[Elecraft] Shack equipment noise
Jim Brown
jim at audiosystemsgroup.com
Mon Dec 12 12:17:10 EST 2011
On 12/12/2011 9:04 AM, "Carl, WCØV" wrote:
> What isn't mentioned is the fact that MOVs are rated by the TOTAL energy
> in Joules that they will absorb in their lifetime before they fail. The
> energy absorbed from spikes is cumulative and when the rating is
> exceeded, boom.
> They are also responsible for nasty noise generated by the crowbar
> action on the spikes in making them square waves - the sum of all
> possible harmonics. I
> used to have to deal with these things when customers would use them
> in front of servoamplifiers. The noise from the square waves drove us
> crazy until we found the MOVs and removed them.
YES! We learned about this in the pro audio world as well, and did the
same thing to search out and destroy them. And yes, they are insidious,
in everything from copy machines to printers and even power supplies. I
found some in an Astron power supply when they failed.
The WORST thing about MOV-based protectors is that they CAUSE
destructive failures of equipment that is interconnected by low voltage
wiring when that equipment is plugged into different power outlets, even
when all of the outlets are "protected." That's because they dump the
surge into the green wire that serves each outlet, the length, and thus
the voltage drop due to the surge to each outlet is different, and the
DIFFERENCE blows up the equipment. That's why MOVs are ONLY a good
thing at the service entrance.
73, Jim Brown K9YC
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