Packing tape as transformer insulator?

[orbanp1]

Hello Everyone,

I picked up a Leader LBO-522 scope which has a faulty power transformer, one of the "HV" windings (~130VAC) for the vertical amp PS is OC.
I will rewind it. I already managed to disassemble the transformer and removed the outer windings.

The transformer, among other things, uses transparent foil to insulate the layers of windings.
I am thinking about using clear packing tape for that purpose.
Reading the markings on the tape, it is "BOPP", bi-axilly oriented polypropylene, though not sure about the thickness:
https://en.wikipedia.org/wiki/Polypropylene

Polypropylene is also used in foil capacitors as insulator:
https://en.wikipedia.org/wiki/Film_c...ilm_capacitors

An advantage, that I can see, of having a sticky tape is that the coil form is open on the side, this will help keep the wires in place till I assemble the transformer.

Any comment on using the packing tape for this purpose?

I have some "electrical insulating paper", a bit thicker, which is OK as outer and inter-winding insulation. I do not have any onion paper.
Before I also used teflon tape (got from a plumbing shop) for insulating layers of transformer windings, but that transformer bobbin had sides. (That transformer is still working...)

Thanks, Peter

[1100 man]

Hi Peter,
Kapton tape seems to be one product that's had several recommendations by people that are in a position to know on the forum. It seems to be available in different widths and is quite reasonably priced on Ebay. I've not used it myself yet, but it's what I plan to use for a transformer I'm doing.

I don't doubt parcel tape might work but the glue is unstable over time which may or may not be a problem. I don't know what it's thermal properties are either?

Considering the huge effort involved in rewinding, personally I would want to use the most suitable materials I could find, within a sensible budget of course!

Good luck with the rewind!

Cheers
Nick

[Terry_VK5TM]

Packing tape doesn't have a very high temperature threshold, it tends to shrivel up with heat.

As above, Kapton tape would be better.

[Diabolical Artificer]

Parcel tape is better than nothing but it is a bit thin, tears easily and the glue isn't brilliant. Kapton tape is not that much thicker but is tough and sticks well.

There is a disparity in price between Kapton tape sourced from reputable suppliers like Digikey and the stuff you get on ebay, meaning the stuff on ebay might not be 100% kosher, however I've bought several rolls off ebay cheaply and used it to wind a few tfmrs, tested it by trying to set fire to it, tested it's insulation properties etc; not had a problem.

Andy.

[kalee20]

Originally Posted by 1100 man

Considering the huge effort involved in rewinding, personally I would want to use the most suitable materials I could find, within a sensible budget of course!

Definitely!

Packing tape OK for a prototype, a transformer with perhaps a few tens of hours of operation. For something you expect to last a very long time, get the proper stuff - or do some trials (wrap some turns of tape around something suitable, put in an over at 110°C for a week, see if it has degraded).

Originally Posted by Diabolical Artificer

Parcel tape is better than nothing but it is a bit thin, tears easily and the glue isn't brilliant. Kapton tape is not that much thicker but is tough and sticks well.

Kapton tape as used by transformer industry is commonly available in 0.001" thickness and also 0.002".

One slight down side of Kapton is, it's high-temperature capabilities means that it needs an adhesive which does not degrade at high temperatures. That limits the chemistries available to the glue-makers, and what is often used is silicone-based adhesive which alt6hough it does stick quite well, does not have the adhesive properties of some of the lower-temperature glues. So, OK for interwinding insulation, but if you use Kapton as final wrap, and dunk it in varnish, expect it to all come loose.

[Ed_Dinning]

Hi Gents, Kalee is the specialist here and the notes should be followed.
If you used Grade 2 insulated wire (double enamel layer) then inter layer insulation for most voltages is not needed, but must be used between windings.
As said, the suitability of tape depends both on the type of tape and the adhesive used. Enamel is usually tough stuff, but the solvents used in non approved tapes may attack it and it can allow the tapes to move with higher temperatures. Even molten candle wax can give problems on some tapes used for outer wraps.

Ed

[Davewantsone]

Just a thought. Has anyone tried the white plumbers tape?

[retailer]

I have some yellow tape in various widths that was sold to me by a company that supplies electrical insulation materials I believe it is mylar.

[orbanp1]

Hello Everyone,

Many thanks for all the replies!
Certainly food for thought!

Wikipedia lists polypropylene (PP) as thermoplastic, with a melting point of 160C.
The fact that PP is also used as insulator in film caps is encouraging though.

I will be checking the various sources for kapton tape.

As mentioned, I did use previously teflon tape as transformer insulation, originally made for plumbing. It was quite tricky to apply as it is so thin, it was hard to use it without any wrinkles.
I also got a bit thicker teflon tape now, also for plumbing, but have not looked at it so far.
Testing the packing tape in an oven sounds also a good idea (though not sure what my wife would think).
I would be worried if the transformer would be warmer than 50C - 60C or so during regular operation...

Thank again for the comments!

Regards, Peter

Handy hint when applying PTFE tape, put the supply reel "backwards" then it has to unwind rather than follow, then you can control the tension by mere finger pressure.

I will try to describe it, assume right handed person...
Object to be wound in left hand, horizontal in front of you.
Reel of tape on right hand index finger with tape coming off the top front, i.e. it dangles away from you.
Hold tape on object.
Wind clockwise looking from the right hand side, i.e. over the top, under the bottom.

Once you get the trick it seems impossible you did it any other way.

[kalee20]

Originally Posted by orbanp1

Wikipedia lists polypropylene (PP) as thermoplastic, with a melting point of 160C.

The fact that PP is also used as insulator in film caps is encouraging though.

Melting point is not the key number, unfortunately.

Polypropylene is a soft plastic, and yields under pressure at temperatures a lot less then MP.

Kapton, Polyester/Mylar, etc don't. So the wires won't gradually penetrate the insulation if you use these.

But, as you point out, it is an excellent insulator, and moreover, has very low AC loss, important in a capacitor. (Used in a rolled-up capacitor, there are no pressure points to worry about).

[Ed_Dinning]

I would beware of plumbing PTFE tape as it tears and punctures very easily

Ed

[Bazz4CQJ]

Taking an ultra-cautious view, when Teflon (PTFE) gets hot enough, it produces some extremely hazardous decomposition products. You would never expect the transformer to get that hot, but it is conceivable that under fault conditions, it might happen.

B

[Jac]

The well known (?) yellow 3M insulating tape is polyester.

Jac

[Diabolical Artificer]

I can let you have a bit of the 3m yellow tape Peter, running short of kapton else I'd send that, no one seems to be posting at present on ebay, should be able to wind a bit on a wood former and pop some in the post. Drop us a PM if interested

Andy.

[kalee20]

Originally Posted by Bazz4CQJ

Taking an ultra-cautious view, when Teflon (PTFE) gets hot enough, it produces some extremely hazardous decomposition products. You would never expect the transformer to get that hot, but it is conceivable that under fault conditions, it might happen.

It does, and it could. For this reason, PTFE insulation is banned for certain applications: confined space, limited ventilation possible, people present, simply because of the consequences of an equipment fire.

In a domestic situation, the minute amount of PTFE present, is unlikely to be a problem though.

And PTFE has super temperature capabilities. I have used it in transformers operating at 220°C (size and weight more important than efficiency - it really does run this hot!!) using Kapton sheet insulation, PTFE-insulated leadouts, and 240°C-rated enamelled wire (10x more expensive than the 'commodity' stuff). It works really well and does not degrade or harden over time.

[orbanp1]

Many thanks for the further clarifications!
Also thanks to Andy for the kind offer from to send me some tape!
Heeding the advise, I did order some wide (50mm) capton tape from eBay.

Regards, Peter

[orbanp1]

Hello Everyone,

Just as a follow-up, now I remember the name of that transparent foil that was used in the transformer for insulation.
I think it is styroflex, or polystyrene.
When I was a youngster (that was way back), I used that foil for making coil forms for making windings on ferrite rods, for my first transistor radios that I built.
Checking out the physical properties of that material, it has an even lower melting point than polypropylene (100C vs. 160C):
https://en.wikipedia.org/wiki/Polystyrene

Here is a picture of that loopstick antenna, I did find it not that long ago. I am now putting together an AM radio from a TCA440 board and LM386 board that I have, just for fun.

Thanks, Peter

[kalee20]

Good comment.

Polystyrene is low loss, rigid, stable, lots of good properties... But temperature and solvent resistance are not its strong points.

Use in RF coils, but not in power transformers!