Topband: Desktop Power Supply Brand/Noise Question

[Tom W8JI]

> I simply purchased 10 Corcom filters on ebay ( I think they were $2 each +
> shipping). I cut a power supply cord (universal plug type) about 6 inches
> from the computer and wired the filter into a plastic electrical box with
> the other end of the cord out the other end of the box. I used standard
> electrical cable clamps to secure the cable to the box and added the
> cheapest electrical plastic cover. The whole thing cost me about $6 each 
> and
> I built 5 of them in a couple of hours.  The boxes live behind the 
> computer
> so the blue color appearance doesn't matter.  When I purchased newer
> computers, I just moved the filtered cord to the new computers.

That's OK, but be aware the corcom appears to be a typical design with 
limited value for choking isolation.

See this page:

http://www.cor.com/pdf/DA.pdf

The windings appear to be linked magnetically, and the ground appears to 
have no isolation.  This seriously limits the effectiveness of the filter, 
making it similar to beads over cords. The worse part is not having 
isolation on the ground path.

The best part is the filter adds bypass capacitors, which when properly 
sized either assist any added iron core cord chokes, establish a low or 
controlled differential and common mode impedance, and often do a better job 
than any practical choke system or core does.

I use a filter like this:
http://www.w8ji.com/images/filters/filter7.gif

It isolates and bypasses all three wires.


One of my tricks in a suburban environment was a three wire plug with two 
.01 uF  250VAC UL/CSA bypass caps. No cord or anything on it. I moved that 
plug around my house until I found a sweet spot that eliminated conducted 
noise from the house next door. They had some battery charger that just tore 
up 80 and 160 through our common sharing of a pole transformer.

73 Tom