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
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