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In Reply to: RE: Transformer Orientation posted by Triode_Kingdom on June 01, 2025 at 08:48:18
Do the same "rules" apply (90deg) if a choke/IT/OPT is in close proximity to a power transformer but separated by an aluminum plate of a chassis?
Follow Ups:
An aluminum plate can provide attenuation from the power transformer, but there are many variables. Minimal coupling really requires the components to be at 90°. I wouldn't place an IT next to a power transformer under any conditions, other than maybe the use of a thick mu metal shield.
I'll keep all the recommendations in mind. Thanks for mentioning mu metal.
I do have sheets that I never put to use, I believe 3M... it's on the thin side so I'll maintain the 90 rule.
I think you might be referring to 3M Ultraperm 80. I've used that myself, but it's only moderately helpful. Like you say, it's thin (.004"), and that just isn't enough to be very effective at 60Hz. The last time I had a coupling problem, I wrapped the output choke (under a cathode follower) with Ultraperm. It attenuated 60Hz at the output about 6dB. That wasn't enough, so I removed it and installed an aluminum plate instead. That reduced the hum about 10dB to 12dB. I didn't try using both, but probably should have.
I suspect mu metal needs to be at least .020" to .030" thick to provide good shielding at these frequencies. I once worked on a RF project where we made the entire chassis and all the sub-compartments from mu metal. The goal was to push unwanted EMI (including 60Hz and 120Hz) into the noise level, which was about -170dBC. IIRC, the panels were about .080" thick. The boxes weighed a ton, but the shielding was excellent.
Mu metal is strain sensitive. If it becomes mechanically strained, it loses permeability. It still shields, but you don't get "max perm". Steel also works well and isn't as expensive. If I remember the curves correctly (don't have them anymore) a "dropped" sheet of mu metal has lower perm than a dropped sheet of steel.
At work, I had them replace some mu-metal shields that were (randomly) not working to spec with steel shields. Problem went away. The "bad" mu-metal shields all looked like they were a "little bent" to me, but looked fine to every else.
Play safe and play longer! Don't be an "OUCH!" casualty.
Unplug it, discharge it and measure it (twice) before you touch it.
. . .Oh!. . .Remember: Modifying things voids their warranty.
"Mu metal is strain sensitive."
Ultraperm is sold as a product that can be bent and shaped. I haven't read anything on the 3M site or elsewhere to indicate that it loses effectiveness when used this way. Maybe it does though, or maybe it's an alloy that alleviates this condition? Would be interesting to know more about this.
The name Ultraperm is used in the past by the German Vacuumschmelze company.
However, they don't have Ultraperm 80.
Looking at the site of 3M I couldn't find ultraperm 80 either.
Vacuumschmelze used ultraperm with a "standard" nickel alloys, this means it is crystalline based. bending means you destroy the permeability.
For good magnetic shielding you need the old fashioned Mumetall and as thick as possible. Of course annealing if you have to shape it in the final form.
You're right it's no longer shown on the 3M Website. No idea why. That's odd, considering "3M Ultraperm 80" is sold all over the Internet. Even Walmart lists it. I guess we have to rely on the specifications published by resellers now. None of them advise against bending it, as far as I know. (Not that this means anything.)
Ultraperm is a brand name of Vacuumschmelze, probably that was a problem for 3M with their Ultraperm 80.Anyway, as far as I can see, Ultraperm 80 is not a very good shielding product to shield a transformer.
* Unknown datasheet, unrealistic.
Edits: 06/11/25 06/11/25
"Ultraperm is a brand name of Vacuumschmelze"
Just curious, do you know when they copyrighted or trademarked this name? 3M sold Ultraperm 80 for many years.
"Ultraperm is a brand name of Vacuumschmelze"
"Just curious, do you know when they copyrighted or trademarked this name? 3M sold Ultraperm 80 for many years."
I know that Vacuumschmelze has been using the name Ultraperm for over 50 years, but I don't know if they have rights to this name.
Thanks for clarifying. 3M doesn't appear to still offer the same product under any name, so perhaps it was just a marketing decision. I have several sheets of this that I bought at a good price about 15 years ago. It was much less expensive than the prices I see now online. I don't know why that is, or even who is making it (maybe it's all old stock). In any event, this material does work to attenuate line noise from transformers, but the effectiveness is limited. It's mostly useful as a "last mile" sort of thing when just a little more cleanup is needed.
I am not 100% sure this was ever a 3M product. I think the 3M branding is simply from the adhesive and is not indicative of the overall manufacturer. Put another way it could simply be a 3M adhesive attached to a Vacuumschmelze nickel by a third party.
There was a large amount of this specific material available from multiple surplus sources about 25 years ago at like $6 a sheet in packs of 100. People started buying them 100 at a time and flipping them on ebay for $20-30 a sheet about 20 years ago and as the surplus market dried up, the ebay price went up.... some kept the price the same and cut the sheets in half.... ahhh shrink-flation.
dave
Didn't think it was quite 25 years ago, but I did buy four or five sheets at about $4 each. I think it came from Electronic Goldmine, which also sells PC drill bits and such (my main need at the time). Anyway, I'm sure I remember looking this up on the 3M site and finding specifications that appeared to describe it as a product of theirs. They do still sell EMI absorption materials, but not this one. It's a mystery :)
Google led me to ad ad for Ultraperm, which said "The alloy contains 80.3% nickel, 14.3% iron and 5.4% molybdenum." I assume that Molyperm is the same thing. 20+ years ago we (Bottlhead) had a power supply transformer in a phono preamp that induced hum. The manufacturer resolved it with a copper flux band overlayed with a layer of molyperm tape.
I don't know which wrap was more effective, but together they worked well.
Thanks Paul, good info!
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