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Making an EMI Proof Inductor

Discussion in 'Electronic Design' started by D from BC, Nov 14, 2007.

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  1. D from BC

    D from BC Guest

    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
     
  2. Bob Myers

    Bob Myers Guest

    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.
     
  3. Guest

    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.
     
  4. John Larkin

    John Larkin Guest

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

    There are lots of shielded inductors in Mouser and Digikey.

    John
     
  5. Joerg

    Joerg Guest

    Try something like this:
    http://specfinder.delevan.com/pdf/sds680.pdf

    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.
     
  6. D from BC

    D from BC Guest

    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
     
  7. Phil Allison

    Phil Allison Guest

    "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
     
  8. D from BC

    D from BC Guest

    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
     
  9. D from BC

    D from BC Guest

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


    D from BC
     
  10. Guest

    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

    http://www.npl.co.uk/

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

    Amazon doesn't have one.
     
  11. neon

    neon

    1,325
    0
    Oct 21, 2006
    look up mu metal
     
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