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Power supply basics
 

Power supply basics  

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Wojciech Banaś
(@wojciech_banas)
New Member
Joined: 4 weeks ago
 

Hi all,

 

I'm planning to set up my own eGPU build but there is one thing that prevents me from doing so- the power supply.

 

As much as multiple guides were made on different connectors between the motherboard and the raiser/enclosure, the topic of supplying the power to the card and riser itself hasn't really been covered at the ground level (at least I couldn't find any, sorry!). 

From what I know, the raiser (the beast, the cheaper PCE164P-N03 and other) have a limit of how much power they give to the GPU via the PCIe long connector. I have GTX 1660 on my mind, so I know I need a PSU, most likely the Dell DA-2, because it's cheap as heck. Now, I know it has an 8-pin power connector and it is compatible with the connector on the riser. However, the riser only supplies a limited amount of power to the GPU, so I need a direct connection to the GPU from the PSU. 

In that case, do I get an 8-Pin to PCIe power connector converter and plug that into the card only? Do I need to power the raiser itself too? Do I need to power both at the same time? if so, how? 

Does my laptop supply some power through the connection too? If yes, how much and how does that work when PSU provides an excess of power? (PSU 200W, GPU requires 130W, does the PSU provide only a proportion of it, the rest my laptop, or does the PSU provide everything?

I would really appreciate if someone could explain all the connection issues, what needs to be supplied by what and how the power runs. It would be of so much help to people starting the eGPU journey! 

Pending: Add my system information and expected eGPU configuration to my signature to give context to my posts


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joevt
(@joevt)
Prominent Member
Joined: 2 years ago
 

@wojciech_banas PCIe wikipedia/2.1.3 Power

A PCIe card requires 3.3V and 12V through the PCIe slot. On most x16 risers, a voltage regulator converts 12V or 5V from your power supply to 3.3V. An M.2 adapter usually gets 3.3V from the M.2 slot. I have an M.2 riser that gets 3.3V from 5V using a voltage regulator. There is an M.2 adapter that gets 3.3V from SATA power connection (usually floppy power is used in M.2 adapters).

A GPU in a PCIe slot uses 3.3V and 12V from the PCIe slot and additional power from the PCIe power connectors (6 pin and 8 pin connectors). A 6 pin connector is supposed to be able to provide 75W. and 8 pin connector is supposed to be able to supply 150W.

The PCIe slot's 3.3V connection is 9.9W. The 12V connection is at least 6W. An x1 card max combined is 10W.
An x4 connection can have 25W at 12V (25W combined).
An x1 card can negotiate 25W (I don't know if this happens with GPUs in risers).
An x16 card can negotiate 66W at 12V (75W combined) (I don't think happens with risers because they don't have an x16 connection).
I guess these negotiations are viewable with pciutils by looking at the PCIe registers.

For an M.2 adapter, your laptop might provide the 3.3V for the PCIe slot (if there's no voltage regulator in the adapter). The riser/adapter requires power for the PCIe slot. The GPU requires power for it's 6 pin and/or 8 pin connectors. The EXP GDC Beast requires a single power connection to a PSU, and then has a connector for the 6 pin/8 pin connectors of the GPU. If you're using a normal ATX or similar power supply, then you can connect the 6 pin/8 pin connectors of the GPU directly to the PSU.

PSUs don't supply excess power. That's not how electricity works. It could supply excess voltage but all the PSUs you'll be using should be 12V. A device takes an amount of current from the power supply. power = voltage x current (P=VI). The amount of current increases inversely with resistance. V=IR; I=V/R. Zero resistance gives infinite current = a short circuit = sparks and melted wires. More current requires thicker and/or more wires so they don't melt (also, thicker wires have less resistance = less voltage drop). A 1200W PSU can power a 10W device but it would operate more efficiently with a higher power draw (like 600W). https://en.wikipedia.org/wiki/80_Plus  So you should match the PSU to the devices you want to connect it to (add some headroom).

A well behaved power supply should shutdown if a device tries to take too much power (or there's a short circuit).

 

Mac mini (2018), Mac Pro (Early 2008), MacBook Pro (Retina, 15-inch, Mid 2015), GA-Z170X-Gaming 7, Sapphire Pulse Radeon RX 580 8GB GDDR5, Sonnet Echo Express III-D, Trebleet Thunderbolt 3 to NVMe M.2 case


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