Dual PSU in practice. How good is it?

JonJ678 · 27 Aug 2021 at 10:58

I'm trying to work out how to power a threadripper multi-gpu workstation. Looks like the CPU can pull 800W and the cards 400W apiece, so that blows past the 1600W limit on the most expensive ATX PSU I can find (a Corsair one).

The options seem to be downclock/power limits or fewer GPUs. There might be something from the world of server power supplies, where form factor is fine but acoustics probably aren't. Difficult to search for too.

So how does two PSU's working together work in practice? E.g. plug one into the motherboard and the other into the PCIE cables and arrange for them both to start together somehow. I remember reading something about them disagreeing on the value of ground so one pushes current into the other. Don't want to kill the system through ignorance.

Thanks!

SonicSW20 · 27 Aug 2021 at 11:21

Its not too difficult to do. You can get adapters to handle the PSU switching on/off side of things, then its just a matter of keeping things sensibly segregated - for example if you have a 2x8pin GPU, don't use power from both GPU's as voltage differences on the 12V rails between the two could cause problems.

In your case the most pragmatic solution would likely be use one PSU for board / processor power and another one for GPU power.

mickyflinn · 27 Aug 2021 at 11:32

Tgis video shows how You badicslly just need a 24 pin dual psu cable.

~Dockie · 27 Aug 2021 at 11:35

its pretty easy if you can use a soldering iron ( the method i used )

its also worth mentioning that you'll probably get a way with a decent 1600w psu, you'd have to check the amperage on the rails and compare them to the components, but you'd be surprised how overspec'd a decent psu is and manufacturers just edge on the side of caution maybe a bit too much. an example is my 3700x+6700xt recommend above a 650w psu, but in reality (measured with a wattmeter) they barely draw above 250w in heavy gaming.

SonicSW20 · 27 Aug 2021 at 12:03

~Dockie said:
its pretty easy if you can use a soldering iron ( the method i used )

its also worth mentioning that you'll probably get a way with a decent 1600w psu, you'd have to check the amperage on the rails and compare them to the components, but you'd be surprised how overspec'd a decent psu is and manufacturers just edge on the side of caution maybe a bit too much. an example is my 3700x+6700xt recommend above a 650w psu, but in reality (measured with a wattmeter) they barely draw above 250w in heavy gaming.

250W is an averaged figure over the reporting period, a GPU can have transient spikes significantly higher than that which is why the PSU requirements from the manufacturers are seemingly high. Sure with a quality PSU you'll probably be fine as long as those transients do not trigger OCP, but those seemingly high requirements are set for a reason.

JonJ678 · 28 Aug 2021 at 20:19

The cable referenced above handles turning them both on at the same time but doesn't say anything about a voltage between the two '12V' rails. I'm having a hard time finding out whether that's a problem in practice, consensus seems to be "looks bad, but doesn't obviously break things".

I have found a couple of 2000W power supplies (one by evga, 'SuperNova 2000 G1+, one by superflower) which don't appear to exist anywhere at present but probably used to. If I can find one, that's probably the way to go. There are also 'dell' or similar branded server ones that look loud, and some 'mining' branded ones that are cheap enough that I don't believe the headline power number.

re: power ratings - anand measured a 3990X plus some nvidia card at 1200W at the wall, so it seems like the threadripper really can pull 800W. Which is an amazing number.

Tetras · 28 Aug 2021 at 20:44

I didn't watch the video, but I don't think those cables actually do anything, aren't they just to detect the power? In the ones I've seen only two pins are connected.

EsaT · 28 Aug 2021 at 20:47

JonJ678 said:
Looks like the CPU can pull 800W

You planning on liquid Helium cooled overclocking?

It's 280W.
https://www.anandtech.com/show/15483/amd-threadripper-3990x-review/2
https://www.anandtech.com/show/1647...ness-the-amd-threadripper-pro-3995wx-review/3
https://www.anandtech.com/show/1680...review-an-upgrade-over-regular-threadripper/2

Grim5 · 29 Aug 2021 at 10:58

The hell Threadripper pulls 800w?

as above, you'd need LN2 to even cool that - that's not a workstation that's a benchmark station

JonJ678 · 31 Aug 2021 at 21:59

Can obviously sink 800W through water. It's a big heatspreader and beyond that it's just having enough radiators to pick the water temperature. I don't know if you can do that sort of heat load through nitrogen, guess it depends how fast you pour it into the pot.

I'm new to this generation. Terminology has changed on me since ~ nehalem. As far as I can tell, the motherboards come with a few settings to increase the power the chip can draw. Called things like PBO and PPT. Idea seems to be to run all the cores at the single core boost freq provided the motherboard vrm can keep up and the chip stays below 90C or similar.

A few reviews have turned the dials up for the 64C chip (3990X) and reported power consumption well over 500W. I haven't been able to determine where diminishing returns kicks in, but on the face of it double the frequency for 3x the power sounds like a win. E.g.
https://www.kitguru.net/components/cpu/luke-hill/amd-ryzen-threadripper-3990x-cpu-review/9/ puts it at 900W at the wall before their cooling gave in.

I've tried quite hard to determine if the threadripper pro has the same more power -> faster control and can't find an answer either way.

Quartz · 31 Aug 2021 at 22:47

OCUK do a 2000W PSU. There are other 2000W ATX PSUs out there.

Grim5 · 31 Aug 2021 at 23:03

JonJ678 said:
Can obviously sink 800W through water. It's a big heatspreader and beyond that it's just having enough radiators to pick the water temperature. I don't know if you can do that sort of heat load through nitrogen, guess it depends how fast you pour it into the pot.

I'm new to this generation. Terminology has changed on me since ~ nehalem. As far as I can tell, the motherboards come with a few settings to increase the power the chip can draw. Called things like PBO and PPT. Idea seems to be to run all the cores at the single core boost freq provided the motherboard vrm can keep up and the chip stays below 90C or similar.

A few reviews have turned the dials up for the 64C chip (3990X) and reported power consumption well over 500W. I haven't been able to determine where diminishing returns kicks in, but on the face of it double the frequency for 3x the power sounds like a win. E.g.
https://www.kitguru.net/components/cpu/luke-hill/amd-ryzen-threadripper-3990x-cpu-review/9/ puts it at 900W at the wall before their cooling gave in.

I've tried quite hard to determine if the threadripper pro has the same more power -> faster control and can't find an answer either way.

It's amazing how much better the larger die of Threadripper cools so much better. To see a 3990x running significantly cooler than a 3950x both CPUs on a 240mm AIO it's just crazy.

3990x at 280w in blender with 240mm AIO = 60c

3950x at 220w PBO in blender with 240mm AIO = 75c

nuts

If I ever buy AMD again I think I'll go Threadripper just to get better temps, my 5950x gets up to 85c on my huge custom loop