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Re: help w/ homade line fil.


> how would I go about designing my own line filter?
> I have boxes of reclaimed caps and inductors and would 
> like to save some money by using what I have on
> hand.a ready to go schematic would be wonderful :)

a full filter:

hot -------+------uuu-----uuu----+-------+-------- transformer
           |      ---     ---    |       |
          === Cx    / L1  |  L2 === Cx  === Cy
           |       /      |      |       +-- mainsground(PE)
           |      ---     ---    |      === Cy
neutral ---+------uuu-----uuu----+       | 
                                 +-------+-------- transformer

Cx is, at least here in europe, most commonly 470nF (.470 microF) at 250
volts AC, sometimes even 1 microF. That gives around 220nF for your line.

Those Cy caps are at most 22nF (0.02 microF) here, to keep hot->ground
leakage current below 3.5mA, so as a guess 10nF could be ok for your

Best to use those caps with lots of certificates printed on them... ;)

The chokes: the wire has to be thick enough for the current (AWG #28 could
work fine). Both L1 and L2 have small ferrite cores. L1 has two windings,
wound in opposite sense or direction. L2 has two windings wound in the
same direction and sense. Values range from 0.5 milliHenry to some
milliHenry. Maybe 1 mH for each of the four parts of the two chokes is ok.
Maybe even more.

Use either ready current-compensated (non-saturating at mains) chokes, or
wind your own chokes on short ferrite rods.

A bleeder resistor of 1MOhm or more accross the Cy caps might be good.

And hey, don't forget to add that obligatory mains fuse... :)

 Jan Florian Wagner