[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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