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Choosing a ferrite bead

Discussion in 'Electronic Design' started by Mike Noone, Jan 10, 2006.

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  1. Mike Noone

    Mike Noone Guest

    Hi - I was recently reccomended to use a ferrite bead to get rid of noise
    on a very noisy supply line. I have never used a ferrite bead so am
    unfamiliar with their usage. The supply line is a 6VDC line and it is
    supplying a GPS module that consumes approximately 70ma. The noise is
    caused by 18 motors running on the same 6V line. I don't care about the
    motors getting a noisy supply but I've been told I need to keep noise under
    100mV on the GPS supply. Can anybody tell me how to choose a ferrite bead
    for this application? I don't have any measurements for just how noisy this
    6V line is as the board is still being designed! So what that in mind, I'd
    like to error very far on the side of caution.


  2. Here's some decent information about using them:

    I'm not sure how much they will help you though, since you're not really
    talking about RF frequencies, but they may help tame the spikes. Make
    sure you have bypass caps on your motor terminals and use some bypass
    and filter caps near the GPS module.
  3. Mike Noone

    Mike Noone Guest

    Oh I should have bypass caps on the motors? I thought bypass caps were
    only for things that were sensitive to large loads. Any idea how large
    of a bypass cap I should use? The motors peak at about 200ma at 6VDC.

  4. Guest

    make sure theyre ceramic plate, not wound types.

    Another approach is to put your gps supply through a diode and
    reservoir cap. The cap function is obvious, half the noise is
    eliminated by the cap staying at 5.4v when the noise is down-going
    (cant think of a better word!) and positive going noise has to face the
    diode resistance then the cap.

  5. Clark

    Clark Guest

    First I would use .047 - .0 uF caps across the motors and then power the
    Gps unit through a choke with a 47 - 100 uF cap across the Gps unit.
  6. Ferrite beads are used to block RF frequencies. The manufacturers
    usually supply a graph of effective impedance versus frequency and the
    limits of DC that will saturate the bead.

    But I am worried that your motors are making lots of noise well below
    the frequencies that are effectively attenuated by a ferrite bead (or
    bead on a lead). I think you need a wound inductor on the order of a
    millihenry. You will still need high frequency bypassing where the
    motor current is drawn from the 6 volt supply and also a fairly large
    capacitor after the inductor that has a low impedance at high frequencies.

    Did you see my last post in your LDO thread that gave specific part
  7. Ideally you'd put caps from each motor terminal to the case of the
    motor. If this is just not possible, at least put one accross the power
    leads right on the motor terminals themselves. Try .1uF ceramic caps.

    Here's some decent advice, but be carefull about heating the motor case
    to solder to it.

    Heres some more stuff on bypass caps in general.
  8. At the points where each motor drive circuit makes connections to the
    6 volt bus and ground, you should bypass across the supply with a
    capacitor that acts as a local supply to keep the current pulses to
    the motor from having to wander around the supply and ground buses.

    For the motor current you mention, I might use something like a
    Panasonic series FC, 470 uF @ 10 volts, 0.117 ohm E.S.R., part number

    A 200mA current step will produce only a 20 millivolt step on step on
    the bus in the short term. That sort of step will be fairly easy for
    the series inductor between that and the GPS supply line to suppress.
  9. Rich Grise

    Rich Grise Guest

    About .001 uF (1 nf) disk ceramics, right across the motor terminals.
    With a high enough voltage rating, of course. It keeps the commutator
    hash from going back to the supply lines. Maybe one from each motor
    terminal to frame, as well.

    I don't know much about ferrite beads, but I'd say get the biggest
    ones you can find.

    Good Luck!
  10. Rich Grise wrote:
    Have you checked, lately, how big you can get them?
  11. I have.


    Very big.

  12. 3 1nf ceramics per motor should be possible. They should be able to be
    squeezed in. Will definitely be a pain to install - but no part of this
    project has been easy - so why let this part be? Argh

  13. Hi John - those are some big caps! I haven't finished work on the boards
    that will connect to the motors - I'll try to fit those on, but not sure if
    I'll be able to. Definitely might have to go smaller. Do there exist
    smaller (footprint-wise) capacitors that would work as well? Price is not a
    major concern.

  14. Hmm - I don't expect RF frequencies on the line. I expectnoise more from
    the combination of a DC/DC switcher (in the khz range I believe) and
    from the motors (I'm guessing 50 hz range - but I could be wrong). Are
    there other types of beads besides ferrites? I was told by the maker of
    the GPS module that a 'bead' would be beneficial.

    If I am reading your post correctly - you're suggesting a 1mh wound
    inductor connected to the +6v line. Bypass cap(s) would be attached to
    the other end of the inductor and ground. The input to the GPS module
    would be then connected to the capacitors. For the capacitors I'm
    thinking maybe a 10uf tantalum and a .1uf ceramic (as I already have a
    large number of both on the board - why not add a couple more?). For the
    1mh inductor - I really don't have any idea how to choose such a part.
    Could you reccomend one that is as small as possible?

    I missed your post in the LDO thread (I have now seen it though). Google
    groups has been acting up and never mentioned it to me. I have since
    switched back to my trusty ol newsgroup server and will no longer be
    missing posts. After reading everybody's posts here I think I will
    actually make an attempt to just smooth the 6V line and forget about
    using an LDO. Would the inductor you mentioned in that thread
    (CDRH127/LDNP-102MC) be a good fit for this application?


  15. The bigger the cap, the lower the frequencies remaining in the bumps
    left by motor operation. I am just guessing how well you need to
    filter the 6 volt supply. You may get by with just removing the
    higher frequency components of the bumps. In that case, a low E.S.R.
    multilayer ceramic surface mount capacitor may be enough. This might
    be as small as an 0805 surface mount cap like a X5R 10 uF, 10 volt
    unit, like Panasonic ECJ-2FB1A106K. I would avoid the Y5V and F
    types, even though they are available in higher capacitance per
    volume. Their E.S.R is higher.

    Do you have a scope to look at how well various options work?

    A lot depends on how well you lay out the circuit board. When you get
    to that point, I would be happy to discus principles and criticize
    your artwork.
  16. We agree on this.
    There are beads of various dimensions, and some with multiple passes
    through the ferrite, for an effect that is like several beads in series.
    Look up part 28C0236 on Digikey. But they are still intended to
    attenuate only 10s of megahertz.
    That's right.
    Are you using surface mount parts or through hole?
    It took 2 days for it to show up.
    I think it would. It is fairly small for the energy stored. You may
    be able to find a slightly smaller part in through hole in an axial
    would part (like a resistor with wire wound around it), but they spray
    field out all over the place, where the part I suggested keeps almost
    all its field inside.
  17. Rich Grise

    Rich Grise Guest

    No, actually, I haven't. I know cores can be had in practically any
    arbitrary size - at what point does it cease to be a "bead" and
    start to become a "core"? :)

    And everybody knows I'm too lazy to do what looks like somebody else's
    homework - I even try to duck my own. ;-) But, here's where I'd start:

  18. Rich Grise

    Rich Grise Guest

    In addition to the other suggestions, if at all possible, split the two 6V
    busses right at the power supply - that way the power supply filter will
    help keep the motor noise out of the GPS. Also, split their return lines
    the same way.

    A ferrite bead is just a little torus of ferrite that strings on the
    power lead just like stringing beads on a string. It adds a little
    inductance, and presents a high impedance to higher frequency noise.

    But with only 70 mA, you might even use an ordinary RF choke - it might
    make it easier to assemble, but only you can determine that. :)

    Good Luck!
  19. how about

    ugh, wrap probs, or something like this

  20. Paul Mathews

    Paul Mathews Guest

    Add this to your toolbox: Provided that you have adequate bulk bypass
    capacitance near the GPS module, you can add series resistance in the
    6V supply line to the GPS. This can be VERY effective at quite low
    frequencies. To decide how much resistance to use, determine a voltage
    drop budget for filtering. Suppose you decide that you can spare 200
    millivolts. That means that you can add 200mV/70mA = ~3 Ohms. The
    combination of 3 Ohms with your bypass capacitors can provide a lot of
    filtering, without resorting to an inductor. The power rating
    dissipation of the resistor is P = I^2*R = 15 mW, so you can use a
    really small part. IF the gps module needs a long surge of current at
    some point, the resistance can be a problem, and you'll have to use an
    inductor instead.
    ]I agree with the other posters who suggested that filtering spikes at
    the motors is your first priority.
    Paul Mathews
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