Topic: Mix riser power from different PSU's? (Read 3340 times)

Hi,

I’ve added a 1200 watt server PSU per 6 rigs. The rigs were made to run at default clock which took about 85% of the PSU capacity, so the PSU’s can’t keep up with overclocking.
I needed to spread some load to a second PSU (GPU high power input is 7.5amp = 90 watt x 2 GPU's = 180 watt per rig x 6 rigs is 1080 watt).
I’ve read a lot what to do and what not, but nobody has a straight forward explanation.


The problem is that the ground differential is conducted though:
1.   The GPU
2.   The motherboard
3.   PSU to PSU wire, 1 card
4.   PSU to PSU wire, max cards.

https://i.imgur.com/jrFxxZX.png

If you would take the same brand and type PSU’s, the chance both grounds are equal is bigger than you take a b-brand desktop PSU and combine it with a b-brand server PSU.
To start of you can measure the difference in current by turning them on without the second PSU connected. Hook up a multimeter and measure the amps running between the two grounds. This is the load your GPU, motherboard or straight wire needs to conduct.

The ground on a GPU circuit board should be pretty strong. If the PSU’s are wired like option 1, the current is spread across the 4 cards. It could run, maybe unstable. It depends on the difference between the PSU’s.

If all is hooked up like option 1, but only one GPU’s high input is powered by PSU 2, all 3 amps will run through 1 GPU. Could be bad.

Let’s say you got 3 amps between PSU’s and wire everything up by option 2, your motherboard needs to conduct 3 amps on top of what it was already conducting and it will eventually burn.

If all is wired by option 3 with a thick wire between PSU’s, the most of the current will run through the wire. Do the same with a thin wire and most of the current will go through the single GPU’s, risers and motherboard.
Make the wire the least resistance and get most of the current run though the wire.

Best option is number 4. Get the current though the cable and spread the rest over the GPU’s.
4 or 6 mm2 (12 or 10 AWG) per amp difference should be good.

A dual PSU setup wired the wrong way could work perfectly because the PSU’s ground voltage is equal.
A dual PSU setup wired the right way could run unstable because the PSU’s ground voltage varies too much.

It doesen't matter if the risers are newer or not. Instead of saying you think you're 'pretty sure', do what I did. While the motherboard is on, get a mulitimeter and measure the voltage on the outer pins of the USB connector coming from the motherboard that plugs in to the back of the riser. If you do that, you will see there is 3.3V coming from the motherboard through the USB cable connected to PCI-E slot. Which means that  supposed 'electical enginner' saying there is NO power connection between the riser and the motherboard is false. There is 3.3V coming from the motherboard to the riser though the USB cable. This post explains why you need to power the risers from the same PSU that powers the motherboard, so the signal voltage and ground is set in refrence to the primary PSU which is also powering the PCI-E lanes on the motherboard.

https://bitcointalksearch.org/topic/m.4878774

That is the suggestion I took and all of my dual/triple PSU rigs are setup up where all of the risers are connected to the primary PSU connected to the motherboard and I only use the secondary PSU's for VGA power inputs on the cards. I've been dual mining 24/7 for close to 10 months without any problems, so for me that is the best way to setup multiple PSU rigs.

I'm pretty sure the riser is creating that 3.3V from its power, if it's a newer riser,
and if the usb has a ground in it going to the mobo, wouldn't that also ground the PSUs together?
If you do a PSU on the gpu and a different PSU on the riser, you might have 12V rails fighting if they are not exactly the same and that could cause a fire, right?

Here is an experienced engineer talking about this:
https://forum.z.cash/t/the-facts-about-gpu-mining-electrical-specifications/19468

After 5 days of trying to get my 8 GPU rig working by splitting riser power evenly among two PSUs I finally switched and powered all 8 risers with the Master PSU (that is powering the motherboard) and MAGIC HAPPENED! it all started to work...

So what is said above is 100% true and highly recommended. The other issue I had is my SLAVE 850w PSU is not sufficient to power 6 cards (I have 2 GPUs powered by MASTER, 6 by SLAVE).

Interestingly I built this rig based on a Youtube video where a guy is using 8 Red Devils on two 750w EVGAs and he is splitting both the riser and video card power evenly. I bumped my PSU wattage to 850 x 2 just to be safe when I built my version of it and it is still way insufficient to power 4 cards on the MASTER. Somehow it works for him, but we can't really tell from the video how long that arrangement actually lasted 

jaja

for me both cases worked, but i always used usb powered risers,
- powered usb risers cut 12v and 5v between pci-e and riser, so card not stress pci-e slot
- if you connect 2 psu to one gpu card (like one to riser and second to erxternal connector) then both psu are grounded together, because grounds are connected in gpu
- you have to connect with grounds, so there is not voltage potential between both psu and will be no other voltage potential than signal in connection between gpu and motherboard
- usb powered risers have ground connected between pci-e adapter and riser, any connection would ground psu together

You are wrong. I checked the outer pins on the USB cable that connects to the riser from the 1x PCI-E slot with a multiimeter. There was 3.3V coming from the motherboard. There is also a ground connection between the riser and the motherboard.

http://www.tomshardware.com/reviews/msi-geforce-gtx-1080-ti-gaming-x-11g,5036-4.html
http://www.tomshardware.co.uk/aorus-gtx-1080-ti-xtreme-edition,review-33896-4.html

At the bottom of the page it shows the power drawn from pcie slot. Msi 1080ti only draws 0.7A max wheres Aorus draws 3.8A.

Using for years without issue two PSU setup connection marked at Your drawings as "not correct". My opinion is opposite, of course:
- I know for sure that there is no power connection between motherboard and riser. Therefore I can freely use different PSUs for motherboard and GPUs.
- I don't know for sure is there power connection in GPU between it's PCI-e slot and PCI-e power connectors. So I don't mix PSUs to power riser and power connectors on single GPU.

- got link for this?

Tom's Hardware reviews show load on motherboard slot for various 1080ti.

Does anyone know how power from risers/cables are distributed? I mean, let's say a 1080 Ti uses 250W and has one 8-pin and one 6-pin.

Will it draw 225W from the cables and the remaining 25W from the risers or will it be pulling 75W from the risers at all times and the remaining 175W from the cables? Or perhaps something else?

I an inclined to believe it is true, at least more preferable when possible as in the new rig I am setting up one of the GPUs on riser powered slave other psu threw code 43 out of blue after running some time. Card runs by itself on my test system, card runs when it is powered from master psu with other risers but did not when powered from slave psu.

It looks like powering risers from different PSUs can/may run but rig is not stable.

I will change all setup to what is suggested here.

Lets see if it will be better.

Yes, you can power the VGA inputs from different PSU's. What's important is that all the risers are powered by the same PSU that powers the motherboard. BBT has built more rigs than many of us put together and in several of his videos he adamantly says you MUST set up multiple PSU rigs that way. Otherwise you will create ground loops. The links in my post also explain why you need to set it up that way.

I'm having some issues now because pretty much ALL power supplies at 1000W or more are sold out all over Sweden, UK and Germany. So I'm now thinking about powering rigs using 2x 850W PSU's.

Your image suggests that the risers needs to be powered by the same PSU that powers the motherboard (for some reason).

Would it be okay to power the rig like this:


That way all risers are powered by the same PSU as in your image. However, that PSU also powers one GPU. The second PSU would only power the remaining five GPU's.

On the other hand I'm reading a lot of different answers. A lot of people are saying that they're dividing the power with one PSU powering 3 risers and 3 GPU's (and the second the rest + motherboard) while others say that you can't do that (though I can't find a single post properly explaining why). So many different opinions and answers.

There are differing ideas on powering the risers and GPUs with dual PSU with respect to using/not using sata adapters, more than one riser on same sata/molex channel and etc.

So far:

 I have 2 Evga 850 G2 and here is my setup. Each has 4 VGA cables. (2 x (6+2) Pins and 2 x(6+2)+ 6 Pins) and 1 Perip (4 Pin Molex)

I have several risers  such as 6 Pin PCIE powered and 4 pin Sata powered.

I aimed to power riser with 6 Pin PCIE as much as I can and then 4 pin molex to avoid sata.

PS1: Master.Powers system. 4 pin molex channel of PS_1 is used to connect add2Psu with PS2
PS2: Slave

In summary:

PS_1: Powers 3 risers and  3 GPUs
PS_2: Powers 2 risers and  3 GPUs.

GPU_0 pluggged into mobo directly and  powered by PS_1 PCIE (6+2) Pins.

GPU_1 on riser. Both riser and GPU_1 are powered as: PS_1 PCIE (6+2) pin to GPU and PCIE 6 Pin to Riser with 6 Pin Male/Female Extension.

GPU_2 on riser. Both riser and GPU_2 are powered as: PS_1 PCIE (6+2) pin to GPU and PCIE 6 Pin to Riser with 6 Pin Male/Female Extension.

GPU_3 on riser. Riser is powered as: PS_1 6 pin (of 6+2 Pin) PCIE cable. GPU_3 on this riser is powerd by PS_2 6+2 Pin VGA Cable. This is the only cross riser/GPU in my setup.

GPU_4 on riser.Both riser and GPU_4 powered as: PS_2 PCIE (6+2) pin to GPU and PCIE 6 Pin to Riser with  6 Pin Male/Female Extension.

GPU_5 on riser. Both riser and GPU_5 are powered by PS_2 as riser is powered by 4 pin molex of PS_2 and GPU_5 is powered with PCIE 6+2 Pin.

This setup leaves me one available VGA on PS2 but I have one missing (6+2)+ 6 Pins VGA cable. I contacted Evga for this and waiting for a response.
When/if I get missing cable I can change cross riser/gpu connection too.

This sounds complicated. I can power oll the risers from main PSU only if I use sata/molex extension cable(s) only.

System runs fine so far.

I am open to suggestions for better power connectivity if I am doing something really bad.

Well it might not be enough for 6x 1080 Ti cards.

2400w is way more than enough power for 6 cards. You could get away with half of that.

Personally, I use a 1200w server PSU for the cards + risers. Then I use a 300-500w psu (whatever is cheapest) to power the board and SSD.

Also, in terms of the picture above, I have definitely done that before and have had no issues with it.

I recently upgraded to a dual power supply setup which had me researching on how to connect them safely.

I came across posts suggesting powering the riser and PCI-E power connector on the card with the same PSU. Others say the psu that powers the motherboard must also power alll the risers. The slave psu should only power the PCI-E power connector on the cards.

https://bitcointalksearch.org/topic/m.17825999

https://bitcointalksearch.org/topic/m.18351224

Ultimately, I did follow the latter suggestion as it seems logical the same power supply that powers the motherboard should also power the risers connected to the PCI-E slots on the motherboard. The second slave power supply only powers the 6 & 8 pin auxiliary power connector on the video cards. According to another post I came across that connecting the PSU's that way ensures they share the same ground, which is important.



https://bitcointalksearch.org/topic/m.4878774

I also noticed BBT in his latest live stream set up a 9 card dual PSU rig and also explained that when setting it up, you need to power the risers with the same PSU that powers the motherboard and only use the second PSU to power the GPU's.

https://youtu.be/pQ-EAunoAqY?t=988

I was under the impression that it is preferred to use one cable per riser.

Why would you do that... you can power the riser and the gpu with one cable + a splitter....