400 micron vacuum, stable and ready to use.
Patrick, Sat Dec 21 2019, 07:29PMIve been able to pump down to 400 microns reliably and now Im ready to see what i can do with it. The vacuum stays at about 390-400-410 ish.
Encapsulating transformer windings is the first priority. But as we all know encapsulating ceramic HV resistors without care of dimensions or thermal expansion can cause cracking. My question is with a plastic bobbin wound with 38 awg wire do the same rules apply ? would a rigid epoxy cause thermal problems and cracking or separating ?
Re: 400 micron vacuum, stable and ready to use.
Sulaiman, Sun Dec 22 2019, 05:44AM
The thermal expansion of epoxy resin is similar to common plastics, but higher than copper.
I think that the main factor will be local overheating (swelling and/or burning) of the epoxy,
as it is a relatively poor conducor of heat.
The crack(s) caused by swelling allow moisture into the windings.
Burning forms conductive residue.
So I think that vacuum impregnation of the windings is good for electrical insulation,
but excessive external potting thickness will result in significant thermal de-rating.
e.g.
Sulaiman, Sun Dec 22 2019, 05:44AM
The thermal expansion of epoxy resin is similar to common plastics, but higher than copper.
I think that the main factor will be local overheating (swelling and/or burning) of the epoxy,
as it is a relatively poor conducor of heat.
The crack(s) caused by swelling allow moisture into the windings.
Burning forms conductive residue.
So I think that vacuum impregnation of the windings is good for electrical insulation,
but excessive external potting thickness will result in significant thermal de-rating.
e.g.
Re: 400 micron vacuum, stable and ready to use.
Patrick, Sun Dec 22 2019, 08:21PM
Yes i was worried someone would make these points. i did some testing with toluene and xylene to water down epoxy, as ive previously done and made it permanently soft by doing so. but this it didnt work on this new epoxy.
I see all the useful information on the master bond site, but they dont sell publicly so id have to find a reseller i guess and it would have to be low viscosity to penetrate a 38 awg coil
i may start with this, but its viscosity is 2500 cPs.
Patrick, Sun Dec 22 2019, 08:21PM
Yes i was worried someone would make these points. i did some testing with toluene and xylene to water down epoxy, as ive previously done and made it permanently soft by doing so. but this it didnt work on this new epoxy.
I see all the useful information on the master bond site, but they dont sell publicly so id have to find a reseller i guess and it would have to be low viscosity to penetrate a 38 awg coil
i may start with this, but its viscosity is 2500 cPs.
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