Questions about encapsulation. / High Voltage / Forums

Forums

4hv.org :: Forums :: High Voltage

« Previous topic | Next topic »

Questions about encapsulation.

Move Thread

LAN_403

Patrick

Tue Feb 16 2016, 10:19PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Id like to get some other opinions here.

I'm considering epoxy. But I have a ton of MAX 1618 epoxy available. Which is a normal structural epoxy. I don't want yet another chemical to store around my lab / house. So id like to use to avoid the electrical version if possible.

here is a pic with some info:

Thus may not be enough info for you guys to answer on, so tell me if so. I'm waiting for a response from Caddock on the requirements of the length and cracking conditions.

EDIT:

Patrick:

Below I am quoting you US OEM Prices for three Caddock Resistor Models rated for 32 kV continuous, the MX485, the MG785-15, and the TG985-15. Note that the minimum resistance for the MG785-15 and TG985-15 is 320 Megohms. For this reason I am quoting the resistance value of 320 Megohms as an example so you can see the prices as a function of the quantity and the model.

Caddock Type MG and Type TG resistors have been used successfully in potted assemblies in Industrial, Military/Aerospace, and Space/Satellite applications for over 35 years. Successful potting of precision high voltage resistors is not a trivial matter, there are plenty of ways to make mistakes. Any epoxy potting compound should have a coefficient of thermal expansion matched as closely as possible to the coefficient of thermal expansion of the resistors, which have a 96% Alumina core. Any mismatch in the coefficient of thermal expansion between the resistor body and the potting compound can result in damage to the resistor and/or delamination between the resistor and the potting material that can lead to field stress damage to the resistor. Thermal cycling of the potted resistor assembly due to power cycling or ambient conditions need to be considered. Design, field stress management, and potting is the responsibility of the user.

The resistors I would recommend depend on the application and the performance required, please refer to the questions I posed in my previous email. Generally speaking, we do not recommend the Type MX resistors for potting in epoxy.

Quote:

We are pleased to offer you the following quote for your consideration.

These prices are based on OEM Consumption in the USA.

Your Caddock Sales contact is Karla Strunk, see contact details below.

1. Caddock P/N: MX485-320M-1%

Order Quantity Unit price (USD)
1 to 9 $55.03 each
10 to 24 36.69
25 to 99 24.46
100 to 249 20.38
250 to 999 18.34
1,000 to 2,499 15.66
Contact me for higher quantity pricing

2. Caddock P/N: MG785-15-320M-1%

Order Quantity Unit price (USD)
1 to 9 $81.53 each
10 to 24 54.35
25 to 99 36.24
100 to 249 30.20
250 to 999 27.18
1,000 to 2,499 22.65
Contact me for higher quantity pricing

3. Caddock P/N: TG985-15-320M-1%

Order Quantity Unit price (USD)
1 to 9 $173.24 each
10 to 24 115.49
25 to 99 77.00
100 to 249 64.17
250 to 999 57.75
1,000 to 2,499 48.13
Contact me for higher quantity pricing

NEXT:
I cant remember if JPsmith used a soft encapsulation HV material . . .

AND:

This one appears to be very soft.

2Spoons

Tue Feb 16 2016, 11:15PM

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

from here

it looks like epoxy expansion is 5 to 8 times that of alumina, so I think pure epoxy is out. What you could do though is add a very high proportion of glass beads or chopped glass fibre to the epoxy. This will reduce the expansion coeff to something close to glass (assuming high solids loading). It will also save epoxy (cheaper) and reduce exotherm problems with bulk potting as there will be less epoxy to generate heat. Using glass beads will maintain pourablity - fibre will make the epoxy thicker. I assume you have vacuum degassing available?

jpsmith123

Wed Feb 17 2016, 02:44AM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

When I was vacuum encapsulating HV secondary coils, I used two different epoxy products. One was MG chemicals clear potting epoxy, and I don't remember the other one off-hand but I can look it up.

One of the things I was concerned with was trying to find an epoxy with a coefficient of thermal expansion that would be close to that of the material of the coil former and the shell (which were CPVC). I was also looking for a product with a low viscosity (for easier degassing) and a low dissipation factor.

I think if I was going to do it again, I would use some of the potting epoxy (with some calcium carbonate filler) available from seller "polymerproducts" on ebay.

And when I potted the voltage multiplier, I used a medium-hard silicone potting compound. Off hand I don't remember which particular product I used, but if I did it again I think I would use some "Sylgard 184" which is sold on ebay. (BTW I know that Sylgard 184 has been successfully used quite a bit in high voltage potting applications).

Ash Small

Wed Feb 17 2016, 09:58AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

As I understand it, a lot of the potting epoxies are just not as strong as the structural epoxies.

They may have various other additives, but if I was potting something, I'd tend to use normal structural epoxy, and add my own additives as required.

Patrick

Wed Feb 17 2016, 05:38PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

I'm looking at using solar cell encapsulation material, That may turn out cheaper and I could verify it as good or not for the rest of you.

dtproff

Thu Feb 18 2016, 04:59AM

Registered Member #57929 Joined: Thu Nov 12 2015, 03:29PM
Location:
Posts: 5

jpsmith123 wrote ...

When I was vacuum encapsulating HV secondary coils, I used two different epoxy products. One was MG chemicals clear potting epoxy, and I don't remember the other one off-hand but I can look it up.

One of the things I was concerned with was trying to find an epoxy with a coefficient of thermal expansion that would be close to that of the material of the coil former and the shell (which were CPVC). I was also looking for a product with a low viscosity (for easier degassing) and a low dissipation factor.

I think if I was going to do it again, I would use some of the potting epoxy (with some calcium carbonate filler) available from seller "polymerproducts" on ebay.

And when I potted the voltage multiplier, I used a medium-hard silicone potting compound. Off hand I don't remember which particular product I used, but if I did it again I think I would use some "Sylgard 184" which is sold on ebay. (BTW I know that Sylgard 184 has been successfully used quite a bit in high voltage potting applications).

I agree with the sylgard 184. If you need a bit more cure time you can use the 182 version. I have had good luck with both. One note of caution. I did run into an issue when attaching the HV output cable. It was a TV-40 silicone wire which I believe is a tin cure whereas the Sylgard is a platinum cure. the Platinum cure silicons will often not cure when certain other silicons are around like the TV-40 wire. The net result is I ended up having to put a couple coats of AC-43 varnish on it first and then pot with the Sylgard.

Tony

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob