Bill, it's one thing to say a coil's reactance is non-linear, but it's
another to assert its inductance varies with frequency.

Both statements are true and easily provable. A simple air core coil
which measures one microhenry at a low frequency may have an inductance
of several millihenries (or even henries) when near its self resonant
frequency. It's a simple law of physics; there is no way around it.
And *above* the self-resonant frequency, the choke actually behaves like
a capacitor, believe it or not.


As I responded
before, the inductance of air coils varies very little with frequency.

That statement is true only at relatively low frequencies. Get near the
self-resonant frequency of an air core coil and you'll find otherwise.
Designers using relatively large coils over a wide frequency range run
into this problem all the time. As I mentioned in another post, the
classic example for Amateur Radio is the plate choke in a tube type
amplifier. Designing such a choke that has enough inductance to work
over the entire HF spectrum without self-resonances is nearly
impossible. Many amplifier designers don't even try; they just switch
inductance in and out of the choke depending on frequency.


Youall seem to be hitting all around the 'problem'. A coil has 3
components, the resistance of the wire, the inductance, and the stray
capacitance. As the frequency is changed from DC to low AC to RF each
component has more or a less effect on how it acts in a circuit. The actual
value of each does not change, just the effect on an external circuit.

For small coils at DC the reisitance is the major item that will be seen by
an external circuit. At low to medium frequencies the inductance will be
the major factor. At very high frequencies the capacitance may be the major
factor. At self resonant frequencies , the tuned circuit effect takes
over.

Bill Turner writes:

On Sun, 07 Dec 2003 10:24:02 +0000, John Devereux
wrote:

Well, just about anything is "non-linear" if you measure it accurately
enough! But is it really true that the *inductance* of a "small air
coil" is "dramatically" non-linear with frequency as you stated?

__________________________________________________ _______

Yes, it really is true. If you graph the reactance vs frequency of any
coil, starting just above DC, it will rise in a near-linear fashion for
a while, but will begin to steepen and when approaching the
self-resonant frequency, will quickly rise to maximum, and at that point
will suddenly drop to the opposite (negative, or capacitive) extreme and
then diminish back to near zero as the frequency continues to increase.

No, you are talking about the *reactance* ("reactive impedance"). We
have been talking about the *inductance* ! They are not the same
thing.

If you model a real-world "coil" as a perfect capacitor in parallel
with a perfect, *fixed*, inductor, it will behave as you
describe. (Well you need a resistor too if you don't want infinite
"Q"!)

At that self-resonant frequency, the coil is behaving like a parallel
resonant circuit, which of course it is, due to the parasitic
capacitance between each winding. This parasitic capacitance is
unavoidable and ALL coils exhibit this characteristic. The truly
strange thing is that above the self-resonant frequency, the coil
actually behaves exactly like a capacitor, believe it or not.

Real "Inductors" do indeed have a self-capacitance too, which will
make the component deviate from that of an ideal inductor in the way
that you describe. But this in itself does not make the inductance
(i.e. the inductive part of the reactance), vary.

SNIP

--

John Devereux

Bill Turner writes:

On Sun, 07 Dec 2003 10:24:02 +0000, John Devereux
wrote:

Well, just about anything is "non-linear" if you measure it accurately
enough! But is it really true that the *inductance* of a "small air
coil" is "dramatically" non-linear with frequency as you stated?

__________________________________________________ _______

Yes, it really is true. If you graph the reactance vs frequency of any
coil, starting just above DC, it will rise in a near-linear fashion for
a while, but will begin to steepen and when approaching the
self-resonant frequency, will quickly rise to maximum, and at that point
will suddenly drop to the opposite (negative, or capacitive) extreme and
then diminish back to near zero as the frequency continues to increase.

No, you are talking about the *reactance* ("reactive impedance"). We
have been talking about the *inductance* ! They are not the same
thing.

If you model a real-world "coil" as a perfect capacitor in parallel
with a perfect, *fixed*, inductor, it will behave as you
describe. (Well you need a resistor too if you don't want infinite
"Q"!)

At that self-resonant frequency, the coil is behaving like a parallel
resonant circuit, which of course it is, due to the parasitic
capacitance between each winding. This parasitic capacitance is
unavoidable and ALL coils exhibit this characteristic. The truly
strange thing is that above the self-resonant frequency, the coil
actually behaves exactly like a capacitor, believe it or not.

Real "Inductors" do indeed have a self-capacitance too, which will
make the component deviate from that of an ideal inductor in the way
that you describe. But this in itself does not make the inductance
(i.e. the inductive part of the reactance), vary.

SNIP

--

John Devereux

Bill Turner writes:

On 7 Dec 2003 04:21:04 -0800, Winfield Hill
wrote:

Bill, it's one thing to say a coil's reactance is non-linear, but it's
another to assert its inductance varies with frequency.

Both statements are true and easily provable. A simple air core coil
which measures one microhenry at a low frequency may have an inductance
of several millihenries (or even henries) when near its self resonant
frequency.


No, it does not. I'm afraid you are using the word "inductance" in a
different way from everyone else :)

It's a simple law of physics; there is no way around it.
And *above* the self-resonant frequency, the choke actually behaves like
a capacitor, believe it or not.

Yes, because at high frequencies the current goes through the
capacitance of the coil rather than the *fixed* inductance.

(Uh, by the way, you do know who you are arguing with, right?)...



--

John Devereux

Bill Turner writes:

On 7 Dec 2003 04:21:04 -0800, Winfield Hill
wrote:

Bill, it's one thing to say a coil's reactance is non-linear, but it's
another to assert its inductance varies with frequency.

Both statements are true and easily provable. A simple air core coil
which measures one microhenry at a low frequency may have an inductance
of several millihenries (or even henries) when near its self resonant
frequency.


No, it does not. I'm afraid you are using the word "inductance" in a
different way from everyone else :)

It's a simple law of physics; there is no way around it.
And *above* the self-resonant frequency, the choke actually behaves like
a capacitor, believe it or not.

Yes, because at high frequencies the current goes through the
capacitance of the coil rather than the *fixed* inductance.

(Uh, by the way, you do know who you are arguing with, right?)...



--

John Devereux

Bill Turner wrote:

Both statements are true and easily provable. A simple air core coil
which measures one microhenry at a low frequency may have an inductance
of several millihenries (or even henries) when near its self resonant
frequency. It's a simple law of physics; there is no way around it.
And *above* the self-resonant frequency, the choke actually behaves like
a capacitor, believe it or not.

Now you have gone and said something that is simply not true. The
small inductor has a nearly fixed inductance with a parallel with a
nearly fixed capacitance. The combined impedance of these two fixed
reactances produces a nonlinear impedance, but there is nothing about
that impedance that implies a large change in either the inductance or
capacitance of the combination, at least not until you get to so high
a frequency that the winding is a significant fraction of a cycle
long. The rise in impedance near resonance does not exhibit the same
phase shift that between voltage and current that a large inductance
would have.

--
John Popelish

Bill Turner wrote:

Both statements are true and easily provable. A simple air core coil
which measures one microhenry at a low frequency may have an inductance
of several millihenries (or even henries) when near its self resonant
frequency. It's a simple law of physics; there is no way around it.
And *above* the self-resonant frequency, the choke actually behaves like
a capacitor, believe it or not.

Now you have gone and said something that is simply not true. The
small inductor has a nearly fixed inductance with a parallel with a
nearly fixed capacitance. The combined impedance of these two fixed
reactances produces a nonlinear impedance, but there is nothing about
that impedance that implies a large change in either the inductance or
capacitance of the combination, at least not until you get to so high
a frequency that the winding is a significant fraction of a cycle
long. The rise in impedance near resonance does not exhibit the same
phase shift that between voltage and current that a large inductance
would have.

--
John Popelish

Bill Turner wrote:

Not only can you *not* measure them separately, they can not be
physically separated either, since the parasitic capacitance is always
present between adjacent windings. I would not call it an artifact of
the measurement method, but rather an artifact of the coil itself.

Agreed.

The inductance should be measured at whatever frequency you plan to use
the inductor, whether a single frequency or a wide band of frequencies.
Otherwise you risk a nasty surprise. To measure a coil at low frequency
and then label it as a "one microhenry" coil, for example, is asking for
trouble when that "one microhenry" coil is used at a higher frequency.
To be accurate, when you specify inductance you must also specify the
frequency of measurement.

Agreed. One usually specifies an inductor that is measured at a
higher frequency than the one being used.

(snip)

--
John Popelish

Bill Turner wrote:

Not only can you *not* measure them separately, they can not be
physically separated either, since the parasitic capacitance is always
present between adjacent windings. I would not call it an artifact of
the measurement method, but rather an artifact of the coil itself.

Agreed.

The inductance should be measured at whatever frequency you plan to use
the inductor, whether a single frequency or a wide band of frequencies.
Otherwise you risk a nasty surprise. To measure a coil at low frequency
and then label it as a "one microhenry" coil, for example, is asking for
trouble when that "one microhenry" coil is used at a higher frequency.
To be accurate, when you specify inductance you must also specify the
frequency of measurement.

Agreed. One usually specifies an inductor that is measured at a
higher frequency than the one being used.

(snip)

--
John Popelish

In article , Bill Turner
writes:

On Sun, 07 Dec 2003 13:55:35 +0200, Paul Keinanen
wrote:

One can still argue that the inductance and inductive reactance are as
well as the capacitance and the capacitive reactance are still there
as separate entities, but we can not measure them separately from
terminals of the coil. Thus, this is an artefact of the measurement
method.

Not only can you *not* measure them separately, they can not be
physically separated either, since the parasitic capacitance is always
present between adjacent windings. I would not call it an artifact of
the measurement method, but rather an artifact of the coil itself.

Nonsense. General Radio had a nice little formula way back
before 1956 for finding the distributed capacity of an inductor.
It was published in the Green Bible (ITT Reference Data for
Radio Engineers, small format, dark green hard cover). I used it
years ago and earlier this year and many times between.

Write on the whiteboard 100 times: Inductance does not change
with frequency...reactance changes with frequency.

Now if someone actually wants to WIND COILS, I have a little aid
for tiny ones wound on common screw thread forms that was
published in Ham Radio magazine. Has measured Qs over
frequency as well as basic inductance. I'll attach it to private e-mail
(PDF) to anyone that requests it.

Using common screw thread formers and solid wire allows a good
repeatability between bench and application. Forms can be anything
from a 4-40 bolt to a common screw-thread lamp base.

Folks in here are getting too wound up...and coiling to strike. :-)

Len Anderson
retired (from regular hours) electronic engineer person

In article , Bill Turner
writes:

On Sun, 07 Dec 2003 13:55:35 +0200, Paul Keinanen
wrote:

One can still argue that the inductance and inductive reactance are as
well as the capacitance and the capacitive reactance are still there
as separate entities, but we can not measure them separately from
terminals of the coil. Thus, this is an artefact of the measurement
method.

Not only can you *not* measure them separately, they can not be
physically separated either, since the parasitic capacitance is always
present between adjacent windings. I would not call it an artifact of
the measurement method, but rather an artifact of the coil itself.

Nonsense. General Radio had a nice little formula way back
before 1956 for finding the distributed capacity of an inductor.
It was published in the Green Bible (ITT Reference Data for
Radio Engineers, small format, dark green hard cover). I used it
years ago and earlier this year and many times between.

Write on the whiteboard 100 times: Inductance does not change
with frequency...reactance changes with frequency.

Now if someone actually wants to WIND COILS, I have a little aid
for tiny ones wound on common screw thread forms that was
published in Ham Radio magazine. Has measured Qs over
frequency as well as basic inductance. I'll attach it to private e-mail
(PDF) to anyone that requests it.

Using common screw thread formers and solid wire allows a good
repeatability between bench and application. Forms can be anything
from a 4-40 bolt to a common screw-thread lamp base.

Folks in here are getting too wound up...and coiling to strike. :-)

Len Anderson
retired (from regular hours) electronic engineer person

I read in sci.electronics.design that Terry Pinnell terrypinDELETE@dial
..pipexTHIS.com wrote (in )
about 'Winding coils', on Sun, 7 Dec 2003:
John Devereux wrote:

Bill Turner writes:

On 6 Dec 2003 13:39:51 -0800, Winfield Hill
wrote:

We're talking a small air-coil here.

Doesn't matter what kind of coil; all coils have a non-linear plot of
either inductance vs frequency OR reactance vs frequency. ALL coils.

Well, just about anything is "non-linear" if you measure it accurately
enough! But is it really true that the *inductance* of a "small air
coil" is "dramatically" non-linear with frequency as you stated?

Intuitively I'd have thought the answer was plainly No, but I'm
certainly not technically savvy enough to be confident about that. But
I strongly suspect that the thread is already ovedue an unambiguous
definition of 'inductance'. Where's John Woodgate when you really need
him...g.

I'm here today, but I was away all last week. I don't think I can add
much; small air-cored coils have inductance independent of frequency up
to about half the self-resonant frequency, when deviation begins to be
noticeable. Low-frequency iron-cored coils are quite another matter; the
inductance varies with frequency, voltage, temperature, previous history
and the state of the tide on Europa. Also, it comes in two varieties,
series equivalent and shunt equivalent, and you'd better get the right
one for your problem. As the inductor gets 'worse', at lower
frequencies, the shunt equivalent tends to infinity, which puzzles
students no end.
--
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
Interested in professional sound reinforcement and distribution? Then go to
http://www.isce.org.uk
PLEASE do NOT copy news posts to me by E-MAIL!

I read in sci.electronics.design that Terry Pinnell terrypinDELETE@dial
..pipexTHIS.com wrote (in )
about 'Winding coils', on Sun, 7 Dec 2003:
John Devereux wrote:

Bill Turner writes:

On 6 Dec 2003 13:39:51 -0800, Winfield Hill
wrote:

We're talking a small air-coil here.

Doesn't matter what kind of coil; all coils have a non-linear plot of
either inductance vs frequency OR reactance vs frequency. ALL coils.

Well, just about anything is "non-linear" if you measure it accurately
enough! But is it really true that the *inductance* of a "small air
coil" is "dramatically" non-linear with frequency as you stated?

Intuitively I'd have thought the answer was plainly No, but I'm
certainly not technically savvy enough to be confident about that. But
I strongly suspect that the thread is already ovedue an unambiguous
definition of 'inductance'. Where's John Woodgate when you really need
him...g.

I'm here today, but I was away all last week. I don't think I can add
much; small air-cored coils have inductance independent of frequency up
to about half the self-resonant frequency, when deviation begins to be
noticeable. Low-frequency iron-cored coils are quite another matter; the
inductance varies with frequency, voltage, temperature, previous history
and the state of the tide on Europa. Also, it comes in two varieties,
series equivalent and shunt equivalent, and you'd better get the right
one for your problem. As the inductor gets 'worse', at lower
frequencies, the shunt equivalent tends to infinity, which puzzles
students no end.
--
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
Interested in professional sound reinforcement and distribution? Then go to
http://www.isce.org.uk
PLEASE do NOT copy news posts to me by E-MAIL!

I read in sci.electronics.design that Bill Turner
wrote (in ) about 'Winding
coils', on Sun, 7 Dec 2003:
Making a plate choke which covers all frequencies from 160
through 10 meters (including the WARC bands) with sufficient inductance
but without self-resonances in any ham band is difficult to the point of
being nearly impossible. Many amplifier designers give up trying to
design such a choke and simply switch part of the inductance in or out
of the circuit depending on which band is selected.

This is a 1920s problem. Just as you parallel capacitors of different
type, electrolytic, metallized foil and ceramic, to get a wideband
component, so you put inductors of different construction in series to
get a wide band component. You can wind them all on a bit of wax-
impregnated dowel if you like. (;-)
--
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
Interested in professional sound reinforcement and distribution? Then go to
http://www.isce.org.uk
PLEASE do NOT copy news posts to me by E-MAIL!

I read in sci.electronics.design that Bill Turner
wrote (in ) about 'Winding
coils', on Sun, 7 Dec 2003:
Making a plate choke which covers all frequencies from 160
through 10 meters (including the WARC bands) with sufficient inductance
but without self-resonances in any ham band is difficult to the point of
being nearly impossible. Many amplifier designers give up trying to
design such a choke and simply switch part of the inductance in or out
of the circuit depending on which band is selected.

This is a 1920s problem. Just as you parallel capacitors of different
type, electrolytic, metallized foil and ceramic, to get a wideband
component, so you put inductors of different construction in series to
get a wide band component. You can wind them all on a bit of wax-
impregnated dowel if you like. (;-)
--
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
Interested in professional sound reinforcement and distribution? Then go to
http://www.isce.org.uk
PLEASE do NOT copy news posts to me by E-MAIL!

I read in sci.electronics.design that John Devereux
wrote (in ) about 'Winding coils', on Sun,
7 Dec 2003:
I'm afraid you are using the word "inductance" in a
different way from everyone else :)

It would be better to say 'equivalent inductance', being the value L -
1/(w^2C). L = true (series equivalent) inductance, C = self-capacitance
(treated as lumped in parallel with L) and w = 2[pi]f = angular
frequency.

Resistance ignored, as irrelevant at this level.
--
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
Interested in professional sound reinforcement and distribution? Then go to
http://www.isce.org.uk
PLEASE do NOT copy news posts to me by E-MAIL!

I read in sci.electronics.design that John Devereux
wrote (in ) about 'Winding coils', on Sun,
7 Dec 2003:
I'm afraid you are using the word "inductance" in a
different way from everyone else :)

It would be better to say 'equivalent inductance', being the value L -
1/(w^2C). L = true (series equivalent) inductance, C = self-capacitance
(treated as lumped in parallel with L) and w = 2[pi]f = angular
frequency.

Resistance ignored, as irrelevant at this level.
--
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
Interested in professional sound reinforcement and distribution? Then go to
http://www.isce.org.uk
PLEASE do NOT copy news posts to me by E-MAIL!

I read in sci.electronics.design that Bill Turner
wrote (in ) about 'Winding
coils', on Sun, 7 Dec 2003:

Both statements are true and easily provable. A simple air core coil
which measures one microhenry at a low frequency may have an inductance
of several millihenries (or even henries) when near its self resonant
frequency.

This is what happens to the *parallel equivalent* inductance. The series
equivalent goes down as the frequency increases, and goes to zero at
resonance.
--
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
Interested in professional sound reinforcement and distribution? Then go to
http://www.isce.org.uk
PLEASE do NOT copy news posts to me by E-MAIL!

I read in sci.electronics.design that Bill Turner
wrote (in ) about 'Winding
coils', on Sun, 7 Dec 2003:

Both statements are true and easily provable. A simple air core coil
which measures one microhenry at a low frequency may have an inductance
of several millihenries (or even henries) when near its self resonant
frequency.

This is what happens to the *parallel equivalent* inductance. The series
equivalent goes down as the frequency increases, and goes to zero at
resonance.
--
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
Interested in professional sound reinforcement and distribution? Then go to
http://www.isce.org.uk
PLEASE do NOT copy news posts to me by E-MAIL!

Bill Turner wrote:

You are speaking in *practical* terms, which is fine. It's true that at
relatively low frequencies, well below the self-resonant point, coils
appear to have constant inductance. No argument there. The discussion
came about because someone asserted that inductance was a constant,
REGARDLESS of frequency, and that is just not true.

I disagree. The inductive component of the impedance remains
essentially constant through resonance. What is non ideal about the
inductor is that it does not exhibit just inductance, but a parallel
combination if inductance and capacitance. Ignoring the capacitance
and calling the effect variable inductance is just not as accurate a
way to describe what is going on.

--
John Popelish

Bill Turner wrote:

You are speaking in *practical* terms, which is fine. It's true that at
relatively low frequencies, well below the self-resonant point, coils
appear to have constant inductance. No argument there. The discussion
came about because someone asserted that inductance was a constant,
REGARDLESS of frequency, and that is just not true.

I disagree. The inductive component of the impedance remains
essentially constant through resonance. What is non ideal about the
inductor is that it does not exhibit just inductance, but a parallel
combination if inductance and capacitance. Ignoring the capacitance
and calling the effect variable inductance is just not as accurate a
way to describe what is going on.

--
John Popelish

Bill Turner wrote:

That will work, no doubt. My point was that it takes some serious
engineering and careful testing; you can't just wrap some wire on a form
and expect it to work correctly across a wide range of frequencies.

This is a generality I can agree with.

--
John Popelish

Bill Turner wrote:

That will work, no doubt. My point was that it takes some serious
engineering and careful testing; you can't just wrap some wire on a form
and expect it to work correctly across a wide range of frequencies.

This is a generality I can agree with.

--
John Popelish

Bill Turner wrote:

On 07 Dec 2003 18:25:51 GMT, (Avery Fineman) wrote:

Write on the whiteboard 100 times: Inductance does not change
with frequency...reactance changes with frequency.

__________________________________________________ _______

Not true. Inductance and reactance are related by the formula
XsubL = 2 pi F L. If XsubL has changed, then so has the inductance, and
vice versa.

How could you possibly define it otherwise?

But the impedance of a coil near resonance is not well described as an
XsubL. It is a combination of XsubL and XsubC, including their
different phase shifts.

You cannot just measure the magnitude of impedance of a coil and
assume that you are measuring pure XsubL. You have to prove that this
is the case by some other measurement, like the phase relationship
between voltage and current.

--
John Popelish

Bill Turner wrote:

On 07 Dec 2003 18:25:51 GMT, (Avery Fineman) wrote:

Write on the whiteboard 100 times: Inductance does not change
with frequency...reactance changes with frequency.

__________________________________________________ _______

Not true. Inductance and reactance are related by the formula
XsubL = 2 pi F L. If XsubL has changed, then so has the inductance, and
vice versa.

How could you possibly define it otherwise?

But the impedance of a coil near resonance is not well described as an
XsubL. It is a combination of XsubL and XsubC, including their
different phase shifts.

You cannot just measure the magnitude of impedance of a coil and
assume that you are measuring pure XsubL. You have to prove that this
is the case by some other measurement, like the phase relationship
between voltage and current.

--
John Popelish

Bill Turner writes:

On Sun, 07 Dec 2003 15:54:05 +0000, John Devereux
wrote:

No, you are talking about the *reactance* ("reactive impedance"). We
have been talking about the *inductance* ! They are not the same
thing.

__________________________________________________ _______

No one ever said they were the same thing. They are related to each
other by the formula XsubL = 2 pi F L. That is a direct, linear
relationship.

The important thing here is the "subL". It applies only to the
inductive part of the overall reactance.

Are you saying that formula is correct as some (low) frequency but
incorrect at another (high) frequency?

No, it is always correct. It is practically the *definition* of
inductance so it had better be!

I'll say it another way: Inductance and reactance are directly related
to each other by the (2 pi F) factor. Given one (inductance or
reactance) you can calculate the other. There is no other way.

No. Because the "reactance" (without the sub-L) now has both inductive
*and* capacitive terms. When you measure the *overall* reactance of a
real life coil you are measuring the effect of *both* terms. You
cannot measure this combined reactance and then just plug the number
into a formula which ignores the capacitive part. You have to use the
general formula which include the self capacitance.

Ignoring the coil resistance (i.e. we have infinite Q) the correct
formula is something like:

Xtotal = 1
--------------
|1/Xc| - |1/Xl|

Where Xc = 1/(2 pi F C) and Xl = 2 pi F L.

Hopefully you can see how Xtotal behaves as you describe, even with
constant L.


--

John Devereux

Bill Turner writes:

On Sun, 07 Dec 2003 15:54:05 +0000, John Devereux
wrote:

No, you are talking about the *reactance* ("reactive impedance"). We
have been talking about the *inductance* ! They are not the same
thing.

__________________________________________________ _______

No one ever said they were the same thing. They are related to each
other by the formula XsubL = 2 pi F L. That is a direct, linear
relationship.

The important thing here is the "subL". It applies only to the
inductive part of the overall reactance.

Are you saying that formula is correct as some (low) frequency but
incorrect at another (high) frequency?

No, it is always correct. It is practically the *definition* of
inductance so it had better be!

I'll say it another way: Inductance and reactance are directly related
to each other by the (2 pi F) factor. Given one (inductance or
reactance) you can calculate the other. There is no other way.

No. Because the "reactance" (without the sub-L) now has both inductive
*and* capacitive terms. When you measure the *overall* reactance of a
real life coil you are measuring the effect of *both* terms. You
cannot measure this combined reactance and then just plug the number
into a formula which ignores the capacitive part. You have to use the
general formula which include the self capacitance.

Ignoring the coil resistance (i.e. we have infinite Q) the correct
formula is something like:

Xtotal = 1
--------------
|1/Xc| - |1/Xl|

Where Xc = 1/(2 pi F C) and Xl = 2 pi F L.

Hopefully you can see how Xtotal behaves as you describe, even with
constant L.


--

John Devereux

On Sun, 07 Dec 2003 04:31:46 -0800, Bill Turner
wrote:

On Sun, 07 Dec 2003 13:55:35 +0200, Paul Keinanen
wrote:

One can still argue that the inductance and inductive reactance are as
well as the capacitance and the capacitive reactance are still there
as separate entities, but we can not measure them separately from
terminals of the coil. Thus, this is an artefact of the measurement
method.

Not only can you *not* measure them separately, they can not be
physically separated either, since the parasitic capacitance is always
present between adjacent windings. I would not call it an artifact of
the measurement method, but rather an artifact of the coil itself.

The problem with circuits containing both inductances and capacitances
is that in one kind of reactance, there is a +90 degree phase shift
and the other with -90 degree phase shift. Thus, when these are
combined, they partially cancel each other, producing different
magnitudes and some phase shift between -90 and +90 degrees. If only
the resultant magnitude is used (and the resultant phase is ignored),
this would give the false impression that the inductance changes with
frequency.

Instead of using the resultant reactance on some specific frequency,
the inductance could be measured in a different way.

When a DC current I is flowing through and inductance L, the energy
stored in the inductance is W = I*I*L/2. This could be used to
determine the inductance L.

One way to measure the energy W would be to cut the DC current through
L and after disconnecting I, dissipate the energy in some kind of
integrating load across L. Even if there is a significant capacitance
across L, no energy is initially stored in C, since during the steady
state condition, the current I would be flowing through L, but there
would be no voltage difference between the ends of L (assuming R=0),
thus all energy in this parallel resonance circuit is stored in L.

After disconnecting the DC current I, the energy would bounce back
between L and C, but finally it would be dissipated by the external
load. The same energy would be dissipated in the external load even if
C did not exist (assuming zero losses).

Thus using this measurement method, the value of L would be the same
regardless if C is present or not.

Thus, getting a frequency dependent L, is a measurement artifact in
the method that you are using.

Paul OH3LWR

On Sun, 07 Dec 2003 04:31:46 -0800, Bill Turner
wrote:

On Sun, 07 Dec 2003 13:55:35 +0200, Paul Keinanen
wrote:

One can still argue that the inductance and inductive reactance are as
well as the capacitance and the capacitive reactance are still there
as separate entities, but we can not measure them separately from
terminals of the coil. Thus, this is an artefact of the measurement
method.

Not only can you *not* measure them separately, they can not be
physically separated either, since the parasitic capacitance is always
present between adjacent windings. I would not call it an artifact of
the measurement method, but rather an artifact of the coil itself.

The problem with circuits containing both inductances and capacitances
is that in one kind of reactance, there is a +90 degree phase shift
and the other with -90 degree phase shift. Thus, when these are
combined, they partially cancel each other, producing different
magnitudes and some phase shift between -90 and +90 degrees. If only
the resultant magnitude is used (and the resultant phase is ignored),
this would give the false impression that the inductance changes with
frequency.

Instead of using the resultant reactance on some specific frequency,
the inductance could be measured in a different way.

When a DC current I is flowing through and inductance L, the energy
stored in the inductance is W = I*I*L/2. This could be used to
determine the inductance L.

One way to measure the energy W would be to cut the DC current through
L and after disconnecting I, dissipate the energy in some kind of
integrating load across L. Even if there is a significant capacitance
across L, no energy is initially stored in C, since during the steady
state condition, the current I would be flowing through L, but there
would be no voltage difference between the ends of L (assuming R=0),
thus all energy in this parallel resonance circuit is stored in L.

After disconnecting the DC current I, the energy would bounce back
between L and C, but finally it would be dissipated by the external
load. The same energy would be dissipated in the external load even if
C did not exist (assuming zero losses).

Thus using this measurement method, the value of L would be the same
regardless if C is present or not.

Thus, getting a frequency dependent L, is a measurement artifact in
the method that you are using.

Paul OH3LWR

On Sun, 7 Dec 2003 19:13:36 +0000, John Woodgate
wrote:

Low-frequency iron-cored coils are quite another matter; the
inductance varies with frequency, voltage, temperature, previous history
and the state of the tide on Europa.

I assume that you are referring to DC biased iron cores (without an
air gap) or some high permeability ferrites with a strong DC bias
current. These do indeed show a variation of inductance depending on
the DC bias current.

Paul OH3LWR

On Sun, 7 Dec 2003 19:13:36 +0000, John Woodgate
wrote:

Low-frequency iron-cored coils are quite another matter; the
inductance varies with frequency, voltage, temperature, previous history
and the state of the tide on Europa.

I assume that you are referring to DC biased iron cores (without an
air gap) or some high permeability ferrites with a strong DC bias
current. These do indeed show a variation of inductance depending on
the DC bias current.

Paul OH3LWR

Bill Turner writes:

On Sun, 07 Dec 2003 20:00:37 GMT, John Popelish
wrote:

The inductive component of the impedance remains
essentially constant through resonance. What is non ideal about the
inductor is that it does not exhibit just inductance, but a parallel
combination if inductance and capacitance. Ignoring the capacitance
and calling the effect variable inductance is just not as accurate a
way to describe what is going on.

__________________________________________________ _______

Your point is well taken, but look at it this way:

Say I give you a black box containing an inductor with two terminals on
the box. If I have you measure the inductance at one and only one
frequency, there is no way for you to know whether it is an inductor
operating well below its self-resonance point, or an inductor operating
near its self-resonance point. To the outside world, at ONE frequency,
they appear identical; same reactance, same inductance.

No, you are neglecting the phase. The two cases would have very
different phase shifts (the current would be out of phase with the
applied voltage, by different amounts), depending on whether you were
below, at, or above resonance.

And yet, at some other (lower) frequency, they will measure quite
differently. This is the basis for my observation that inductance does
indeed vary with frequency, based on the parasitic capacitance present
in all inductors.

And yes, if you can factor out the self-capacitance, then the inductance
would indeed be constant with frequency. The problem is, no one has
ever figured out how to do that with an actual coil. It can't be done.

Yes it can. This is what a network analyser or impedance bridge does,
(as I understand it, I've never actually had to use either!).

At low frequencies the black box would be inductive. The current would
lag the voltage. At resonance the voltage would be in phase with the
current (the black box would appear resistive). At high frequencies
the current would lead the voltage. It would appear capacitive.


--

John Devereux

Bill Turner writes:

On Sun, 07 Dec 2003 20:00:37 GMT, John Popelish
wrote:

The inductive component of the impedance remains
essentially constant through resonance. What is non ideal about the
inductor is that it does not exhibit just inductance, but a parallel
combination if inductance and capacitance. Ignoring the capacitance
and calling the effect variable inductance is just not as accurate a
way to describe what is going on.

__________________________________________________ _______

Your point is well taken, but look at it this way:

Say I give you a black box containing an inductor with two terminals on
the box. If I have you measure the inductance at one and only one
frequency, there is no way for you to know whether it is an inductor
operating well below its self-resonance point, or an inductor operating
near its self-resonance point. To the outside world, at ONE frequency,
they appear identical; same reactance, same inductance.

No, you are neglecting the phase. The two cases would have very
different phase shifts (the current would be out of phase with the
applied voltage, by different amounts), depending on whether you were
below, at, or above resonance.

And yet, at some other (lower) frequency, they will measure quite
differently. This is the basis for my observation that inductance does
indeed vary with frequency, based on the parasitic capacitance present
in all inductors.

And yes, if you can factor out the self-capacitance, then the inductance
would indeed be constant with frequency. The problem is, no one has
ever figured out how to do that with an actual coil. It can't be done.

Yes it can. This is what a network analyser or impedance bridge does,
(as I understand it, I've never actually had to use either!).

At low frequencies the black box would be inductive. The current would
lag the voltage. At resonance the voltage would be in phase with the
current (the black box would appear resistive). At high frequencies
the current would lead the voltage. It would appear capacitive.


--

John Devereux

Bill Turner wrote:

Your point is well taken, but look at it this way:

Say I give you a black box containing an inductor with two terminals on
the box. If I have you measure the inductance at one and only one
frequency, there is no way for you to know whether it is an inductor
operating well below its self-resonance point, or an inductor operating
near its self-resonance point. To the outside world, at ONE frequency,
they appear identical; same reactance, same inductance.

Not if I can measure both the magnitude and phase relationship of the
device.

If I can only measure the magnitude of impedance at one frequency, I
can't even tell if the device is predominately inductive, capacitive
or resistive. So it would be a bit silly to call that magnitude an
inductive impedance.

And yet, at some other (lower) frequency, they will measure quite
differently. This is the basis for my observation that inductance does
indeed vary with frequency, based on the parasitic capacitance present
in all inductors.

Only because you are willing to confuse complex impedance with
inductive reactance.

And yes, if you can factor out the self-capacitance, then the inductance
would indeed be constant with frequency. The problem is, no one has
ever figured out how to do that with an actual coil. It can't be done.

You are projecting your limitations onto others.

--
John Popelish

Bill Turner wrote:

Your point is well taken, but look at it this way:

Say I give you a black box containing an inductor with two terminals on
the box. If I have you measure the inductance at one and only one
frequency, there is no way for you to know whether it is an inductor
operating well below its self-resonance point, or an inductor operating
near its self-resonance point. To the outside world, at ONE frequency,
they appear identical; same reactance, same inductance.

Not if I can measure both the magnitude and phase relationship of the
device.

If I can only measure the magnitude of impedance at one frequency, I
can't even tell if the device is predominately inductive, capacitive
or resistive. So it would be a bit silly to call that magnitude an
inductive impedance.

And yet, at some other (lower) frequency, they will measure quite
differently. This is the basis for my observation that inductance does
indeed vary with frequency, based on the parasitic capacitance present
in all inductors.

Only because you are willing to confuse complex impedance with
inductive reactance.

And yes, if you can factor out the self-capacitance, then the inductance
would indeed be constant with frequency. The problem is, no one has
ever figured out how to do that with an actual coil. It can't be done.

You are projecting your limitations onto others.

--
John Popelish

On Sun, 07 Dec 2003 11:10:46 -0800, Bill Turner
wrote:

On 07 Dec 2003 18:25:51 GMT, (Avery Fineman) wrote:

Write on the whiteboard 100 times: Inductance does not change
with frequency...reactance changes with frequency.

_________________________________________________ ________

Not true. Inductance and reactance are related by the formula
XsubL = 2 pi F L. If XsubL has changed, then so has the inductance, and
vice versa.

How could you possibly define it otherwise?

I don't understand this all-important formula you keep quoting. Kindly
explain what "sub" is and clearly re-state the formua in unambiguous
terms.

--

"I expect history will be kind to me, since I intend to write it."
- Winston Churchill

On Sun, 07 Dec 2003 11:10:46 -0800, Bill Turner
wrote:

On 07 Dec 2003 18:25:51 GMT, (Avery Fineman) wrote:

Write on the whiteboard 100 times: Inductance does not change
with frequency...reactance changes with frequency.

_________________________________________________ ________

Not true. Inductance and reactance are related by the formula
XsubL = 2 pi F L. If XsubL has changed, then so has the inductance, and
vice versa.

How could you possibly define it otherwise?

I don't understand this all-important formula you keep quoting. Kindly
explain what "sub" is and clearly re-state the formua in unambiguous
terms.

--

"I expect history will be kind to me, since I intend to write it."
- Winston Churchill

On Sun, 07 Dec 2003 16:16:26 -0800, Bill Turner
wrote:

On Sun, 07 Dec 2003 21:35:22 GMT, John Popelish
wrote:

You are projecting your limitations onto others.

_________________________________________________ ________

I do have one limitation: I don't take insults from people I'm trying
to have a discussion with.

Bye.

--
Bill, W6WRT


Hello John, Hello Bill,
c'mon chaps, kiss and make up.
This sort of thing happens all the time.
Someone asks an innocent question and later on down
the discussion, two highly respected fellows fall out.
So sad, because readers like me and others, who are
trying to learn something, miss out when the discussion
stops because of a silly personal remark. What a pity! :-(
Regards,
John Crighton
Sydney

On Sun, 07 Dec 2003 16:16:26 -0800, Bill Turner
wrote:

On Sun, 07 Dec 2003 21:35:22 GMT, John Popelish
wrote:

You are projecting your limitations onto others.

_________________________________________________ ________

I do have one limitation: I don't take insults from people I'm trying
to have a discussion with.

Bye.

--
Bill, W6WRT


Hello John, Hello Bill,
c'mon chaps, kiss and make up.
This sort of thing happens all the time.
Someone asks an innocent question and later on down
the discussion, two highly respected fellows fall out.
So sad, because readers like me and others, who are
trying to learn something, miss out when the discussion
stops because of a silly personal remark. What a pity! :-(
Regards,
John Crighton
Sydney

Bill Turner wrote:

On 07 Dec 2003 18:25:51 GMT, (Avery Fineman) wrote:

Write on the whiteboard 100 times: Inductance does not change
with frequency...reactance changes with frequency.

__________________________________________________ _______

Not true. Inductance and reactance are related by the formula
XsubL = 2 pi F L. If XsubL has changed, then so has the inductance, and
vice versa.

Say what???? You have two variables that satisfy the equation:
XsubL and F
The equation does not mean that L varies!!!!!!!!!!!





How could you possibly define it otherwise?

--
Bill, W6WRT