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Common Mode Choke question

Discussion in 'Electronic Basics' started by [email protected], Nov 8, 2006.

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  1. Guest

    Hi everybody,

    I have a 1mH, 10A common mode choke. Can I use this as an inductor with
    the same specs?
    If yes, how? Like (a) connecting one winding and leaving the other open
    or (b) paralleling both.

  2. Phil Allison

    Phil Allison Guest

    ** No. It is a essentially low current inductor of 1 mH.

    A genuine 1mH, 10 amp inductor will have a laminated iron core and be about
    the size of your fist.

    ....... Phil
  3. Guest

    Thanks for the reply. But I don't understand. How is it a low current
    inductor? The choke uses a toroid and is about 25mm(dia)x20mm.

    Please be kind,
  4. Phil Allison

    Phil Allison Guest


    ** Go look up "core saturation" on Google.

    A " common mode " choke avoids the problem by having equal and opposite
    currents in two coils.

    A genuine, high current choke must be of reasonable core size and have an
    air gap in the magnetic circuit.

    ......... Phil
  5. Sorry, no. A common mode choke is wound such that the load
    current (the one rated at 10 amperes) trough one coil
    magnetizes the core in one direction, while that same load
    current, returning back out the other winding, magnetizes
    the core in the other direction. So the fields of these two
    currents cancel, preventing the core from saturating, and
    producing very little inductance.

    The device shows 1 mH only for current that goes through one
    or both windings in the same direction, so that the fields
    do not cancel. In line filter applications (where these
    devices are usually used) this common mode (same direction
    in both sides)current is very much smaller than the load
    current (that goes in one winding and out the other).

    For instance, the common mode current may be a result of
    voltage that is in common on both line wires, like signals
    picked up from local AM stations by the line wiring acting
    as an antenna, or generated by diode switching or switching
    regulator operation on the load side.

    The core of a common mode inductor is generally a solid,
    high permeability ferrite (with no air gap) to maximize the
    winding coupling and inductance, since the common mode
    signals are generally very much smaller than the load
    current. They are really a form of transformer with two
    equal windings.

    Normal mode inductors (two lead devices) generally are made
    with distributed gap powdered iron material that takes a lot
    more turns to produce the same inductance, but are
    saturation resistant, because of the nonmagnetic gaps in the
    flux path that store energy when a magnetic field is forced
    through them.
  6. Greg Neill

    Greg Neill Guest

    If you can get at all the leads there should be
    no reason why a suitable cross connection couldn't
    effectively parallel the windings.

    * *
    1----+ +----2
    ) (
    ( )
    ) (
    ( )
    ) (
    3----+ +----4

    In common-mode configuration the input connections
    are to terminals 1 and 2, with the load connections
    being 3 and 4. Current entering at 1 will return
    via terminal 2, so the fields oppose and cancel.

    Connect 1 & 2 together and 3 & 4 together to form
    a two terminal device and you should end up with
    the fields reinforcing rather than canceling.
  7. Yes, the zero current inductance will be about 1 mH, and the
    windings will handle about 20 amperes. Unfortunately, the
    core will saturate with something like a fraction of an ampere.

    You could also connect 2 to 3, and between 1 and 4 you would
    have about 4 mH zero current inductance, and the windings
    would carry about 10 amperes. but the saturation current
    would be about half of the first case.
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