adjustable inductors

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Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>

Looking for ideas on adjustable inductors for a primary.. The idea 
being that you have a turn or two tightly coupled to the secondary, 
and the rest of the primary L is in an adjustable L.

There's rafts of roller inductors around with suitable values, but I 
don't know that they'll handle the voltage, and the losses might be too high.

So, here's some ideas as grist for speculation:

1) Two nested coils, one on a positioner.  As you slide one in or 
out, the mutual coupling changes, changing the overall L of the 
pair.  A modified version has 3 coils, one moving, and two wound with 
opposite sense. The two are stacked end to end, so the position of 
the moving coil determines the effective L.  (think LVDT)

2) A coil within a coil, with the moving coil rotating (a goniometer).

Both of these require brushes or flexible leads, I think

3) A coil with a permeable core that is moved in and out.