Modify linear supply to make less electrical noise? - Page 1
Forum section: electronics / repair

author: apblog
author_rank: Regular Contributor
timestamp: April 02, 2019, 06:11:57 pm
content: I have an almost 50 year old 13.8V linear supply that I use to power some stuff on my bench. Years ago I modified it to put out 9V instead of 13.8V, so I basically know what is inside.It's just a transformer followed by a full bridge rectifier, some caps, and a discrete linear regulator.I recently moved a bookshelf stereo into my office and whenever I turn the supply on or off, I get a loud pop from the speakers. It's especially bad if I have a load connected when I turn it on or off. I've verifiedthat the noise is being conducted through the AC power line.One thing that helps quite a bit is plugging the P/S into a filtering power strip that has a bunch of beefy chokes inside.I was just wondering if there was any simple mods I could make to reduce this noise. Is it possible that the old filter caps need to be replaced?And to answer the obvious question of why I don't just replace it: I've had it since I was a kid and it's nice to still have it on my desk and in day to day use.Thanks.

author: mariush
author_rank: Super Contributor
timestamp: April 02, 2019, 06:32:36 pm
content: Maybe add some NTCs to limit current at start (or a resistor which you short out after a few seconds using a relay?there's ac input filters (seehttps://www.digikey.com/products/en/connectors-interconnects/power-entry-connectors-inlets-outlets-modules/301?k=ac+filter&k=&pkeyword=ac+filter&sv=0&pv21=215&pv21=216&sf=1&FV=ffe0012d&quantity=&ColumnSort=0&page=1&pageSize=25) but don't think they'd help much

author: apblog
author_rank: Regular Contributor
timestamp: April 02, 2019, 06:57:58 pm
content: I like the NTC idea, because it's somewhat gradual.I've also seen FET circuits for soft start that I occasionally have thought about playing with. I've got another board that I made that could benefit from that too.

author: iMo
author_rank: Super Contributor
timestamp: April 02, 2019, 07:39:12 pm
content: You may try with a snubber (R and C in series) on the primary.Like R=47ohm/2W and C=68nF/650V or something like that.

author: apblog
author_rank: Regular Contributor
timestamp: April 02, 2019, 07:49:37 pm
content: Quote from: imo on April 02, 2019, 07:39:12 pmThat's the sort of thing I was hoping might be possible. Especially since I get the noise even with no load (except the DC filter capacitors) on the secondary.

author: David Hess
author_rank: Super Contributor
timestamp: April 03, 2019, 12:29:07 am
content: I agree with Imo; start with a snubber across the primary.

author: soldar
author_rank: Super Contributor
timestamp: April 05, 2019, 05:58:59 pm
content: Yes, switching the power supply is sending a transient up the mains and you can attenuate that but, also, the stereo is at fault because it should not be amplifying out the speakers the transients from the power line. I would look into correcting the stereo as well.

author: pinyoro
author_rank: Contributor
timestamp: April 06, 2019, 09:44:32 am
content: Would 0.1uf 400V AC capacitors across the mains switch help?Sent from my WAS-LX1A using Tapatalk