It’s often useful to have standalone power supply for power supply troubleshooting on various videocards. Using motheboard for this purpose is not optimal and even dangerous when critical failures occur. Best solution for this issue is to make easy plug-in power connector for powering videocard without anything, just with bare PSU.

Connecting just PCI-E power connector to videocard and powering PSU is not enough for power supply operation. Main power +12V rail is used from PCI-E slot. Also +3.3V voltage used for auxiliary curcuits and digital domain. So the idea is simple: take PCI-E plastic connector from motherboard and apply power to card thru it. This will allow us to have versatile power adapter to power different videocards without any extra soldering.

Step 1:

Get ATX power connector (type Minifit JR 24pin) for using as power input from standard PSU (you need +12V and +3.3V voltages). Then desolder PCI-E slot from some dead motherboard.

It can be any width, not only limited to x16 wide PCI-Express graphics slot. Just make open side with mill or sharp cut for smaller slots, power section before mechanical key is just same on all slots.

Step 2:

Assemble adapter according to next wiring diagram:

All +12V pins connect to +12V pin on input connector, same for +3.3V and similar for grounds.
Take care not to short rails and non-related signals.

Step 3:

Check assembly

Step 4:

Power up PSU with adapter connected. I use jumper placed between GND and green-wired PS_ON signal with 24-pin connector for power-on / power-off control.

Measure voltage on slot – if everything okay – connect it to videocard

Assembled adapter with videocard connected

Step 5:

If videocard have extra PCI-Express power inputs – connect them as usual to PSU. Hook adapter to videocard

Everything now fine, all voltages are present on videocard.
GPU chip and memory are powered as usual, staying in 2D/uninitialized operation mode.

This adapter is useful for repairs and power supply modifications.