Where to scavenge mu-metal?

"Arfa Daily" wrote in news:e5fTh.3134$ snipped-for-privacy@newsfe6-gui.ntli.net:

Ah,yes,I remember TEK 650 series monitors having a metal CRT shield.

--
Jim Yanik
jyanik
at
kua.net

"Long Ranger" wrote in news:UvlTh.136717$ snipped-for-privacy@newsread2.news.pas.earthlink.net:

that was SCMOO-metal.

--
Jim Yanik
jyanik
at
kua.net

My bench power supply's transformer is coupling hum into my breadboard. Yes, I can move my circuit farther away, but I'd rather solve the problem (short of replacing an otherwise good supply) than the symptom.

It's a 70's vintage lam transformer with half a dozen secondary windings. I'd prefer a toroid, but it will be cheaper to simply shield this one.

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Sparky

Mu-metal probably won't help. A copper shorting strap around the transformer would. You can make that out of sheet copper, or even bare wire and a lot of solder.

Or, um, move it farther away.

John

Flashing material can be obtained at almost every major hardware store. But not the Al stuff, as that won't solder...

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Regards, Joerg

http://www.analogconsultants.com

My understanding is that it should be high-nickel content metal, such as is mu, and that without that content magnetic fields aren't much effected by metals.

Or does solder serve as the poor-man's nickel?

Sounds like DIY mu-copper :-) You say "strap" which I interpret as not full-height of the transformer. Should it be less-than the height of the lams? Or is taller better?

Isn't that a lot of wire (a tertiary winding)?

Wrap the sheet around and bolt it under the feet of the xfmr? (Only need to shield in 1 direction, really: forward.)

Yeah, thanks for that, John... :-)

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Sparky

Can use Al if I don't need to solder? Or is it solder that makes it effective against the magnetic field?

Thanks,

--
Sparky

foot

Hey, Michael - just because _you_ can't figure out which name I'm using, doesn't mean that _I_ can't. ;-)

I use the different names to distinguish between whether I'm being technical or goofing around.

Some people don't afford such simple considerations. :-)

Thanks! Rich

As soon as you cut it or otherwise stress it, mu metal becomes worthless.

-- Many thanks,

Don Lancaster voice phone: (928)428-4073 Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552 rss:

email: snipped-for-privacy@tinaja.com

Please visit my GURU's LAIR web site at

Had a big roll we used to do a couple of things. Of course, after bending i had no way to measure its effects. It still did the job. Had to shield some rather large boxes of about

4 cubic feet, and I shielded a power supply. The supply was open on one side, but the shield really hepled out out 60 Hz noise getting into sensitive photodiode amplifiers. I think we also got some samples of different materials and different thicknesses, but I think it cost something. Probably close to 10 sq feet.

greg

Al won't be very good but to be of some help against EM fields it would at least need to be riveted. After a while it'll oxydize though.

Think of it this way: In order to muffle a magnetic field coming from the inside you need to short out any induced currents. A work of caution if you ever do that around toroid transformers: If there is a metallic stud that holds them in place never, never let a shield touch that. It might result in molten metal sparkling all around you.

--
Regards, Joerg

http://www.analogconsultants.com

It goes right on the outside of the lams, and around the windings, in the same direction as the windings, like this:

It acts as a shorted turn; it has something to do with "leakage inductance".

Cheers! Rich

Yep, that's it. Just dawned on me where to buy the sheet stock: Hobby shops or arts and crafts suppliers. The ones that cater to artists who build stuff like Tiffany lamps or do other stained glass work. That's where you can also buy copper tape, brass sheets and all that.

--
Regards, Joerg

http://www.analogconsultants.com

Not necessary to ground it?

Is wider (as wide as the windings) best? Dimension confusion...

Thanks,

--
Sparky

then

you

Quote from the text in your link (in german) "Aussparungen zur Leitungsführung oder Befestigung sollten daher sehr vorsichtig und ohne Verbiegung oder Verspannung der Gehäuse vorgenommen werden, um optimale Abschirmfaktoren zu erreichen." Openings to pass conductors or for bolts should be made very carefully and without bending or stressing the shield, so that you have maximum shielding factor. So you're right, you're not allowed to bend or stress it.

-- Tzortzakakis Dimitrios major in electrical engineering mechanized infantry reservist dimtzort AT otenet DOT gr

High MU metal is not a particularly rare material. I assume you are attempting to shield something from a low frequency magnetic field. You may find the cast-iron door of an old stove or wood furnace will be all you need.

That transformer in the picture is a ferrite cored one from a switch-mode power supply, and is designed to run at a frequency of around 100kHz. A copper band around such a tranny, is a whole different kettle of fish to putting one around a low frequency power transformer. Some such trannies have a copper shield right around the windings, inside the lamination limbs, but if I recall correctly, this is more for electrostatic shielding purposes, than magnetic. The first thing I would try, before getting all complicated with mumetal shields, is to rotate the tranny through 90 degrees in the horizontal, and if that doesn't work, doing the same in the vertical, or any combination. I've found that this will often reduce or even eliminate stray field pickup by adjacent circuitry. The tranny mounting holes may even still line up in the best case, but even worst case, it's only a case of drill 4 more, or make up a right-angle bracket.

Arfa

Not if your problem is magnetic field leakage.

Think of it as a single, shorted secondary turn but *outside* the laminations. Any field that leaks out of the transformer induces a current into the strap, and it in turn generates a field that fights the excitation. It should be thick, wide, and well-shorted, which means well soldered. Copper is best.

John

Are you a solipsist, Rich?

Regards Ian

;-)