Noise from AC->DC - this seems extreme - Page 1
Forum section: electronics / general

author: bitman
author_rank: Unknown
timestamp: July 15, 2020, 09:24:48 pm
content: I'm trying to get a relatively stable 5V power supply for at least 2amp to power a bunch of TTL 7400 I am using for a project. To replace the bench power supply. I found the following IRM-15-5 on digikeyhttps://www.meanwell.com/Upload/PDF/IRM-15/IRM-15-SPEC.PDFwhich would do nicely and seems packaged in a relatively useful way for me. But when I do some tests, I see quite a noisy output - the higher the load, the more noise I see - the "peak to peak" is close to 0.5v at 2 amps (the image capture from the scope is when the output is 5v with a load of 2amps). I have a DC load and the meter attached to the output, and an isolated 120v supply on the input.I don't expect no noise - but this just seems to be off by a lot? Is it just a bad "Chinese AC-DC" component or is this expected? The datasheet says 200mV but as I show, I definitely see a lot more. Of course if I only put a small load on it, the Ripple is much less.

author: Bud
author_rank: Super Contributor
timestamp: July 15, 2020, 09:30:29 pm
content: Did you read Note 2 in the datasheet? Are you measuring ripple same way?

author: bitman
author_rank: Unknown
timestamp: July 15, 2020, 10:00:52 pm
content: Quote from: Bud on July 15, 2020, 09:30:29 pmI'm measuring the same way except I didn't use capacitors as that seems to sorta defeat testing the unit by itself. I do expect to use capacitors to smooth things out, but 500mV ripple is too much. I have other supplies that barely makes it to 50mV ripple at 2A using the same setup. I went through the AC-DC I had in the parts drawer and none of them come close to this one when it comes to ripple at "high" loads.What I'm trying to find out is if I need to look for a better PSU or if this is within expected behavior. As I said I have others (3.3v) and it barely got close to 50mV at 2A - I had to increase resolution on the scope to even measure it.

author: David Hess
author_rank: Super Contributor
timestamp: July 17, 2020, 11:08:30 am
content: Make sure you are not measuring common mode noise by shorting the probe tip and ground lead at the measurement point.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 17, 2020, 01:11:59 pm
content: May be of interest:https://www.meanwell.com/Upload/PDF/EMI_statement_en.pdfToo brief though. Not obvious how they would mount or ground a PC-mount module.If nothing else, the fact that the load is on a plate suggests that they should still have to worry about high frequency filtering. Levels may be elevated at lower frequencies, in an application with a grounded load.Last time I used that part, it needed a little external Y-cap between input and output, with a 100uH CMC between output and load (where the Y cap connects). Don't think input CMC was needed?Tim

author: bitman
author_rank: Unknown
timestamp: July 19, 2020, 06:22:23 pm
content: Quote from: David Hess on July 17, 2020, 11:08:30 amI don't understand this comment. If I shorted the ground and probe I would be shorting the output of the supply too. If I'm probing against the something where the probe ground is on too, I won't see a voltage (at least not 3.3+v).I probably should repeat that with the same setup, testing other AC->DC supplies, I see far far less ripple (+/- 50mV) - this unit is in the 500mV range.

author: bitman
author_rank: Unknown
timestamp: July 19, 2020, 06:26:50 pm
content: Quote from: T3sl4co1l on July 17, 2020, 01:11:59 pmThis statement looks like a response to a lot of complaints. It really make no sense to me. They have a datasheet that says X but testing says Y. It's not about that some units will have more noise than others and units have particular use-cases they're meant for. I get this - but as best I can tell from the first few paragraphs this seems to be an address their "support" can hand out to people who complains about noise being too high. I wouldn't have reacted if I just saw numbers slightly past the data-sheet. Double it's numbers (that are already bad compared to others) is another situation.So I guess I found my own conclusion - by testing other units it's clear that from what I have in my small collection, this is an outlier. I've marked it as such and plan to use something else for now. Thanks for the link.//Bit

author: David Hess
author_rank: Super Contributor
timestamp: July 19, 2020, 07:19:39 pm
content: Quote from: bitman on July 19, 2020, 06:22:23 pmWhat I mean is attach the probe's ground lead as normal, and then before making the measurement with the probe, attach the probe tip to the same point as the probe's ground lead and see what shows up. So only the probe is shorted at the ground point.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 19, 2020, 10:24:36 pm
content: Quote from: David Hess on July 19, 2020, 07:19:39 pmMore specifically, what you're doing is:If there's a common mode voltage (voltage with respect to scope ground):That voltage will create an AC drop across the probe ground lead.Which you then probe with, well, the probe.It looks like magic. You "short" the probe tip to its ground clip, it should read zero, period, right? Then you touch this grounded probe to circuit ground, and it's spikes all over.The answer lies in the equivalent circuit. The ground clip is about 0.3uH, so it has considerable impedance at RF. CM voltage might actually be more like a square wave or something, but from some oddball impedance (usually capacitive from the power transformer), so of course when that impedance divides down against your ground clip impedance and everything else, it ends up as spikes.Tim

author: ocset
author_rank: Super Contributor
timestamp: July 20, 2020, 09:02:33 am
content: Yes i agree with David Hess and T3sl4co1lWhat you are seeing is just "pickup".You need to measure ripple with a home made coaxial probe (i will send it to you soon, a picture)You say you see more ripple with this psu than others.......well, maybe the switching stage of it is noisier, and that noise is coupling into the scope probe....or just maybe the ambient noise was worse when you measured this psu.Dave Jones has an EEVBLOG video on pickup................................oops...i dont have a picture of the home made coaxial probe...but just order some 50 ohm or 75 ohm coax ..obviously one end with a BNC connector on...the other end....hack it up so you can solder the braid and inner to your power rail...then measure away with scope on x1I went to an RF place once, and when I made my home made coax probe for them…they altered it, to improve it…..they had a bit of coax and they were able to put a “DC blocker” into the coax itself….we then simply viewed the ripple with the scope on “DC coupled” (instead of AC coupled), and this in itself was an improvement……as the DC blocking cap in the scope is none too great a thing to be using.The DC blocker was from a well known RF vendor…name something like “RF circuits” or something like that.

author: David Hess
author_rank: Super Contributor
timestamp: July 20, 2020, 11:11:24 pm
content: Quote from: T3sl4co1l on July 19, 2020, 10:24:36 pmIt will show both the common mode noise and flux coupled into the loop formed by the probe tip and ground lead. The later can be removed by discarding the ground lead and using a coaxial connection to the probe tip and I have done this many times. Waving the probe around with the ground lead attached to the tip can be useful for just inspecting the magnetic field component.