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RE: Anybody using dc-filters on the 12V, 5V and 3,3V DC power supply lines?

Smoothing caps on the P4 line [is] a way of shunting frequencies.

Yes, it was me who cited Ivor Catt's book - I'm very glad you found it interesting. As you know, the circuit you show is taken from his chapter on mains filters. I'd hesitate before using it willy-nilly in other applications as it's designed for AC mains with a safety earth, not low-voltage DC supplies.

Any power supply, be it SM, linear or battery, has an internal resistance. (In the case of lead-acid batteries, this is quite high and, obviously, increases as temperatures fall. That explains in part why cars are harder to start in cold weather - V is fixed, R goes up so I goes down and the starter motor struggles.)

If you place capacitors across e.g. a PC-PSU's 12-volt P4 line, you are in effect creating an RC low-pass filter (i.e. one that passes DC but tends to filter out HF) where the internal resistance of the PS and its cables provide the "R" bit.

As the values of R (and L) are, to put it mildly, imprecise, the effect is unpredictable but that doesn't matter as the aim is simply to filter out as much HF crud as possible at minimal cost. For a variety of reasons, I'd keep the safety earth well away from the motherboard and rely on the SMPS to provide competent earthing.

Several others here have forgotten more about all this than I'm ever likely to know and, if I've got it wrong, will hopefully explain why but that was the thinking behind my advocating the tweak way back when (though I was by no means the first to do so).

Dave


Edits: 12/15/09
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