Electronics > Beginners
is it worth chaining mains filters?
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exe:
Hi!

I'm building a lab power supply and I wonder if I can have better filtering of mains noise by adding more filters. Currently I have one filter directly in the inlet (schurter 4304.4001, bought on flea market), and a discrete one (filtercon 1FP414-1R). Shall I use both or only one of them?

I tried to do some measurements with my AD2. May be I'm doing measurements wrong, I just attached signal gen to input, scope probes to output and measured amplitude. I expect to see differential mode rejection :).

schurter 4304.4001 datasheet: http://www.farnell.com/datasheets/2343272.pdf
filtercon 1FP414-1R datasheet: https://www.tme.eu/en/Document/c532f99b805f3981a0ea6c4609c8138b/FILTERCON-FP.pdf
T3sl4co1l:
Sort of.

How much filtering do you actually need?

A filter is useless without a spec to meet!

Just as bad is, a filter without a test to prove it's meeting spec.

If you don't have a spec or a test... why bother? :)

Tim
exe:
Can it be a best-effort approach? :). Less noise is better. I have total noise budget of 1mV p-p noise in, say, 100MHz bandwidth, but I don't know if this is achiavable or not. The psu itself is linear, but I doubt that it will have good rejection above 100KHz, esp. above a few MHz due to caps self-resonant frequency (but never did any measurements, I judge on impedance curves from the manufacturer).

Looking closer to graphs I see that schurter alone gives more rejection than two devices together. Probably, I should just use one...
T3sl4co1l:
Okay but what are you doing?  For an SMPS, you'll probably need one of the better filters, like the Schurter or even a 4- or 5-pole one.  Preferably something similar on the output side as well, with the SMPS chassis grounded to the enclosure, and using a metal enclosure.

More Y-caps means more ground leakage, which can be a problem, particularly for medical equipment.  YMMV.

For a linear supply, it seems doubtful that you'll notice anything at all, with mains ripple being the dominant noise output.  Preferably, you'd have a power transformer with an electrostatic shield between primary and secondary, but honestly I wonder how the price compares between one of those (which may not be very standard parts) and putting a line filter out front (which are rather cheap).  Don't forget an R+C (maybe 10R + 10n) across the rectifier diodes to deal with possible recovery noise.

Tim
exe:
Sorry for the late answer. Tim, are you sure that rc-snubber is needed for diodes in bridge? Mr. Rod Elliot says it's not needed due to very low mains frequency. Here he provides some measurements showing that fast recovery diodes behave better: http://sound.whsites.net/articles/psu-snubber2.htm (schematic of his "hight voltage probe" is in the first part of the article)

(I'm gonna use LT4320 and some fetsas bridge, not sure if this still need a snubber, I'd expect it to be a relatively slow circuit)
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