Making inductors adjustable?

Sorry, no. The magnetic field is almost completely contained inside the core. If you pass anything through the hole, it acts as another turn, and 1 turn's worth of voltage will be induced in that material from one side of the hole to the other. You would have to cut a gap in the core or break it to alter the total magnetic field and, so, the inductance.

On Tue, 2 Dec 2003 7:14:35 -0800, John Popelish wrote (in message ):

Thanks, John.

So, let's say I cut a thin channel in each of the toroids, right between the leads. How can I put these two together to make an adjustable inductor?

Thanks,

Interesting Idea

Carefully cut a bare toroid in half taking care to polish the ends smooth. (not sure it can be done with less than a diamond tile cutting saw)

Wind two coils on each half, phase properly and connect in series, and find some way to vary the gap and you'd have a variable inductor.

There's bound to be lots of easier ways to do it though. There used to be adjustable pot cores around in the 70's, they may still make them. UTC had a line of high/variable inductance coils on the market.

In the dark ages when TV's had long CRT's, there would be powdered iron half-toroids that went around the deflection coils. Great for varying inductance in sizes useful to the audio frequencies.

What were you trying to achieve; what would it be used for?

You would have to physically vary the thickness of the air gap. Toroids are not very well suited to such things. Gapped pot cores with a screw in ferrite plug that crosses the gap are available in some sizes.

Adjustable inductors have air cores with an adjustable plunger to affect the inductance. Typically these can be purchased as cans from companies like muRata. Toroids absolutely are not adjustable.

Oops I have to correct that.... I do recall once seeing a "center tapped" inductor that looked very much like a toroid rheostat however that was a special build and I haven't seen another since.

FWIW, variable inductors like you describe can be found at any AM Radio transmitter site and are used for tuning the antenna array.

They are typically air-cored and can be quite large, designed to handle the full output power of the transmitter...but that's a job for a plumber, not an electrician ;-)

Cameron:-)

I recall that the h-size control on old CGA monitors was almost always an inductor wound around a hollow plastic core, with a threaded ferrite slug inside that core, which could be screwed in and out using a plastic Allen key. Is that not a variable inductor of the type the OP is trying to make?

No, the OP and the response you quoted were talking about a toroid...

It would be helpful to know the application and inductance value, adjusting range, max current, AC, DC bias etc.

Thoroid are ill suited to "magnetic" adjustment, however you can tap the turns either discretely or imitating a variac style of winding.

MG

Need to create a series resonant circuit for several different capacitive loads. In other words, I need a variable inductor which can be attached in series to a capacitive load and tuned to resonance. Average capacitance is about 1500 pf. Inductor should be adjustable, oh about 10 percent?

Forget about the toroids. They were just my starting point, after looking in my "whatzits" box to see what I had. I can buy surplus, new, or hopefully, wind my own with some helpful instructions from you knowledgable folks. I have a few surplus winding bobbins with adjustable slugs. I just need to know inductance needed for resonance, turns of wire required, etc. to make an inductor.

Thanks,

What power level and frequency range?

tHAT IS not exactly true. Depending on the amount of the total bh loop you are using, if it is small comparte to the total bh loop a little dc will move you aroung the loop and you can often vary inductance that way. . . I DO NOT FOLLOW MANY OF THESE NEWS GROUPS To answere me address mail to snipped-for-privacy@aol.com

On Tue, 2 Dec 2003 18:49:07 -0800, Michael A. Terrell wrote (in message ):

less than 1A (probably less than half that); 20KHz; 40 KHz; 60 KHz; 80 KHz -- a different inductor for each F (I presume an adjustable one couldn't cover that entire range...)

Thanks,

Here's one method that hasn't been explored by the "experts". Move a bismuth disc or cylinder closer to the inductor to be varied. I found that this works fairly well (and also you can make a wide range variable inductor by supergluing a piece of ferrite or dust core and a bismuth cylinder on top of each other).

-A

What about a tapped inductor? Would that suit your needs? It's not adjustable, but the inductance is selectable via a rotary switch - or by moving an alligator clip. (But you'll have to change your frequency - see below)

Frequency: - at those low frequencies LC is not your best choice. To begin with, you would need a MUCH greater capacitance to make it workable, and even then you'd neet a LOT of inductance - unless my math is off. I figured a

If you can go to 2.0, 4.0, 6.0 and 8.0 mHz instead, then LC works well.

On Fri, 5 Dec 2003 23:08:18 -0800, snipped-for-privacy@bellatlantic.net wrote (in message ):

Is L in Henries? C in Farads?

Thanks,

The 1/f turn-to-frequency relationship is cute. Of course DaveC can't do fractional turns, but I'm sure the nearest whole turn value will be close enough.

Thanks, - Win

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