I understood his saying ""driving the transformer to saturation and then reducing the drive to zero."" as being a DC thing. Upon reexamination, it appears he meant AC. Which is what we do here.
However, I still don't see any cores of any kind remaining anywhere near saturation after the current is removed, at any point in the waveform.
jn
a few cut and pastes..
3.1 Ferrite Temperature Limits
The Curie temperature (the temperature at which the ferrite becomes nonmagnetic) can be
relatively low. The Ferroxcube 3E5 ferrite may have a Curie temperature as low as 120o
C.
Other Ferroxcube materials have Curie temperatures up to 300o
C. If there is any possibility
of the ferrite device operating at a temperature above 120o
C due to internal losses, a ferrite
material with an adequate Curie temperature must be selected.
The copper wire used in winding inductors or transformers remains mechanically stable
at temperatures far above the ferrite Curie temperature, so the wire itself is not of concern.
However, the insulation on the wire may fail at relatively low temperatures. Polyvinyl chloride
(PVC) insulated wire typically has a maximum temperature rating between 80o
C and 105o
C,
for example. Magnet wire has a somewhat higher rated temperature, such as the Belden polythermaleze coating rated at 180o
C. Belden also makes Teflon coated wires rated at 200o
C and
260o
C.
Solid State Tesla Coil by Dr. Gary L. Johnson October 29, 2001
and:
Soft magnetic materials of high Curie point, little affected by temperature; soft magnetic materials with low Curie point, are more sensitive to temperature changes, affected by temperature. For example, Mn-Zn ferrite of the Curie point is only 215 ℃, low, magnetic flux density, permeability and losses are changes with temperature, in addition to the normal temperature of 25 ℃ suits, but also gives 60 ℃, 80 ℃, 100 ℃, the various parameters of the data. Therefore, the Mn-Zn ferrite core temperature is generally limited to below 100 ℃, which is the ambient temperature is 40 ℃, the temperature must be below 60 ℃. Co-based amorphous alloy Curie point of 205 ℃, is also low, with temperature limits below 100 ℃. Fe-based amorphous alloy Curie point of 370 ℃, at 150 ℃ ~ 180 ℃ the following use. High-permeability permalloy Curie point of 460 ℃ to 480 ℃, at 200 ℃~ 250 ℃ the following use. Ceramic nano-crystalline alloy Curie point of 600 ℃, oriented silicon steel Curie point 730 ℃, can be 300 ℃ ~ 400 ℃ to use.
jn
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Yes, he meant AC.
I don't recall anyone saying they would. Only recommending you begin the demagnetization at saturation in order to be sure of getting all the domains scrambled again.
se
Ah, I was just paying attention to the bit about low level transformers.
Yeah, that doesn't make any sense to me either.
se
The industrial demagnetisation practice for magnetically soft materials is to apply progressively diminishing sinusoids (and sometime pulses), starting from the highest saturating level.
The same for magnetically harder materials (or soft ones with complex shapes) , repeating it a few more times, while banging the piece with a mallet. Check the end result with a magnetometer. (*)
For magnetically really hard materials, heating over their curie temperature.
(*) Note: It is difficult to demagnetise a circularly magnetized object (e.g. a torroidal core) and almost impossible to reliably check it for remaining magnetisation(Edit>using direct methods). You need to magnetize it to saturation in the axial direction and then to demagnetise it as per above. Now, for to achieve proper axial magnetisation, a cylindrical or toroidal piece need to have L/D>5:1. If the component is shorter than this, magnetic extenders are used.
George
The same for magnetically harder materials (or soft ones with complex shapes) , repeating it a few more times, while banging the piece with a mallet. Check the end result with a magnetometer. (*)
For magnetically really hard materials, heating over their curie temperature.
(*) Note: It is difficult to demagnetise a circularly magnetized object (e.g. a torroidal core) and almost impossible to reliably check it for remaining magnetisation(Edit>using direct methods). You need to magnetize it to saturation in the axial direction and then to demagnetise it as per above. Now, for to achieve proper axial magnetisation, a cylindrical or toroidal piece need to have L/D>5:1. If the component is shorter than this, magnetic extenders are used.
George
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A few years ago I read a claim that line level audio transformers "sound" better if their magnetic field is not restricted by the transformer being mounted in a metal casing ( as is standard ) I seem to remember the transformers in question were EI.
Of course this arrangement would radiate magnetic fields and be prone to pick up but putting these obvious disadvantages to one side can any of you guys who have a very good technical understanding of transformer theory see any logical explaination that would support this claim.
I wonder if mounting a transformer in the centre a larger metal casing would be a more ideal environment for a transformer to work than than the normal tight fit casings.
mike
Of course this arrangement would radiate magnetic fields and be prone to pick up but putting these obvious disadvantages to one side can any of you guys who have a very good technical understanding of transformer theory see any logical explaination that would support this claim.
I wonder if mounting a transformer in the centre a larger metal casing would be a more ideal environment for a transformer to work than than the normal tight fit casings.
mike
EFM
Cool, Fas42! You just rediscovered EFM as physically represented on a CD! No wonder they sound so good! (ducks and sprints towards the door) I'm getting the Barkhausen outta here!
Howie
Howard Hoyt
CE - WXYC-FM 89.3
UNC Chapel Hill, NC
www.wxyc.org
Cool, Fas42! You just rediscovered EFM as physically represented on a CD! No wonder they sound so good! (ducks and sprints towards the door) I'm getting the Barkhausen outta here!
Howie
Howard Hoyt
CE - WXYC-FM 89.3
UNC Chapel Hill, NC
www.wxyc.org
None whatsoever in transformer theory. Plenty in psychological theory (see "pathetic fallacy"). Fields gotta be free!
Almost correct in my opinion. See below.
This will always be a correct answer to any (not only) audio problem.
SY, we have never asked them, so we don’t know what they like.
I understand you haven’t mention fields for no reason.
I would agree that the radiated fields have to do with Mike’s question and some of the effects are not exactly fallacious, they are difficult to isolate and they differ from case (literally ) to case.
The main point is the source of the emittion.
This is the transformer but think also the driver plus the driven circuits.
These three together will determine if the radiation impedance (RI) of each coil will be high or low
Radiation impedance means E/H i.e. electric/magnetic field.
RI changes appreciably in near field.
As frequencies in concern are low, think only near field here.
The more E/H>377 Ohm the more electric field dominates. The more E/H<377 Ohm, the more the magnetic field dominates.
What is where, will determine the shielding, confinement method, the degree of interaction btn field and enclosure and subsequently the effect.
E.g. With a strong electric field, the capacitive coupling to the grounded case will have a strong effect, with a strong magnetic field the effect will be weak.
Think over if you will ground the case and on how many pick-up points.
Think over the ground point (it's impedance and if it is a signal ground) of the case.
Think over what is the impedance of nearby electric circuits.
Think over that jneutron, Ed and Richard will jump in
Think low frequency EMI way.
George
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Keep in mind that the leakage inductance will have been minimized by design. And that the case material will generally have a very high permeability. Likewise, a quick reading of the transformer specs (and a few measurements) will show a very low capacitance to the case (e.g., 100pF for the JT-11P).
Two possibilities.
1. If the core has external fields, any conductive metal will both eddy and do that Lenzy thing...it will try to exclude time varying magnetic fields.
If it Lenz's, then the total inductance will lower.
If eddies occur, the effective resistance will raise, and there will be non linearities due to the dissipation losses in the external conductive surfaces.
Both of these can be spotted by measurement. With a good meter capable of Ls/Rs, watch the Ls for decrease when the metal enclosure is put into place. Watch for increase in Rs, the meter will think dissipation is a part of the inductance model because it is 90 degrees away from the reactance.
2. If any of the metal closes around any of the flux, it'll act as a shorted turn. I do not think anybody would do that however.
George, was there a question in there for me?
jn
The input transformer demagnitization stuff came from Princton Applied Research PAR 1900 manual: http://www.sunnytek.net/admin/xiazaifiles/201011415836697.pdf pg 1-8.
The big toroid story I got from the transformer vendor (Major toroid vendor) as to why sometimes the fuse would blow even though it was way higher than the load required was the one about the transformer retaining a high flux level. Saturation was probably an exaggeration. However if it was magnetized one way and the first cycle of the ac wave on turn on was in the same direction it could be close to the equivalent of saturation plus on the first cycle (or at least it sounds good). This was the rationale behind adding a turn on inrush limiter that I was asked to to for an audiophile amp that was blowing fuses and circuit breakers. (2 KVA transformer for a 250W woofer amp.)
SY
I thought that mikel’s question was for transformer’s open construction.
Now I understand and I have to agree that for a properly constructed canned small signal transformer, any claimed effect due to additional metal casing is a fallacy.
I know you are generous enough to provide valuable knowledge (No 1) for free without been asked to.
No 2 happens everytime transformer is canned and I have noticed that this shorted turn provides a lot of damping, asking for low impedance driving circuit.
Thanks again.
George
Followup: the Qvortrup favorite 6463 is the Telefunken, I just verified.
"
... I also called on Coates. He was afraid he had mislaid her notes. He took his article from a steel file: "It's accurate. I have not changed her style. There's one misprint --- not that it matters much: Mountain, not fountain --- the majestic touch."
"
from Vladimir Nabokov, Pale Fire
Thanks. And for the record, I had no argument with you regarding transformer demagnetiziation.
Sure the fuse wasn't being blown by the inrush current into the reservoir capacitors? Toroids are quite efficient and wouldn't get much in the way of the reservoir caps. And as jn noted, there's not much residual magnetization to speak of in the cores of the solenoids he works with.
se
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