Sunday, March 27, 2016

Hacking GPU PCIe power connections


Until recently, I never thought much about PCIe power connectors.  Three 12 power and three ground wires was all I thought there was to them.  I thought it was odd that the 8-pin connectors just added two more ground pins and not another power pin, but never bothered to look into it.  That all changed when I got a new GPU card with a single 8-pin connector.

My old card had two 6-pin connectors, which I had plugged a 1-2 18AWG splitter cable into.  That was connected to a 16AWG PCIe power cable, which is good for about 200W at a drop of under 0.1V.  My new card with the single 8-pin connector wouldn't power up with just a 6-pin plug installed.  Using my multi-meter to test for continuity between the pins, I realized that it's not just a row of 12V pins and a row of ground pins.  There was continuity between the three 12V pins, and between three of what I thought were five ground pins.  After searching for the PCIe power connector pinout, I found out why.
Diagram edited from http://www.overclock.net/a/gpu-and-cpu-power-connections

Apparently some 6-pin PCIe cables only have 2 12V wires, 2 ground, and a grounded sense wire (blue in the diagram above).  With just two 12V wires, a crap 18" 20AWG PCIe power cable would have a drop of over 0.1V at 75W.  Since the 8-pin connector has three 12V pins, it can provide 150% more power.  My 6-pin 16AWG PCIe cable would have voltage drop of only 40mV at 75W, so I just needed to figure out a way to trick the GPU card into thinking I had an 8-pin connector plugged in.  The way to do that is ground the 2nd sense pin (green in diagram above).

I didn't want the modification to be permanent, so soldering a wire to the sense pin was out.  The PCIe power connectors use the same kind of pins as ATX power connectors, and I had an old ATX power connector I had cut from a dead PSU.  To get one of the female contacts out of the ATX connector, I used a hack saw to cut apart the ATX connector.  Not pretty, but I'm no maker, I'm a hacker. :-)  I stripped the end of the wire (red in the first photo), wrapping the bare part of the wire around the screw that holds the card bracket in the case.  I powered up the computer, and the video card worked perfectly.

Looking for a cleaner solution, I decided to make a jumper wire to go between the sense pin and the adjacent ground.  I also did some searching on better ways to remove the female contacts from the connectors.  For this, overclock.net has a good technique using staples.  When the staples aren't enough to get the contacts out, I found a finish nail counter-sink punch helps.

Here's the end result, using a marrette (wire nut) to make the jumper: