slide_capacitor

Slide Capacitor

Designing a Worm Driven Capacitor for Slow Tuning VFO

This is a "work-in-progress" instead of a finished design. It is published here because others may find it interesting and want to do a parallel design effort.

I am also working on several variations of PTO design. This screw-driven variable capacitor was a side thought developed from that effort.

Initial pictures show how I built the first prototype. It is not beautiful, but does work pretty much as anticipated. Next steps are to use thinner dielectric for increased capacitance range, and to scale it up a bit in size for more mechanical robustness and also to increase the maximum capacitance.

At this point I feel the concept has been proven, and others should be able to duplicate my effort. You could make this from a section of single-sided PCB material with the copper etched off one half of it. The slide plate could be either tin-plate from a can, or another small square of PCB material. Capacitance is determined by plate size, spacing, and dielectric constant of the insulating material.

Final thoughts on this project are that the concept could be used to make multi-plate variable capacitors or possibly differential capacitors.

The idea is for a threaded rod to drive the grounded plate forward and back above dielectric separating it from the ungrounded plate. This provides a slow-tuning capacitor with excellent resetability to specific values.

This is the insulated plate view.

In this prototype version the adjustment range is 10 to 30 pf. in 33 turns of the drive shaft.

A Delrin (Plastic) end plate has been added and the tensioning method changed to just pressure on the front panel. The threaded rod seats in a hole bored half way through the plastic block. Tuning is smooth, with no backlash.

Because I bent the movable plate down on the edges to eliminate any slop in adjustment, now the added air gap has reduced the total capacitance. Tuning range is straight-line 4 to 12 pf in 33 turns.