Help! Issue with risers not fitting my larger PCI slots/Dual PSU question

[BreakYourFrame]

Well that escalated quickly...

[joaocha]

I have 10 rigs for more than 1 year, some of them are mixing with 3 psus, never had any problems that is power related.

Ps: im using paper clips jumper in all Psus that are not powering motherboard

The rest are just bullshit to sell some gimnicks acessories/psus

#peace

[SCSI2]

I have 10 rigs for more than 1 year, some of them are mixing with 3 psus, never had any problems that is power related.

Ps: im using paper clips jumper in all Psus that are not powering motherboard

The rest are just bullshit to sell some gimnicks acessories/psus

#peace

So you're not synchronizing PSUs when powering-on/off? I'm just wondering because I don't do that either and I have up to 4 (four) PSU's on some of my rigs.

One day I will probably measure currents flowing between PCI-e and the risers, but I don't think there is much if anything at all. 3.3V seen on the rizer USB connector is likely signals, not VDD.

PCI-E lanes that carry data are usually AC coupled via small capacitors on all GPUs. They are not isolated (like optically), but not DC coupled either.

-scsi

[SCSI2]

Just checked some risers i have laying around (v006 and v006c) and they don't conduct neither 3.3VDC nor 12VDC power from the motherboard to the 16x slot on the riser. Though your mileage may vary...

All ground lines on my risers are connected through to the 16x slot as they should, therefore powering risers/GPUs from whatever PSU should be acceptable as long as all grounds are reliably connected directly or otherwise via USB3.0 cables.

This ground connection does not have to carry high currents as all power hungry devices, such as GPUs, get their own high capacity GND and 12VDC lines from respective power sources. In this way, the common GND connectivity throughout the system is supposed to carry small currents necessary to equalize GND levels from different PSUs.

This of course is an ideal situation assuming quality PSUs and risers being used without any miswiring or shorts. In real world anything can happen, but I can imagine worse outcomes possible in the most commonly recommended wiring scenario with all risers being powered from the main PSU and some GPUs powered from a slave PSU(s).

In such situation those GPUs will receive one chunk of their 12VDC power (up to 6.25A or 75W) from main PSU via riser and the rest (100W?) from the slave PSU via PCIe 8-pin connector. These 12V levels can not be exactly the same because they are coming from different PSUs. You can imagine some horribly designed GPU board where these different 12V buses meet somewhere on the board and then smoke comes out.

In my opinion, the most important thing to understand here is that feeding each particular power hungry device (GPU) from the same source is much safer, therefore they need to have their 12VDC lines via PCIe connector (from the riser) and via 8-pin PCIe power connector to be coming from the same PSU.

One other common misconception I see being thrown around is "Ground Loops" and the myth which says that you somehow can avoid it. These things are impossible to avoid in dual PSU configuration simply because GND lines do connect inside each GPU. Even if we feed power to them from different sources (riser and 8-pin), grounds from those sources still meet on the ground plane on the GPU board. They also meet on the motherboard, when more than one GPU is involved and in many other situations.

In general, ground loops are bad, but in our situation with PCI-Express interconnect, we can mostly ignore them because the bus uses differential signaling which is largely insensitive to the noise and garbage caused by ground loops.

-scsi

[Subw]

In my opinion, the most important thing to understand here is that feeding each particular power hungry device (GPU) from the same source is much safer, therefore they need to have their 12VDC lines via PCIe connector (from the riser) and via 8-pin PCIe power connector to be coming from the same PSU.

brainless idiots won't listen, they have "guru" on youtube with 15K followers (lol) telling them to power risers only from main PSU