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Inuctor and ferrite bead

Discussion in 'Electronic Basics' started by eeh, Mar 31, 2005.

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  1. eeh

    eeh Guest


    Just a question: Is ferrite bead actually an inductor? Or a filter?

  2. The ferrit bead is a core for an inductor, which can be used in a filter,
    or as a filter.

    When you send an AC signal through an inductor the higher frequencies
    will be more attenuated than the low frequencies.

    A loudspeaker filter often consists of an inductor and a capacitor. The
    signal comes to the loudspeaker, is connected to an inductor which is in
    series with the bass speaker, it is also connected to a capacitor which
    is connected in series with the treble speaker.

    The inductor lets the bass sounds through to the bass speaker but stops
    the treble signals. The capacitor lets the treble signal through to the
    treble speaker but stops the bass signals, which would damage the treble

    The inductor in this case has a core of ferrite or iron powder which
    makes the inductor more effective, without the need for many more turns,
    which would increase the resistance and steal power from the bass

  3. Both, really, and a bit more.

    The bead is intended to form a filter, in conjunction with
    the impedances surrounding it.

    The bead can be modelled as an inductor coupled to
    current paths within the bead itself. This makes it a
    lossy inductor, which you can see if you look at the
    complex impedance carefully.
  4. I think it is more correct to see the bead as the core of an inductor,
    the inductor is the wire going through the bead, and it all works a a
    filter, stopping high frequencies and allowing lower frequencies, as all
    inductors do.

    That the core has a shape of a hollow bead or cylinder does not change
    its function as a core for the inductor.

    That the inductor in this case is a piece of straight wire through the
    bead does not change its function as an inductor.

    The core makes the inductance value a lot higher than in another straight
    wire. Iron powder or ferrite cores always make inductance values higher.
  5. Whenever an AC signal travels through a wire it creates a magnetic field around the wire, which effectively makes an inductance of
    the wire.

    This inductance is like many small coils along the length of the wire.

    To increase the inductance we can arrange the wire in a coil, or put iron/ferrite material near it, around it, close to it, to strengthen the
    magnetic field, which increases the inductance.
  6. It is a one turn inductor (the wire passes once through the hole in
    the core). It is usually used as part of a filter. That is, it is
    used as a series impedance that passes most of the desired low
    frequency part and gets in the way of or absorbs some of the high
    frequency part of whatever signal enters it. The absorb comes into
    play because it if a lossy inductor at higher frequencies. Often its
    effect is increased by connecting a capacitor between the signal line
    and ground to act as a load at those higher frequencies, so that the
    impedance of the wire passing through the bead drops voltage.

    Many beads have two or more wires passing through them (or whole
    bunches of wire). The bead couples them all together and acts as a
    filter for any high frequencys that are shared in common. Any
    currents that pass one way through one wire and back out the other way
    through another wire make no net magnetic field in the core, so are
    not affected by it.
  7. Jim Gregory

    Jim Gregory Guest

    I have seen op-amp audio i/ps fed from a source wire not thread just the
    once, but re-looped in and out again 2 to 3 times, through the hole of a
    small ferrite bead. Does this method re-filter its unnecessary HF each pass?
    And is that practice kosher?
  8. A wire is an inductor in itself, a certain number of nanohenries for each
    millimeter of wire. That inductance is so small that we usually ignore it
    and it has no practical effect on the signal, as long as the frequencies
    we are interested in is below the GigaHertz range.

    If we arrange the wire in a loop, a turn, it becomes a coil, and these
    small inductance values multiply and the wire attenuates frequencies in
    lower ranges, like the hundred MegaHertz range.

    If you make more turns, like a hundred turns, it attenuates frequencies
    in the MehaHertz range or even lower, like the 100kHz range.

    Putting ironpowder/ferrite material close to the wire multiplies the
    inductance because it amplifies the magnetic field around the wire or
    coil. This lowers the affected frequencies a lot.

    A straight wire attenuates or affects the gigahertz range, put a ferrite
    bead on it and it affects the hundred megahertz range.

    Using both methods simultaneously, arranging the wire in a coil and
    putting ferrite material close to it, around it, or inside the coil,
    increases the inductance even more.

    Taking the wire several times through the same hole in a ferrite bead is
    to make a coil and to put ferrite around it, inside the coil and outside
    it, which makes the wire into a much bigger inductance, which affects
    lower frequencies.

    One sort of core is called a pot core, because it looks like a pot where
    the coil is put inside a pot of ferrite/iron material. You can put
    another pot core on top of it, like a lid of a jar, so the coil is
    completely surrounded by magnetic material, both inside and outside the
  9. Terry

    Terry Guest


    The wire threaded through the bead (either once or a number of times)
    becomes (or already is depending on frequency) an inductor.

    Inductors in the presence of AC current through the wire have reactance
    (often designated as Xl) which is measured in ohms. The higher the
    frequency the greater the reactance.

    In one sense an inductor alone is not a 'filter'. (Draw a line across piece
    of paper and put some coils/squiggles in the middle of it to indicate
    inductance and it should demonstrate that is it not 'filtering' anything in

    Filters are traditionally thought of as circuits designed to;
    a) Allow all signals/voltages above a certain frequency - High Pass.
    b) Allow all signals/voltages below a certain frequency - Low Pass.
    c) Allow all signals within a certain bandwidth - Band Pass.
    d) Prevent or bypass a certain specific frequency signal/voltage which is
    desired or causing a problem etc. - Often called by some such name as a
    'Stop' or 'Spike' or 'Notch' Filter.

    True 'filters' require design of all the components and impedances involved.
    Filter circuits can be very simple comprising little more than one inductor
    and/or one capacitor, or very complicated with many components.

    However if the requirement is to stop or attenuate higher frequencies while
    allowing lower ones to pass an inductor including one using a ferrite bead
    as the magnetic core could be considered to 'filter out' those higher

    Thus a ferrite bead on the AC power lead of a computer monitor would allow
    the 60 hertz AC power to go through unimpeded. But high frequencies which
    might be causing interference to/from nearby equipment could be greatly

    Inductance 'L' is measured in Henries. Reactance is measured in ohms.
    Frequency is measured in hertz.
    Reactance Xl = 2 x pi x frequency x L.
    Thus Xl at 1000 hertz for 1 henry inductor is; 2 x pi x 1000 x 1 = 6284 ohms
    (Inductive reactance).
    Xl at freq. 20,000hz for a 0.01 henry inductor is; 2 x pi x 20,000 x 0.01 =
    1256 ohms
    At very, very high frequencies the inductive reactance of the wire itself,
    without any extra inductors (beads etc.) can be most significant.

    Any help?
  10. If I read you correctly, your correction would not be
    necessary if I had said "The two terminal device consisting
    of a short length of wire going thru the bead can be ...".
    While I would be hard put to deny the correctness of that
    correction, I doubt that anybody failed to understand what
    object my "can be modelled" statement applied to.
    That is a correction I cannot accept. In response to the
    question, "Is ferrite bead ... a filter?", I wrote:
    The bead is intended to form a filter, in conjunction with
    the impedances surrounding it.
    Your addition of the usually expected gross frequency response
    goes beyond the OP's question. I don't dispute that it usually
    forms a LPF, but that new issue is not "more correct".

    [More new and uncontroverted facts cuts.]
  11. Passing a wire more than once through a core multiplies the effective
    inductance produced by approximately the turns count squared. So 3
    passes gives almost 9 times the inductance and so, 9 times the
    impedance for all frequencies that the device acts as an inductor.
    Unfortunately, it also increases the shunt capacitance that jumps
    signal around the inductor at high frequencies and lowers the
    frequency of self resonance, at which the device ceases to act like an
    inductor. So it helps improve noise rejection or whatever inductive
    effect you may be after at the lower end of the spectrum, while giving
    up lots of effect at the high end.
  12. mike

    mike Guest

    First question to ask is, "What do I mean by ferrite bead?"

    Ferrite bead is a generic term applied to donut shaped pieces of
    SOME material. If you dumped a bunch of electronic componetry on the
    bench and asked a dozen people to pick out the ferrite beads, you'd
    probably get very good agreement among the group.

    But if you aske those same people about the characteristics of those
    devices, you'd get a lot of disagreement.
    The correct answer is, "Where's the spec sheet for this device?"
    has some spec sheets.
    Also ran across this interesting article


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