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Re: [TCML] Corcom EMI filter
Gary Lau wrote:
The filter case should be connected to the mains green-wire ground. I've
written a web page on this very non-intuitive topic:
http://www.laushaus.com/tesla/emifilter.htm
I generally agree with Gary's observations on the linked page..
What's interesting is that these filters are designed not to filter out
interference coming from the outside, but to reduce noise being
propagated out from your circuit.. so the "use it backwards" is entirely
the wrong way to use them. If you think about it, they get used because
manufacturers want to meet a FCC or EU "conducted emissions" test, not
because the mfr cares about transients from the outside world. So how
the mfr intends it to be used and how we intend it to be used in
filtering the supply for a TC is exactly the same.
A couple things to watch out for..
If you add a capacitor between line and greenwire ground (on the input)
it has to be appropriately rated. If it fails because of a line
transient, it will sometimes fail semi-shorted, and just draw a lot of
leakage current, get hot, and that is bad. (10mA at 120V is a watt, and
a little disk ceramic gets mighty hot at a watt)
You may not want a capacitor to green wire ground on the input anyway..
A cap that is big enough to shunt 100kHz interference to ground is
probably big enough to draw enough current to trip a GFCI. The leakage
spec on the filter is probably around 1 mA.
The Corcom filter that Gary shows has the ground point on the load side,
which is the typical scheme. (http://www.cor.com/) Unfortunately, the
F2411 is an obsolete type, so there's no datasheet on the site. But I
found a "K series" RFI filter which looks similar (in terms of
schematic), so I looked at a 30VK6
http://www.cor.com/pdf/K.pdf
That filter has an attenuation (in a 50 ohm system!!!) of only 10 dB at
100kHz for common mode, 20dB for differential mode. It really doesn't
start to "work" until you get up around 1 MHz.
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