[Dalewyn]

Except when the "expected tolerance" is unreasonable.

Even if the connectors and wires are to spec, the design leaves next to no margin for play. You need that margin to account for reality: Handling by casual end-users rather than trained professionals, the ambient temperature of the average room or office, dirt and grime that might get lodged and go unnoticed, wonky supply/draw of power, and more.

Running 8.3A through connections rated for 8.5A is "expected tolerance", it's also fucking stupid in no uncertain terms.

[clhodapp]

There are at least two issues here:

1) The designed safety margin is unacceptably low. It should be set such that any cable that complies with the expected safety tolerance for carrying current is safe to use.

2) The late-model Nvidia cards in particular have no feedback system to discover unbalanced current on 12v wires that make up the connector and no circuitry to keep the current balanced even if they did. That is, they forgo any digital control and depend on the physical properties of the conductors to be perfectly balanced.

Overall, Nvidia failed to learn from the melting connector issues in the RTX 4000 series and doubled down by increasing the power draw while further cost-cutting the safety circuitry.

See:

    * High-level demonstration: https://youtu.be/Ndmoi1s0ZaY?si=bkv12pXG4K5T72YN
  
    * Low-level explanation: https://youtu.be/kb5YzMoVQyw?si=Bl5aowND4uXoI8s6

[caycep]

Ok, I have to say this, but the narrator sounds a lot like Jim Henson/Kermit the Frog....(my understanding it is a maryland/mississippi accent combo)

[sozforex]

~23A had been measured going going through one of 6 wires/pins in this test: https://youtu.be/Ndmoi1s0ZaY?t=927

Standard requires these connectors to handle 9.5A per pin (9.5 A × 12 V × 6 pin = 684 W).

A detailed explanation of this mess: https://www.youtube.com/watch?v=kb5YzMoVQyw

[leeter]

I'm curious, if there are any high level electrical engineers reading this please respond.

I wonder if that vertical (as far as the PCB goes) power connector will always ensure that this sort of imbalance will always occur. While we like to pretend that current is even in any given current plane that's not what happens. The impedance of the wires and copper is not perfectly ideal. This is why these connectors have equal number of grounds, so they have an ideal shortest path and balanced return current path. So I'm curious if electrically it's just impossible to have a vertical connector like that (on that shorts all the pins for 12V together instead of current balancing them) and have it balance current across the pins. The pins closest to the board should in theory have the greatest currents as they are the shortest path electrically. Based on the pictures that appears to be the case. It appears that the pins under the most stress are likely those with the lowest impedance.

Assuming my SWAG above is correct... I'm curious if this is affected by the per pin impedance on the PSU too. Where if certain folks are just unlucky get a situation where some pins in the connector have a significantly lower impedance than the rest.

If my second SWAG is plausible, my third and really bad SWAG is that removing the two ground pins nearest the PCB could actually "balance" the current better by forcing the current to use a slightly longer path for the power pins. But, my guess is this will just cause EMI issues. So please don't test this unless you're an EE and know what you're doing.

This is pure speculation on top of what Buildzoid, the posts above this have said, and what I've learned from Robert Feranec's videos. I'm in no way an electrical engineer, just a humble hobbyist and person that loves to learn.

[exmadscientist]

Paralleling wires is stable because the TCR of copper is positive. When one connection carries too much current compared to its peers, it will heat up. This will increase its resistance, causing it to accordingly carry less of the current. So the system is self-balancing.

Do not remove ground wires. That is stupid. You'll just be raising the current in the remaining wires. EMI should not be a major concern as we are talking about DC power delivery here (also why I'm saying "resistance" instead of "impedance") and so the potential for trouble by changing the number of conductors making a connection is limited. Yes, anything could happen, but that's just the nature of EMC problems.

[leeter]

Yeah I realized that was the worst way to go about testing that anyway right after I went to bed last night. If (big stress on if) that was the issue a ferrite bead would be a better way to test it. Based on what you're saying my SWAGs were wildly off. I'd still like to see the sims of it however to see if they provide any illumination on the issue. What makes me think something weird is going on is that it's two out of six wires heating up to absurd degrees. Of the other four two are carrying normal currents and the last two (based on Roman's video) are carrying practically nothing. Buildzoid makes the convincing argument that clearly Nvidia engineers were aware of something like this could happen on the 3090. But, then didn't carry that over to the 4090/5090.

[toast0]

> This is why these connectors have equal number of grounds, so they have an ideal shortest path and balanced return current path.

These connectors have an equal number of grounds and 12v because the same current flows on both sides, and the required current justifies at least 6 wires at the specified current.

Pci-e 8-pin power is a bit weird, because it's 3 12v and essentially 5 grounds; but that's because it's pci-e 6 pin and a promise that the power supply makers know what they're doing... The extra 2 grounds signal that the PSU designers are aware of the higher current limit, even though the wiring specifications are the same.