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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 21st Sep 2020, 2:34 pm   #1
vinrads
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Default What to use for potting transformers.

I have made aluminium boxes to house the mains tx's , I was thinking if they get warm I need a potting compound to conduct the heat ,any suggestions , apart from the compound on Ebay which is very expensive . Mick.
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Old 21st Sep 2020, 2:46 pm   #2
G6Tanuki
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Default Re: What to use for potting transformers.

I'd imagine that most potting-compounds would - if anything - retain the heat. Free-air convection would seem preferable.
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Old 21st Sep 2020, 3:55 pm   #3
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Default Re: What to use for potting transformers.

I'm afraid G6Tanuki's right. Thermally conductive potting compound is expensive, but it sells because there isn't a cheap alternative. It should be possible to embed lumps of metal, so the thickness through the compound that the heat has to travel isn't far. But lumps of metal aren't cheap either.

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Old 21st Sep 2020, 5:06 pm   #4
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Default Re: What to use for potting transformers.

A liquid would be best, fun to get leakproof.
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Old 21st Sep 2020, 5:35 pm   #5
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Default Re: What to use for potting transformers.

some of the industrial tool transformers we had at work were potted in some type of clear resin mixed with sand.
Pete
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Old 21st Sep 2020, 5:43 pm   #6
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Default Re: What to use for potting transformers.

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Originally Posted by G4_Pete View Post
some of the industrial tool transformers we had at work were potted in some type of clear resin mixed with sand.
Pete
I think if you're selling it, you call it "mineral-loaded"....
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Old 21st Sep 2020, 5:44 pm   #7
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Default Re: What to use for potting transformers.

The Parmeko Neptune series are liquid (transformer oil) immersed of course. But prices for those these days make conductive polymer look cheap ! I don't think any DIYer works with oil unless he absolutely has to. It's not just the liquid spillage issue. Getting the best out of it involves stopping it absorbing atmospheric moisture. And there's also the smell ...

(Voice of experience - from the early 80's to the late 90's I was responsible for various bits of oil-immersed kit, peaking at about 15 tonnes of the stuff.)

Cheers,

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Old 21st Sep 2020, 7:01 pm   #8
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Default Re: What to use for potting transformers.

Thanks G4 Pete . clear resin mixed with sand sounds like a good idea , Mick.
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Old 21st Sep 2020, 7:21 pm   #9
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Default Re: What to use for potting transformers.

Getting the potting compound into the right places before it cures is also a problem. Vacuum impregnation is needed.

David
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Old 21st Sep 2020, 7:43 pm   #10
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Default Re: What to use for potting transformers.

Hi Paraffin wax will work well and it is not too difficult to seal the container against it.
It also has the marked advantage that it has quite a high latent heat of liquefaction, which can save the windings against a short term overload. Bitumen, as in roofers will also work, but melts ata higher temp and is very messy.

At Reyrolle a sand/ epoxy resin mix was often used to cool HV divider resistors. This had a pretty high proportion of sand.

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Old 21st Sep 2020, 8:50 pm   #11
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Default Re: What to use for potting transformers.

Thermal insulation and resistance go together with most materials, so getting a thermally conducting but electrically insulating material is not very easy. Think of the (highly toxic) Berylium oxide used on some 'scope output transistors.
A few vent holes makes best sense, if necessary backed up by a small fan.
Alternatively, loadsa cash for something a bit exotic.
Les.
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Old 21st Sep 2020, 9:27 pm   #12
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Default Re: What to use for potting transformers.

Sand and some sort of resin/glue would be my suggestion. My 110V isolating transformer is certainly potted in something like this. It isn't light weight, though
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Old 21st Sep 2020, 11:08 pm   #13
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Default Re: What to use for potting transformers.

Depends what temperature range you need.

Epoxy is good, but at low temperatures its thermal expansion, being different to the transformer materials, can lead to stresses and cracks. The same goes for high temperatures of course, but epoxy tends to be softer (above its glass transition temperature) so can absorb the stresses.

For a domestic temperature range, this shouldn't be a problem, but much beyond 0 to 100 deg C needs care.

Thermally conductive epoxies usually have a generous amount of filler added. Often aluminium oxide, but could be silicon dioxide (a major constituent of the sand that's already been mentioned). Aluminium oxide (alumina) is a better heat conductor.

Actually potting the transformer isn't trivial - with enough filler to make it conduct heat well, the stuff tends to be sludgy and viscous. Heating makes it flow better, but also accelerates curing, so there is an optimum temperature for doing it.

The best approach is to use something formulated for the purpose, by resin specialists. It may be a bit expensive, but rest assured that if there was a cheaper mixture which worked, they'd be offering it!!
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Old 25th Sep 2020, 5:34 pm   #14
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Default Re: What to use for potting transformers.

In the late 60's / early 70's a friend of mine made some discreet "logic" modules.. JK and "D" Flip flops and the like. These were potted with fibre glass resin..... the heat given off from the curing was serious, resulting in pots of water being used to cool the modules until cured.
I have no idea if this is the same today..but back then it was a problem
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Old 26th Sep 2020, 12:46 pm   #15
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Question Re: What to use for potting transformers.

Quote:
Originally Posted by vinrads View Post
I have made aluminium boxes to house the mains tx's.
Why have you chosen to house those transformers in a metal box? I'm sure that there is a good reason: just curious.

Al.
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Old 2nd Oct 2020, 8:49 am   #16
vinrads
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Default Re: What to use for potting transformers.

Hi Al is all to do with looks ,I did the same with the output tx the transformers were from a scrap Ekco record player ,tell me which you prefer ,Mick.
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Old 4th Oct 2020, 4:47 am   #17
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Default Re: What to use for potting transformers.

I'd listen to Ed - paraffin wax sounds a good plan. I use it for lubricating bicycle chains and it takes quite some effort on the Rayburn to get it all to melt. Cheap, too.

Really though, are the transformers going to get so hot that it will matter? Surely in use they shouldn't get more than vaguely warm, and not hot enough to affect insulation or require heatsinking to the outside of your box. I wouldn't have thought any high-tech or expensive material will be required.
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Old 4th Oct 2020, 6:01 am   #18
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Default Re: What to use for potting transformers.

Those transformers in the photo look to already have been impregnated with pitch or something similar.

If so, that would suggest continuing with something similar, or using the boxes for decorative reasons, but making sure they're ventilated. Or else putting the transformers under the chassis.

The mains toroidal transformers in my hifi amp are acoustically very noisy, so they got vacuum impregnated twice, then finally potted in a thermally conductive silicone rubber. It was remarkably expensive.

If the industry knew of anything which worked and was cheap, they'd be using it.

David
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Old 4th Oct 2020, 9:56 pm   #19
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Default Re: What to use for potting transformers.

This reminds me of the old "Mr punch's advice on those about to marry.... Don't!)
Ok joking aside, the transformers you have are probably already quiet and a check in situ will confirm this.
So. as david RW and others advise I'd simply ventilate the covers and maybe add a bit of acoustic damping to the covers, probably more effective than pouring a kilo of epoxy in.
And as an aside from expensive experience I now know what an "audio grade" toroid is. Its one that's got an epoxy plug in the middle and is possibly also vac impregged with the same stuff up to the point of being totally encapsulated. And only to stop vibes, the transformer core and windings have to be uprated to compensate for heat bulid up. The point being its not an audiophool concept, rather than for a normal listener the faintest hum from a noisy transformer can really distract and spoil a musical or dramatical experience ( I like radio4 plays and R3 poetry).

No such thing as a free ride as they say.

A.
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Old 4th Oct 2020, 10:23 pm   #20
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Arrow Re: What to use for potting transformers.

Quote:
Originally Posted by vinrads View Post
Hi Al is all to do with looks ,I did the same with the output tx the transformers were from a scrap Ekco record player ,tell me which you prefer ,Mick.
Point taken, Mick.

Al.
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