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Dots on inductors

Discussion in 'Electronic Design' started by Raveninghorde, Nov 26, 2012.

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  1. I read many years ago (National app note from memory) that one should
    connect the inner layer of an inductor to the switch in a buck
    regulator and the outer layer to the capacitor as the outer layer will
    then act as a screen.

    I don't know if it makes a difference in practice but it's free.

    With many modern inductors it is impossible to see how they are wound
    so the only indication of "polarity" is the dot. Is the dot arbitary
    and down to the winder or does it actually relate in anyway to the way
    the inductor is wound?
     
  2. legg

    legg Guest

    It indicates phasing alone. Constructional details, where critical,
    will only show on the winding sheet.

    You won't get the winding you want without including winding
    instructions in the transformer drawing and then batch inspecting each
    purchased lot.

    RL
     
  3. SoothSayer

    SoothSayer Guest


    The dot is for phasing of a TRANSFORMER. In such a case, it indicates
    the winding START point. In many cases, it is already part of the
    bobbin. In some cases, it gets painted on as part of the winding process.
    There is no need to mark the winding start of an inductor.

    An inductor is a non phased device.

    No, it matters not which 'way' it gets utilized. AT ALL.
     
  4. Joerg

    Joerg Guest


    Nope, no standard there. Fastest way to find out is crack one open and
    look. Watch out for ferrite shrapnel. The gentle method is to measure
    the RF leaking out with one end grounded and the other energized, then
    reverse. Then talk to the mfg whether their production procedure (SOP)
    calls out to always connect in this particular way. Typically they are
    but it can be tough to obtain something in writing about it.
     
  5. Bill Sloman

    Bill Sloman Guest

    Wrong. If the outermost layer of a multilayer winding is connected to
    ground at one end, the inductor as a whole will radiate appreciably
    less than it would if one end of the outermost layer were connected to
    a high AC voltage.

    It only the stray capacitative coupling that changes, but that can
    matter quite a lot.
     
  6. I've just checked with a German manufacturer and they confirm the dot
    on their inductors is the start of the inner winding.

    I appreciate not all manufacturers will necessarily be as consistent.
     
  7. Joerg

    Joerg Guest


    Great, then at least you found one where they have a system to it. If
    this is for a product design I'd still get that in writing though.
     
  8. legg

    legg Guest

    It is highly unlikely that a dot will occur on a magnetic drawing that
    has only one winding. Inductors with multiple, tapped and
    phase-coherent windings do exist and are constructed with all of the
    precautions required for transformers. The are called inductors simply
    as a convention to indicate their intended use.

    Every line-powered computer you've ever seen probably has one of the
    more common examples; a common-mode choke to assist with EMC
    compliance. Such a part becomes unpredictable (if not completely
    useless) if phasing or winding instructions are ignored.

    There are many intentional constructional details in magnetic
    components that are not immediately obvious, even to the experienced
    technician, that can determine their effectiveness in many ways.

    RL
     
  9. Joerg

    Joerg Guest

  10. What they said by email was:

    The dot signifies the start of the winding on the inductor, thus this
    would be the inner layer of the windings which would connect to the
    hot side of the output of the DC/DC converter. The outer winding of
    the inductor in effect self shields the emi on the inner windings.
     
  11. Joerg

    Joerg Guest


    Perfect, then you got it in writing. I thought it was just a phone
    conversation across the Channel.
     
  12. legg

    legg Guest

    Interesting; both of them. At a quick glance, they are unique in the
    range of both manufacturers.

    For Coilcraft, the 'dot' isn't actually associared with either solder
    junction, but appears equidistant between the two......so you'd have
    to crack one open to figure out what they mean, and hope that the
    meaning isn't subverted in the part's sales lifetime. It's not
    guaranteed that the coil winding personnel have even a working
    familiarity with the language of the spec......

    The instruction 'winding direction' is unfortunately ambiguous in the
    English language, as winding is both a noun and a verb. When
    indicating prefered wire layout, I always indicate the direction of
    the coil former's rotation (ie the verb sense of the winding machine's
    function) and illustrate the movement of the physical part, to avoid
    this (hopefully....).

    RL
     
  13. Joerg

    Joerg Guest

    It's not that unique, even in the distant lands of China they do it:

    http://xfmrs.com/pdf/2xfs1m.pdf

    But it's always good policy to obtain written confirmation about the dot
    and winding orientation.
     
  14. legg

    legg Guest

    and, of course, there was actually a noticeable effect on the EMI
    plots, when checked........making the effort worthwhile?

    There'a difference between anecdote and application. There's a
    difference between a 200W toroid and a shielded bobbin core.

    The costs of playing it safe, just in case, can whittle a budget past
    the bare bone.

    RL
     
  15. Joerg

    Joerg Guest

    It already applies at much lower power levels. I finished a flyback
    design a few weeks ago. If you get the winding orientation wrong on
    those then all hell breaks loose at the EMC chambers.

    The word "shielded" in ferrites is often more a marketing term. A closer
    looks typically reveals a fairly large potting area. The potting
    compound is colored almost the same as the core.
     
  16. Joerg

    Joerg Guest

    True, but fairly easy to test when you have a near field probe kit. That
    kit is probably the most worn tool out here. Other than the bottle
    opener :)
     
  17. legg

    legg Guest

    The function of the rf-conductive core body has also to be taken into
    account in the radiation process. Capacitance to the core from
    different portions of a winding (or windings) has to be taken into
    account, as well as possible core body and screen current paths.
    A shielded bobbin core, on the other hand, specifically indicates the
    employment of a sleeve structure with a controlled gap, whether SMD or
    through-hole. This controlled gap is where the filled adhesive is
    present.

    RL
     
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