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RM 10 cores

Discussion in 'Electronic Design' started by CampinGazz, Feb 18, 2004.

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  1. CampinGazz

    CampinGazz Guest

    I need an RM 10 core that can handle 500 KHz it seems, this is for that
    inductor in the sepic psu i've built, where a 300KHz RM10 core sings, and
    i've got bad efficiancy,

    The coil is 2 windings of 13 turns with 0.7mm copper wire (enamaled), and i
    need a .003 inch air gap between the 2 halfs of the RM10 core,

    RS components sell 2 RM10 cores that can handle 500KHz, but they are
    adjustable cores,

    What exactly is adjusted.. as in why.. to change the inductance, core
    saturation level etc??
    i can see there's another part of the core that's is screwed down the centre
    of the hollow main core, so by doing that it looks like i can't have the air
    gap i need? is this correct, or am i wrong again.

    I.e. can i use an adjustable RM10 core for an aplication that requires a
    ..003 inch air gap?
     
  2. Usually adjutable cores have a tiny ferrite screw in the hollow
    center. It allows to adjust the inductivity by a few percent.
    They are used for LC filters to adjust the passband and stopband
    attenuation.
    Unless you need to tune the inductivity, there is no need for an
    adjustable core. I doubt an adjustable core has a 3 mil gap.

    You best get a selection of cores and go through some tests.

    Rene
     
  3. R.Legg

    R.Legg Guest

    These core sets are pre-gapped, with a ferrite-tipped rod part that is
    adjusted to bridge the gap. Remove the rod entirely and you almost
    have an RM10 with a fixed gap - no spacer needed if it's the right one
    - .006 gap required if concentrated in one location (your .003 gap
    actually occurs twice in spacer-forced gap assembly). The largest
    fixed gap in an adjustible RM10 is 0.9mm (.035), the smallest is
    0.15mm (.006) - so you'll have to pick the smallest gap (highest
    gapped L) version with a book Al of 630nH/root-turn.

    One problem may be the core material, which is unlikely to be a
    low-loss power conversion type - more likely chosen for easier
    fabrication, highest ungapped permeability and possibly tempco (though
    this is probably only critical in the unused original rod part). It
    will be identified in the part number.

    In a DC inductor with moderate flux swing, a low loss material may not
    be required. If you have some idea of the duty cycle or %ripple, it
    would help determine this. The fact that the part was ringing audibly
    does not neccessarily indicate that high AC flux will be present, at
    the conversion frequency (which is inaudible) in a normal, stable
    circuit.

    RL
     
  4. CBarn24050

    CBarn24050 Guest

    The easiest way to get a small gap is to use a shim.
     
  5. CampinGazz

    CampinGazz Guest

    got no probs doing the gap, i use a stick mailing label,
    it's just getting an air gap with an adjustable core, which it seems i can't
    do,
    looks like i'll have to order this one item from farnel or someplace else, i
    have a lot of other stuff to get from RS, so adding another core would have
    been no problems.
     
  6. qrk

    qrk Guest

    Beware of shim idea. It works, but also allows magnetic goo to ooze
    from the core. This can couple into other circuitry and cause
    problems. It will also cause problems if you need to pass radiation
    tests.

    Your audible noise is probably due to stability issues or pulse
    skipping. 300 kHz is way above human hearing ablity, so, going to 500
    kHz won't do anything unless it changes your feedback loop.

    Mark
     
  7. R.Legg

    R.Legg Guest

    Gapping material should be stable and not be degraded by heat.
    Adhesives can form a considerable portion of the initial effective
    material thickness, and will flow and deform under pressure.

    In a commercial application, you'd want virgin sheet stock of a
    recognized material of known flammability. For breadboarding, paper
    may be adequate - but there's no guarantee that a mailing label even
    uses paper as the stock material.

    RL
     
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