George,
I discuss problems with powerline-spikes in chapter 8, Designing Power
Supplies, of my book Electronic System Design: Interference and Noise
Control Techniques (Prentice-Hall, 1987, now out of print). For
equipment that will be used indoors, you should try to design your
equipment to be immune to 6kV spikes. That is approximately the voltage
at which our wall outlets arc over.
A quick and dirty test is to plug a vacuum cleaner into the same wall
outlet as your product, and try to run your product with the vacuum
cleaner running. Vacuum cleaners use universal motors, which have the
rotor windings connected to the field winding through a commutator. The
arcing at the commutator puts a tremendous amount of hash on the
powerline, which will sneak through linear or switching power supplies
if you aren't careful.
One of my first business trips after I got out of college was going to
Chemical Bank in New York City, because one of the Sycor 250 terminals
(for which I had written the firmware) would lock up every night. The
hardware designer and I installed some hardware and software monitors on
this unit, and left for the evening. Next morning we returned, and
discovered that it had died shortly after 11pm-- the very time that the
cleaning people were making their rounds! We discovered that the
cleaning people were plugging their industrial vacuum cleaners into the
same wall outlet as our terminal because it was convenient. I think
that the bank changed to a simplex wall outlet there, and that solved
the problem.
A very basic precaution is to put ceramic capacitors in parallel with
the bulk electrolytic capacitors in your power supply, to short out the
high-frequency components of the spikes. A common-mode choke on the
input can also help a lot. To see if this would help, you can put a
snap-on ferrite sleeve around the power cord next to the power supply.
John Barnes
dBi Corporation
http://www.dbicorporation.com/
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