EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: Neukyhm on April 21, 2018, 12:35:41 pm
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Hi there, I have built a 5.5kV transformer using AWG30 wire. I used epoxy resin to fix the windings and isolate the transformer.
I applied the resin to the secondary using a brush. The problem I have is that the curing time is very much and the resin will slide and fall from the transformer. Heating it won't help because the heat will speed up the curing process but the resin will become less viscous and slide anyway.
The question: is there a better sealant/adhesive/resin to use?
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Try applying the resin in more than one layer, so it's not thick enough to drip off.
Note: this is not an easy thing to do. Making the insulation between the windings good enough to hold off 5k5V, is not easy. Take care to ensure the voltage between each layer is no more than a few hundred volts and keep the wire clean and dry. If you wind by hand, then use cotton gloves.
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Place in a vacuum chamber, submerge in a pot of resin or varnish, pull vacuum, wait to stop bubbling, release vacuum. (You may need a long cure resin (hours, or thermally activated) to do this.) If there's time, cycle vacuum once or twice more just to be sure. A vacuum pump is desirable here, not just a venturi kind (which is hard pressed to do better than 1/10th atmospheric).
Painting it on the outside isn't going to do very much (assuming you need it at all). Or if you've used enough tape to deal with it as-is, it'll be fine -- at least in the short term, until corona and partial discharge eats it up.
If that's your concern, then all the more reason a vacuum is required, to fill the voids that PD occurs in. :-+
Tim
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I agree with T3sl4co1l; pot it under a vacuum. For an air wired voltage multiplier, I might accept brushing on corona dope and then potting in wax under atmospheric pressure but not for a high voltage transformer.
T3sl4co1l, what do you think about applying a couple atmospheres of pressure after vacuum potting?
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Also, it might be worth opting for one of the low-viscosity, infusion grade epoxies. They are used for making carbon fiber items, laying up the fiber in a plastic bag in a mold, then sucking the air out of the bag and letting this thin resin in. Because of the low viscosity it is able to penetrate the weave of the carbon fiber, under vacuum, and make a solid matrix with no trapped air. This would make it ideal for getting right into the windings of a transformer. I think the temp requirements for most CF layup are generally more stringent than potting resins, but you'd have to check that.
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applying a couple atmospheres of pressure after vacuum potting?
That’s is called vacuum impregnating and the process isn’t easy.
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applying a couple atmospheres of pressure after vacuum potting?
That’s is called vacuum impregnating and the process isn’t easy.
What is the difficulty? Use a pressure vessel instead of a vacuum chamber, apply a vacuum, release the vacuum, apply pressure, release pressure, and then remove the item from the bath of potting material.
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Pressure would have to be maintained until the material cures. Which would work for resin, just heat the pressure vessel to activate, and let it sit until solid. I guess there might be some oddness about heating a pressure vessel, just make sure it's good for it.
Then you have trapped bubbles under pressure, which will eventually diffuse back to ~atmospheric depending on material properties, though that could take very long indeed. Well, at least if they're under pressure, they'll have higher breakdown voltage, which is a good thing. Although they'll be smaller too, but, that means more E-field, which still works out. Preferable to remove voids of course, but una-void-able (ha) ones under pressure is better than not.
It wouldn't work so well for varnish, where the pressure will reduce evaporation rate, and that's if you have ventilation in the chamber at all (say, a cold trap off to the side?).
Tim
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You evacuate the chamber before submerging it in resin. Then you apply pressure. And remove left over resin to prevent curing it as a block.
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As others have said, vacuum impregnation of a resin will help. But transformer design is also very important, if you are just randomly adding wire to the bobbin, then you risk having a potential between adjacent wires that exceeds the breakdown voltage of the wire enamel (in reality polyurethane or polyester), no amount/grade of resin is going to stop that happening.
You're probably close to the voltage where I would vacuum pot the whole transformer in RTV silicone or similar - it doesn't sound very big, probably just use a small potting box.
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if you are just randomly adding wire to the bobbin, then you risk having a potential between adjacent wires that exceeds the breakdown voltage of the wire enamel
All turns are well placed just next to the previous one. I have also tested the enamel to ensure it will resist the voltage between every turn.
Well the epoxy is now cured and the transformer is complete, I will test it today, but the resin has a lot of bubbles. I hope that won't be a problem, I don't know if 5kV is enough voltage to cause ionization of the bubbles. I forgot to say that the frequency ~110kHz, that will probably be a problem.
Edit: and the transformer is a ETD59, using its coil former.
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Sorry, should have said earlier: if you've got a large solid mass of epoxy over ferrite, you can get cracking of the ferrite due to CTE mismatch (Coefficient of Thermal Expansion). Epoxy on the windings and bobbin is probably fine for CTE.
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Sorry, should have said earlier: if you've got a large solid mass of epoxy over ferrite, you can get cracking of the ferrite due to CTE mismatch (Coefficient of Thermal Expansion). Epoxy on the windings and bobbin is probably fine for CTE.
That's one reason to use gel type potting compounds. Not sure how they compare to the solid type for dielectric strength.
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Then you have trapped bubbles under pressure, which will eventually diffuse back to ~atmospheric depending on material properties, though that could take very long indeed.
Trapped bubbles of what? Trapped bubbles of the remaining air from when vacuum was initially applied? If so, then the same bubbles would exist from only using a vacuum.
I thought there might be some advantage if atmospheric pressure was not enough by itself to push the resin into the areas under vacuum inside the assembly.
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There's a wealth of practical information about building transformers (and many other electronic topics) on Manfred Mornhinweg's ludens.cl website.
https://ludens.cl/Electron/Electron.html
https://ludens.cl/Electron/trafos/trafos.html
https://ludens.cl/Electron/tek310/tek310.html
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Trapped bubbles of what? Trapped bubbles of the remaining air from when vacuum was initially applied? If so, then the same bubbles would exist from only using a vacuum.
I thought there might be some advantage if atmospheric pressure was not enough by itself to push the resin into the areas under vacuum inside the assembly.
Yes -- there will always be bubbles, because there's no perfect vacuum, nor perfectly nonvolatile resin (though a separate degassing step might help, also, I suppose the components could be degassed with heating to remove volatiles efficiently?), or perfectly permeable assembly (how much gas will exit in the time given, how much resin will flow in?).
The question is, is it good enough? For sure, using vacuum at all is a big start, and the bubbles should be very small indeed after pressurization (reduced by whatever the pressure ratio is). I wonder if they might even disappear, because of surface tension and solubility.
It's probably down in the realm of empirical results at this point: how good is good? Do a burn-in test for different conditions, and there you go. Of course, that only helps if you need a known lifetime, for a production run. One-offs? :-//
Tim
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Yes -- there will always be bubbles, because there's no perfect vacuum, nor perfectly nonvolatile resin (though a separate degassing step might help, also, I suppose the components could be degassed with heating to remove volatiles efficiently?), or perfectly permeable assembly (how much gas will exit in the time given, how much resin will flow in?).
We did this when potting the cavities in load cells where the strain gauges are mounted although I an not sure what the justification was. Degassing the epoxy gel after mixing produced crystal clear instead of a fogged translucent result but I think we did the same thing for non-transparent epoxies implying that it was for more than aesthetics.
The question is, is it good enough? For sure, using vacuum at all is a big start, and the bubbles should be very small indeed after pressurization (reduced by whatever the pressure ratio is). I wonder if they might even disappear, because of surface tension and solubility.
I suspect if you did it right that the gas remaining after a vacuum is applied would completely dissolve in the degassed epoxy. Most of the advantage however comes from applying a vacuum since the ratio of pressures (gas volumes) will be much greater. I was more concerned with getting the epoxy to completely displace as much of the free space as possible.
It's probably down in the realm of empirical results at this point: how good is good? Do a burn-in test for different conditions, and there you go. Of course, that only helps if you need a known lifetime, for a production run. One-offs? :-//
I have been anticipating very small production runs, accelerated testing at higher than operating voltage under worst case conditions, and destructive testing on samples in lieu of a long term testing program. The last time I did destructive testing like this I could not get the assembly to fail but I am sure that would not be a problem when high voltages are involved.
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The transformer died almost instantly with a visible arc inside the epoxy :'(
I have to find a better insulator. I got that epoxy from RS Components, so it is supposed to be high quality, but it was very difficult to apply it to a ETD59 coil former, it had lots of bubbles.
Notice the core I'm using: ETD59.
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The transformer died almost instantly with a visible arc inside the epoxy :'(
I have to find a better insulator. I got that epoxy from RS Components, so it is supposed to be high quality, but it was very difficult to apply it to a ETD59 coil former, it had lots of bubbles.
Notice the core I'm using: ETD59.
Did you have layers of insulating material between winding layers? This is standard practice. You wind a layer on the coil form, then lay down a layer of insulating paper. Then, wind the next layer of windings, and repeat. This creates an additional amount of insulation between each winding layer. The insulating paper is longer that the circumference around the winding, so the ends of the paper overlap.
Jon
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Hmm, "paper" sounds very old fashioned, but then I remember, there are plastic fiber products (usually polypropylene felt, I think?) intended for transformer winding -- the fiber allows resin in.
Just don't forget that this exists, and fall back on ordinary (cellulose) paper -- those days are long over! ;D
Tim
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Not all enamelled wire is the same (!)
These guys make the dogs-danglies of transformer wire: http://www.tex-e.com/ (http://www.tex-e.com/)
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If higher leakage inductance is OK, a special bobbin with a sectioned winding (I.e. split the winding into multiple smaller lumps) might help.
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The transformer died almost instantly with a visible arc inside the epoxy :'(
I have to find a better insulator. I got that epoxy from RS Components, so it is supposed to be high quality, but it was very difficult to apply it to a ETD59 coil former, it had lots of bubbles.
Notice the core I'm using: ETD59.
Did you have layers of insulating material between winding layers?
Yes, I left a film of epoxy dry before applying the second layer of wire, but that film failed, see picture, where I have removed part of the epoxy.
The arc I saw happened at the burnt area, It's a critical part because the highest voltage between the first layer of wire and the second happens there.
Not only that, It's also the area with less epoxy, because as I said, part of it drifted and fell from the transformer, leaving that zone (the upper part) with less epoxy than other parts of the coil.
Also notice the bubbles |O
I'm going to try again, I will remake a new transformer but this time I'm going to apply more layers of epoxy between the first and second coil.
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Is that ptfe plumbers' thread sealing tape that I see?
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Is that ptfe plumbers' thread sealing tape that I see?
Yes, I put a bit of it to make easier to wind the second layer of wire, so between the first and second layer of wire I had that and epoxy.
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PTFE tape has nil thickness to begin with, and is porous and cold-flows under pressure. In short, it's useless here!
Actually using masking tape isn't a showstopper here -- as long as you only need class A insulation, and take adequate precautions to deal with the possibility of it catching fire (it's not exactly 94V-0 grade!).
"Electrical tape" is definitely not the thing here, as it is also very soft, especially when hot. Or duct tape, or anything else rather on the gooey side.
It's well worth getting some legit yellow transformer tape! It's only $20, and a small roll will supply many transformers. I forget which Scotch part number is usual, but any moderate temperature (105C+), polyester tape will do. The adhesive is usually acrylic based.
It goes on kind of loose, not the stickiest thing, but the adhesive softens on heating -- usually when the assembly is impregnated with varnish, then baked to cure it. But even if you've just got solid layers of polyester film separating the layers, without resin, you'll stand a much better chance of success!
Tim
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We use AT4004.
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A trick that is used in some HV transformers is to use a bobbin that splits the winding into multiple side by side windings that are in series. When dealing with high frequency it becomes even harder to prevent corona and arcing, you may want to pot the entire winding using a casing similar to those used for CRT flyback transformers. For prototyping and experimentation immersing in mineral oil is a practical approach. It can be messy but it is a very effective insulator. I've wound HV transformers that I placed in a glass jar of mineral oil with the leads feeding through insulators in the lid. Just don't use stranded wire leads, the oil will wick up the wire.
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Bank wound construction (or "pi wound" like the old chokes) is fantastic for decreasing capacitance. Downside: more leakage inductance. Well, in short, it's a method to raise the characteristic impedance of the winding. At high turns ratios, it's impossible to get a Zo anywhere near what you're using it for, so every little bit helps. (Such a construction might achieve Zo ~ low kohms, whereas you might be using it for ~mA at ~kV = Mohms!)
CCFL transformers are cheaply available, constructed in this way. I don't know how much OCV they generate, but it could be a few kV. I've seen small Tesla coils made with them.
Tim
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I have finished winding the first layer of wire, I have left 3 layers of epoxy dry (the first attempt had one). Now I'm thinking what to do before starting the second layer of wire, more epoxy layers? What about applying some layers of PVC black tape? Is it a good insulator?
Notice that the first layer of wire covers the 41.2mm of the coil former, but the second layer just takes less than the half of it. I have estimated the voltage difference between the first and second layer of wire at it highest value to be around 3kV.
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You seem to be specializing in tape that plastic deforms (after your plumbers tape). PVC is also soft and stretchy, although thicker than the ptfe, so will also tend to thin out under compression, temperature performance is poor too. What you really need is Polyester/PET/Mylar. Try ebay - even if you don't have it immediately to hand, it's better to wait and do the job properly.
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I wound some transformers a few years ago and used polyester tape, I think I got it from tapecase.com.
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I forget which Scotch part number is usual
I think you are talking about Scotch 23, is it good?
I would wind a few layers of it above the 3 layers of epoxy. The epoxy and the Scotch tape should be strong enough to withstand around 3kV@110kHz.
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Ok I have now some Scotch 23, before using it I would like to hear your opinion, is it good? What's the best way to use it? If I wrap some layers of it around the epoxy, it should be able to withstand those 3kV.
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I suppose if you're going to use epoxy at all, it might be nice to apply a layer of goo, then wrap it with tape so the tape smooshes it in place. But that's messy and hard to do.
I'd just recommend winding dry, cover each layer with two overlapping layers of tape, and make sure the tape goes past the edge of the winding so you don't get arcing around the edges (creepage).
Tim
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I suppose if you're going to use epoxy at all, it might be nice to apply a layer of goo, then wrap it with tape so the tape smooshes it in place. But that's messy and hard to do.
I'd just recommend winding dry, cover each layer with two overlapping layers of tape, and make sure the tape goes past the edge of the winding so you don't get arcing around the edges (creepage).
Tim
Will do. Also I need some assistance understanding the Scoth 23 datasheet (https://docs-emea.rs-online.com/webdocs/0c01/0900766b80c01aa4.pdf), page 3, graph "Dielectric Strength Versus Thickness", I don't understand why dielectric strength (intensive magnitude) depends on thickness (extensive magnitude) :-// . Also I don't know how this tape's dielectric strength is better when thinner.
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Dielectric strength is better when thinner, something about the molecules being lined up, tighter, stronger -- but mind that the thickness drops too (that's where it's being elongated from), so the total voltage per layer of tape may be less!
Oh wait, splicing tape? This isn't the yellow mylar stuff.
Try... uhh... Scotch 56?
Tim
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Dielectric strength is better when thinner, something about the molecules being lined up, tighter, stronger -- but mind that the thickness drops too (that's where it's being elongated from), so the total voltage per layer of tape may be less!
Oh wait, splicing tape? This isn't the yellow mylar stuff.
Try... uhh... Scotch 56?
Tim
Scotch 23 seems better than 56 (also 23 is thicker, but it's not a problem to me)
Damn, 3M has a lot of tapes.
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You might check out the forum at 4hv.org, lots of guys there making HV transformers. I wouldn't bother using epoxy at all unless you vacuum pot the whole winding. HV transformers require careful technique to prevent arcing and corona, and the higher the voltage and/or frequency the more difficult it gets.
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23 is squishy, a property several posters have enumerated is undesirable!
Tim
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You might check out the forum at 4hv.org
People over there are suggesting me to use Kapton tape, uh... now I really don't know what to use XD
It seems that, If you search on the internet, you find people using everything. Some say winding a lot of layers of PTFE tape works fine, others use Kapton, others prefer Scotch. :-/O
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Well, if it helps, I'm an engineer, and most of them are not... :=\
Kapton(R) (polyimide) is even better, having higher operating temperature (if the kind with silicone adhesive) and better corona resistance. Major downside is ease of tearing and cracking, but that shouldn't be a problem in a transformer, where the wires don't have to move.
Tim
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This Engineer agrees with you. Kapton is good stuff for high voltages. There are other materials also. Note if you use oil insulation it's best to pull a vacuum on it to get the bubbles out.
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I like to refer to the silicon adhesive Kapton tape as "aerospace tape" to go along with my "aerospace epoxy". It is marvelous stuff but I have seen it replaced with Mylar tape (but not the stuff for transformers) to save in costs and the yellow Mylar tape they use for transformers seems pretty good also.
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Polyester (Polyethylene terephthalate) sheet is usually used for isolation in between transformer winding layers.
https://www.google.com/search?q=polyester+transformer+tape (https://www.google.com/search?q=polyester+transformer+tape)
Most parcel packaging tapes are made from the same material and can be used for that application (at least I used it)
Most important aspect is not to wind completely to the end of the transformer core so that no discharge can happen at the air gap (as dielectric breakdown voltage of air is much lower than the one of PET polymer):
good:
-----------------------------------------------------
°°°°°°°°°°°°°°°°°°°°°°°°
-----------------------------------------------------
bad:
-----------------------------------------------------
°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°° ZZZ
----------------------------------------------------- Z
°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°ZZZZ
-----------------------------------------------------
If the tape isolation wrap gets warped, wrinkled and ugly, you can use a heat gun to shrink it back into niceness, btw.
For isolation of complete transformer, I wouldn't use epoxy, much too expensive, difficult to apply, hard remove for repairs, modifications or to reclaim parts. Go with plain candle/paraffin wax and simply cast it into the transformer housing. Prior to casting, heat it well to boil off any moisture and then cast it while it's nice and hot and has low viscosity, so that it can seep into all the small spaces. If mechanical strenght/melting point is too low for your application, you can actually dissolve polyethylene plastic (plastic bottles etc) into your molten paraffin wax (takes some time). The long polyethylene molecules (PE is like parafin, but with much longer hydrocarbon chains) increase strenght/melting point.
If something goes bad, simply use a heat gun or dump the whole assembly into a pot of already molten wax.
Used this to for isolation on capacitor/diode voltage multipliers, up to maybe 100kV (judging from spark lenght) and it worked great.
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Polyester (Polyethylene terephthalate) sheet is usually used for isolation in between transformer winding layers.
I assume the common yellow polyester (same thing as Mylar in case anybody was unaware) transformer tape uses acrylic adhesive. I have noticed before when unwinding it after it is old that the adhesive is dry and flakes off.
For isolation of complete transformer, I wouldn't use epoxy, much too expensive, difficult to apply, hard remove for repairs, modifications or to reclaim parts. Go with plain candle/paraffin wax and simply cast it into the transformer housing.
Epoxy has another problem which may have to do with water absorption. As Tektronix discovered when they changed from wax to epoxy during the era of tube oscilloscopes, some epoxies become lossy as they age which can completely disrupt operation of self oscillating inverters. My first choice is wax as well for all of the reasons you identify.
Prior to casting, heat it well to boil off any moisture and then cast it while it's nice and hot and has low viscosity, so that it can seep into all the small spaces.
I have thought about doing this with wax under a vacuum.
If mechanical strenght/melting point is too low for your application, you can actually dissolve polyethylene plastic (plastic bottles etc) into your molten paraffin wax (takes some time). The long polyethylene molecules (PE is like parafin, but with much longer hydrocarbon chains) increase strenght/melting point.
There are a lot of different grades of wax also and combining two in the right proportions can tailor its characteristics. The problem I have usually seen is the wax separating or cracking if it is part of a large volume but I do not have much experience with using it yet. There is a fair amount of information online about it.
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Some of you are suggesting to use Kapton. I have found some cheap polyamide tape, but it's not genuine DuPont Kapton, will it do the job?
Genuine DuPont Kapton is really, I mean, REALLY expensive. I will take a look to polyester tapes as well.
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Polyamide != polyimide (and yes it matters, and yes chemists are that pedantic..).
Though I don't actually know what polyamides would be made in tape form. Probably the most common is the aramid fiber Kevlar (and relatives). Hmm, there's probably fiber-reinforced tapes though.
Anyway, I don't have a problem with cheap tape, as long as it'll do what's needed. Test it if you like. Polyimide doesn't melt, it just kind of chars -- at nearly red heat!
Mind you aren't getting the full temp rating if it's only acrylic adhesive.
Tim
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Polyamide != polyimide (and yes it matters, and yes chemists are that pedantic..).
Thanks for that, I didn't realize, even big suppliers like RS use polyamide when referring to Kapton :palm:
I have bought that fake Kapton and I will test a few layers of it with 20kV before using it in my transformer.
I'm also investigating these polyester tapes, they are very affordable compared to genuine DuPont Kapton, and they seem good according to the datasheet (https://docs-emea.rs-online.com/webdocs/1439/0900766b81439724.pdf)
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2.5 and 3.5 mil? You may find that tape hard to work with, actually, compared to thinner stuff like #56. You can always put more on, but you can't shave it down.
Tim
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Thanks for that, I didn't realize, even big suppliers like RS use polyamide when referring to Kapton :palm:
I have bought that fake Kapton and I will test a few layers of it with 20kV before using it in my transformer.
I'm also investigating these polyester tapes, they are very affordable compared to genuine DuPont Kapton, and they seem good according to the datasheet (https://docs-emea.rs-online.com/webdocs/1439/0900766b81439724.pdf)
These guys have what you need: https://www.ibselectronics.com/permacel/ (https://www.ibselectronics.com/permacel/)
Including Kapton tape: http://www.ibselectronics.com/ibsstore/catalogsearch/result/index/?searchKind=http%3A%2F%2Fwww.ibselectronics.com%2Fibsstore%2Fcatalogsearch%2Fresult%2Findex%2F&s=p-222&q=p-222 (http://www.ibselectronics.com/ibsstore/catalogsearch/result/index/?searchKind=http%3A%2F%2Fwww.ibselectronics.com%2Fibsstore%2Fcatalogsearch%2Fresult%2Findex%2F&s=p-222&q=p-222)
Also, Nomex is good stuff: https://www.eis-inc.com/paper-type-roll/p-ph21-nomex%20410-rl (https://www.eis-inc.com/paper-type-roll/p-ph21-nomex%20410-rl)
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Well the fake Kapton tape is still on its way. I have bought polyester tape as well: https://docs-emea.rs-online.com/webdocs/1439/0900766b81439724.pdf
It's a 3M T1350, with a 5.5kV breakdown voltage. However, there is another tape, T56 from 3M, that is designed for layer insulation according to the datasheet, but it has 5kV breakdown voltage (less that T1350). Also it's more expensive.
You will probably say that there is no reason to be concerned because both tapes will do the work and you are right, it's just that I don't want to kill another transformer xD :-/O
T1350 should be fine, the main difference is that this tape's adhesive is acrylic and T56's is rubber, I don't know what's best to stick to full cured epoxy.
This tape thing is an entire new and huge world dude.