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A Variable Inductor

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A variable af inductor is described. The inductance is controlled by the movement of a magnet to and from the cores. The industor can be equipped with a calibrated dial. Using two cores through the coil with the flux from the magnet flowing in opposite directions through the two cores results in no increase in harmonics over the amount generated by the same cores with air gaps. This method of inductance control eliminates -electrical backlash.- In other words, the inductance returns to the same value for a given dial setting. Q's of over 100 at 1,000 cps and 30 at 100 cps are possible. Inductance changes of more than 30 to 1 are obtainable. Two limitations have shown up in the experimental work. First, the permeability increases 0.66% per degree centigrade on the ferrite material that is being used; second, the ferrite material saturates at 3,400 gauss, limiting the power handling ability of the coil.

JAES Volume 1 Issue 3 pp. 262-264; July 1953
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Scott Dorsey
Scott Dorsey

Comment posted January 28, 2021 @ 17:41:33 UTC (Comment permalink)

Ingenious idea: a high value ferrite-core inductor that can be moved into saturation by a permanent magnet to reduce the value of inductance.  This acts like a magnetic amplifier but with the control flux provided mechanically rather than electronically.

I am skeptical of the author's claim of 1% distortion at 100mw... it would seem to me that any tiny asymmetry of the core would be magnified as the bias flux is increased and that even harmonic distortion would be a real problem.

It does not look as if this design thread was ever carried on any farther, unfortunately.  It's an interesting design and might have been useful for many things in the days before tunable state-variable filters became available.

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