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Why does the inductance go down?

Prodigy 20 points

Replies: 2

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I'm don't understand why the inductance goes down (and freq goes up) when ferrous metal comes near the coil of the demo board.  What am I missing?  Is there something about the structure of a flat coil that I'm not getting?

In Solenoids the inductance always increases when iron is moved into the coil, and decreases with aluminum, etc.  (I found that out years ago when all I had was an exacto knife to shift the inductance of a large coil.)

In the flat coil, I guess it's more like putting the iron up against the bottom of a solenoid.  Even when you put it thru the hole the edge of the traces is probably a minor player in the overall dynamic of what's going on.  The bottom of the coil creates a more complex flux pattern vs the solenoid, where the flux is all the same direction.  I guess the bottom would create flux circulations in all directions as the spiral wraps around and they would produce all kinds of interesting eddy currents that may all be fighting each other causing any kind of conductor to look less permeable than air.  That would probably make for a very frustrated inductor that would have put all that energy into the metal, but not be able to retrieve any of it later. (Kind of like our retirement funds right after the meltdown ;)

Can anyone briefly explain or point me to a link that will help me wrap my head around this?  Uh, the magnetic part, not the retirement part.

Thanks,

  • In short, magnetic field does not penetrate the metal (be it ferrous or not) at 3.5 MHz. Thus, inductance increase due to field magnification in ferrous material is insignificant.

    However, eddy currents are generated in either case, and they have a negative coupling coefficient with the coil thus bringing down the L of the system.

    Your solenoids I bet operated at much lower frequencies, where penetration was much deeper.

    With best regards,

    Evgeny Fomin

    Pr. Systems Architect

    SSP PSP SVA

  • In reply to Evgeny Fomin:

    Thank you for the quick response.  I will sleep better tonight.

    yes, you win the bet.  The solenoids operated at 12KHz and 20KHz. 

    I do intend to try the board using those coils.  Does that mean I should see much more action out of the L chart in my application?  I was (until now) a bit surprised that there was so little change in inductance.  It's the delta-L that we've been using for our signal in the past.  It will be interesting to see how the delta-Prox responds and if I can figure out how to make use of the additional information.

    Will your inductance measurement be as accurate at such a low frequency?  And will that slow down the effective sampling rate much?  Maybe I'll set up an experiment to sweep thru the freq range of the chip just to see what happens.  Maybe I'll get lucky and find a sweet spot that gives me more information than I'm getting now.

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