Help using a server PSU

Archaic0 · December 8, 2022

Country: USA

I would like to use a rack server PSU for a build I'm working on and I'm wondering if anyone else has done this successfully.

I only need 12v out of the PSU and I do have 12v coming out of the PSU, but when I put a load on that rail the PSU cycles and shuts off after 5-10 seconds. I'm assuming/hoping this may be because I haven't 'triggered' the PSU to actually be in an ON state (like shorting the pins on a 24-pin connector). I'm looking for a wiring diagram for the PSU, but I'm not confident that the diagram can be found or that it will show an obvious 'trigger' pin.

When I probe the PSU connectors, I can find the following voltages: 12v, 11.9, 11.8, 2.7, & 4.9. The lack of a good 5.0 and 3.3 seemed curious to me, as well as the couple that aren't quite 12, but that may be because of the PSU not being truly in an ON state.

Has anyone else used a rack server PSU for anything or know how to trigger them on (if that is needed)?

Thanks in advance for any insight!

SpookyCitrus · December 8, 2022

You can use the paperclip method to jump the 24pin and see if it power cycles like it does when it's plugged into what you're trying to power. If the PSU has issues using the paperclip method I'd say there is an issue with the PSU. What is it's make and model of the PSU you are trying to use?

Archaic0 · December 8, 2022

There is no 24 pin. The server PSUs just have a backplane connector that slots into the server motherboard. I salvaged the connector and have soldered onto the 12v output to run a PICO PSU that takes care of the 24 pin connection carrying the correct motherboard voltages.

Server PSU [+12v/Ground] -> PICO PSU input -> MB 24pin.

The PSU is HP P/N 643954 from an HP ProLiant DL360p Gen8 server. It's a 460W hot-swap PSU from a 1U server.

==== EDIT ====

I was mixing up Amps and Watts in my post there. The 12v rail is rated at 38.8 AMPS, which would be ~450 WATTS. So a 65W CPU plus motherboard would be fine. However, there are 2 12v rails noted on the label so maybe I'm just connected to the smaller rail rated at 2.5 AMPS, which would be 30 WATTS, and that would be too small for the board and CPU and could explain the reset behavior I saw. Definitely need to find the wiring diagram!

==== EDIT ====

...hmm... May have just found my issue though. The PSU may not need to be triggered on, I may just be overloading the 12v rail. The label says 460W max, but also says only 38.8A Max on the 12v rail. My Ryzen 5 3600 CPU is 65W max and probably is at least in the 30-40 range during boot if not 60, plush the motherboard itself. I bet I'm easily maxing out 38.3A on the 12v rail during boot even without the GPU being on there.

There are 1200W versions of this PSU, I wonder if the 12v rail on those is up to the task...

Archaic0 · December 8, 2022

Discovered that VSB on that PSU label means voltage standby. So it makes sense that I am finding 12v even when the PSU isn't triggered/on. The VSB limit being 2.5A / 30W lines up perfectly with what I am seeing when trying to run the PC.

I'm back to trying to figure out how to trigger the PSU to full power on out of standby. If anyone knows if that's possible like with typical ATX PSUs, please chime in!

Archaic0 · December 8, 2022

I may have found the answer! These people have a different use case, but show how to bring the PSU out of standby and that's all I need. We'll find out after some experiments later tonight!

https://pa0fri.home.xs4all.nl/Diversen/DPS-800GB A Server sypply/DPS-800GB A PSU eng.htm

Also found people selling a breakout board to help use these PSUs for non-server uses. In my case, I want to keep the original motherboard connection as it has the aestetic I'm going for as well as providing solid mounting for the conector.

https://www.wizardofwire.com/product/hp-1200-watt-server-power-supply-breakout-board/