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Why do they put those ferrite things on computer monitor cables?

Discussion in 'Electronic Design' started by Martin Brown, Mar 16, 2012.

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  1. Martin Brown

    Martin Brown Guest

    To prevent the cable radiating RF interference like an aerial.
     
  2. Guest

    Why do they put those ferrite things on computer monitor cables?
    I'm referring to those round cylinders molded onto the cables.

    Thanks
     
  3. Winston

    Winston Guest

    As a 'choke' to convert 'conducted emissions'
    or electronic noise into harmless heat.
    http://computer.howstuffworks.com/question352.htm

    www.ee.calpoly.edu/~darakaki/Paper2.pdf

    See the chart on the top right corner of page 3.
    Just by adding series inductance, the conducted emissions
    from a switching power supply are quieted down below the
    'compliance line'. (An additional cap on the filter improves
    things a bit more.)

    --Winston
     
  4. They act as a common-mode choke- like putting a little single-turn
    inductor in series with each line of the cable, but using the same
    core. So the inductance impedes net current through the cable, but not
    differential currents between conductors in the cable.

    They put them there so their product can pass EMC tests.
     
  5. miso

    miso Guest

    Shouldn't both sides of the cable get a ferrite choke? I often see
    cables with just one choke and this never seemed kosher to me.
     
  6. Winston

    Winston Guest

    Both share the same core so both sides get a choke, yes?

    --Winston
     
  7. Tim Williams

    Tim Williams Guest

    The radiation impedance of six feet of cable isn't very low, so the
    incremental benefit of the second is much smaller.

    Imagine the wire as a piece of antenna. If it's 1/4 wave long from the
    driving source, it acts as a whip antenna, when the far end is "open
    circuit" (read, sufficiently high impedance).

    If you have a wire grounded at both ends, it can resonate at 1/2 wave,
    basically an inverse dipole (which is grounded in the middle and has
    antinodes at the open ends).

    The quality of the ground matters; a computer monitor would have to be very
    large, physically, to make a good ground against a 6' quarter or half wave
    resonance. Since it therefore does not make a great ground, it develops
    some voltage on its exterior, which means it serves, in part, as the antenna
    as well, and so on to anything connected to it.

    The quality of the wire is most important at the nodes, where current is
    highest. For any arbitrary frequency at any arbitrary position, this is
    difficult to define -- the nodes could be distributed anywhere along the
    wire. However, nodes are almost guaranteed at the ends (where the monitor
    or whatever serves as something of a ground plane, even if a poor one), so
    increasing the impedance there with a ferrite bead does an excellent job at
    dampening wideband resonances, and thus propagation of noise.

    Damping at the antinodes is also possible, but less likely to help, because
    you need a ground to work against, and dielectric loss rather than core
    loss.

    Tim
     
  8. Proves you lack in electronics knowledge.

    The ferrite is a single turn inductor, and the location on the wire(s)
    it is an inductor for does not matter, and there does not need to be "one
    at each end".

    You obviously know how to use the word choke, but also obviously have a
    limited grasp of what it means in electronics, much less the effect it
    has.

    Have you EVER seen one where the idiot put one at both ends? EVER?
     
  9. miso

    miso Guest

    Here ya go, needle dick:

    "Chokes may be needed at both ends of cables longer than about λ/8. "
     
  10. Guest

    My reason in posting this is because I got a LCD monitor at a local
    auction and it came without the cables. The power cable is just a
    common computer cord. But the data cable is what I needed to buy. Most
    monitors have the data cable permanently wired right into the monitor.
    This one dont. The data cable has a male plug on both ends and plugs
    into computer and monitor. Same plug on both ends. The cable I ordered
    has the ferrite on BOTH ends. That's what lead to this posting.

    I will say that every monitor I've ever seen with the permanent data
    cable only has one ferrite and always near the computer end. But the
    cable I ordered has two.

    Thanks for all replies!
     
  11. Jasen Betts

    Jasen Betts Guest


    as it's the same plug both ends they need a ferrite at the other end
    also incase it's installed backwards :)

    most flat panel monitors have detachable video cables, especially now
    that most do both VGA and DVI connections.
     
  12. Stupid audiophool graphtard. Zero emissions data. all simple "series
    resistance" readings. Big deal.

    Hahaha! $10.00 for a 6 foot long printer cable because it has
    micro-thin gold plated connectors and DUAL ferrites? BULLSHIT!

    Almost as bad as "Monster Cable". I'll bet you defend their stupidity
    as well, eh?
     
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