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Re: adjustable inductors



Original poster: Ed Phillips <evp@xxxxxxxxxxx>

Tesla list wrote:

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.


Go with 1) if you aren't going to adjust it a lot. To calculate inductance you need to know the inductance of both coils and the mutual inductance between them. 2) is fine also but a little more trouble to make. Called a variometer - goniometer is usually reserved for an RF equivalent of a resolver used in a DF application. Some of us playing with LF transmission have used both 1) and 2) with good results. Problem with 3) is saturation in the permeable core. [Primary has "lots of volts" and "few turns".]

I have a simple compiled Basic program to calculate Lm. Not as elegant by far as some of the others offered here but works fine for coaxial solenoids, with better accuracy than you can control the dimensions. If you want it I'll send it off line.

Ed