Why, WHY could I not find this forum months ago?Awesome reply: thanks a lot; the latter build is approximately what I'm try to emulate ! So with a gtx 750 ti, 4x will be totally fine. Will try to post some pics once it's done I found a french site dealing with pci-e 3.0, and they found a difference of about 1-2% only with gtx 980 cards. So cards are slowly pushing towards the maximum capacity of x4 lanes, but nowhere to the x8 lanes. So x16 is only useful for professional cards ? Now I understand why QinX isn't worried at all plugging his 970 onto a 4x riser...
The x4 riser may be you problem. The PCIe slot can provide up to 75W through bus power, but only for the x16 interface, with the limit being lower for x4/x8 and x1. Depending on how your motherboard's BIOS/EFI is configured, the card may request 75W with its 4X link, and the motherboard may simply say "NO". It's entirely down to whether you motherboard sticks to the letter of the specifications, or will go over and above what they require.
Second point of suspicion is the external DC brick. If it;s a laptop brick, those are designed to provide a sustained constant load to charge a battery.
For a normal PC power supply this is not an issue at all, the PSU is designed to handle that as a requirement of being ATX compliant. A laptop brick is not, so may shut down when it encounters a power spike.
All slots can draw up to 25W by default (10W for 1x). If a card is a 16x full-height (half-height cards are limited to 25W) card, then it can request that be updated to 75W. The Motherboard is responsible for responding to that request. Technically, to comply with the PCIe standard if the interface is 4x then the motherboard should not agree to switch the slot to high-power mode. This is regardless of the number of physical power traces connected, which will be the same for all bus widths.My interpretation was that all the power pins were still in the x4 pins, that only PCIe graphics cards can request more then 25w and since it is indeed a graphics card it can still request 75w, and since it is the only PCIe slot that it might actually be able to supply the 75w to the x4 slot. Again, that's my interpretation, so I could definitely be wrong.
FSP's spec page is very clear on a 7.9A maximum current limit for this PSU, which at 19V is 150.1W and with no mention of a higher peak current capacity.
Technically, to comply with the PCIe standard if the interface is 4x then the motherboard should not agree to switch the slot to high-power mode. This is regardless of the number of physical power traces connected, which will be the same for all bus widths.
Also keep in mind that the mainboards traces will could only be wide enough to handle 25W, not 75W, so even if the pins could handle such an amount of power to be drawn, the board could fry itself if it allowed that to happen.
Are you still working on this project or a similar SFF build?