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,
