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Confused about Ferrite Beads

  1. Aug 26, 2008 #1
    I'm currently looking at a datasheet for ferrite-bead style inductors, specifically, http://search.murata.co.jp/Ceramy/CatalogAction.do?sHinnm=BLM31A601S&sNHinnm=BLM31AJ601SN1&sNhin_key=BLM31AJ601SN1B&sLang=en&sParam=blm31aj601 [Broken] . I understand that its impedance changes as a function of frequency and it is primarily used to combat EMI, but how would I determine the inductance value of said inductor? Or is that a parameter that is not normally given on this style of inductor? I know I could set up a test circuit and measure it experimentally, but that's not really an option at this point.

    Thanks for the help!
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Aug 26, 2008 #2
    Actually...now that I think about it a little more, wouldn't I just be able to solve Z=sqrt(r^2 wL^2) and get the inductance that way?

    Edit- No, probably not, because I don't have the purly resistive value. Hmpf.
  4. Aug 26, 2008 #3
    When I was working to pass some equipment through sec.15 in the FCC regulations, I discovered quite quickly that the math varied from reality.
    The design parameters suggested a value but from the actual testing a sometimes very different value turned out to produce the lowest RF leakage.
    The design team would scratch their heads at the data and try to recalculate the values, but the test values were inevitably used in production.

    Fun times.
  5. Aug 26, 2008 #4


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    Staff: Mentor

    EMI supression beads are different from regular inductors. If you just used a regular inductor, it will go through a resonance (where the impedance is max) with the parasitic capacitance, and its impedance decreases after resonance. So you really don't get much in the way of an effective "high impedance" bandwidth with regular non-lossy inductor beads.

    So, special materials are used for EMI supression beads, like material 43 for example. The material is lossy at RF frequencies (so it would make a lousy inductor, BTW), so what you see on the impedance analyzer is a rising impedance with frequency up to about where resonance would be, and then the impedance flattens out for a broad RF frequency range after that before falling off. The "impedance" or "resistance" spec for the EMI beads is usually the value of that flat spot in impedance, with a test frequency specified.

    Check out the Fair-Rite website for more info on EMI supression beads and materials:


  6. Aug 26, 2008 #5
    Thanks! So, in terms of a circuit experiencing resonance, would this mean that there is a wider band of frequencies that will cause resonant behavior than if we used a standard inductor?
  7. Aug 26, 2008 #6


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    Staff: Mentor

    Not sure I understand the question. The EMI bead's impedance plot looks like a series LC with a parallel R damping resistor, which gives the flat-top Z(f) characteristic. You sacrafice peak impedance in order to get a wider bandwidth with a useful impedance (usually in the 100-200 Ohm range).

    Look at the relatively wide bandwidth of impedance for some of these beads:

    http://www.fair-rite.com/newfair/pdf/CUP Paper.pdf

    You don't use EMI supression beads in resonant circuits. You use them as RF impedances to block or divide down RF energy, while still passing your lower-frequency signal energy (like in datacom).
  8. Aug 26, 2008 #7
    Okay, I see. The problem wasn't so much about trying to achieve resonance as much as it 'just happening' and causing unexpected problems.
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