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Has anyone tried this, any thoughts? I am doing radiology as part of my degree and it occured to me that this might be a significant tweak.
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There are two types of fields which need to be dealt with.Magnetic fields
and
Electric fieldsFor magnetic shielding, a high magnetic permeable material should be used. The best is mu-metal. It is expensive and difficult to work with. Next best is 'soft' iron. It needs to be around 10 times the thickness of mu-metal, for the same effectiveness.
For electric fields, a high conductivity material should be used. That means, silver, copper, or aluminium. Copper is best, thanks to it reasonable cost and ease working.
Lead is not much good at either of these jobs. It is also quite poisonous.
Trevor
The often mentioned mu-metal is in MOST cases NOT the best material for the power transformer. While it works exceptionally well in shielding the receivers (things like the CRT, or the microphone transformer), where fields are low, it usually saturates very quickly (due to its high mu) in the strong fields usually found next to the power transformer cores.As result, its efficiency drops like a brick to virtually zero, and the "proper" and cheaper soft iron sield is far, far more effective.
There are various alloys that are positioned between theh traditional mu-metal and soft steel (things like Conetic), but their makers will also warn you about the saturation problems as well, so they need to be tried in every case.
So as a rule - use mu-metal for the sensitive components, use soft steel for power transformers.
Also, don't forget a very cheap and effective solution - a short copper turn around the transformer, easy to add and works like a charm.
"Also, don't forget a very cheap and effective solution - a short copper turn around the transformer, easy to add and works like a charm."Does this amount to just winding, say 12 gauge copper wire, once around the transformer parallel to the coil windings? or perpendicular? Do you then ground the turn?
Thanks!
You can certainly use some bare wire, but the problem is when you connect the end you have all that wire lenght - with its extra resistance - reducing the effectiveness. That is why the copper strip is usually used, as its resistance is very low.You can still use the wire, but in that case I would also solder few pieces across the new wire stack, to short them more effectively (connecting all turns together).
And yes, you wind the wire parallel to the coil windings.
Correct, Victor. Just to clarify, though, the copper strap must NOT be wound directly over the windings, as this will create a shroted turn, Much smoke will result. The copper strap should be wound around the entire transformer. That is, around the outside of the laminations.
Trevor
What about the grounding issue? Where would you attach the shield?Thanks in advance.
...as this is not a shield, but a field drain. So usually you just let it touch the core and brackets, most likely grounding itself in process. I have never seen any with a ground wire, and never put one there muself.
nt
What does that mean?, am curious....thanks
It is actually quite common, you can see it in many older audio products. Here is how it is constructed - if I can successfully put it in words.Imagine you have the standard E-I type power transformer. Look at how the winding is done - pretend it goes horizontally. Let's say, the bobin is 2" high - so the winding takes that 2" space.
Now, take a strip of copper (the heavier gage the better) of 2" wide and wrap it around the transformer in the same direction as the direction of windings. Usually you bend it as you wrap, so the copper follows the shape of the transformer. Thet you solder the ends of the copper strip together and you are done.
This short turn works as a shunt absorbing the energy from the stray magnetic field and converting it into heat (think of it as your French drain if you will). It is not gonna get hot, as the energy levels are low, but it can be extremely effective.
Best of all, if you have a piece of gear and would like to reduce the magnetic field leakage, that copper turn can be added easily without removing the transformer - I have done it many times.
So would this strip go around just the windings, or the laminations as well?
Sorry - I though it was clear from the description, but as Trevor rightly clarified it, it goes around the whole shabang. DEFINITELY NOT around just the windings, as this will simply burn the transformer in no time at all.Often it will go under the bell caps, and be clamped by them, so you woudl only see it between the caps.
I presume many of you have seen it without paying attention to it.
How close should the copper strap be to the windings? Is the plastic that is already there enough of an insulator, should I add more, or should I leave a small gap?
Simply treat it as any other grounded or chassis part. In some cases the existing insulation may be sufficient, but it also depends on the trnasformer application. For instance, if the top winding was the floating filament supply that worked at high voltage, I would add a layer of insulation.The copper doesn't need to be super-close to the windings, and usually you just make it snug over the structure. What I have done usually is to solder the ends, and then you can shape it a bit more tightly.
I'll give it a try next time I get the urge to do some tweaking.
no text...
no Pb plse. Just use mu-metal.Quest ;^>
Lead is good for blocking x-rays, but not so good for magnetic fields. The best thing to use for magnetic fields is iron or MuMetal (nickel/iron alloy). I'm have been working on a similar project for a while and will post results when I'm done. I posted (below) some of my initial findings under the thread "having fun with my Trifield Broadband meter".
Thanks for the input, I look forward to reading about your results.
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