great information;

The multiplier is just diodes and caps on some perf board. I've taken great care in laying out the leads such that there are no corona points. The multiplier is about 1.5" x 3", What type of varnish can be used for insulating? I am also investigating permatex RTV

Some of their products are advertised as sensor safe; and they don't smell like vinegar when they cure, still investigating...

statik

Tue Jul 06 2010, 06:55PM

Registered Member #2967 Joined: Mon Jul 05 2010, 12:49PM
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Posts: 3

Here is the proper link to the RTV I was thinking of; at $7 a tube the price is right...

radiotech

Wed Jul 07 2010, 08:47PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
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Posts: 1546

Glyptal coating paint may still be available at motor repair shops. It can be baked to cure and works at 15kV AC. The issue with high voltage insulation is even potential gradient. In big stuff it can be done by layering and shape or by semiconductive insulation. Potting rather than dip insulation would help sharp edges from getting exposed, but if they are smoothed first, insulation is easier.

2Spoons

Thu Jul 08 2010, 04:07AM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615

Dissolve hot-glue sticks in molten paraffin wax, and use that as a potting compound. Just beware of shrinkage in thick sections. The glue sticks make the resulting solid a lot tougher. Good thing about it is you can melt it out again if something breaks, which you cannot do with silicone or other thermoset resins.

IntraWinding

Thu Jul 08 2010, 11:53AM

Registered Member #2261 Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581

2Spoons wrote ...

Dissolve hot-glue sticks in molten paraffin wax, and use that as a potting compound. Just beware of shrinkage in thick sections. The glue sticks make the resulting solid a lot tougher. Good thing about it is you can melt it out again if something breaks, which you cannot do with silicone or other thermoset resins.

I haven't heard of that trick before - nice one

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