Using Magnets for EMI Shielding

Sure. Keeps the electrons in proper orbit >:-} ...Jim Thompson

-- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at

| 1962 | I love to cook with wine. Sometimes I even put it in the food.

Are you sure they were ceramic magnets you saw, and not ceramic shielding material? The ceramic for cores and shielding tiles looks an awful lot like the ceramic in magnets, the difference is that if you want to build an inductor you use a ceramic that doesn't hold magnetization (it's "soft"), where if you want a magnet you use a ceramic that does (it's "hard").

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com

Do you mean apply magnetic bias field to increase permeability? Indeed this could make substantial difference, depending on particular steel.

Vladimir Vassilevsky DSP and Mixed Signal Designs

Yes, they were actual ceramic magnets. They magnetize the entire steel enclosure once applied.

I wonder, in terms of electrophysics, how this might work.

Glen Lewis

Hard drive makers place additional steel crescents on the covers, situated over the platters, for vibrational damping, audio damping, and EMI radiating from the heads, one would presume. Or to keep an external magnetic field spike from introducing an error on the written surface by way of said spike making its way into a head, and onto the platter.

But that is mere speculation.

No field "containment" gains whatsoever, IMO.

That could only make things worse. Likely they were for something else (e.g., microwave circulator).

Tim

-- Deep Friar: a very philosophical monk. Website:

If there's a large magnetic field that's always in the same direction, e.g. from nearby electromagnets or a big SMPS, the magnetic bias provided by the permanent magnets might help prevent the shield from saturating.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 USA 
+1 845 480 2058 

hobbs at electrooptical dot net 
http://electrooptical.net

No. By shifting the B-H curve towards saturation the magnets can only reduce the steel's shielding effect.

--
Mike Perkins 
Video Solutions Ltd 
www.videosolutions.ltd.uk

Very interesting. But I could not find much theory on microwave circulators online.

For example, what would be the effect, on a micorwave beam, of a circular steel plate with a strong magnet at its center?

Glen Lewis

Depending on how thick the plate is, mostly reflection and attenuation (i.e., it's a shield).

Circulators are based on the Faraday effect (rotation of polarization), using a material with a particularly high coefficient (microwave ferrites). The casing is usually steel (helps concentrate the magnetic field), but plenty of magnetic field is present outside the casing (enough to make a screw stand on end).

I have no idea if this is at all relevant to what you're looking at.

Incidentally, circulators depend on physics, and don't have a linear (RLC) equivalent that you might build, say, if you wanted an audio frequency circulator.

Tim

--
Deep Friar: a very philosophical monk. 
Website: http://www.seventransistorlabs.com/ 

 wrote in message  
news:si1mu854c13o1on7leti0tm2431f46fd1p@4ax.com... 
> On Sat, 20 Jul 2013 09:54:42 -0500, "Tim Williams" 
>  wrote: 
> 
>>That could only make things worse.  Likely they were for something else 
>>(e.g., microwave circulator). 
>> 
> 
> Very interesting. But I could not find much theory on microwave 
> circulators online. 
> 
> For example, what would be the effect, on a micorwave beam, of a 
> circular steel plate with a strong magnet at its center? 
> 
> Glen Lewis 
>

This might be true for some steels. IIRR it isn't true for all high-permeability ferromagnetic alloys, and the Wikipedia example graph shows a material with a permeability that peaks at a finite magnetic field.

The first high permeability material on their list, Metglas

has incremental permeability of about 500,000 at 0.3T, but only about 60,000 at 0.01T.

--
Bill Sloman, Sydney

some thing like this?

Place them on your body to cure EMI induced cancer and global warming.

Please study DC bias magnetic recording, it worked reasonably well. Then think about the fields in SMPS very carefully.

?-)

DC biased recording on tape was all about using a more linear part of the BH curve, away from the grossly non-linear part at zero bias.

Perhaps you should study AC bias, which was deemed an improvement?

Hmm, I have thought and I don't see the relevance between hi-fidelity tape recording and hi-efficiency SMPS.

--
Mike Perkins 
Video Solutions Ltd 
www.videosolutions.ltd.uk

Thank you for your concern, but i already understand AC bias quite well.

And the external field from the inductor has a DC bias component. Thus magnets nay help keep thinner (and thus cheaper) shields out of saturation.

HTH

?-)