Noisy SMPS putting 100Hz buzz into mains? - Page 1
Forum section: electronics / beginners

author: dazz
author_rank: Frequent Contributor
timestamp: September 04, 2018, 05:37:10 pm
content: Something tells me this is one of those instances when I should be able to find answers on google, and I've tried to no avail, so please excuse me.Anyway, I got this SMPS to power an audio (guitar) amplifier a while back, grounded the output to minimize noise (see picture) and the amp is reasonably quiet when powered by it, but it seems to be inducing a loud 100Hz buzz into the mains that an AM/FM battery powered radio picks up several meters away in a different room. The buzz is also audible (read loud AF) through the guitar amp when I turn the guitar's volume pot all the way down, which grounds the pickups. For what I could gather, the buzz comes from the full wave rectifier. Any suggestions as to how to troubleshoot it, please?

author: HB9EVI
author_rank: Frequent Contributor
timestamp: September 04, 2018, 05:44:56 pm
content: Hmm, it looks like there several essential parts missing, like common mode choke, X cap and Y-caps, so no wonder that everything possible ends up on mains

author: SeanB
author_rank: Super Contributor
timestamp: September 04, 2018, 05:49:35 pm
content: Simplest fix is a new SMPS, one with actual input filtering, the right class x capacitors across the mains, the right Class Y capacitors to shunt mains noise to ground and LC filtering on the output side. Otherwise you have to add it to this one, and fitting this into the same size housing will not be possible.

author: Gyro
author_rank: Super Contributor
timestamp: September 04, 2018, 05:50:29 pm
content: It looks like a dodgy PSU to me...Firstly, No evidence of EMC filtering on the mains input, so it could well inject switching noise (rectifier and other) into the mains.Secondly, it seems to have the primary side switching transistor and the secondary side rectifier clipped to the same heatsink (top edge). I can see that there is a ground wire running from the mains socket input to the output (bottom left) but I'd want to examine it more closely to ensure that the heatsink is safely grounded.

author: dazz
author_rank: Frequent Contributor
timestamp: September 04, 2018, 06:15:00 pm
content: And here I thought the transformer labeled JHT-4096 was the common mode choke. I'm so cluelessIsn't CY1 the Y cap? but anyway, I understand there's no way to fix it, so unless it's unsafe for me or my equipment I'll leave it for other purposes other than the amp.Or maybe one of thesemains EMI filters would help?

author: Gyro
author_rank: Super Contributor
timestamp: September 04, 2018, 06:50:12 pm
content: You get less clueless the more of them you see, you get to the point where you can pretty much evaluate general quality after about 30 seconds of observation.Yes CY1 isaY cap, it helps to deaden switching noise across the transformer windings. Unfortunately its connection to ground, via the wire, is too long and inductive to help with noise radiation into the mains.Regarding the safety, if you can do a continuity check and confirm that the mains ground is solidly connected to that shared heatsink then I wouldn't have the same safety worry.Yes, the mains EMI filter you linked includes the necessary components (X cap, two Y caps and common mode choke - the problem issafelyattaching it to the input of the PSU.

author: dazz
author_rank: Frequent Contributor
timestamp: September 04, 2018, 07:14:46 pm
content: Quote from: Gyro on September 04, 2018, 06:50:12 pmWell, no, the heatsink (at the top of the pic) the mosfet and that other chip are attached to is not groundedQuote from: Gyro on September 04, 2018, 06:50:12 pmI see, thanks, although it's looking more and more like I should get rid of this crap, right?

author: Gyro
author_rank: Super Contributor
timestamp: September 04, 2018, 07:22:28 pm
content: QuoteNope, I wouldn't trust it in that case, even with a grounded negative output terminal there's too much risk of something very nasty turning up on the positive one! I would normally ask to see the track layout on the underside of the PCB but it's pretty much redundant in this case.Yes, I'd ditch it and find a decent one. Look for a known brand such as Meanwell, Delta, or others that I can't remember off hand but I'm sure others will fill in.

author: dazz
author_rank: Frequent Contributor
timestamp: September 04, 2018, 07:57:58 pm
content: Quote from: Gyro on September 04, 2018, 07:22:28 pmApparently the heatsink is connected to Vdc, and what I think makes it even worse is that the heatsink is L shaped, extending all across the bottom of the PCB, with only a thick paper/thin cardboard sheet acting as an insulator. So I'm guessing if that sheet got ripped or torn I could potentially have mains voltage at the outputDamn, I payed like 17€ for this piece of $%/t. No big deal, it's just that there are decent laptop adapters for much less than that for all I know.

author: Gyro
author_rank: Super Contributor
timestamp: September 04, 2018, 08:15:25 pm
content: Yes, that sums up the situation. You could always try to get a refund from the seller on safety grounds, maybe referencing this thread.I don't know what voltage and current you need, but yes, ebay second hand genuine brand name laptop adapters (eg, Dell, Lenovo, HP), as opposed to the new "for xxx model" clone ones, are an excellent and usually cheap purchase. They are engineered andtestedto meet very strict EMC and Safety approvals.

author: dazz
author_rank: Frequent Contributor
timestamp: September 04, 2018, 08:28:32 pm
content: It's a TPA3118 based amp that outputs about 20W @ 20V 8 ohms, but with distortion pedals might go above 30W. So I think a 20V 3.3A adapter would suffice. Actually I think I have an old Dell Inspiron 6000 adapter in a box somewhere