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# Making inductors adjustable?

D

#### DaveC

Jan 1, 1970
0
I have a couple of toroid inductors. Can I place some solid core through them
to increase mutual inductance? If varying the position of this core, can I
adjust the total inductance?

http://tinyurl.com/xcxe

If this is possible, what can the core be made of? Can it be a common bolt?
Must it be the same stuff the toroids are made of?

Thanks,

J

#### John Popelish

Jan 1, 1970
0
DaveC said:
I have a couple of toroid inductors. Can I place some solid core through them
to increase mutual inductance? If varying the position of this core, can I
adjust the total inductance?

http://tinyurl.com/xcxe

If this is possible, what can the core be made of? Can it be a common bolt?
Must it be the same stuff the toroids are made of?

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.

D

#### DaveC

Jan 1, 1970
0
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.

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,

D

#### default

Jan 1, 1970
0
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?

J

#### John Popelish

Jan 1, 1970
0
DaveC said:
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?

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.

E

#### EEng

Jan 1, 1970
0
I have a couple of toroid inductors. Can I place some solid core through them
to increase mutual inductance? If varying the position of this core, can I
adjust the total inductance?

http://tinyurl.com/xcxe

If this is possible, what can the core be made of? Can it be a common bolt?
Must it be the same stuff the toroids are made of?

Thanks,

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.

E

#### EEng

Jan 1, 1970
0
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.

C

#### Cameron Dorrough

Jan 1, 1970
0
EEng said:
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

L

#### Lewin A.R.W. Edwards

Jan 1, 1970
0
Sorry, no. The magnetic field is almost completely contained inside
the core. If you pass anything through the hole, it acts as another

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?

W

#### William P.N. Smith

Jan 1, 1970
0
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...

M

#### MG

Jan 1, 1970
0
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

D

#### DaveC

Jan 1, 1970
0
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,

M

#### Michael A. Terrell

Jan 1, 1970
0
DaveC said:
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,
--
DaveC
[email protected]
This is an invalid return address

What power level and frequency range?
--
23 days!

Michael A. Terrell
Central Florida

D

#### DaveC

Jan 1, 1970
0
What power level and frequency range?

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,

C

#### coolmos

Jan 1, 1970
0
default said:
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?

These are called gapped toroids. Ferroxcube
(http://www.ferroxcube.com) does offer these.

A

#### Andre

Jan 1, 1970
0
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

E

#### [email protected]

Jan 1, 1970
0
DaveC said:
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,

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
4,000 uH inductor and a 15,000 pF cap would get you areound
20.5 kHz [It's late, I'm tired, so you 'll have to check
my math: freq = 1/(2*pi*sqrt(LC)) ] Those values are not
practical.

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

D

#### DaveC

Jan 1, 1970
0
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
4,000 uH inductor and a 15,000 pF cap would get you areound
20.5 kHz [It's late, I'm tired, so you 'll have to check
my math: freq = 1/(2*pi*sqrt(LC)) ] Those values are not
practical.

Is L in Henries? C in Farads?

Thanks,

W

#### Winfield Hill

Jan 1, 1970
0
[email protected] wrote...
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...)

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
4,000 uH inductor and a 15,000 pF cap would get you around
20.5 kHz [It's late, I'm tired, so you 'll have to check
my math: freq = 1/(2*pi*sqrt(LC))] Those values are not
practical.

4mH and 15nF are certainly practical with the appropriate
technologies. I see that Jameco's data sheet shows their
150uH toroid inductor has 51 turns. Rewinding it for 4mH
requires only 51 * sqrt (4000/150) = 263 turns, which isn't
hard by hand. The toroid core constant is A_L = 57.7 nH/t^2
and I can derive a formula t = (1/f) sqrt(25.33E6 / C A_L)
for the taps that DaveC will need as he winds. Using a
0.015uF cap we get t = 5.41E6 / f and so this table:

80kHz 67.6 turns
60kHz 90.2
40kHz 135.2
20kHz 270.5

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

whill_at_picovolt-dot-com

E

#### [email protected]

Jan 1, 1970
0
Winfield said:
[email protected] wrote...
Michael A. Terrell wrote:

What power level and frequency range?

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...)

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
4,000 uH inductor and a 15,000 pF cap would get you around
20.5 kHz [It's late, I'm tired, so you 'll have to check
my math: freq = 1/(2*pi*sqrt(LC))] Those values are not
practical.

4mH and 15nF are certainly practical with the appropriate
technologies. I see that Jameco's data sheet shows their
150uH toroid inductor has 51 turns.

That's great info - I didn't know Jameco had data sheets
at all. You sent me scampering to the web site, and lo and
behold there it is under Tech Doc when you search on the
choke. Thanks!
Rewinding it for 4mH
requires only 51 * sqrt (4000/150) = 263 turns, which isn't
hard by hand. The toroid core constant is A_L = 57.7 nH/t^2
and I can derive a formula t = (1/f) sqrt(25.33E6 / C A_L)
for the taps that DaveC will need as he winds. Using a
0.015uF cap we get t = 5.41E6 / f and so this table:

80kHz 67.6 turns
60kHz 90.2
40kHz 135.2
20kHz 270.5

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

I think the OP rejected toroids a couple of posts back,
but maybe he'll re-consider. Still, it's a no go. He's
got a 1500 pF average load, and he can't resonate that
with an inductor to the frequencies he wants. He'd need
something like 40 Henry to get his 20 kHz. I was
trying to point out that he would have to have a much
higher load capacitance - at least ten times higher,
and even then he is in the realm of large inductances.
I should have inquired if he could place a parallel
cap across the specified load.

If he's starting from scratch where the load capacitance
can be whatever he wants to make it, then he can go with
far more practical values. (I wouldn't call 263 turns
practical - but that's not really relevant.) I fiddled
around with the values and came up with a .22uF cap and
the following more practical (i.e. less winding, fewer turns)
inductances:

~80 kHz 18 uH (79.946 kHz)
~60 kHz 32 uH (59.959 kHz)
~40 kHz 72 uH (39.973 kHz)
~20 kHz 288 uH (19.986 kHz)
~20 kHz 272 uh (20.566 kHz) !!!!!!!!!

272 = 150 + 72 + 32 + 18 !!!!!!!

The 150 uH inductor can be purchased, and series connected
to a wound, tapped 72 uH - or the 72 uH can be wound for
88 uH, tapped for 18, 32, 72, and series connected with
the 150 uH - all to save on how many turns he has to wind.

If it were me starting from scratch and just experimenting,
I'd buy off the shelf inductors and pick capacitors to get
the frequency, perhaps using a variable cap in parallel with
the series cap.

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