Multi rail power supplies - can you link rails?

[JerryTheC]

I see that some PSUs allow you to choose between multi rail and single rail operation - which makes me wonder whether it's safe to link individual rails (of the same voltage, obviously ) in a multi rail PSU that doesn't have the single rail option. (Or, if you have a graphics card that has multiple 6 or 8 pin power connectors, is it safe to plug power from different rails into the power connectors?)

[kalniel]

Doing it yourself.. not recommended, but absolutely fine to use different rails into two connectors for the GPU.

[ik9000]

ditto what Kalniel says. It's just how the PSU is set up to manage power delivery. It does not care how that power is then delivered to items within the PC. Occasionally you can get sensitive elements to switching impulses/ripple etc that benefit from being on a different rail to something electrically noisy, but most stuff these days behaves itself and separation matters less. Key thing is not to try and pull too much current from one rail.

[JerryTheC]

Hmm.. If you can plug different rails into a GPU. does that mean you could get away with using one PSU for the computer andPCIe Slots etc, and a completely separate PSU for the GPU extra power connectors? Or would evil, unpleasant and terminal things happen to the GPU (or the rest of the PC)if one PSU was on while the other was off?
the nonstandard 625W PSU in my workstation box only supports up to 150W for the GPU extra connectors, plus 75W for the PCIe slot for a total of 225W - which would be borderline for an RTX3070 and not enough for an RTX3080 if they become available sometime in the future .

[ik9000]

buy a new PSU is my advice. Sry but not worth trying to run 2 dif psus like you describe, and for those kind of power hungry (and very expensive, and rare) cards i just wouldn't risk it.

[JerryTheC]

If the box took a standard ATX PSU, that'd be the obvious fix. Unfortunately, it's a nonstandard size (and I think the motherboard may be a nonstandard layout too, so not sure if swapping the case as well as the PSU is an option). May have to just stick with a lower powered GPU and defer thinking about a 30x0 GPU for a new build in the future where I can use a standard size PSU .

[ik9000]

If the box took a standard ATX PSU, that'd be the obvious fix. Unfortunately, it's a nonstandard size (and I think the motherboard may be a nonstandard layout too, so not sure if swapping the case as well as the PSU is an option). May have to just stick with a lower powered GPU and defer thinking about a 30x0 GPU for a new build in the future where I can use a standard size PSU .

new machine time?

[JerryTheC]

That's the plan eventually - I'd started working out what I wanted to end up with, decided that it was going to be rather expensive, and got a refurbished 8 core 32GB ram windows 10 pro xeon workstation (which is a nice improvement over my ancient core 2 quad system which maxed out at 8GB). and cost less than the PSU and cheaper case option I was looking at for the new build. In the long run, I'll probably do a new build with a fancy graphics card for gaming and use the workstation for music production. In the meanwhile, I'd like to find a decent directX12 GPU to put in the workstation and add some more disk space and at least one SDD over the next few months.

[DanceswithUnix]

You do have to be a bit careful on what rails you pull power from, but I have done it on my son's Dell workstation. I ended up using something like this for one connector as he only had a 6 pin PCIe cable:

https://www.amazon.co.uk/gp/product/B012HGBWHE/

and then something like this to power the other connector:

https://www.amazon.co.uk/gp/product/B07QFSL8VW/

Those Dell workstations have decent power supplies in them, just as well given how completely non standard they are. At one point it didn't like how I had wired it, and just refused to power up.

So if you are prepared to spend money on various adaptors until it works then you can probably cobble something together, but the problem is I don't think anyone is going to be able to give you a configuration that will definitely work with your machine. I would try and keep the power level down a bit for now because of that.

[kalniel]

The main problem with running two power supplies is you might need some dummy load on the 24pin connector to get the PSU to wake up. In days gone by you could use a paper clip but now I think they are a bit more sophisticated. Add on the inefficiencies of running a second supply and it's just not a great idea. 625W should be enough for a modern GPU but having only 12.5A available through the all the PCIe connectors in total is very low - where's it sending all those extra watts?!

I think aiming for a more efficient GPU is the most practical solution for now - I am expecting the next process node improvement GPUs to be better on that front.

[Scryder]

Older video, but still relevant, I think:



Also, this Gamers Nexus vid came related to this on YT... was an informative vid (for me): Talking Catastrophic PSU Failures with JonnyGuru

[Xlucine]

The main problem with running two power supplies is you might need some dummy load on the 24pin connector to get the PSU to wake up. In days gone by you could use a paper clip but now I think they are a bit more sophisticated. Add on the inefficiencies of running a second supply and it's just not a great idea. 625W should be enough for a modern GPU but having only 12.5A available through the all the PCIe connectors in total is very low - where's it sending all those extra watts?!

I think aiming for a more efficient GPU is the most practical solution for now - I am expecting the next process node improvement GPUs to be better on that front.

The ATX standard hasn't changed, so a dumb link should still work. I got a seasonic PSU last year that came with a 24 pin female connector to power the PSU sans motherboard, and it was just a wire in a plastic case

[ik9000]

The ATX standard hasn't changed, so a dumb link should still work. I got a seasonic PSU last year that came with a 24 pin female connector to power the PSU sans motherboard, and it was just a wire in a plastic case

ditto, also a seasonic owner and mine came with the same. The instructions actually said to test the unit with it prior to installing! It is basically a big jumper connecting two pins of the 24, the rest are all blanks.

[matts-uk]

The ATX standard hasn't changed, so a dumb link should still work. I got a seasonic PSU last year that came with a 24 pin female connector to power the PSU sans motherboard, and it was just a wire in a plastic case

ATX PSUs are held on by grounding the green wire. That's not the end of the story.

Some PSUs have +5V and/or +3V sense wires that need to be loaded before the more powerful +12V rails will power up. Sometimes marketed as under-volt protection. A 10W resistor around 5 Ohms connected to ground should defeat it, or a 12V incandescent bulb.

Another thing to watch out for is the balanced circuit. Insufficient load on the +3V and +5V rails causes the 12V rail to swing and blow the motherboard. More of an issue on old AT PSUs and very cheap ATX PSUs that lack decent regulation.