What's the difference between splitting 8-pin power inside vs outside the PSU?

marcosscriven · 14 Dec 2020 at 06:58

Long story short, I was using an 8-pin splitter for my 3080FE and seeing shutdowns. Moved to two separate cables, and all is now fine.

What I'm struggling to find now is a really good explanation as to why - there's a lot of posts out there taking it as common knowledge.

It *sounds* to me like the PSU is cutting out (presumably to protect itself) - but protect against what?

The gauge of the wires is ample for a single cable, so I don't think it's temp
The SF750 has a single 12V rail, so I don't think it's over power (especially as fine with two cable)
I don't think there's per-plug monitoring on the PSU

All I can think is that, purely electrically, there's a slightly greater voltage drop when using one 8-pin with a splitter, vs two. But I'm not really sure why? The power has to split from a single rail either inside the PSU or outside via the y-cable. I don't know enough to know why that's different (and clearly enough to go from assured shutdowns to completely stable).

gpuerrilla · 14 Dec 2020 at 22:19

Can you share a picture of this splitter?

EsaT · 15 Dec 2020 at 17:00

Transfer losses:
Because no real world conductor is super conductor it has some resistance.
And when you double the current wire is carrying, you double the voltage loss.
(V=IR)

That means 12V graphics card gets has twice the drops at same load and RTX 3080 is pretty extreme power hog.
That forces GPU's VRMs to do more work to provide stable output voltage risking stability.
Same voltage loss actually happens also on current's return path and "ground" what graphics cards sees rises doubled amount above actual 0V.
Which certainly doesn't help with stability.

And in precise scale since input voltage of graphics card's VRM drops, it needs to draw higher than original current when outputting same power.
So wires have to actually carry little more than twice the current.
Which excarberates the problem.

marcosscriven · 17 Dec 2020 at 12:32

EsaT said:
Transfer losses:
Because no real world conductor is super conductor it has some resistance.
And when you double the current wire is carrying, you double the voltage loss.

Great explanation, thank you.

While I'm familiar with the essential school-level physics here, I hadn't thought the resistance of using one cable would be anywhere near *double* the resistance of two when considering the rest of the circuit in the PSU itself. That last point being the confusion here for me. If the PSU is itself one rail for 12V, isn't itself splitting that 12V across multiple sockets?

It seems to be it's the PSU that shuts off, presumably due to over-current protection.

EsaT · 17 Dec 2020 at 19:54

Well, technically also resistance of the wire rises little bit, because of higher temperature increasing resistance.
Power loss in resistance rises to square of current:
P=UI and U=IR and hence P=IIR
So doubling the current quadruples power loss in wire and how much it heats up.

But it won't trigger PSU's overcurrent protections any more than two cables.
Again PSU stays powered only as long as motherboard keeps pulling PS_ON signal to ground.
And unstable craphics card transmitting erroneous signals could make motherboard/CPU crash.

pp111 · 19 Dec 2020 at 21:28

I am guessing here that it's a temperature thing. There is a firm recommendation to use two separate cables with the 3080's and this is probably why. I remember a recent test that monitored the cable temperatures to a gigabyte 3080 gaming and they got up to 50 degrees.That was with two! Not screaming hot by any means but that's enough to alter their resistance a measurable amount. Using one it's going to be a lot worse.