Making an EMI Proof Inductor

My app uses a 1uH inductor. Problem: It's going to be near some other power inductors with strong E and M fields and I'm concerned about interference by E or M coupling.

Guessing... Make the inductor with a pot core? Maybe put foil on the pot core? Put the inductor in a ferrite box covered with foil? Perhaps use something like those IF transformers found in AM FM radios? Put the inductor in a grounded steel box?

Is there something in production?

D from BC

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D from BC
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Making it a toroid and shielding it would be the standard fixes that first come to mind. If the shielding itself is of a magnetic material (steel, ferrite, mu-metal, etc.), you're likely going to have to fiddle a bit to get the inductance value dead-on, if that's important to you. Might not even really need "shielding" per se, though.

Bob M.

Reply to
Bob Myers

Toroids with astatic windings have no external magnetic field and are insensitive to external magentic fields.

In theory, a pot core is topologically identical to a toroid, but the slits for the leads mean that the external field rejection isn't as good.

Shield against electric fields with a box around the toroid. If you put a can over a toroid and a saucer underneath it, don't clamp them together with a metal bolt running through the central hole in the torod, because that creates a shorted turn - nylon bolts are fine, but a couple of metal bolts or clips out side the toroid are even better.

If you can get away with a pot core, you can get pretty good electric field shielding with a ground copper foil screen wrapped around the outside of the coil - overlap the ends, but keep them separate with a bit of insulating tap (you really don't want the screen to form a shorted turn here either). It will push up the stray capacitance of your coil to ground, but I think it reduces the parallel capacitance of the coil - I was never in a situation where this was important enough that I had to measure it, which means that my opinion on the subject isn't all that reliable.

If you want chapter and verse, Ralph Morrison's "Grounding and Shielding Techniques" is the book to read. It's up to its fifth edition now, published in March 2007 ISBN-10: 0470097728, ISBN-13:

978-0470097724.

The earlier editions that I bought for various employers over the years were largely aimed at 50Hz and 400Hz interference. The fourth edition - which I bought for myself - does have a chapter on RF which is useful, if not up the standard of the low frequency advice. Amazon gives the table of contents for the fifth edition and it does seem to have a lot more on high frequency techniques.

-- Bill Sloman, Nijmegen

Reply to
bill.sloman

What's the application? Frequency? A little coupling may not matter.

There are lots of shielded inductors in Mouser and Digikey.

John

Reply to
John Larkin

Try something like this:

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Digikey has them. The traces connecting to it may actually pick up more than the inductor itself. If that's a problem I'd place a shielding can over it. The regular cheap thin metal stuff.

--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

Astatic winding? I haven't heard of that... Does that mean wound normally?

Neat... a toroid has no M coupling to another toroid.. :)

The nearby power inductors are toroid. So that helps. But, the inductors also resonant a bit and put out small 'dings' at 25Mhz. It's broadcasting. I guess that would be EM(RF) radiation as opposed to just pure M.

That means the little 1uH inductor just needs shielding from RF and E field. The grounded foil should help with both. I'll watch for shield capacitance..

Thanks

D from BC

Reply to
D from BC

"D from BC"

** That's tiny - only 10 turns on a 0.5 inch former.
** There will be no such problem.
** You must be off your head.

....... Phil

Reply to
Phil Allison

I'm trying to decide whether to experiment for tolerated interference or just foil the inductor to dodge experimenting.

Being really lazy, I like dodging experiments :)

The inductor is near an interference source with this spectra: An RF (25Mhz sine) and E field only (1Mhz square).

The little inductor is for a butterworth low pass filter.

((()))) Vin--R1-----L---+---+-----Vout (((()))) | | C R2 | | G--------------+---+-----G

R1=R2 is not known yet but will be between 2 to 200 ohms.

Vin is sine around 500Khz 14mV. Tolerated interference on Vout is ~1.75mV Fc ~ 2Mhz to 5Mhz.

It's all up to proximity and how strong the interference sources are. So, I'm planning on foiling just to be sure. :)

D from BC

Reply to
D from BC

Ok..maybe I asked a dopey question but my posts are still better than penis enlargement spam.. :)

D from BC

Reply to
D from BC

Probably not. If you wind a toroid with single layer winding in the simplest possible way, you create the equivalent of a loop of wire in the plane of the toroid.

This is discussed in Kibble and Rayner's "Coaxial AC Bridges" ISBN

0-85274-389-0 in section 4.2.1 on designig transformer windings.

One way making an astatic winding is a two layer winding, where your first layer goes only halfway arond the toroid, the stops and you start the second layer going back on top of the first layer, then keep on going until you have got back all the way around the coil, passing your starting point on the way. When you got to poit where you intially turned around, you turn around again, and wind your way back to the start, so that there is no loop running all the way around the toroid, and no external magentic field, and no sensitivity to any magnetic field theading the toroid.

The "coaxial AC bridges" discussed are used in standards laboratories, mostly at relatively low frequencies, and the "coaxial" refers to the way they are made completely shielded. The book is out of print in the conventional sense, but you could still buy a copy from the British National Physical Laboratories at Teddington in England for 45 UK pounds a few years ago.

It doesn't appear on their web-site any longer

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but you could e-mail them to find out if it is still available.

Amazon doesn't have one.

-- Bill Sloman, Nijmegen

Reply to
bill.sloman

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