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Old 29th September 2010, 05:10 AM   #1
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Default mains filter

Will a mains filter stop a piece of equipment from emmitting noise onto the mains? For example a noisy pc plugged into a filter.

ALso will the mains filter create more noise onto the surrounding mains supply?
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Old 29th September 2010, 06:33 AM   #2
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Filters are designed to block certain frequencies. That means any noise or other signals in the stop band will be attenuated. But that's only on the wire. Radiated noise won't be eliminated by filters. If you want the most suppression of noise, you need a filter on each lead and a shield over the unit.

Filters don't create noise.
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Old 29th September 2010, 11:00 AM   #3
AndrewT is offline AndrewT  Scotland
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most mains filters work both ways.
They attenuate the frequencies above 50/60Hz, but do it better in the forward direction.

The PC should already be compliant with EMI regulations.
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Old 29th September 2010, 11:29 AM   #4
DF96 is offline DF96  England
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With the emphasis on "should be" - far too often they are not.
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Old 29th September 2010, 12:05 PM   #5
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One of my neighbors found that when I turned on my PC it killed his AM radio! I opened my PC power supply and found the place for the power line inductor had
been bypassed and the caps just not installed at the factory. Good cost cutting!!

I found in my case using an isolation transformer kept the noise off the power line and my neighbor can now use his AM radio.
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Old 29th September 2010, 12:20 PM   #6
DF96 is offline DF96  England
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Yes, this is surprisingly common, especially on Chinese-made goods. I assume they get the PSU through its EMC testing with all components in place, then omit the suppression components for production. It happens too often for it to be an accident. The cheaper factories probably don't even bother with EMC testing, just make up the paperwork and slap a CE mark on the equipment. This is the wonderful world of self-certification!

In the UK this is supposed to be policed by local Trading Standards officers, but they are too busy doing other things. We hear of a shop or importer being fined, usually for bad paperwork, only once every few years so the risk of getting caught is very small.
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Old 29th September 2010, 01:51 PM   #7
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Quote:
Originally Posted by woody View Post
One of my neighbors found that when I turned on my PC it killed his AM radio! I opened my PC power supply and found the place for the power line inductor had
been bypassed and the caps just not installed at the factory. Good cost cutting!!
That's classic, seen that a lot They probably were "some" components short at the "end" of their production batch

Quote:
I found in my case using an isolation transformer kept the noise off the power line and my neighbor can now use his AM radio.
A cheaper option (not for the faint of heart) is to harvest the inductor & caps from a discarded PC psu and implant them in the offending unit.
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