Power cables within test labs/your workbench? - Page 1
Forum section: electronics / testgear

author: NickKUK
author_rank: Contributor
timestamp: November 27, 2022, 10:27:57 am
content: I'm a beginner, but with the scope, power supplies, sig gen etc I'm starting to find my workbench has 8 power cables - 99% of them are 3 core (L/N/E). I've noted quite a noise at the mains 50Hz so I'm thinking putting the cables in a shielded box/conduit thus reducing the number of exposed mains cables but interested in what others do to minimise line noise?Naturally shielded signalling cabling works well but I can't always operate in a closed metal box.So what do you do to minimise the noise for your test equipment?

author: unknownparticle
author_rank: Frequent Contributor
timestamp: November 27, 2022, 04:04:36 pm
content: Where is the noise coming from?

author: NickKUK
author_rank: Contributor
timestamp: January 08, 2023, 09:40:01 am
content: Sorry for the late response - the mad crush for December projects getting into production before the freezes and then COVID over the xmas and new year.In short I believe this is due to having eight power blocks that are extension leads (plastic with non shielded cords) with the 2x SMPS, scope and sig gen all squirrelled into a corner, I also have the Mac mini and monitor close to that corner. LED main light.I know the SDG1032X sig gen has phase noise and that seems to be riding on what seems to be gaussian noise with some periodic (I've been learning about clocks as I'm building my own) but given the Brymen 896s with leads can detect 50Hz mains in the air I suspect it's a case of shielding down the cables - either replacing the mains block with a shielded cable and shielded extension block or by slieving the mains cables for the instruments (or perhaps replace with after market shielded mains cables?).

author: jonpaul
author_rank: Super Contributor
timestamp: January 09, 2023, 01:41:19 am
content: in 55 years of work as an EE with dozens of instruments and power cables randomly across my bench, never had the problemI suspect your instrument is misconmected, not probing correctly or bad Chinese instruments CMRRj

author: DavidKo
author_rank: Frequent Contributor
timestamp: January 11, 2023, 06:42:14 am
content: How does your noise looks like? It is sometimes or still there?It can be cell phone, wifi from your mac mini, bad switching power supply, LED lamp etc. Not like in old good days when it was usually from commutator motor without noise suppression (dead doggy capacitor was threw away without replacement).

author: NickKUK
author_rank: Contributor
timestamp: January 11, 2023, 01:52:09 pm
content: The CMRR of the kit may be the issue. The siglent scope radiates some noise for example.

author: Stray Electron
author_rank: Super Contributor
timestamp: January 11, 2023, 03:19:28 pm
content: Turn off each of the instruments and devices on your work bench one by one and see which ones are causing the noise. You might try plugging the noisy ones into a different power circuit and see if that makes a difference. If the noise is coming through the power lines, then it should help. If you can get your hands on an isolation xformer see if that helps.In the recent past, the biggest noise source that I've found was due to the extremely noisy CF and LED lightbulbs. Try tru I've also heard a lot of complaints about Chinese TE but I don't own any so it hasn't been a problem for me.You can make a simple isolation transformer by connecting two same-ratio transformers back to make. i.e. a 120 to 12v transformer connected to the 12v side of an other 120 to 12 transformer. Just watch your current draw.

author: BillyO
author_rank: Super Contributor
timestamp: January 11, 2023, 05:26:55 pm
content: If you have any LED lights, they would be your primary suspects. Some can create thoroughly awe inspiring noise, both radiated and on the power lines.

author: tautech
author_rank: Super Contributor
timestamp: January 11, 2023, 08:29:37 pm
content: Quote from: NickKUK on January 11, 2023, 01:52:09 pmAny instrument with LCD display will but normally bad local RFI can be traced to shitty wallwarts.Seen this several times once customers buy instruments with better sensitivity able to detect all the crap spewed from some wallwarts so best advice is to focus on them initially.Don’t discount it might even be in another room.