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Old 8th April 2007, 05:53 PM   #1
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Join Date: Jan 2004
Location: Minnesota
Default line filter design.

I have seen line filters with caps across the H/N and caps across the H to G and N to G.

what is the difference and should I do both?

Should all the filter caps be really symetrical about the G?

i.e. cap from H to G and same from N to G? then maybe a cap across the H and N?

CMC chokes in the H and N lines 50-100uH <0.03DCR and another cap where?? across the H and N or 2 caps from H and N to G?

a little confused... whats the best way... and whats the difference?

Thanks!
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Old 8th April 2007, 10:54 PM   #2
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Join Date: Mar 2007
Location: Canandaigua, NY USA
You might try a Google search on LISNs. These are used in EMC testing to be sure that line noise doesn't get into the test, and to provide a sample port (that you don't need). There are a couple sites with schematics. Don't build one, just read the logic of why they design them the way they do. Then, look at the schematics for Corcom (or other) commercial line filters. I tend to use these as the power input for my projects, rather than brew my own. You can get them with both switches and fuses built in. IMO, noise can appear on both hot and neutral, so caps from both to ground are good. They also do the job of a cap across the two, but you can add it anyway. You *must* use caps rated specifically for this kind of use, not just caps that appear to have sufficient rating, lest you cause a hazard!
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Old 9th April 2007, 03:30 AM   #3
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What Conrad said. Unless you design filters for a living and you're up to date on all the possible ways things can go wrong, you're much better off buying an off-the-shelf line filter instead of rolling your own.
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Old 9th April 2007, 07:20 PM   #4
Tweeker is offline Tweeker  United States
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Ive seen some claim Y caps (line to ground) contaminate the ground with noise. That said, be sure any such cap is small and rated for this duty, it puts leakage currents on the ground. X caps (line to neutral) should also be rated for this duty. Using an oversize Y cap is not safe, and may be sensed by GFCI.

Ive seen some that are pi filters- ei. X cap, CMC, X cap. The caps not not necessarily the same size.

Many of the line filters in use are more about protecting the line from noise from the equipment the filter serves rather than vice versa.
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