[RFI] Surge Suppressors and EMI
jim at audiosystemsgroup.com
Thu Mar 18 10:44:52 PDT 2010
On Thu, 18 Mar 2010 07:23:27 -0400, Pete Smith wrote:
>Has anyone explored the EMI suppression capabilities of the Staples
>Model 16949 Surge Protector? The spec sheet promises "150 KHz -100 MHz,
>up to 58 dB." Of course the "up to" renders the spec meaningless, but
>the strip otherwise seems like a good value.
>Is there another surge protector strip that is known to be particularly
>effective in suppressing EMI at HF?
Virtually all consumer surge suppressors use MOVs to short out the surge.
An MOV across the line is OK (that is, from Line to Neutral), but an MOV
from Line to Ground (the green wire) has the potential to cause destructive
failure of equipment because it raises the potential at the outlet where it
is connected by virtue of IR drop (and IZ drop due to inductance) of the
green wire. If there is a signal line connecting equipment plugged into
different outlets, one or both protected by MOVs to the green wire, a high
voltage exists between those two pieces of gear and is likely blow up
circuitry in one or both units.
If you want EMI suppression on a power line, a FAR safer (and likely more
effective) way to do it is with a dedicated power line EMI filter. Corcom
and Delta are two respected brands. Both can be found at "closeout" prices
with a google search, and their published data sheets are easily available
on line. The link below shows part numbers and my plots of manufacturer's
specs (NOT my measurements) for a variety of such filters that I found at a
Silicon Valley surplus house for under $10 (some as cheap as a buck!). Put
one of these in a standard electrical backbox with outlets and you've got a
FAR better line filter than almost anything you're likely to find at
consumer retail. There are photos in the pdf to give you ideas. I've built
up a bunch of these, and plug all my consumer gear into them (hi-fi, TV,
wall warts, etc.)
Since I wrote this up, I put one of the largest of these (the one on the
top right in the photo) in a six-gang box with connectors to use it at the
output of our Honda Field Day generators.
Another VERY important point. The place for a power line filter is at the
SOURCE of the noise -- that is, where those noise-generating power supplies
are plugged into the power system. That's because the trash they generate
is RADIATED by power wiring directly to your antennas. It is NOT usually
conducted via the power line to your equipment. The only thing that a power
line filter in your shack does is keep YOUR RF trash (including that
generated by noisy power supplies) off of the power line.
Jim Brown K9YC
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