Where to scavenge mu-metal?

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...

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... :-)

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

Thanks,

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

-- Many thanks,

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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.

Not necessary to ground it?

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

Thanks,

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

On Thu, 12 Apr 2007 16:08:23 GMT, SparkyGuy Gave us:

Make sure you simply twist your output lines from the banana jacks, right up to the circuit under test. Your noise figure should drop a lot on that act alone. A couple turns per inch... not too damn loose or it does no good.

Now *there's* a mental picture that deserves framing!

No need to ground, I presume...

On Fri, 13 Apr 2007 09:01:56 -0700, SparkyGuy Gave us:

Have you formed the leads going to your project into twisted pair yet? I guarantee that will reduce injected noise a LOT.

That is a completely garbage overstatement. Just the same, any tight bend (bending r less than 10 thicknesses) will require annealing in a reducing atmosphere. It is really quite intolerant of cold working if you wish to maintain the magnetic properties. Do not even flex it unnecessarily.