Making inductors adjustable?

[James W]

your timing is perfect. Run, don't walk to your nearest magazine stand
and get a copy of the December QST ( ham radio magazine ).. There's an
article for a home built condensor microphone in this issue.

Simple to build. Works great.. Have fun

- jim

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.

 

[[email protected]]

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?

Yes - L in Henries, C in Farads


This might be helpful if you haven't been exposed to it
before:

1 picofarad (pF) = .000000000001 farads (10^-12)
1 nanofarad (nF) = .000000001 farads (10^-9)
1 microfarad (uF) = .000001 farads (10^-6)
1 micromicrofarad (uuF) = 1 picofarad
1000 picofarads = 1 nanofarad
1000 nanofarads = 1 microfarad
1 "mickeymike" = 1 micromicrofarad

Same is true for Henries

You may see notations like 10E6 - that E means raised to
the negative power, so for example, 10E6 = 10^-6

 

[DaveC]

your timing is perfect. Run, don't walk to your nearest magazine stand
and get a copy of the December QST ( ham radio magazine ).. There's an
article for a home built condensor microphone in this issue.

Can I presume that this post was mistakenly put here?

 

[BFoelsch]

I think you might have a decimal point in the wrong place.

By my calculations, 1500 pf will resonate with 42mH at about 20 kHz.

A perfectly practical set of values for audio work, you can probably buy the
inductor pre-wound.

Not sure what the OPs actual project is, but at, as you said, at those
values I might be inclined to adjust the frequency with a variable
compression capacitor, as surplus they are likely to be less expensive and
more available than a variable inductor.

Or have I completely missed something here?

Thanks in advance.

[email protected] wrote...

DaveC 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.

whill_at_picovolt-dot-com

 

[N. Thornton]

Hi


I missed what the app was. For some apps changing the coil Q works:
this can be done with a swinging metal plate, swing it towards or away
from your inductor.

Regards, NT

 

[[email protected]]

I think you might have a decimal point in the wrong place.

I hate it when that happens! You're right - thanks for
spotting it.

By my calculations, 1500 pf will resonate with 42mH at about 20 kHz.

A perfectly practical set of values for audio work, you can probably buy the
inductor pre-wound.

Not sure what the OPs actual project is, but at, as you said, at those
values I might be inclined to adjust the frequency with a variable
compression capacitor, as surplus they are likely to be less expensive and
more available than a variable inductor.

Or have I completely missed something here?

Thanks in advance.

[email protected] wrote...

DaveC 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.

whill_at_picovolt-dot-com

 

[als]

Dunno what your space constraints are, but my old GR Impedance
Measuring Box does it with an air-cored inductor swivelling
inside another one. IIRC, I have had it resonate at 10 kHz
and 80 kHz, but it's been a looonnnggg time. The variable
cap was considerably larger than 1500 pf, tho. (also IIRC).
I have toyed with toroids (pun intended); if you can reasonably
add a DC component, you can vary the inductance by varying the
current. May be more than you wanted to fool with, tho.
<als>