Winding coils

[John Popelish]

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 Devereux]

_________________________________________________________

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.

 

[Paul Keinanen]

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

 

[Bill Turner]

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.

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.

 

[Paul Keinanen]

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

 

[John Devereux]

_________________________________________________________

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 Popelish]

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.

 

[Bill Turner]

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.

 

[John Crighton]

_________________________________________________________

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

Bye.

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

 

[John Popelish]

_________________________________________________________

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

Bye.

Bill, I sincerely apologize for hurting your feelings unintentionally
with my clumsy comment. I should have kept strictly to inductors and
away from anything that could have been interpreted as a personal
attack.

You may not have an impedance bridge (a limitation) but I and others
do have one and they separate the components of an impedance,
especially if you take two or more readings at different frequencies
and solve a bit of math.

 

[John Woodgate]

I read in sci.electronics.design that Paul Keinanen <[email protected]>

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.

Not only that, the inductance can vary with the AC voltage applied, most
notably when saturation is approached, but it can also happen with
silicon iron at very low inductions. Nickel-iron alloys don't normally
show this 'bottom bend' effect.

 

[Steve Nosko]

1) you guys are just arguing symantics. You both know what really happens.
".... "Inductance" vs. the total reactance measuring as inductive...." call
it what you like.

1a)
You are also both using (some might say mis-using) the term "linear" to mean
"varies linearly with..." rather than the more common meaning that
superposition applies. RLC sircuits are linear. Any given parameter may
not vary linearly as the frequency is varied. Also, this use of 'linear'
depends upon the type of scale being used--log or linear.

2) John,
You better re-think your last statement about the series equivalent of a
practical coil. It implies that there is some way to measure a low Z at the
resonance of the coil under discussion. You say:
"The series

equivalent [impedance ? Steve] goes down as the frequency increases, and goes to zero at
resonance. "

While a series resonant LC exhibits this behavior, the series equivalent
of a practical coil does not do this. The series equivalent must do the
same thing as the parallel equivalent -- namely go to a high impedance at
resonance. That's why it is called *equivalent*--the total, or terminal
impedance is equal for the two representations (at a single frequency).

Pretty sure I got that right....
Steve
k]9]d]c]i


A practical coil usually goes to parallel resonance - at which the series
equivalent does not go to zero

 

[Steve Nosko]

Gents,

Another practical consideration. Another area where caution is
advised--paralleling bypass caps. In solid state Power Amplifier design,
such a configuratin can cause problems because there is a point where one is
above self resonance and acts like an inductance in parallel with the other
cap which is still capacitive-- thus, resonance and no bypass. Been there,
done that. We put a small Z in between. Frequently a small bead or
resistor if possible.
Seems there is an equivalent problem with series inductors.

 

[Steve Nosko]

There'something sour here. Way down ...

[snip]

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.

I don't quite follow where you are going here. below the self resonant
freq the angle will be +90 (minus a little for what ever resistance is
there).


The rest of this about measuring the energy from DC, I don't think is at all
practical.

[snip] the inductance could be measured in a different way.
... the energy
stored in the inductance is W = I*I*L/2. ...
...cut the DC current...dissipate the energy in some kind of
integrating load across L. Even if there is a significant capacitance[snip]
...the energy would bounce back
between L and C, but finally it would be dissipated by the external
load. ...
Thus using this measurement method, the value of L would be the same
regardless if C is present or not....

Paul OH3LWR

OK. so then, how do you propose to measure this energy? I don't think
it is practical.

 

[Jan-Martin Noeding, LA8AK]

A rule of thumb says that the screen around an oscillator coils could
be 3 times the coil diameter, and Radiotron Designer's handbook, 3rd
edition, 1941 says that the screen should go at least one coil radius
above the coil. But how far from the chassis should the coil winding
start?

73
Jan-Martin
LA8AK, http://home.online.no/~la8ak/c.htm

 

[Jim Thompson]

A rule of thumb says that the screen around an oscillator coils could
be 3 times the coil diameter, and Radiotron Designer's handbook, 3rd
edition, 1941 says that the screen should go at least one coil radius
above the coil. But how far from the chassis should the coil winding
start?

73
Jan-Martin
LA8AK, http://home.online.no/~la8ak/c.htm

Wouldn't you presume one coil radius above the chassis?

...Jim Thompson

 

[John Woodgate]

I read in sci.electronics.design that Jan-Martin Noeding, LA8AK
) about 'Need more details for mounting high-Q oscillator coils?', on
Wed, 11 Feb 2004:

A rule of thumb says that the screen around an oscillator coils could be
3 times the coil diameter, and Radiotron Designer's handbook, 3rd
edition, 1941 says that the screen should go at least one coil radius
above the coil. But how far from the chassis should the coil winding
start?

The 'at least one radius' thing applies at both ends. The magnetic field
of a solenoid is axially symmetrical.

 

[Jan-Martin Noeding, LA8AK]

About the same distance. The old Radiotron book (I have a CD
for one of the editions as a keepsake) has charts showing the
inductance reduction and Q reduction for various relative diameters.
The closer the enclosure, the more reduction of both values. That
sould be accounted for in any resonant applications, especially those
for tracking as in superhet or TRF front-ends.

The conducting enclosure or shield contains the magnetic field on the
coil and, ideally, should have the same clearange from both top and
bottom of the winding. It will still work with an assymetric clearance
but L and Q are spoiled by the closer spacing.

Well, I forgot to say that I would leave the top side open, so it is
somewhat more distance from the hot side of coil to the metal surface
than in the bottom, but thanks to all for the reply

Toroidal cores are much less affected by conducting enclosures due to
the magnetic field contained largely IN the torus' core material.

Len Anderson
retired (from regular hours) electronic engineer person

on this side you you are put in an office landscape and all the tool
is a PC with Microsoft programs, soldering iron is not permitted any
more, you just have to be constructive, hi, so I now prefer thermionic
valves at home

73