I'm partially rewinding a toroidal transformer just for the pure thrill of controlling the output voltage. I'm sure I'll get over it, but for the moment that prospect is pretty exciting for some reason :)

I'm leaving the 120V primary in place (I removed a second 220V "extension" primary winding) which is good because it's about 1500 turns. I'll wind a new secondary or two and then wrap it back up.

On the outside and between the primary and secondary windings, the transformer is wound with a clear plastic tape (no adhesive backing) that is 12mm wide and 3 mils (0.003") thick. I'm sure it performs a mechanical protection role and given its inter-winding appearance an electrical insulation role as well.

toroidal transformer showing winding "tape"

I notice the tape is pretty stiff (not floppy or stretchy to speak of) and holds the kinks from its original winding position pretty clearly. It occurred to me that it might be some sort of heat-shrinkable tape that stiffens and holds bends once it's been shrunk. That would make some sense I suppose because the shrinking would insure the winding underneath was tightly bound and less likely to become noisy or whatever.

Does anyone know what this stuff is and whether it's available in modest quantities for prototyping purposes etc.? Or perhaps whether it's okay to reuse it?

  • \$\begingroup\$ It probably differs among manufacturers what exactly they use, but you could easily figure out if its heat shrinkable by heating it up, it surely didn't reach its minimal length yet \$\endgroup\$
    – PlasmaHH
    Commented Jan 7, 2016 at 20:35
  • 2
    \$\begingroup\$ @PlasmaHH - If it were heatshrink it wouldn't be taped in place. At a guess it's mylar. \$\endgroup\$ Commented Jan 7, 2016 at 20:37
  • \$\begingroup\$ @PlasmaHH - Ooh, great idea. I'm off to fire up the heat gun! :) \$\endgroup\$
    – scanny
    Commented Jan 7, 2016 at 20:38
  • \$\begingroup\$ @WhatRoughBeast: not if it was a big heat shrink tube, but if it was heatsrhink tape, you would not want loose ends. From the images low resolution and bad color depth it is hard to determine what is taped where. \$\endgroup\$
    – PlasmaHH
    Commented Jan 7, 2016 at 20:39
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    \$\begingroup\$ @scanny: If it's heat shrink, why would it not already have been? My vote, since it's clear, is that it's polyester (Mylar) tape which is slightly heat shrinkable, although that's not generally a property which is important except when building electrostatic loudspeakers and, perhaps, capacitors. \$\endgroup\$
    – EM Fields
    Commented Jan 7, 2016 at 22:50

2 Answers 2


Ok, I think I pieced the answer together from the comments I received, I'll lay out what I found here for anyone else that may find this on search.

I'm very confident the material is Amide-modified Mylar heat-shrinking tape, commonly used for inter-winding insulation and outer covering of toroidal transformers.

enter image description here

I found a supplier on eBay, which was the only supplier I could find anywhere, although I gave up looking after about 20 minutes :)

The pictured item is 40 yards of 3/4" width, 5 mil thick, generally used on toroids somewhat bigger than the one I'm working on at the moment. Here's the link to that item in case anyone is curious.

According to DuPont, the dielectric strength of (their) mylar is about 4kV/mil at this thickness, so half-lapped and double wound as mine was, it provides a heck of an insulating layer, almost 50kV nominally.

I ended up reusing what I unwound from mine, here's how it turned out:

enter image description here

I ran over it with the heat gun afterward and it tightened right up the few edges that were not quite flat. I can't tell the difference between this one and the others I have that are factory fresh. Except for the yellow mylar tape which I went a little crazy with; I was worried it wouldn't hold when I shrunk it again but that didn't seem to be any problem at all. I'll probably pull most of that off before I wind the secondaries; might as well keep it looking professional :)

Apparently the way you cure this stuff is by putting it in the oven at 320 degrees F for an hour. I have a thermal fuse wound in that trips at 105C so wasn't sure I wanted to do that :)

Anyway, that's what I found out. Hope that's a help to someone. Many thanks to @PlasmaHH and @WhatRoughBeast and others for their helpful comments :)

  • \$\begingroup\$ Nice work with the Mylar. Did you use a shuttle for winding? \$\endgroup\$
    – gsills
    Commented Jan 9, 2016 at 3:50
  • \$\begingroup\$ Thanks @gsills :) - I did at the start. It was slippery stuff and tricky to keep it from "erupting" off of the shuttle. I missed the grab when there was about 1m of tape left. After that I just pulled through the unwound tail and that worked well enough. I expect the fresh stuff is a bit easier to work with :) \$\endgroup\$
    – scanny
    Commented Jan 10, 2016 at 2:08
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    \$\begingroup\$ Thanks to your post I was able to find this. DuPont calls it DuPont HS65 for the 65-gauge heat-shrink Mylar. Hobbyists have to stick to ebay though apparently. \$\endgroup\$
    – Rag
    Commented Apr 2, 2019 at 23:30

I know this is a long time after the question was answered but I just came across it.

I manufacture toroidal transformers commercially. All the toroidal transformers we make and the ones I've seen from other manufacturers use a clear mylar film for insulation. It isn't heat shrinkable by design or during the manufacturing process. The stuff we use is 50um thick and usually wound with a 50% overlap.

I have no idea where a hobbyist would buy it - I have trouble buying it in suitable quantities as a manufacturer. My supplier wanted me to order 1000 rolls of one size - I negotiated it down to 300 by telling him to throw the rest away (since the material is cheap but the freight is not). Perhaps try asking a local toroidal transformer manufacturer if they will sell you a roll.

  • \$\begingroup\$ Try toroid.com/transformer-kits. They sell transformer winding kits which includes the Mylar tape,and you can probably buy it separately. \$\endgroup\$
    – PStechPaul
    Commented Aug 30, 2023 at 15:47

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