Variable inductor

IIRC it was about 25%-30% of the maximum. They were good for trimming the frequency only.

See TOKO Coils

Mark

I was reading this thread, and was just about to suggest saturating the core with a second winding. This way you can control the inductance electronicly.

Recently I have been playing around with something kind of like this. I have been characterizing inductors to find out where and how they saturate, by putting a DC current in one winding and measuring the inductance on a second winding. The complication is the impedance of the DC current source appears on the measurement winding. To get a decent measurement I have to make sure the DC source impedance is very high relative to the indutance I am trying to measure.

The inductance of the parts I am looking at varies from about 1.5mH to less than 50uH at 100kHz. (About 30-1 inductance change)

There are some tricks to make the DC source impedance less of an issue. These are old magnetic amplifier techniques. You can use matched inductors in series with the DC windings coupled in opposite directions so the coupling cancels out. Something else I have been meaning to try is to put two windings on a flat core on quadrature axis so they don't couple. This is kind of like the variometer with the inner winding turned to 90 deg.

I would also suggest a core with a lumpy shape rather than a torroid. An E core or something. If the core saturates unevenly the inductance will roll off more gradually and be easier to control.

This is all probably more complicated to impelment then what you are looking for, but hopefully is still interesting.

Ethan

The only trick really needed is to make the second coil orthogonal to the inductor coil. For instance, your controlled coil would be wound as usual, through the hole, then the controlling coil wound over the entire toroid. This arrangement allows you to have the two coils almost completely insensitive to each other.

Another method uses two cores canceling each other, with the controlled coil again wound through the holes of each core, but in opposite directions; then the controlling coil wound through the pair. This way, the controlled coil's current causes fields opposing each other in the controlling coil, and vice versa.

John Perry

In article , Ethan wrote: [...]

It also makes a dandy microphone, thermometer, phase of moon and customer nearby detector. I'd avoid this method if you can. It can be real trouble.

To extend the comments a bit: You need two inductors or some funny windings.

• * ---------))))))---------------))))))----------- Inductance ======== ======= ---------))))))----- ----)))))))---- * ! ! * ! ! ---------------!------- Control -------------------

OR:

Get an "E" core and wind like this:

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! !!! !!! !!! !!! !!! !!! A------------ C--------- E--------- ----- ----- ----- ----- ----- ----- B------- !!! D---- !!! F---- !!! !!! !!! !!! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

The (C-D) winding is the inductor. A is wired to F. B-E is the control.

Ken,

Don't you want A wired to E? As drawn, via the right-hand rule, the left and right coils produce flux in the "downward" direction, no?

I'm not sure about that (OTTOMH Ken's right, a) because he usually is, and b) because that looks right), but here are a few books on saturable inductors:

Geyger, Nonlinear Magnetic Devices Storm, Magnetic Amplifiers Attura, Magnetic Amplifier Engineering LaFuze, Magnetic Amplifier Analysis Geyger, Magnetic Amplifier Circuits Platt, Magnetic Amplifiers: Theory and Application Reyner, The Magnetic Amplifier Neeteson, Square Loop Ferrite Core Switching Quartly, Square Loop Ferrite Circuitry

The first two are IMHO the best.

Cheers Terry

Oooops! Yes, I got it backwards. The (ABEF) combination needs to make the field flow around the outside of the E and not through the middle leg.

No, it looks like I messed up in this case. The current in the (A,B) winding and the (E,F) winding should cause fields that cancel in the middle leg. This way, there is no coupling to the winding on the middle leg.

Actually "no coupling" is a bit of an overstatement. They won't cancel exactly and you will see the 2nd harmonic of the signal on the (C,D) winding on the control winding.

It isn't as easy as just winding a DC coil to saturate the core, a properly constructed saturable reactor has double E ferrite cores one winding is on the centre pole and the other is shared between the side limbs, I forget the exact details but its done to eliminate polarisation and prevent asymmetric change in inductance as the signal adds/subtracts to/from the saturating field.

Ken,

I realize I actually have a fundamental misunderstanding of how this is supposed to work. You feed current into the control winding, and since the magnetic field generated in each one points in the opposite direction, don't you end up with zero net flux *everywhere* throughout the coil and therefore haven't changed the net inductance the controlled coil sees?

Or isn't the schematic above intended to produce a variable inductance?

Thanks,

---Joel

Not with seperated cores. You do develop a nasty voltage on the control windings (and thus capacitive loading), though, which is why cancelling flux is a better idea than cancelling voltage.

Tim

What's it for anyway? Maybe there is a way to avoid the requirement for this thing.

Chris

My original query:

" So are variable caps with a 4:1 range.

What sort ot of dumb wank is this ? "

did not extract a satisfactory reply out of the OP..

But did reveal his character.

........ Phil

It's for reactively coupling a 200 Ohm output impedance amplifier, into a 50 Ohm load. Although many interesting comments have been made (and thanks for all of you who've contributed), the acceptable solution is to change the inductor for different frequency ranges.

Good question.

Probably due to the offensive nature of your posting style.

Seems pretty normal to me. Certainly revealed (confirmed) your character.

"Mike Tosser "

One f*****ad is just like another.

....... Phil

The Plessey PR155 series from the sixties used core saturation for rough tuning of a 37.3 - 67.3 Mhz synthesised first local oscillator + varicap for fine tuning. Interesting design in that it used the ssb phasing method in the syntheser and was one of the first (1965) hf receivers to use a hot carrier diode quad for the first mixer, upconverting to a 37.3 Mhz first if.

You could use a small toroid - conventional turns for the oscillator, with turns wound across the outside of the core for saturation control...

Chris

There is an uncanny degree of homogeneity among certain mentalities on NG- very dull.