[sdiy] case shielding properties

[harrybissell]

If the steel is in a continuous path around the circuit... it will make the
external
magnetic flux flow in the steel... stopping it from getting to the circuit
inside, That
circulating current can (and will) induce a voltage drop across two ground
points
on the box.  Aluminum will do the same btw.

So the right answer is a continuous steel shield... with one and only one
ground point.

I got burned very badly by this in a high gain circuit once...  I ended up
using MuMetal
(a nickel cobalt chromium alloy) known for its extreme affinity for magnetic
flux, to shield
the transformer (it was inside my box)

Know whats even better... keep the transformer outside of the box (wall wart
- on the
wall 6 feet away... ;^)

H^) harry

Paul Higgins wrote:

> Isn't there also an issue with steel actually conducting magnetic
> fields, say from very large transformers, and injecting it into the
> circuit?  I think I remember reading something like this in an old book
> on tube amplifier construction.  It is not surprisingly the biggest
> problem in low-level signal paths, especially those that use magnetic
> materials (e.g. microphone input transformers).
>
> -PRH
>
> On Thursday, May 27, 2004, at 05:24 PM, Tim Parkhurst wrote:
>
> > As a mechanical designer, I would have to go with the steel for the
> > best
> > shielding properties, followed by the thick aluminum and then the thin
> > aluminum. Some other factors will influence the 'quality' of the shield
> > (rectangular is better than square, use fewer parallel surfaces, round
> > openings are better than squared-off openings, etc.), but if you look
> > at the
> > material used in RFI shielding you will find a lot of beryllium-copper,
> > plated copper, nickel-steel, or steel with various platings, and brass
> > with
> > various platings. Aluminum is rarely, if ever, used for shielding
> > purposes.
> > Unfortunately, most of the shielding materials I mentioned above are
> > not
> > suitable for making enclosures, but given the choice between steel and
> > aluminum, I'd have to go with steel.
> >
> > Another choice would be an aluminum case with internal shields made of
> > thin
> > steel, copper, or brass. I've done this quite a bit for RF amplifiers,
> > receivers and the like (I've designed enclosures for a lot of gear like
> > this). This is a very common industry practice.
>
> Paul Higgins
> email: higg0008 at tc.umn.edu