Common mode choke definitions

Discussion in 'General Electronics Chat' started by tmac, Nov 20, 2012.

  1. tmac

    Thread Starter Member

    Oct 28, 2008
    10
    0
    Hi there!

    I have a question about common mode chokes. I found a 8mH 3.2A common mode choke, and I wonder what inductance and current specs I will obtain if I wire it as an inductor in different ways.

    As I understand it, the common mode choke lets a differential current pass unhindered by arranging the coil orientations so that there is no net induced magnetic field within the choke, thereby presenting a very low inductance. Common currents would, on the other hand, induce net magnetic fields in the core, thereby experiencing a non-zero inductance.

    But I'm not sure how the inductance or current tolerance of a common mode choke is defined... Is the inductance simply defined as the inductance measured in one coil while the other is disconnected? Does the current specification mean that it can support a common current of 3.2A, or a differential current of 3.2A, without the core saturating and the wires melting?

    The specs ought to state independently what amount of common current and differential current that the choke supports, right?

    Thanks!
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,023
    3,236
    I think the inductance is with both windings in parallel. The current is the differential (normal) current through the choke. It's limit is determined by winding resistance and temperature rise of the choke.
     
  3. tmac

    Thread Starter Member

    Oct 28, 2008
    10
    0
    Thanks!

    In that case the magnetic field in the core is not strong, so core saturation is not really the primary issue for the common mode choke current spec.

    On the other hand, core saturation will influence the amount of common mode current that the choke can suppress (i.e. the amount of noise that it is designed to suppress). But that is probably much lower than 3.2A, which explains the small size of the thing. It seems to be quite a bit smaller than a typical 10mH 3.2A inductor.

    Essentially, it means that the current spec is probably much lower if I wire it as a simple inductor, so I have to look elsewhere...
     
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