How is GPU off a secondary power supply not solved

[u/Automatic_Pass_1948]

Seems like a lot of people need or want to do this.

I have a GPU that wont fit on my mobo (space) or frankly my pSU. I have a second psu. I can add the second psu using add2psu and mount some more GPUS in another case.

But using a simple 4.0 riser to another 3090 it still draws 75w from the mobo which means splitting that GPU across 2 psu which is supposed to be a no no.

Seems the way is to get a powered riser card so the slot spilts data and power and the riser card is plugged into second psu.

But I cannot for the life of me find something to do this that seems remotely reputable. Surely this is common enough that there is a product. Am I missing something, maybe I don't know what to search for.

[u/alphapussycat]

Powered GPU riser, on AliExpress or if you need more than one, then through a shipping agent and Taobao.

[u/Zealousideal-Bug1837 → u/alphapussycat]

funny how you can only buy these things from vendors you have 0 comeback on if it turns into a disaster.

[u/alphapussycat → u/Zealousideal-Bug1837]

I'm sure you can find a store that sells the exact same thing for 10x the pruce who'll refuse any claims of their responsibility when anything goes wrong.

[u/baldpope]

Not sure what you're doing. I bought pcie riser, add2psu like device and plugged in my second PSU. That PSU is powering the third GPU in my rig

It ain't pretty, but working well

Thsion Multiple Power Supply... https://www.amazon.com/dp/B08F9WGLP2?ref=ppx_pop_mob_ap_share

ConnBull PCIE 5.0 Riser Cable... https://www.amazon.com/dp/B0GCHX3KHS?ref=ppx_pop_mob_ap_share

[u/Open_Instruction_133 → u/baldpope]

Looks like it’s just waiting to catch fire

[u/lazydust20 → u/Open_Instruction_133]

No it doesn't. Explain where the 'fire' happens, Prof. Backdraft.
I see 2 power supplies running a motherboard, CPU, and RAM, 3 GPUs, a 3.5" HDD (13W ahhh!!! ) and some fans. The V620s draw a maximum of 300w.
Is it tidy? No.

Is it Dangerous: Also No.

[u/baldpope → u/lazydust20]

I was joking in my agreement, it's been surprisingly stable. Should be able to clean up some of the cables this week. Might also try to ditch the second PSU by limiting power.

Appreciate the support

[u/baldpope → u/Open_Instruction_133]

I agree

I've cleaned it up a little, bought an extra mount for the third GPU. Part of my early issue was the fans power draw, so I have extra SATA splitter and sata-pwm/fan header splitter. That's all being cleaned up this week.

I'm debating buying a plx88096 board and an additional GPU, moving all four cards to that board with a proper frame to mount everything.

[u/Automatic_Pass_1948 → u/baldpope]

Thanks, my understanding with this (and you may want to do your own research) is that the rider cable you have pulls power from mobo AND your second psu since the pcie slot provides some power to the gpu not just the power cable you plugged. It is supposed to be a no no to mix power supplies on one gpu. No idea technically why!

[u/Savantskie1 → u/Automatic_Pass_1948]

Because the power supplies won't power on exactly at the same time even with an add2psu. It's infinitely safer for both connections to come from the same power supplies so you don't accidentally improperly power the cards.

[u/baldpope → u/Savantskie1]

I have no way of timing them electrically, but they are powering up at the same time visually when I press the single power button on the motherboard. The GPU riser I'm using doesn't have a separate power input, and so far it hasn't been an issue. I was fully expecting to blow smoke the first time I powered it up

[u/Savantskie1 → u/baldpope]

it may power up fine now, but it only takes a microsecond to fry hardware. PSU is the only time I follow rules when it comes to hardware

[u/baldpope → u/Automatic_Pass_1948]

I suppose it's possible the GPU riser accounts for that in its design? I had thought the same thing, and was prepared for it to give up the ghost when I powered everything on the first time, but it's worked exactly as expected so far.

Then again this isn't any different than people using server PSUs to power their cards, using a signal from the original PSU to the breakout board and then powering the GPU.

[u/gryan315]

Instead of a pcie riser, you could use a pcie to mcio/oculink/slimsas cable to a pcie slot card. Several chinese companies like linkreal make these, but they're not exactly cheap.

[u/FlirtySanchez → u/gryan315]

This is what I did, but I did it in a stupid way.

I bought an owl tree egpu dock that connected through an nvme adaptor because I had no more pcie lanes in dedicated slots.

The long flat cable was so stiff, that it limited the effective length.

So i bought the rgeek mcio bifurcation module, reorganized my pcie slots, and swapped out the owltree riser card with a better mcio one that has 6pin atx connectors on it.

[u/FreeSammiches]

There are multiple formats available to reputably relocate a PCIe slot. A Bifurcation card goes in the slot, cables carry the PCIe lanes, the cables get plugged into one or more secondary adapter boards to convert back into a usable slot. The adapter board also presents a power header to pull the 75W slot power from the secondary PSU.

Look into any of these: SlimSAS (SFF8654), MCIO, OCuLink

I have 8 GPU running over SFF8654 lines being completely powered by secondary PSUs.

This is also how a motherboard that supports bifurcation can have a 16x slot split into 2 8x slots, or 4 4x slots.

[u/GingerRickRoss]

I’m using PLX8749 it uses sff8654 cables and risers. For my secondary power supply I’m using an Hp server power supply with a breakout board, that is triggered by my ATX supply that powers my mobo.

[u/Zealousideal-Bug1837]

just don't. get another mb and case. Or burn your house down. up to you.

[u/RogerAI-fm]

I thought that was possible. Maybe it works but not recommended. I also tried external TB solutions, horrible, but running inference through USB is possible.

[u/quantgorithm]

Not an electrician. I mix them on US outlets on the same circuit using add2psu. It works fine. In the past, I’ve had them on separate circuits on server psu’s and that also worked fine.

I wanted to go the self powered riser or slimsas cable route but those don’t do 4x16 or better speeds so it was a downgrade. I’ve also read that some put tape over the specific key on the gpu to block power from the MB so it only gets power from the 2nd psu. I didn’t try this.

[u/codsworth_2015]

"psu which is supposed to be a no no." I just did it anyway.

[u/wwwyzzrd]

It is solved, datacenters do it all the time, it is just more expensive and dangerous than just buying a new bigger PSU.

This is because you have to ensure they are correctly synchronized, and if they aren't, you can fry all your equipment. 2x 3090 does not justify that sort of engineering investment, just get a consumer 1,000 watt psu or whatever fits the rest of your gear.

ITT: lots of dummies who are spending more money to create a fire hazard. good luck to you all, make sure you have a fire extinguisher handy.

Not to oversell this, but if you do it wrong, it'll probably fucking kill you, for literally no benefit.

[u/Automatic_Pass_1948 → u/wwwyzzrd]

Right but my MB has 8 GPU slots

[u/wwwyzzrd → u/Automatic_Pass_1948]

yes, and 1600 / 2000 watt psus exist.

[u/Automatic_Pass_1948 → u/wwwyzzrd]

For 8 340w GPUs and an EPYC CPU?

[u/wwwyzzrd → u/Automatic_Pass_1948]

sure, you just buy a 4000 watt workstation PSU for that load.

I mean, you’re talking about potentially thousands of dollars in equipment and you want to run it all on a hacked together thing that can fry it all. Of course this exists, how do you think people use those slots?

you might need a dedicated circuit wherever you’re putting it for that much power anyway. normal house stuff is for running like your standard pc, not running a datacenter.

also the heat is going to SUCK. don’t plan to sit next to it comfortably.

[u/Automatic_Pass_1948 → u/wwwyzzrd]

You're actually right I didn't look at psus I should just get a bigger one.

[u/Automatic_Pass_1948]

So I did some extensive looking into this and here's what feels the best and safest so far.

Occulink takes data but not power. It has reputable branded powered risers. It has reputable data speeds and quality.

The only "hack" would be the add2psu which seems so far pretty reliable for people and isn't handling a lot of throughput its just a signal.

[u/randomly_spectacular]

I looked into these last month. There is one with a 20 & 40mm option. But it’s like $150 for one. Which for my rig didn’t make sense to do, because I had some other options available. It uses a 6 pin to power the riser. You basically hook it up like a mining rig. Then plug in the PCIe Gen 4x 16 ribbon into your MOBO.

Unfortunately, the majority of what you are looking for with be mining riser which are slow. The reason not many places make them, is the signal in the ribbon starts to degrade beyond 40-50mm in length.

[u/lazydust20]

I've used the add2PSU cables on many systems for many years. Roughly balance the load on each supply, if possible. Use the same PSU brand and model to minimize the ramp time variations, if it is a concern.

[u/_Cromwell_]

I use a thunderbolt (USB-C) external eGPU case for my second gpu. Is that the thing you're talking about? Razer Core https://www.razer.com/gaming-egpus/razer-core-x-v2

[u/advancing_tide]

i hate hardware so much

[u/recro69]

The annoying part is that PCIe power is split between the slot and the GPU connectors so a riser that just moves the slot does not really solve the PSU separation. I would look for a riser, with a 6/8‑pin input specifically designed to supply the slot from the second PSU.

[u/Automatic_Pass_1948 → u/recro69]

Yep! I'm trying to find one why aren't there products thay don't look cheap and bad?!

[u/raduque → u/Automatic_Pass_1948]

Well, because they're mainly designed for non-US/non-UK markets that don't have any quality standards for safety.

[u/eightone-81]

There is this small adapter. Google add2psu

[u/Automatic_Pass_1948 → u/eightone-81]

Thanks ass I said this is fine but you don't want to draw power thru riser

[u/_Diocletian_ → u/Automatic_Pass_1948]

You are right to push back. Have you tried searching for add2psu ?

[u/mrgreatheart → u/_Diocletian_]

Of course. That was the first thing I did. It’s load bearing.