[sdiy] Fixing humming transformers in 1u rack units

[Scott Nordlund]

> Did you check if your added metal shield gets hot, or if the  
> transformer gets warmer than without shield? Somewhere the energy  
> that used to vibrate the case cover must go. Some really nasty things  
> can happen when a toroidal transformer is improperly shielded.  
> Probably not an issue at all in your case, but I'd check nevertheless.

In the case of the "E-Core" transformer, any divergent flux that couples
into nearby circuits or causes mechanical vibration should be quite 
small, and not a big deal to eliminate with shielding.  It will create 
eddy currents in the shield, but it shouldn't be much cause for concern.

Toroidal transformers are a different matter.  Due to the shape of the 
core, they have much lower divergent flux.  BUT if you happen to, say,
mount it with a bolt that runs through the center hole and connects to
the chassis on both sides, this effectively adds another winding to the 
transformer.  You really don't want that to happen.
 		 	   		  
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