Tuesday, November 08, 2005


A different way of making variable inductors

From a note by Ed Tanton, N4XY, to QRP-L 08 Apr 2003:

"OK... I took one of these [surplus SMT] inductors and measured it at 525nH (0.5mH) [...] and brought the inductor near the 'face' of the 1.5' x 3.5' x 5/16' (3.5cm x 8.9cm x 0.8cm) magnet. Interesting: the inductance changed to ~ 200nH (0.2uH). Then, I moved the tweezers/inductor to the end of the magnet. Wow. The inductance went from its usual 525nH to 43nH (0.043uH)!!!"

Merton Nellis, W0UFO, commented:

Yes, inductors with magnetic material as cores change inductance from a maximum with no biasing magnetic field to a lower value as a bias field is applied. [...] The bias field can be applied to a core with a separate winding carrying d.c. rather than with a permanent magnet. This principle is used to make saturable reactor controls and magnetic amplifiers.

Mounting a magnet near a toroid with a screw for adjustment would let you build variable inductors without having to locate a slug-tuned coil form. Also, it could be used to build an SMT variable inductor with an adjustment big enough to be useful. Attaching a man-sized knob directly to a eensy SMT inductor is begging for the leads to be torn off the board.

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