Alternatives to waxpaper for transformer coil isolation? - Page 1
Forum section: electronics / beginners

author: atos
author_rank: Contributor
timestamp: May 12, 2015, 04:17:59 pm
content: Looking to rewind and fix a transformer. Finding a bobbin with the right dimensions was hard enough, so I made a new one out of presspan. All the original laminates are intact, hence there is no way I'm throwing away something I can fix.Just need to have some kind of confirmation regarding the info I found on isolation of windings.Google told me mylar (of any type) and PTFE-tape should a acceptable alternative to the old school thick wax paper. Checked the material properties and it looks like a proper match.Since I'm doing the primary coil 230VAC as well I don't expect any approval, but for those concerned, it will be enclosed in an insulated IP44 box as well.Also using this as a referencehttp://ludens.cl/Electron/trafos/trafos.html

author: SeanB
author_rank: Super Contributor
timestamp: May 12, 2015, 05:20:17 pm
content: Kapton polyamide tape ( wide enough to do the height of the bobbin) is the thing to use, as a direct replacement for the paper. Mylar tape is good so long as the transformer is not going to run over 60C internal core temperature, otherwise it will melt. If you are going to do it though the polyamide is best, as it will survive the normal operation temperature.For the outer wrap the mylar tape is fine, as it will be cool, but inside you want a better material.

author: Mechanical Menace
author_rank: Super Contributor
timestamp: May 12, 2015, 06:22:58 pm
content: +1 to Kapton. It's what I was thinking when I read the topic title lol.

author: Fsck
author_rank: Super Contributor
timestamp: May 13, 2015, 01:14:42 am
content: Quote from: Mechanical Menace on May 12, 2015, 06:22:58 pmditto. I don't think anyone still uses wax paper for xformers. the polymer revolution happened a long time ago.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 13, 2015, 01:47:58 am
content: Paper is good to 85C or so (class A non-flame retardant etc.). Mylar is good to 105C (class B flame retardant or something like that), whatever the datasheet says (where do you get 60 from..??), and polyimide (Kapton) is good to 200C (class F or something like that, i.e., way more than any other materials you'll be using).Use two or more complete (overlapping) layers of tape between windings (including creepage distance around the edge of the tape layer) and you will have "reinforced" insulation (should pass >2.5kV hipot no problem), which is perfectly safe for use on the AC line.Tim

author: PeterFW
author_rank: Frequent Contributor
timestamp: May 13, 2015, 02:21:24 am
content: Quote from: Fsck on May 13, 2015, 01:14:42 amThere is a whole wing in the audio voodoo asylum were they still preach the gospel of the beeswax sealed, paper wrapped, silver enhanced transformers and coils.Those are state of the art, no way capton and mylar can compare to that

author: Sigmoid
author_rank: Frequent Contributor
timestamp: May 13, 2015, 07:46:42 pm
content: Quote from: PeterFW on May 13, 2015, 02:21:24 amWon't the beeswax result in a BUZZ? xD Or make the sound "stingy"?

author: ConKbot
author_rank: Super Contributor
timestamp: May 13, 2015, 10:39:17 pm
content: Oh here comes the anti-audiophool people to show us how smart they are! lets hear it guys!Seehttp://en.wikipedia.org/wiki/Electrical_insulation_paperandhttp://en.wikipedia.org/wiki/Fishpaperfor 2 types of paper used for electrical applications. As stated in the first article, paper can be de-gassed and absorb oil so it can be used in oil submerged applications, and if youre winding your own HV transformer thats going to be oil submerged, its still a valid method. Polymers are more suitable for dry transformers. If you do use kapton, get the heavier stuff. 2 mil or more, 1 mil, may be suitable, but definitely not 0.5 mil stuff. It should be very orange, or orange brown, the really thin really light yellow stuff would be easier to chafe though. Seehttp://www.iasa.com.au/folders/Safety_Issues/Aircraft_Wire/RAFKapton.htmlfor some information. If you dont have sharp corners on your core (file them, or use a former) And you use enough wraps of sufficiently heavy tape, you should be fine.

author: atos
author_rank: Contributor
timestamp: May 15, 2015, 09:00:55 pm
content: I was told mylar was the thing used in HV caps, hence I figured it would be suitable for transformers as well, especially combined with mylar. Then again the mylar I have is the reflective type, I might have to run some tests. No insulation tester though, would a ZVS suffice? That I could build.Regarding Kapton, yeah, it's probably well suited. In terms of thermal properties and isolation PTFE has way better material properties, and that's what I have in my inventory atm. But I'm kind of ashamed not having Kapton, will place an order on that right away.About having a width matching the coil, what's wrong with overlapping tape layers? Sure the coil gets a bit thicker at one point, but that small difference should be negligible, besides the middle part is the one that will be the warmest point in a worst case overheating scenario.For outer shell and aestetics, one might as well use electrical tape? If it gets overheated wouldn't it be the inner layers that is of concern? Unless it is supernatural in some way, lol. Is it??Thanks for the input guys. Oh about the polymer revolution, the bobbin is another issue. Managed to wreck some vital parts of that. Is injected molded bobbins feasible? If it is, that would be one of the few real uses for 3d-printed stuff I've encountered thus far. Means I have to learn openSCAD... Then again that has been on my todo list quite some time, maybe this is the excuse I've been looking for?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 15, 2015, 09:26:01 pm
content: Injection molded, yes, I don't think any on the market right now *aren't*. Maybe custom one-offs made painstakingly from hunks of G10 and glue?Typical materials range from cheap phenolic to various high temperature, liquid crystal, and/or glass-filled, resins. Nylon probably being the most familiar thermoplastic material, or whatever the stuff is that shrouded headers are made of.3D printing would work, but it has to be highish temperature to be useful. If it starts softening by 100C, it's not very good, and if it's the kind of plastic that gooifies and turns into napalm when it gets really hot... UL isn't going to be very happy with it. (Not that UL has anything to do with 3D printing right now, but hey, rise of consumer printers, rise of consumer-printed parts... you know they're going to have something to say about it *eventually*. And for your own safety, when it comes to line-operated equipment, if that's the sort of purpose this transformer is intended for.)Here's what a good transformer looks like:Classic yellow, 3M Scotch something or other (I don't remember the number, it should turn up quickly on a search though), mylar core, acrylic adhesive. Overlapping layers are fine, as long as the overlap satisfies creepage. (Less important if you're varnishing it, which fills gaps. Maybe.) A layer or a few around the outside keeps the windings safe. And a band around the core keeps it in place.I happen to have a modest supply of that size core and bobbin, for which it just so happens 1/2" tape is the perfect width for wrapping the core. A little wide for two overlapping wraps around the bobbin, but that's okay either slitting the tape or just letting it get sloppy.Tim