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PostPosted: August 21st, 2013, 9:29 pm 
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Joined: July 24th, 2013, 11:11 am
Posts: 21
Shielding a transformer is something I have been interested in but I have only looked into ready made cases which are hard to find and usually not sold individually. Now I am planning on fashioning some myself. Looking at step ups that show when I do a search I see they are usually aluminum cases with shielded transformers inside or on top and rca connectors for the in and outs. I've decided instead of making small cases for individual transformers I will make a larger case/shield combo with rca connectors shielding both transformers. There will be some shielding seperating the transformers so there is no crosstalk between them. Not sure if there would be but I will add this as a precaution. The shielding will consist of different types of metals layered to increase their effectiveness. This is an article I found from 1938 called Magnetic Shielding of Transformers at Audio Frequencies.

http://www3.alcatel-lucent.com/bstj/vol ... -3-416.pdf

Lots of useful information in there. This is where I found the layering of different types of metals to increase efficiency. They say to use layers of Permalloy and Copper. This just happens to be what the modern Texas Instruments TI-Shield is made with. The TI-shield uses 49 Permalloy which is 49% nickel. I have ordered two sheets of 8" x 10.50" x .004" Ultraperm 80. That should be plenty for multiple layers. The Ultraperm 80 is 80% nickel which will give it a higher amount of shielding. I will buy some copper sheet from the hardware store. I would like to have the outside case made from steel if I can find what I need.

Attached is a diagram of the TI-Shield.

Any thoughts or input will be appreciated.
Scott


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PostPosted: August 22nd, 2013, 4:45 am 
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Joined: February 28th, 2013, 1:19 pm
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Mu-metal is what is typically used for transformer (magnetic) shielding, which is a 77% nickel alloy, roughly corresponding to the Ultraperm alloy you mention above. Mu-metal is more effective than permalloy (which was developed and marketed by Western Electric, BTW). It does not tolerate bending or forming without loss of magnetic shielding properties, so has to be re-annealled after mechanical forming. I do not know if Permalloy is subject to a change in it's properties if you form itm, but this is something you should be aware of before trying to use it for shielding. You can wrap a transformer in a rolled tube of mu-metal, taking care not to actually bend it, if you have sufficient shielding material extending beyond the transformer (about 1 diameter or so), it will be as effective as if it were completely enclosed. At least that is what I have been able to run across on shielding.

David


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PostPosted: August 22nd, 2013, 4:52 am 
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Joined: February 28th, 2013, 1:19 pm
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I would like to add one other thing. Cross talk between MC transformers at the low signal (& flux) levels we are talking about should be non-existant, so long as the cores are not in physical contact with each other, so no real reason to provide additional shielding between transformer. Usually, transformers are individually shielded, so you get the shielding between them anyway.

David


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PostPosted: August 22nd, 2013, 11:09 pm 
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Joined: July 24th, 2013, 11:11 am
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The Mu-metal and Ultraperm 80 both have the need to be re-annealled after bending or cutting. Supposedly the 49% permalloy does not. The change in magnetic permeability is only where the metal is bent or cut thou. The rest remains the same. So a sharp right angle bend would only loose some permeability at the crease but even there it would still retain some shielding ability. I am planning on cutting the Ultraperm into pieces and overlapping the edges creating a box inside of a box. The boxes will be alternating layers of Ultraperm and copper. Depending on if I can find a suitable steel box or not, I may use steel sheet also. I will do some tests this weekend to get an idea of what this stuff can do.


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