Topband: Desktop Power Supply Brand/Noise Question

[Jim Brown]

On 7/21/2013 8:49 AM, Tom W8JI wrote:
>
> I can't understand why throwing some ferrite beads at a problem, or 
> changing the supply, are the only two solutions.
>
> Many times, if not most times, a few .01 uF line voltage rated bypass 
> capacitors are significantly better than a sting of cores, or a 
> winding through cores. 

Yes, IF the problem is differential-mode coupling to the power line. But 
often it is NOT -- if the trash is coupled to the power cable as a 
common mode voltage on the green wire, those capacitors won't do a 
thing. There are also times that because of the way a product is built, 
it is not practical to modify it. Or because the owner might wish to 
avoid a warranty issue. If the problem is common mode, a ferrite choke 
is by far the best solution, and often the only solution.

A very common way that trash ends up as common mode on the green wire is 
when the green wire fails to contact the chassis (or shielding 
enclosure) where it enters the unit, but instead wanders around circuit 
common before it MAY or may not find the chassis. All three of my linear 
Astron supplies have this problem -- the green wire goes to the 
retaining lug of a traditional terminal strip, that is insulated from 
the chassis by paint.  Circuit common also goes to that lug. More often 
the problem is intentional, and the result of cost-cutting in the design 
process.

Yes, opening up the unit and correcting it fixed those Astrons, but many 
products with this sort of problem cannot be fixed without major 
surgery.  For all of these reasons, I continue to recommend the use of 
common mode chokes that are optimized for the frequency range of the 
interference.

73, Jim K9YC