[Boatanchors] Computer/ Switching noise

[Robert R. Kerr]

I work in an office environment where everyone has a PC on their desk 
and there are racks of server PC machines here and there.  All these 
machines have switching regulator power supplies, which are plugged into 
the AC power lines.  The noise from the switching regulators gets 
coupled to the AC power line, which functions as an antenna for RF 
(sound familiar?) and radiates from there.  Things got so bad in our 
building for a while that it was impossible to listen to any local 
broadcast AM radio station.  Unfortunately, we also had an electronics 
laboratory where we needed to be able to make accurate high frequency 
measurements, and the hash radiating from the power lines interfered 
with this, too.

The FCC has regulations for noise coming from PC power supplies, and 
manufacturers models have to get type acceptances for all their models 
before they can sell them in the US.

What happens is that the power supply manufacturers, most of whom are 
located in Southeast Asia, also build power supplies for PCs that go to 
countries that don't have an FCC or any regulations regarding power 
supply noise.  In order to save a dollar or so on the cost of each, they 
substitute jumper wires where the series inductors in the line filters 
are supposed to go, and do not install the shunt capacitors.  These 
supplies can be sold cheaper than the ones with filters, and a lot of 
them do make their way to the US anyway, where they are rarely checked 
after the initial type acceptance test for the particular model.  This 
is the sort of stuff you find in the really cheap computer component stores.

If you've got a noisy PC power supply, have a look inside where the 
wires from the AC line attach to the PC board and you'll usually be able 
to see where this has been done.

I've found the best cure for this is to buy "lab grade" power supplies.  
These often cost several times what the power supplies from the cheap PC 
stores cost, but they are usually quiet.  Personally, I've had good luck 
with the power supplies with the brand name "Enermax", which actually 
advertises low noise (No, I don't own their stock), but I'm sure there 
are others.

The distressing thing to me is that this problem seems to be getting 
much more common, especially with "wall wart" type power supplies, which 
seem to be mostly switching regulator units these days.  Right now, I 
have to turn off my cable modem in order to use the radio because it has 
a noisy wall wart power supply.

Bob K4ERR

Dave Maples wrote:
> All: I have battled this several times over the past few weeks.  Best thing
> I have found yet is the Radio Snack split cores.  I will take one and wind
> as many turns of the offending cable as I can around the split core, and
> then reassemble it.  Two of them removed the noise from a cordless phone
> base station.  One removed noise from an LCD monitor.
>
> It's going to cost me a bit, but I'm eventually going to get both ends of
> every computer, entertainment equipment, and telephone lead I have.
>
> My next move is to sample some of the same product from Mouser (a LOT
> cheaper).
>
> All suggestions here welcome!
>
>
> Dave WB4FUR
>
>
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