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Ferrite bead on LM1084 adjustable regulator

Discussion in 'Electronic Basics' started by Dummy, Nov 20, 2003.

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  1. Dummy

    Dummy Guest

    The regulator will be used in an environment which is exposed to RFI.
    I would like to put some ferrite beads on the regulator PCB. Any
    comments? Should I put the ferrite bead at the 3-terminal regulator
    input, which is 12V or regulator output which is 8.5V? Any criteria in
    choosing ferrite bead? Max load current is 5A. Thanks.
     
  2. Robert Baer

    Robert Baer Guest

    Well, if you thing a bead will help, then one at the input and one at
    the output leads.
    Better yet, use SMD regulator and SMD capacitors close to the leads
    and forget beads near the regulator; use RFI shiiielding and suppression
    at widget electrical lines going thru the box.
     
  3. Ban

    Ban Guest

    Dummy wrote:
    || The regulator will be used in an environment which is exposed to RFI.
    || I would like to put some ferrite beads on the regulator PCB. Any
    || comments? Should I put the ferrite bead at the 3-terminal regulator
    || input, which is 12V or regulator output which is 8.5V? Any criteria
    || in choosing ferrite bead? Max load current is 5A. Thanks.

    A good idea, but keep the small decoupling capacitors close to the
    regulator, the bigger electrolytics can go behind.
    .-------------.
    | |
    ___ | Regulator | ___
    o--+-----UUU--+----| |----+-----+---UUU-o
    | + bead | | | | | bead
    ### --- | | .-. ---
    --- --- | | | | ---
    | 0.1u | '-------------' | | | 0.1u
    | | | '-' |
    === === | | ===
    GND GND +-----------+ GND
    | |
    | .-.
    --- | |
    --- | |
    | '-'
    | |
    === ===
    GND GND
    created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

    ciao Ban
     
  4. The beads are probably best placed on the input lead, before the high
    frequency bypass capacitor. Keep in mind that most beads saturate
    with am anp or two DC, so you would probably be better served by some
    wound coil rated for the max. current than a bead. The series
    inductance will increase the regulators tendency to oscillate (at the
    frequency where the coil and bypass cap resonate) so oversize the
    bypass cap, to lower the resonance to where the Q is lower.
     
  5. Dummy

    Dummy Guest

    Any problem by sprinkling more than 1 bypass cap,
    0.1uF//51pF//100pF//680pF at input and output?
    My idea is to bypass the RF freq from HF to UHF. I know the caps won't
    be covering a large range like that, but it worths a try right?
    Then there will be multiple resonant frequencies.
     
  6. Probably no problem, but I doubt that it is worth using more than 2
    caps. A 1 uf stacked film and a .01 uf film should do it.

    If you want to eliminate frequencies not handled by those, you will
    probably have to do it with a feed through cap that passes through the
    box that contains the supply.
     
  7. Robert Baer

    Robert Baer Guest

    More is *NOT* better.
    Use *one* capacitor close to the regulator at the input, and *one*
    capacitor close to the regulator at the output.
    If you are really worried, add a bead at the wire going to the input
    capacitor bypass, before the regulator (ie: *not* between the cap and
    the regulator).
    If the current is high enough that one turn (wire passes thru bead
    *once*) may saturate it, then get a toroid ferrite of same lossy
    material, or use a carbon composition resistor, or a flat film resistor,
    as ordinary film resistors are inductive.
     
  8. Robert Baer

    Robert Baer Guest

    Stacked design, whether film or ceramic are good; this includes the
    chip caps as well.
    If one is to use the old flat, round ceramic cap, then wrap copper
    foil around it and solder the foil to the ground lead: the foil acts
    like a shorted turn and can extend the resonant frequency to near 1GHz.
     
  9. Dummy

    Dummy Guest

    What are the drawbacks by putting more bypass caps at the input and output?
     
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