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I have an ATX PSU that has two 6pin connectors for PCI Express power. Both have Gnd on the lock bar side, and +12V on the opposite side. I lost modular cables that came with the PSU, and it made me to start to dig.

According to this answer on our sister site, in 6-pin configuration pin 5 is Sense A. In 8-pin, pin 6 is sense A (same position as Sense A in 6-pin config) and pin 4 is Sense B.

But what are these pins supposed to do? I know that most people just connects Gnd there and calls it a day, and I could too, but I wanted to ask about the meaning behind it. It's called "Sense" for a reason, I suppose. So, what is that reason?

Image that shows pinout

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Sense pins are connected to ground at power supply or adapter cable. This allows the PCIE card to detect if a supply cable is connected or not, and whether a 6-pin supply is connected to 8-pin socket to indicate less power is available. It is not used by the power supply for "remote voltage sensing" to compensate for voltage drop over the wiring.

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    \$\begingroup\$ OK, now I see two answers that basically oppose each other, and I do not see any source to indicate which one is right. I'm inclined to believe you given that my PSU just connects all 3 pins to the Gnd, but on the other hand, it might be just a lazy PSU design, right? So, do you have any source to back up your answer? \$\endgroup\$ – Mołot Nov 4 '19 at 13:18
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    \$\begingroup\$ @Mołot Justme is right. Unfortunately, the PCI express specs are not freely available, but you can check en.wikipedia.org/wiki/PCI_Express#Power \$\endgroup\$ – dim lost faith in SE Nov 4 '19 at 13:30
  • \$\begingroup\$ @dim well, that's source enough for me. At least until I'll be ready to pay for official documents, and I don't see it happening any time soon. \$\endgroup\$ – Mołot Nov 4 '19 at 13:34
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These are used by the power supply to compensate for voltage drop in the cables and connector. These are low-current returns to the power supply. The PS senses the actual voltages on the PCI board and adjusts the outputs so that the yellow pins are as close to 12 volts as possible and the black pins are as close to zero volts as possible.

For example, say that the +12v has a 0.25v drop because of oxidation on the connector pin producing resistance. The green sense pin is not affected by oxidation due to the very low current flow so the PS senses this and raises the +12v output by 0.25v so that the board receives 12.0v.

Similarly, the blue sense pin allows sensing voltage drop on the black wires, allowing the PS to drive the black wires slightly negative to compensate.

Looking at the location of the sense pins, I believe @justme is correct. This allows the PCI card to detect what size connector is plugged in and so know how much power can be drawn safely.

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