Using a Pot to Tune a Coil

Nope. To illustrate : It is RF right ? Make it work with 100 uF. Ain't happening.

Sounds logical to me. I will try it on the bench using a range of component values and frequencies.

Thanks to everyone who respondied to my OP.

Kevin Foster

MaryJane might work better...

PERFECT! Do not think you missed any methods other than stretching coil or moving a metallic end cap (which changes the capacitance).

I disagree. I made a copper shielded coil resonant in the FM band and a moveable (aka "variable-geometry scheme") end-plate for tuning over the whole FM band; strictly hobby-type construction. Yes, that is still a "DC" frequency. 500MHz is not that much higher, so the dimensions are a little less, but the idea and scheme is not that different. And if one wants the Q to be in the 1000-plus range, a little silver plating goes a long ways.

Rotating one coil between two others (with the whole set wired in series) lets you tune the inductance over a range L+M to L-M.

Cheers

Phil Hobbs

In the times gone, it was called a variometer.

Right, I used to know that. (I found the place I forgot it out of.) ;)

Cheers

Phil Hobbs

Parallel would work too!

I want that in a surface-mount package, the size and shape of a trimpot.

With low TC.

Cheers

Phil Hobbs

Use a ferrofluid core with a separate DC coil to change its shape.

Cheers

Nowadays, you can 3d-print one, of course. As for 'low TC', you don't need that, you just get compensating temperature coefficient capacitors to resonate it with.

I'm gonna claim that anything you come up with, is surface-mountable if it can stick to conductive glue-dots. And 'size and shape of a trimpot' can mean almost anything; I've seen trimpots with quarter-inch screwdriver adjust shafts that come with a 2W rating.

I'm really tired of doing that. The NTC caps have to be special ordered, by the reel, and take forever to arrive. Nasty to iterate on.

And I assume that a variable inductor might well have a variable TC.

Cool. Post some pics of your PC boards.

One proposal here, to generate isolated power for multiple acquisition channels, is to have a motor on the PC board, driving multiple generators along an insulated shaft. All pick-and-place, preferably.

And 'size and shape of a trimpot'

The old spherical TRF radio variometers were often beautiful, and generally huge.

On Mon, 15 Jun 2015 11:36:12 -0400, Phil Hobbs Gave us:

Hey man... I smoked some POT this morning and I had no trouble tuning this antenna coil!

(not really... there was no POT involved).

I did smoke a POT in an LED driver circuit though.

On Mon, 15 Jun 2015 09:06:23 -0700, John Larkin Gave us:

Maybe it will work with a negative resistance POT.

People do sell variable delay lines, and they do tend to look like pots, like old A-B pots with a shaft, or like the long flat PCB mount pots with the screw thungie on one end.

I guess a wirewould pot is a variable inductor.

Or a variac!

And no resonances.

Common-shafted trimcaps. Printed in.

Well, if the dots are *big* enough...

A systems integrator I useta work at incorporated this US$700.00 high-pow er-capacity brake-clutch controller into its conveyor system products. One day my company received a letter from the manufacturer telling us they were no longer going to sell the controller because they couldn't source the hu ge green-enameled wirewound screwdriver-adjusted pots in them any more. The owner of my company buttonholed lowly me to design a transistorized replac ement for the controllers, one that we could make in-house.

The original controller came in a ten-pound foot-long punched-and-bent st eel chassis like a 1950's High School radio shop kit. My prototype (made up out of what was on my bench) used two PFETs, a few resistors, and two tiny ten-turn trimpots on a scrap of PCB. It fit in the hand and costed out at about twelve bucks (plus two days of my bench time). The largest soldered c omponents were the terminal blocks for the solenoid and PLC connections. Th e production version was a little bit more robust, but not much.

I got a nice bonus.

Mark L. Fergerson

Auxiliary to #2, a brass or Al core will lower the inductance. For ex.:

Piglet already provided one. If the coil is part of an active stage you can always use a pot to change the threshold / bias so it switches sooner or l ater.

If all you care about is winning a theoretical argument, there is a simpler option. Take 2 coils with differing resonant f, and use the pot to select one or the other, or a mix of both. Q will be a mess, but if you only need a narrow f range it might be acceptable.

NT