Magnetic Shielding Materials

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Magnetic Shielding Materials

Postby Harry Dalek » Tue May 29, 2018 9:16 pm

I was expecting mumetal to be the best but looks like not !

youtu.be/oxg_hCvXSrw
The electromagnetic spectrum has no theoretical limit at either end. If all the mass/energy in the Universe is considered a 'limit', then that would be the only real theoretical limit to the maximum frequency attainable.
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Re: Magnetic Shielding Materials

Postby Klaas Robers » Wed May 30, 2018 8:14 pm

Mu-metal is very effective, but you should not magnetize it. What this guy did: placing the shield directly on the magnet is the worst way to do this. And then with a super magnet....

In most cases the magnetic fields to be shielded are low intensity fields, e.g. the stray fields of a mains transformer at quite some distance. Then mu-metal is effective. But there are more precautions to do when using mu-metal.

- see that it is not mechanically stressed. If you bend mu metal, the effectiveness drops. You may repair this by heating the metal while it is in the form you want.

- demagnetize the mu metal. When you place a magnet against the material, as this guy did, the shielding properties are spoiled. Using a slowly decreasing AC magnetic field may demagnetize the mu-metal. In our old Colour TV monitors there was a large coil between the cone of the picture tube and the mu-metal shield. With an NTC and PTC resistor network. The coil was energized by 220 volt AC each time the monitor or TV was switched on. You could hear it by a decreasing humming sound of about one second.

Oh, this was nice in those days. If you had a strong magnet (ferrite loudspeaker magnet) and held it close to the screen of a TV, all colours mixed up, due to the magnetisation of the shadow mask. This might lead to panic for the owner of the TV, fast switching off and on, nothing helped.... on the short term. However, after five minutes of staying switched off and THEN switching on, re-demagnetized the screen and everything was Ok again. Those were days.

I still don't know if the trick was demagnetization of the mu-metal shield, or freezing in the local magnetic field of the Earth. This was important. If you replaced a working shadow mask monitor or TV, there were problems with the purity of the colours. After demagnetization of the picture tube this disappeared.
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Re: Magnetic Shielding Materials

Postby Harry Dalek » Wed May 30, 2018 8:44 pm

Oh i see Klaas
I found the thickness logical stopping more of the magnetic field but larger surface area was new to me.
Saw some where that aluminium can also be useful but it has to be pretty thick i recall.
i always wondered why some old transformers were wrapped in a casing to the shape of that transformer that would be mumetal have spotted those in old televisions.
These are passive shied's to magnetism is a active shield possible ? i think you used copper wire wrapped around your sstv power supply transformer ?
The electromagnetic spectrum has no theoretical limit at either end. If all the mass/energy in the Universe is considered a 'limit', then that would be the only real theoretical limit to the maximum frequency attainable.
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Re: Magnetic Shielding Materials

Postby Klaas Robers » Thu May 31, 2018 7:18 pm

Yes, but that works only for AC magnetic fields. The copper winding is shorted, the beginning is firmly connected to the end by soldering. Then you have a short circuited winding around the transformer as a whole. This induces a magnetic field opposing the magnetic stay field of the transformer.

Sometimes you also see a not too thin copper foil wrapped around the transformer, comparably to my winding. That copper foil is also soldered to form a short circuited single windinding. I had no foil, but I had yellow plastic insulated wire. However the insulation serves no goal at all. I could also have used uninsulated copper wire, but had that not laying around.
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Re: Magnetic Shielding Materials

Postby Harry Dalek » Thu May 31, 2018 8:13 pm

Klaas Robers wrote:Yes, but that works only for AC magnetic fields. The copper winding is shorted, the beginning is firmly connected to the end by soldering. Then you have a short circuited winding around the transformer as a whole. This induces a magnetic field opposing the magnetic stay field of the transformer.

Sometimes you also see a not too thin copper foil wrapped around the transformer, comparably to my winding. That copper foil is also soldered to form a short circuited single windinding. I had no foil, but I had yellow plastic insulated wire. However the insulation serves no goal at all. I could also have used uninsulated copper wire, but had that not laying around.


For us main interest is pretty much shielding CRT's is of interest , knowledge on this is lacking i have only picked it up this reading odd bits on the forum you and Steve mentioned
I have seen also a thick copper foil wrapped around outer winding of the transformer you talk about ,you look at things differently when you don't know what they are .
In my little trio scope transformer in its case the transformer is also angled ,must be to direct any fields left away from crt ,must have the copper shielding idea need to have a closer look.
The electromagnetic spectrum has no theoretical limit at either end. If all the mass/energy in the Universe is considered a 'limit', then that would be the only real theoretical limit to the maximum frequency attainable.
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Re: Magnetic Shielding Materials

Postby Klaas Robers » Tue Jun 05, 2018 11:41 pm

The trick is that a magnetic field perfectly aligned with the centre of the CRT does almost no harm. So if you can place the transformer by rotating such that the external magnetic (stray) field of it is coinciding with the direction of the electron beam, you don't see the disturbances. And because you don't know how those stray fields are exactly directed, it is wise to rotate the transformer experimentally. I also have read in the past that initially the transformer is connected with long leads and you move the transformer by hand such that the picture is best. And then try to fix it in that position. Some compromises are in most cases unavoidable.

In general placing the transformer directly behind the CRT is a good plan. But then your cabinet is going to be even deeper than the CRT on its own needs. I also have seen two transformers with the CRT between them. This assumes that the transformers give equal magnetic stray fields.....
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Re: Magnetic Shielding Materials

Postby Harry Dalek » Wed Jun 06, 2018 3:40 pm

Klaas Robers wrote:The trick is that a magnetic field perfectly aligned with the centre of the CRT does almost no harm. So if you can place the transformer by rotating such that the external magnetic (stray) field of it is coinciding with the direction of the electron beam, you don't see the disturbances. And because you don't know how those stray fields are exactly directed, it is wise to rotate the transformer experimentally. I also have read in the past that initially the transformer is connected with long leads and you move the transformer by hand such that the picture is best. And then try to fix it in that position. Some compromises are in most cases unavoidable.

In general placing the transformer directly behind the CRT is a good plan. But then your cabinet is going to be even deeper than the CRT on its own needs. I also have seen two transformers with the CRT between them. This assumes that the transformers give equal magnetic stray fields.....



That's good knowledge to know any one interested in Cathode ray tube technology this is gold.
A good procedure to follow for sure Klaas .
What you mentioned is what must be happening with my trio scope transformer it doesn't look like its any different to a normal transformer no shield just set at an angle to one side of the crt across from the electron gun area .
The electromagnetic spectrum has no theoretical limit at either end. If all the mass/energy in the Universe is considered a 'limit', then that would be the only real theoretical limit to the maximum frequency attainable.
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Harry Dalek
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