Erik, here is link for your reference that illustrates the whole concept quite neatly. If you look at the 2 stage filter in the middle picture you will see a common-mode core combined with 2 seperate cores, to filter out 2 different kinds of interference. Also the matter of proper wiring is illustrated in the bottom pic.
http://www.emcuk.co.uk/awareness/Pages/MitigationTechniques/Design/Filtering.htm Regards, j. On Mon, Oct 17, 2011 at 9:21 AM, Erik Christiansen <[email protected]>wrote: > On 16.10.11 10:50, Kirk Wallace wrote: > > I happened to have the filter left over from another project, and got to > > thinking, in a pinch, it would be nice to be able to make one up from > > typical junk bin parts. If anyone can shed light on this, I'd appreciate > > it. I noticed some small filters (power cord socket / switch / filter > > unit) on some junk telecomm equipment I have, that may be a good source > > for filters too. (Also old PC power supplies?) > > Those in my junkbox, including one built into an IEC mains socket, have > a full circuit diagram on one side. Before I forget, the capacitors > connected across the line must be approved for the role, i.e. be proper > "X" or "Y" denominated capacitors. (I think the category includes > self-healing dielectric, but definitely indicates they're low > flammability, and won't spontaneously exude their guts like a non-X > capacitor I found in this role, in some cheap equipment. Fortunately it > didn't catch fire, but the fumes corroded adjacent exposed metal. I > wouldn't want to breathe that.) > > The "core" of the filter is usually a common-mode choke, i.e. two > windings on one ferrite core. We've probably all seen them on the input > or output of a SMPS, with two diametrically opposite quadrants of the > core carrying maybe a dozen turns of fairly thick wire. (At low voltage > anyway.) An "X" capacitor across the inductor ends connected to line > provides the input leg of what is a balanced pi filter, and two "Y" > capacitors are connected in series across the load ends of the > inductors. A resistor across the load discharges all the capacitors, so > you don't suddenly drop the thing on your foot, after accidentally > touching the input or output leads. > > OK, those capacitors are a bit fat, but here's an ascii art depiction: > > * > --------------------@@@@@@@@@@@--------------------- > | | | > | XXXXXXXXXXX | --- Y > | X % --- > LINE ___ --- X X Common % R |___ Earth LOAD > | --- X Core. % | > | | X % --- Y > Earth | XXXXXXXXXXX | --- > | | | > --------------------@@@@@@@@@@@--------------------- > * > > The coils are connected straight through, but counterwound, i.e. one > side is CW, the other CCW wound, in the several I've just fished out of > the junkbox, so I'd draw the phasing dots as above. That seems > consistent with the intended rejection of common-mode signals, so I > think I have that right. > > The "6ET1" 6A,120/250v 50-60Hz line filter in front of me uses: > > L = 18.5 mH (Per leg, I figure) > X = 47 uF (X2 type) > Y = 4 nF (Y type), 2 off > R = 330 kOhms > > That's about all I can think of, off the top of my head. It should be > nearly enough to start with. The commercial jobs are built in a > shielding enclosure, but the open ones on SMPSs do a lot of good too. > > You could just use separate inductors, and coils wound on straight > ferrite rods have also been used, but they're, not as good. > > Erik > > -- > In the bathtub of history the truth is harder to hold than the soap, > and much more difficult to find." - Terry Pratchett > > > > ------------------------------------------------------------------------------ > All the data continuously generated in your IT infrastructure contains a > definitive record of customers, application performance, security > threats, fraudulent activity and more. Splunk takes this data and makes > sense of it. Business sense. IT sense. Common sense. > http://p.sf.net/sfu/splunk-d2d-oct > _______________________________________________ > Emc-users mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/emc-users > ------------------------------------------------------------------------------ All the data continuously generated in your IT infrastructure contains a definitive record of customers, application performance, security threats, fraudulent activity and more. Splunk takes this data and makes sense of it. Business sense. IT sense. Common sense. http://p.sf.net/sfu/splunk-d2d-oct _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
