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Old 11th March 2011, 06:31 PM   #11031
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Originally Posted by SY View Post
Well, don't the coils have Johnson noise? So that, except at 0K, the core is getting excited.
his premise is that there will be coupling to the coil despite zerofield, and that it's possible to distinguish cores based on the possibility...
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edit: To be clearer, the fact that the box is isothermal means that there has to be energy removed from it by external means.
Hence my goin ta sweden. Always wanted to meet King Gustav..

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I was thinking of something similar.

If what jcx says is true, i.e. that that noise would be independent of signal level and would be manifest even in the absence of a signal, then to quantify noise due to eddy current losses in the core it should be a simple matter of measuring the high frequency noise in an existing transformer and compare that to the calculated noise based on simple winding resistance.

se
Exactly...and it would mean that swapping core types will show a difference.

Yet, there is no coupling mechanism between the two if there is no current..

Hence the conundrum..

Cheers, John
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Old 11th March 2011, 06:36 PM   #11032
jcx is offline jcx  United States
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the wire "skin effect noise" example is a good use of "reductio ad absurdum" - with the twist that the physical principle is "sound" but the absurd conclusion is that it is large enough to have an effect on sound

I suspect the same is true in MC xfmr


- just a thought but maybe cable lifters work by keeping the wires at more nearly the same gravitational potential - after all electrons do have mass

anyone want to trademark "frame acceleration compensated" cables? - apparently it is always good to reference Einstein in your marketing

Last edited by jcx; 11th March 2011 at 06:41 PM.
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Old 11th March 2011, 06:36 PM   #11033
gpapag is offline gpapag  Greece
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Quote:
Originally posted by jneutron

Practical problems:

Cleaning the glue off the laminations.

Packing density.

Noise from the current source.

Proximity effect based noise in the wires (the increase of Rs with frequency.)

Cleaning the glue off the laminations: It can be managed.

Packing density: Correct. The stripped-off insulation has a thickness that can be made-up with paper or plastic inserted between bobbin and core(I donít think it will be more than 5-6 mills).

Proximity effect based noise in the wires (the increase of Rs with frequency.) and Noise from the current source. : Both will be the same on the 3 measurements.

They are meant to be comparative measurements, not absolute ones.
And it was proposed as a doable, not as a thought experiment

Regards
George
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Old 11th March 2011, 06:39 PM   #11034
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Originally Posted by jneutron View Post
Exactly...and it would mean that swapping core types will show a difference.

Yet, there is no coupling mechanism between the two if there is no current..

Hence the conundrum..
There are noise currents in the windings.

se
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Old 11th March 2011, 06:51 PM   #11035
jcx is offline jcx  United States
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Originally Posted by jneutron View Post
his premise is that there will be coupling to the coil despite zerofield, and that it's possible to distinguish cores based on the possibility...

Hence my goin ta sweden. Always wanted to meet King Gustav..



Exactly...and it would mean that swapping core types will show a difference.

Yet, there is no coupling mechanism between the two if there is no current..

Hence the conundrum..

Cheers, John
but we're pretty sure there is thermal noise current in the core material - some components of it will have "the right" physical/spatial structure to couple to the winding

again the simplest version of the question is can you distinguish by noise measurement at the primary different resistances connected to a xfmr secondary?
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Old 11th March 2011, 07:10 PM   #11036
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Originally Posted by jcx View Post
the wire "skin effect noise" example is a good use of "reductio ad absurdum" - with the twist that the physical principle is "sound" but the absurd conclusion is that it is large enough to have an effect on sound
I have coils on my desk which are made using 700 or so turns of 15 guage solid copper, and some made using equivalent guage litz. Measurement of both from 20 hz to 50 Khz clearly shows the equivalent series resistance of the solid goes nuts, exceeding 300 to 400 ohms at 20Khz.

Am I being told that both will have the exact same noise should I push 20 Khz 1 ampere into them?

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Originally Posted by jcx View Post
- just a thought but maybe cable lifters work by keeping the wires at more nearly the same gravitational potential - after all electrons do have mass
Ah, but when they flow downhill they pick up speed therefore mass, and uphill, the opposite, so net neutral..

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Originally Posted by gpapag View Post
Cleaning the glue off the laminations: It can be managed.
Packing density: Correct. The stripped-off insulation has a thickness that can be made-up with paper or plastic inserted between bobbin and core(I don’t think it will be more than 5-6 mills).
The lams have to be electrically connected here to change the eddies..we'll gain packing factor if they touch, but they must..
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Originally Posted by gpapag View Post
Proximity effect based noise in the wires (the increase of Rs with frequency.) and Noise from the current source. : Both will be the same on the 3 measurements.
True if reluctance is exact. Eddies fight flux penetration, so one would expect a variation in penetration therefore proximity effect...given the level one wishes to measure, I'm not sure how close the cores must be to eliminate their influence.
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They are meant to be comparative measurements, not absolute ones.
And it was proposed as a doable, not as a thought experiment

Regards
George
Doing it requires someone with equipment, time, and something to gain by the measurements. Maybe a xfmr manu somewhere? I point out that I will most certainly provide whatever assistance I can should anybody care to setup these experiments.

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There are noise currents in the windings.

se
Yup..

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Originally Posted by jcx View Post
but we're pretty sure there is thermal noise current in the core material - some components of it will have "the right" physical/spatial structure to couple to the winding
Actually, the entire core structure will indeed be setup such that it will couple to the windings. Buuuuuut....

Thermal molecular movement will be totally random in direction. There will be NO net carrier movement, therefore there will be no net fields generated at the macro level.


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Originally Posted by jcx View Post
again the simplest version of the question is can you distinguish by noise measurement at the primary different resistances connected to a xfmr secondary?
That was not the question I was discussing. I was discussing the distinguishing of the core via the primary (with no secondary load), but simply because the eddy losses will generate noise.

Cheers, John

Last edited by jneutron; 11th March 2011 at 07:15 PM.
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Old 11th March 2011, 07:18 PM   #11037
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To complete my previous thought (the coil inducing current in a plate) the only added noise is due to the increase in the plates temperature.This mechanism is present at equilibrium with or without signal. Take my sub-sqrt(2qI) noise maker, a noiseless 1000V battery connected to a 1M resistor as an example. The only increase in noise is due to heating of the resistor (sqrt(4KT/R)).

Remember excess noise would be present in a plate full of defects, etc. and is different.

I rest my case awaiting the experiments.
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Old 11th March 2011, 07:25 PM   #11038
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Originally Posted by jneutron View Post
I have coils on my desk which are made using 700 or so turns of 15 guage solid copper, and some made using equivalent guage litz. Measurement of both from 20 hz to 50 Khz clearly shows the equivalent series resistance of the solid goes nuts, exceeding 300 to 400 ohms at 20Khz.

Am I being told that both will have the exact same noise should I push 20 Khz 1 ampere into them?
Shouldn't you have the same absolute amount of noise whether at 20 Hz or 20 kHz?

I mean, the only source of noise in the wire is the thermal energy in the wire, yes?

So if we leave out joule heating for the moment, then you should have the same amount of noise at 20 Hz as 20 kHz, yes?

Now, if the resistance of the wire goes up with frequency due to skin effect, then you'll have some attenuation of the signal because of it and therefore some reduction in signal to noise ratio, but the noise itself should remain the same, correct?

se
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Old 11th March 2011, 07:33 PM   #11039
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Originally Posted by Steve Eddy View Post
Shouldn't you have the same absolute amount of noise whether at 20 Hz or 20 kHz?

I mean, the only source of noise in the wire is the thermal energy in the wire, yes?

se
My first thought would be no. The shorted coil would exhibit a current noise that would have to obey the same physics. The open circuit voltage noise would be due to the bulk R (there is no current i. e. no skin). Experiment No. 3 :0
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Old 11th March 2011, 07:42 PM   #11040
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Originally Posted by scott wurcer View Post
To complete my previous thought (the coil inducing current in a plate) the only added noise is due to the increase in the plates temperature.This mechanism is present at equilibrium with or without signal. Take my sub-sqrt(2qI) noise maker, a noiseless 1000V battery connected to a 1M resistor as an example. The only increase in noise is due to heating of the resistor (sqrt(4KT/R)).

Remember excess noise would be present in a plate full of defects, etc. and is different.

I rest my case awaiting the experiments.
Makes sense to me.

And I've been thinking about John's Q measurements.

John is assuming that Q dropping with frequency is due to some increase in resistance and therefore there must be a commensurate increase in noise.

But eddy currents in the core cause the core's effective permeability to drop.

All else being equal, as permeability goes down, so does inductance.

Would this not also result in a lower Q with increasing frequency?

se
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