SMPS transformer varnish


Simple question: What would you use for SMPS-transformer varnish?

I guess it is not required, but it will probably help on noise and add a little extra insulation. ( I think, I got a pretty good understanding of the required isolation, including creepage-distances and materials)

I thought about using polyurethane on spray or can and apply it during the winding-process. It would probably make a mess, but would it work?

Kind regards TroelsM
And what do the pros use to impregnate their SMPS trannies? Can this agent be dissolved in order to disassemble a transformer without destroying the core?
Best regards!

If you want to disassemble a ferrite core I have found that heat is the best option. I use a current-limited AC-source around 100Vac that is applied across the core ( from one half to the other).

Conduction will be very limited at first, but enough to heat up the core. When the core heats up, conduction rises ( thus: current-limiting) and it melts the varnish.

Heat up slowly to avoid stressing and cracking the core.

Its risky and you will break a core once in while, but it works.

You avoid the nasty solvents but please be very carefull with high AC-voltage and the heat. - there is a real risk of fire.

Kind regards TroelsM
Varnish protects from moisture and dust, but increases largely the capacitances and the resistive looses. I don't use it neither in ferrite nor in iron transformers/inductors.


Please explain how it affects the resistive losses?

I can see that the capacitance may be affected when air is replaced by an plastic-like material, but I dont get the resistive change?

Kind regards TroelsM
The lacquer is non conductive and does not change the resistive properties. Filling an air gap with a lacquer does not affect the capacitive leak in any way. It does not decrease the gaps anywhere and if, in the unlikely event it did, the designer of the smps will take that into account. That is why the manufacturer produces detailed specifications.

The vacuum just removes the air and lets the empty air spaces fill more easily with lacquer.


2006-09-08 2:04 pm
Varnishes can be alkyds like ordinary varnishes, oleophenolic, polyester, silicone, epoxy, etc., depending on the use and temperature class.
Once set (polymerized or reticulated), they become insoluble.
Some solvents like dichloromethane, Panasolve can soften them, but they also affect the enamel of the wire and the coils former (and their use becomes increasingly restricted).

Modern impregnation products have no effect on DC insulation, but some markedly increase parasitic capacitances and dielectric losses.
Here there is a product called "Cloroformo técnico" (Technical chloroform) that is available in glass bottles at special chemical sale houses that dissolve almost every transformer I tried. It is very powerful with most varnished and resins, but do not Walther substantially bobbins and wire insulation. PVC wire insulation become unusable again, but it usually don't care. I recovered several iron and ferrite core with this element.
I would use a marine lacquer or varnish which remains a little elastic.
Dont use epoxy on ferrite core traffos since it becomes too hard and the core can break during use.
Air has dialectric constant of about one, lacquer and paints have at least 2, probably 3-4. So after lacquer the capacitance will increase by the same factor. That probably does not matter but must be concidered.
You should not get a more quiet SMPS b/c the frequencies are too high to hear anyways. But there are good reasons to impregnate a traffo. Google.
I canabalized a vacuum pump from an old refrigderator. A small bucket just big enough for the traffo. Fill up varnish. Have a lid with a tube to the vacuum pump. Vacuum. The air bubbles out of all crevises and varnish is sucked into all gaps. Done.
Varnish impregnation certainly does affect the intra and interwinding capacitance of a transformer. It has been documented at my workplace countless times by measuring the transformer primary resonant frequency before and after varnishing. The resonant frequency always shifts substantially down after varnishing, indicating an increase in winding capacitance.

If I were going to varnish a transformer using stuff just available from the local hardware store, I would just use a good polyurethane varnish with no stain.. For a "poor man's vacuum impregnation" method, heat the finished transformer up to ~100C for an hour or so, then plunge it into the vanish. The hot air inside the transformer will cool, shrink, and draw the varnish father into the windings than a simple room-temperature dip would accomplish. I try not no vacuum impregnate using a solvent-based varnish, as it tends to boil off the solvent and make a mess.


Paid Member
2003-06-12 7:04 pm
Maine USA
> I canabalized a vacuum pump from an old refrigderator.
Do you mean the fridge's compressor?

The standard Freon pump in a refrigerator will pull a pretty good vacuum. It is not the greatest vacuum pump around, and may not last long sucking dirty air, but "dead reefers" (switches or wires; often the compressor still works) are nearly free in wealthy countries.