Off the shelftransformer cans?

Does anybody have an off the shelf solution for transformer cans mu metal or otherwise? Something that would be used in small input transformers?

I find lots of manufactures by googling, but so far, everyone of them what to sell 100's at a time.

The only ones I've seen for reasonable prices are the ones sold by Vigortronix. They are limited in their size ranges, but are cheap. They worked with some Edcor input transformers I had, with a little modification using a Dremel tool.

Sometimes you can get lucky and find the old UTC shield cans (A and O sizes) on eBay for $25 - 45, but more often they are ridonkulously expensive.

In the end though, I finally just bought some mumetal sheet stock and used a little Harbor Freight bending brake to start making my own, Lundahl style. Works very well for my needs. Way cheaper that way, even with the up-front brake investment.

BT

OEP offers cans for their transformers, maybe something fits?

I'm no expert on that, but I vaguely remember that drilling / bending mu metal does kill the effect of the mu treatment, doesn't it?

Michael

rackmonkey said:

In the end though, I finally just bought some mumetal sheet stock and used a little Harbor Freight bending brake to start making my own, Lundahl style. Works very well for my needs. Way cheaper that way, even with the up-front brake investment.

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That's great, but once you machine mu-metal, its high-mu properties get greatly diminished, and you have to do a high temperature, hydrogen atmosphere annealing to regain the grain oriented properties of mu-metal. That kind of annealing is not simple to do at home.

So, maybe test the finished product to make sure you're getting the permeability that you expect, or maybe just use a lot more material to make up for the diminished permeability?

A location in your profile would be cool ...

https://www.buerklin.com/de/mu-metall-hf-abschirmgehaeuse/p/70h1402
https://www.buerklin.com/de/mu-metall-hf-abschirmgehaeuse/p/70h1412

Yes, it's worth a more detailed discussion than my quick initial reply.

I did a lot of poking around for information on this when I was looking for a solution and in the end, I took a practical approach. We're not doing MRIs here. The goal is to reduce induced noise as much as possible, given the tools and materials available to us.

Yes, mu metal and other hi nickel alloys (supermalloy, etc) are very sensitive to deformation. Even dropping them has a significant effect on their shielding effectiveness. The process required for re-annealing isn't practical for the DIYer.

Here's a chart from a research paper on the relative shielding effectiveness of various materials:

https://drive.google.com/open?id=0B__Vr0ik2GcgWVJGdmdFQkRHdkE

Nickel (expensive) and stainless steel (reasonably cheap) do have some degree of magnetic permeability, but the hiNi alloys are >20 times more permeable.

Here's a chart showing the permeability loss after dropping from various heights:

https://drive.google.com/open?id=0B__Vr0ik2GcgdVplZGdJaWRuMms

I couldn't find any data on the effect of cutting (with shears) and 90 degree bending, but if we use the drop data as a rough reference point, the drops from 30.5 and 61 cm reduce permeability by half (single drop). If the effect of cutting/bending is roughly equivalent to a single drop from one of these heights, we still have a material that's at least 10 times more effective than nickel or stainless steel.

The question in my mind then becomes, is the cost of mu metal sheet worth it for the level of shielding I can expect? For most of the sheet material I found, the answer was "no" (for me anyway). But I did find a couple of sources on eBay that were relatively cheap, which made it attractive to try it. The results have been good enough for me - I have definitely seen a difference between shielded and unshielded input transformers in my crude tests. So I settled on this path as good enough.

Joints are critical in any magnetic shielding application, as gaps allow magnetic leakage into the shielded space.  Again, we're not doing MRIs here and I just did the best job I could. Results were good enough for me.

Here's the full slide deck I took the drop data from:

https://drive.google.com/open?id=0B__Vr0ik2GcgZW40eVBQVElkWGs

It has a nice little chart in that compares cost of various materials to effectiveness. This might give some folks some ideas on other materials to try.

https://drive.google.com/open?id=0B__Vr0ik2GcgRWM5cHF3YUl0ZWc

The other approach I've tried that works about as well is to use 3M magnetic shielding tape. It's the same stuff I think Edcor uses on some of their transformers. It's not cheap. In my experience it's about the same in terms of cost per transformer as the mu metal source I found. Easier to work with, for sure. Maybe a combination of the 3M tape and mumetal would more fully address the joint leakage issue. I don't know. Someone more obsessive about this stuff than me could maybe try it.

BT

Looking back at the cost/effectiveness chart, it reminds me that good old iron  has pretty decent qualities at low cost. The Kenyon/UTC/Halldorson, etc guys obviously knew that because all those old transformers from the 40s and before had iron cases on them. Probably before hiNi alloys were economical.  Cheap iron transformer covers from China are all over eBay.  None are small enough for Input transformers though (where shields matter most). But a sheet of 18 gauge iron or ferrite steel is very cheap and is probably worth experiementing with. Just a thought.

BT

other threads besides this if you search DIY

https://groupdiy.com/index.php?topic=63740.msg807053#msg807053

Exotic materials are better for the same dimensions. But if space is not limited, we can simply use 10x the thickness soft iron, and get similar magnetic shielding. For inputs, soft iron water pipes - you can get short pieces already threaded, and get off-the-shelf end caps too. Works fine, but looks threatening.

Jakob E.

Remember that 2 walls of unit thickness separated by air/plastic/paper/wood is more efficient than a single wall of double thickness.

Link to the Vigortronix cans I mentioned up top. Bottom cover included, I think.  Looks like the OEPs are slightly cheaper. Appear to be the same cans.

http://www.newark.com/vigortronix/vtx-102-000/screening-can-mu-metal-silver/dp/33P8723?brand=vigortronix&DM_PersistentCookieCreated=true&mf=100236&anyFilterApplied=true&ddkey=http%3Aen-US%2FElement14_US%2Fc%2Fenclosures-racks-cabinets%2Fenclosures-boxes-cases%2Fmetal-enclosures

BT

Great information, thank you to all.

More questions.

Interwinding shielding tape? Copper + mylar or some other type of non-conductive backing? All the copper tape I'm finding so far is just copper or has conductive backing.

Trusted and used potting compound? So many different types Which one has someone used and likes?

mkiijam said:

Trusted and used potting compound? So many different types Which one has someone used and likes?

Click to expand...

Beeswax.  Cheap, easy to melt, non-toxic.

Yeah, good idea.

Any leads on copper tape?

I had some good results by ripping off the copper wire from a toroidal transformer and wrap it around the power trafo of my preamps. Should work the other way around ! Hopefully you have a burnt one that's already useless, or you sacrify a cheap one . I think that made the difference necessary to bury the buzz in the noise.

Used it succesfully in some THAT1512 preamps, now I'm currently experimenting it in Jonte Knif's Therapy Preamp (tube). That's about 12dB on the strongest harmonic, with the amount you see on the joined picture, spread on the whole transormer side.

https://drive.google.com/file/d/0B6K-0QVHN7zUX2tPS2Y2TUFsOEk/view?usp=sharing

Copper tape works nicely for electrostatic problems, but not at all on electromagnetic.

Jakob E.

Right, I want copper tape with a plastic/mylar/ non-conductive backing.

I want to use it for an electrostatic shield in-between the primary and secondary windings to shield HF coupling.

To solder onto it, it cant have isolated backing. The copper tape I've seen in transformers is non- isolated but protected between layers of isolating tape..

Jakob E.

mkiijam said:

Right, I want copper tape with a plastic/mylar/ non-conductive backing.

I want to use it for an electrostatic shield in-between the primary and secondary windings to shield HF coupling.

Click to expand...

Google foil inductors. The one I unwound out of curiosity was made like that, I think people unwind them and use them as speaker wire. Gyraf's right though, you're not going to be able to solder on it without destroying the insulation.

Is the HF shielding really needed? I built a prototype that sounds fine and seems to work great.

Does the copper cause some loss? It must have pros and cons?