is it worth chaining mains filters? - Page 1
Forum section: electronics / beginners

author: exe
author_rank: Unknown
timestamp: February 04, 2019, 07:26:52 pm
content: 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.pdffiltercon 1FP414-1R datasheet:https://www.tme.eu/en/Document/c532f99b805f3981a0ea6c4609c8138b/FILTERCON-FP.pdf

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 04, 2019, 09:40:42 pm
content: Sort of.How much filtering do you actuallyneed?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

author: exe
author_rank: Unknown
timestamp: February 04, 2019, 09:56:32 pm
content: 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...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 05, 2019, 01:06:48 am
content: 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

author: exe
author_rank: Unknown
timestamp: February 16, 2019, 06:52:37 pm
content: 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)

author: tkamiya
author_rank: Super Contributor
timestamp: February 18, 2019, 04:32:18 am
content: Say one filter reduces "noise" by 60dB. Having two will reduce the same noise by 63dB. I would think it's a kind of pointless gain. Also, those specs are to be taken with grain of salt. In reality, it is nearly impossible to achieve even close to the kind of number that shows up in spec sheet. Once inside the case, it will get re-radiated also. You'll have to go through your practice with fine tooth comb. Grounding, seam in case, wire routing, etc, all become very critical.In my former life, I was part of a team that designed a micro computer driven equipment that will interface with large LARGE factory machines. I think we had 3 layers of filters. At input, in the case, and in the power supply module. For acceptance testing, we ran a pulse generator at 1KV and overlayed it on top of mains input. Pretty much everything went THROUGH. From there, we brought in scopes and bypassed where we suspect was picking up noise. We got it done but not by adding more of the same. By the time more layers are necessary and effective, you are talking about serious targeted approach. You got to know the nature of the noise.I would just put one good filter and leave it at that.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 18, 2019, 05:07:57 am
content: Where does 63dB come from..?The specs are at 50 ohms (and some give odd impedances, like, 1Ω/100Ω, which can be more representative of some situations -- or, at least, hints at what you might expect at very different impedances), and yeah, in a typical application you'll have something very different from that at most frequencies (SMPS being a typical example, where the source impedance is essentially the capacitance of the switching transformer, no resistance there).If the pulses were IEC 61000-4-5 surge or similar, the frequency content of that is pretty low (a peak in the, uh.. 50kHz range?), and the amplitude quite high, so that you may expect the common mode choke to saturate and let pretty much everything through, slightly delayed. Or let it through regardless because the load side has a high common mode impedance (= higher cutoff frequency). This and other reasons are why the line-to-ground voltage rating is higher (2.5kV?) than line-to-line. You need to be able to ride through something like that.Tim

author: tkamiya
author_rank: Super Contributor
timestamp: February 18, 2019, 05:15:52 am
content: Did I screw that up? If one filter reduces whatever by 60dB (ficticious number), having it done twice and measured that in power will be -3dB, so 63dB?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 18, 2019, 05:26:49 am
content: 60dB is one millionth power, or one thousandth voltage or current; if the filters stack perfectly (they won't!*), the output is one millionth of one millionth the power at the input, or 120dB.*We can figure this because, judging by the internal diagram, the filter's port impedance will not be a constant 50 ohms. A given filter is measured into 50 ohms, but two filters measured together will only give the product when they both see 50 ohms, which means one or both (if the filters are identical, then both of course) need to be constant-resistance filters.Which are perfectly possible to create -- but I don't think you'll ever see one in a stock mains filter.In practice, you can expect more, if maybe not actually double (in dB's), and give or take peaks and dips and shifting of cutoffs. You can expect closer to ideal if you add some damping inbetween, to get the impedance back closer to 50 ohms -- at the midpoint between filters, an R+C to ground (R = 47R say, and C of some nF's, comparable to what Y caps are on there already) from each line will help with this.Tim

author: Calvin
author_rank: Regular Contributor
timestamp: February 18, 2019, 07:46:28 am
content: Hi,filters complex output and input impedance interact.So the chance is high that peaks develop where attenuation is wanted.You should verify the result before trusting blindly.regardsCalvin