Computer psu ground loop ? - Page 1
Forum section: electronics / beginners

author: Sjokolade
author_rank: Regular Contributor
timestamp: April 30, 2016, 12:07:36 am
content: I have a Dell T3600 that I need to modify as a file server and since it only have two drive bays I bought a adapter for the 5.25" bay to hold extra ssd. Since the Dell powersupply only provides 12V the motherboard converts 12V to 5V for drives needing that voltage, as I need 4-6 ssd's I dont think the motherboard will be able to provide all the extra juice.So I bought a MeanWell SD-50A-5 isolated stepdown converter to covert 12V to 5V to solve the power problem.Since I have no graphics card I made a pcie cable adapter and using that connection to supply 12V to the mw psu, as you might be able to see the 4 pin molex gets 5V and ground from the mw supply and 12V and ground is coming from the pcie connector powered by the Dell psu.I have grounded the mw supply to computer chassis.That means that 5V ground and 12V ground has different potentials - Will this cause a problem when powering a 3.5" drive that uses both 12V and 5V ?I'm fitting a raid card to control the ssd's and I'm also worried since the 5V ground has a different potential then sata signal ground on the raid card.Would it be a stupid idea to bridge mw psu chassi to the 5V ground connection as I have drawn in the picture ?Meanwell spec -http://www.meanwell.com/mw_search/sd-50/sd-50-spec.pdf

author: Monkeh
author_rank: Super Contributor
timestamp: April 30, 2016, 12:30:28 am
content: Quote from: Sjokolade on April 30, 2016, 12:07:36 amNot once you connect both of them firmly together via the ground planes of your loads..QuoteNot at all. That's what's going to happen anyway, no harm making it more permanent.

author: Delta
author_rank: Super Contributor
timestamp: April 30, 2016, 12:36:56 am
content: Sorry to sound dumb, but does the computer's PSU not have a 5v rail?

author: uncle_bob
author_rank: Unknown
timestamp: April 30, 2016, 12:44:33 am
content: HiThere is no need to tie the 5V to ground at the power supply. Simply run the 5V minus side over to the motherboard and tie it in there. If you want to get fancy, splice it into one of the existing power supply <-> motherboard ground leads.Bob

author: Sjokolade
author_rank: Regular Contributor
timestamp: April 30, 2016, 12:46:28 am
content: Thanks for the answer Monkeh.Quote from: Delta on April 30, 2016, 12:36:56 amThat's not a dumb question, sadly this Dell psu only provides 12V rails.

author: Monkeh
author_rank: Super Contributor
timestamp: April 30, 2016, 12:48:18 am
content: Quote from: Delta on April 30, 2016, 12:36:56 amNope!A lot of these smaller Dell units are fully 12V. Some still have +5VSB rails, but no main 5V supply. They're pretty pointless these days, 5V consumers include:HDDs, a dribble2.5" HDDsSSDs, and I seem to recall a few I've looked at only using it if 12V's not present.USB.. Well, damn, that's about it.Absolutely everything on the motherboard runs on lower voltages, and there's little if any direct use of 5V even in the few consumers (2.5" HDD motors, some USB trinkets..). 3.3V and 5V rails on PSUs are long obsolete, people just won't let go of ATX. Single-rail 12V supplies are simpler, cheaper, more robust, and more efficient. The standby rail is easily glossed over, makes little difference if it's 5V or 12V, with 5V you get standby charging of USB devices for free I guess.Quote from: uncle_bob on April 30, 2016, 12:44:33 amSo, run.. loads of extra wire?Honestly, easiest thing to do is just link the two V- terminals and the 'earth' terminal.

author: Delta
author_rank: Super Contributor
timestamp: April 30, 2016, 12:52:42 am
content: Quote from: Sjokolade on April 30, 2016, 12:46:28 amTakk skal du har! (eller ha? Min Norsk grammatikk er veldig dritt, beklager!)

author: Sjokolade
author_rank: Regular Contributor
timestamp: April 30, 2016, 12:59:02 am
content: Hehe, det skal være 'ha' og Norsken din er ikke noe verre enn Norsken til oss innfødteThank's guys.

author: uncle_bob
author_rank: Unknown
timestamp: April 30, 2016, 02:03:27 am
content: Quote from: Monkeh on April 30, 2016, 12:48:18 amHiIf you look at the picture, he has a jumper from the V- terminal on the 5V supply to the chassis of the power supply. That wire is not needed.Bob

author: jitter
author_rank: Frequent Contributor
timestamp: April 30, 2016, 08:28:53 am
content: Nah, that looks to be drawn in. But I wouldn't make that connection.As mentioned, 5V ground and 12 V ground will get interconnected anyway by the devices that run on both (sharing the same ground), so putting that jumper in place between -V output and case ground may only promote the creation of a groundloop rather than avoid one.A groundloop isn't a problem per definition, though, but it depends on what's in it. If there isn't a hum source in the path (think of stereo RCA leads), it's not a problem. If there is, like e.g. this DC/DC converter, grounding at both ends might indeed create a groundloop.

author: Monkeh
author_rank: Super Contributor
timestamp: April 30, 2016, 10:28:14 am
content: It's a PC, ground loops are abundant anyway.HDD ground connections: Data cable (SATA or PATA..), power cable, chassis. Both data and power will be grounded via multiple paths back to the PSU, and via the motherboard mounting screws..