Converting ATX PSUs to Bench Power supply major issues. - Page 1
Forum section: electronics / beginners

author: ziplock9000
author_rank: Regular Contributor
timestamp: December 24, 2016, 10:37:26 pm
content: I have few computer PSUs of various configurations laying around that are generic lower quality crap that I'd never use in an expensive new system. So (as it's obligatory) I decided to convert them into a bench power supplies, or at least make a project box they can plug into that would expose the different voltage rails as female banana plugs. I would maybe add some extra smoothing and possibly some extra precision voltage regulation and a few LEDs etc etc. You get the idea. The process looked simple, basically it's just a small checklist of things to do that should work with even the most obtuse PSUs. Boy was I wrong...DELL NPS-300GB B REV:04Connector:Non-Standard 24-PinI knew being dell that it would have non standard pin outs, so the internet once again came to the rescue.Simply connecting PS_ON (Grey) to Ground (Black) turned the PSU on, no loads or anything else was needed. 5v rail read as 5.1v, 12v rail as 11.8v. (I'm assuming regulation is better under normal load, never had time to test this). Everything was good and confidence was highFSP Group Inc (Generic crap) FSP350-60MDN Rev AFSP Group Inc (Generic crap) FSP215-60PNA(PF)Connector:Standard 20-Pin ATXThis is where I hit problems with both of them:1. I shorted PS-ON (Green) with Ground (Black).. Nothing, but I know I needed a load2. I added a 10ohm 5w WW resistor between +5 (Red) and Ground (Black).. Nothing.. Now I'm puzzled. Thisshouldhave worked as it does with 90% of the videos and tutorials on the internet.3. I did further research. It seems on rare occasions there's a "soft" power on, so I added a 25K resistor between +5 VSB (Purple) and Ground (Black)... Nothing... Head scratching began.4. Tried a larger 112K resistor on +5v VSB (Purple), nothing5. I added an old hard drive to another +5v rail for an additional load.. nothing.6. I added a 10ohm load to the 12v rail as on even rarer occasions a load is needed on the +12v rail, nothing.7. Checked that I hadn't somehow killed the PSUs, but both powered on when plugged into a computer8. Went back to basics, maybe stupidity had kicked in.. Checked and rechecked.. nothing.9. Stumped and mildly pissed off that something so simple got the better of me, So I went to the pubNotes:1. PSUs power on when connected to a computer. The PSU's fan rotates and connected HDD spins.2. The +5v VSB registers as ~5v with a MM3. PG (Gray) isnotconnected as it's just a power on indicator for an LED or other logic.4. I tried momentary as well as constant connections on the PS-ON (Green) and also the +5v VSB (Purple lines)Whatever I'm missing, it's got to be something really simple. I've spent too much time looking all over the internet and nothing has worked.Help please! and Merry Christmas and a Happy New Year to all!

author: Codebird
author_rank: Regular Contributor
timestamp: December 24, 2016, 11:27:24 pm
content: I know this will sound silly, but it's worth a try... the ATX connector has quite a lot of ground wires. Eight of them, which should all be connected together at the mainboard end as well as within the PSU. Should be. It's possible the PSU uses these to sense if it is properly connected, so maybe if you were to just link all the blacks together and then try connecting PS_ON to them all?I can't think why any designer would do this, but when you've tried everything else you have to assume something weird is going on.

author: Marinated
author_rank: Contributor
timestamp: December 25, 2016, 12:29:51 am
content: If the supplies work when connected to a PC then I'm sure you'll be able to sort it out before long. Try a subtractive approach, giving it small loads on all rails and the indicator. If it works, keep removing loads until it fails.I'm going to be the obligatory doomsayer. You mentioned adding fancier voltage regulation, LEDs and so on. You didn't mention the most important thing, though. Even if it's a little more involved, please add current limiting. ATX power supplies will be happy to spot weld for you if invited, and everyone makes mistakes.

author: ziplock9000
author_rank: Regular Contributor
timestamp: December 25, 2016, 03:15:22 am
content: Quote from: Codebird on December 24, 2016, 11:27:24 pmI'll give that a try mate. Thanks.

author: ziplock9000
author_rank: Regular Contributor
timestamp: December 25, 2016, 03:19:25 am
content: Quote from: Marinated on December 25, 2016, 12:29:51 amThanks for reminding me /facepalm

author: Monkeh
author_rank: Super Contributor
timestamp: December 25, 2016, 04:09:59 am
content: ATX PSUs have a sense wire for the 3.3V rail. Usually, this is crimped into pin 11/13 with one of the 3.3V wires. Sometimes, especially on low power supplies, it has the pin all to itself. Make sure that's not the case.

author: ziplock9000
author_rank: Regular Contributor
timestamp: December 25, 2016, 06:50:56 am
content: Quote from: Monkeh on December 25, 2016, 04:09:59 amAs you mentioned, Pin 11 has both the +3.3v (Orange) and 3.3v Sense (Brown) crimped together on both PSUs. So It can't be that :/

author: joseph nicholas
author_rank: Frequent Contributor
timestamp: December 25, 2016, 11:42:35 am
content: It seems the power supplies you want to convert are unknown to you. Do they function normally? Try using them the way they were designed, then if they work ok they should work without the connection to the motherboard. I scored a bunch of the power supplies from the local metal recycler and half worked OK and the rest did not start up.

author: ziplock9000
author_rank: Regular Contributor
timestamp: December 26, 2016, 08:23:35 am
content: Quote from: joseph nicholas on December 25, 2016, 11:42:35 amAs I mentioned in my opening post; They work in a computer, but I can't get them to work outside of a system. I've done extensive Googling and from roughly 15 videos and websites, what I've tried *should* have worked. It's very puzzling.What I've not tried (which almost all videos do) is crimp or electrically connect all wires together by colour. The reason I've not done this is because I'd like to make what I'm doing into a project box that I can connect to any computer PSU instead of hard wired to a specific one. The potential issue here may relate to what @Monkeh mentioned earlier.

author: Canobi
author_rank: Regular Contributor
timestamp: January 08, 2017, 04:15:36 pm
content: Here's the plan I followed which worked for me:This is a WIP shot of my setup which shows the wire and load arrangements:Good luck in your endeavour.

author: ziplock9000
author_rank: Regular Contributor
timestamp: January 09, 2017, 06:10:54 pm
content: Thanks mate. I tried that but it didn't work. The next step is to unfortunately cut off the plugs and electrically connect all common coloured wires together to see if that makes a difference. I wanted to avoid that so I could use the "magic box" on any PSU.

author: jdraughn
author_rank: Regular Contributor
timestamp: January 10, 2017, 01:07:07 pm
content: Just see if all the wires of the same color have no resistance between them, no sense in chopping them.

author: ziplock9000
author_rank: Regular Contributor
timestamp: January 11, 2017, 02:44:50 pm
content: Quote from: jdraughn on January 10, 2017, 01:07:07 pmI already tried that, all of the wires of the same colour had zero resistance as you'd expect. But then again so did some of different colours. So essentially it wasn't conclusive and therefore unhelpful.Besides I've already clipped the wires and crimped them together. I need to test if this new configuration makes any difference.

author: Gus
author_rank: Newbie
timestamp: August 08, 2019, 07:47:32 am
content: Did you get it working in the end? As I'm acquiring similar results and nothing seems to be working...

author: sj2wmorris@roadrunner.com
author_rank: Newbie
timestamp: April 16, 2025, 09:09:00 pm
content: Hello All.Good morning everyone.This is my first post, and I see that this problem is not unique, but im pulling my hair out (and there’s not much left) trying to solve this issue. I have read many posts on this and not really see a good fix for this. I am not a electrical engineer, but I am knowledgeable in electronics for my hobbies.I have an old Astec ATX250-3506 PSU that I had converted to bench power supply 15 years ago. For those 15 years it worked beautifully for my hobbies. I decided I wanted to upgrade this to a nice cabinet and such, so when I modified the housing to fit into my box, now there is no output on any rail. The 5VSB LED does come on, the fan will spin for a few seconds until the capacitors are depleted, and then nothing. No power on light either. I have a (2) 10 ohm, 10W resistors in series (20 OHM total) on the 5V rail with no result. I have moved the brown load sense wire to the 3.3 rail, all with no change. I have even tried placing these resistors in parallel for 5 OHM total, with no change. I assumed that I cooked the PSU, and found an identical NIB unit on E*** and converted, with the same result, which tells me the original unit is ok. The only change that I had made was to cut the case to fit my box.Any ideas as to what has happened and how to rectify it?Thank you.

author: tunk
author_rank: Super Contributor
timestamp: April 16, 2025, 11:25:45 pm
content: How did you cut it? - just guessing/speculating, it could possibly bethat the case itself is a conductor between different parts of the PSU.

author: sj2wmorris@roadrunner.com
author_rank: Newbie
timestamp: April 17, 2025, 01:36:59 pm
content: HI Tunk. Thank you for your reply.I did cut the case way back, to almost just the base holding the board in place. There were no connections to the case that I saw. The top is off the PSU and the board is exposed in its new case. The only thing attached to the case was the fan, but it was not grounded to the case. I have considered this to be a problem, but I cant see why this would be an issue.I have one idea left. Since there are 2 10ohm resistors, im going to run a wire to both 5V rail and another to the 3.3V rail and see if that will trick it into coming on.I may smoke it, but i have another to try.

author: SteveThackery
author_rank: Super Contributor
timestamp: April 17, 2025, 02:09:35 pm
content: Have a look at this: I think you'll find it of interest.https://www.ebay.co.uk/itm/405718024426?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=mjdxqmwrr7e&sssrc=4429486&ssuid=sArKqIhrTN-&var=&widget_ver=artemis&media=COPY

author: macboy
author_rank: Super Contributor
timestamp: April 17, 2025, 02:37:39 pm
content: Quote from: sj2wmorris@roadrunner.com on April 16, 2025, 09:09:00 pmOne thing I can think of is that these are designed to be mounted in a grounded metal case, and the PCB mounting screws do make electrical contact with that case. This may seem odd, but some products use these connections to the case to provide ground reference to various parts of the circuit, and they won't function without these connections. If your new case and mounting method does not electrically connect all of these screw mounts together (and to ground), then the supply may not work.

author: sj2wmorris@roadrunner.com
author_rank: Newbie
timestamp: April 17, 2025, 08:53:47 pm
content: Hmm so I wonder if I run a jumper from the case to the unit ground It may work? Ill give it a try! Ill get some pictures this weekend of it.

author: sj2wmorris@roadrunner.com
author_rank: Newbie
timestamp: April 17, 2025, 08:55:34 pm
content: Ive seen these in my searching for alternative power sources if i CANT GET THE psu to spin up. How do they get thier power?

author: Whales
author_rank: Super Contributor
timestamp: April 17, 2025, 10:02:46 pm
content: This is quite strange. Every ATX PSU I ever modded worked fine the moment I connected green (PS_ON) to any black (GND). It's been a few years though, maybe they've changed some things?For everyone encountering this issue please:(1) Provide the exact model name of your power supply(2) Provide photos of all of its connectors(3) Confirm that 5VSB (purple wire) is providing power at all times (even when off). If this is dead then the PSU is dead.(4) Confirm that the PSU actually works in a computer (isn't blown)Many modern PSUs don't turn on their fans until they need to, so make sure you're not relying on a spinning fan as an on-indicator.Minimum loads shouldn't be needed for turn on. In modern computers so little 3.3V and 5V might get used during startup that it would be silly to rely on them for any sensing. 12V maybe, but then it would have to pulse power over the 12V rail and see what happens, which could violate the ATX specs about risetime or other behaviours.

author: SteveThackery
author_rank: Super Contributor
timestamp: April 18, 2025, 05:34:09 pm
content: That device I linked to on eBay works fine on the three ATX PSUs I've tried it on.https://www.ebay.co.uk/itm/405718024426?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=mjdxqmwrr7e&sssrc=4429486&ssuid=sArKqIhrTN-&var=&widget_ver=artemis&media=COPY

author: sj2wmorris@roadrunner.com
author_rank: Newbie
timestamp: April 21, 2025, 01:58:58 pm
content: I have bittersweet news on this PSU. I followed advice given here on this forum and I thank everyone!So, when I was grounding the load resistors before, I was going to the board ground wires. There is a short, 16GA wire from the case to the board. As was suggested by Macboy, I grounded the load resistors to the case and VIOLA! Everything came on. I tested all rails and was getting the correct voltage (3.3, 5.0, 12), the fan was spinning, and was very happy! I then disconnected my sketchy wiring to go back and make it permanent, and when I went to power up, once again, nothing. No 5VSB lights, nothing. Im farther back now than I was before, but for a brief time, I did see it work.Im afraid I smoked it, so now Im looking for alternative sources of 3 channel power for this little project.