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Power a dGPU inside a PC with external PSU


davide445

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Got for free a PC with limited non replaceable PSU, want to use my GPU connected to his mobo but powered also with an external secondary power supply.

Owning the PE4C v2.0 as adapter for eGPU with my laptop, my idea was simply to use it as power adapter with the  Dell power brick as PSU, but testing with PC off the PE4C is not powering the GPU.

I didn't understand the problem and so the possible solution, how is decided the power supply balance btw PCIe slot and the 6+6pin? I need in some way the two PSU communicate? Purchasing a complete ATX PSU will resolve the problem?

Edited by davide445
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Prebuilts are nasty. Last time I got one I took a hammer to it and chopped up the motherboard so that I could fit in an ATX PSU and a GTX 980.

I'm assuming what's happening here is that you've got the power pins from the PE4C connected to the GPU, and that's it.

 

This is obviously not going to work. The PE4C requires a signal from the ExpressCard/mPCIe adapter before it will activate the power supply.

 

Let's just solve the problem instead of trying to make this contraption work. Essentially all you really need is to figure out how to power on the PSU.

You don't even need the PE4C.

Lazy people will jumper pin 14 and 15 on the ATX connector and then the power supply will be on permanently. While that isn't a terrible solution, it isn't a clean one.

 

Smart people will slave the second power supply to the first, so that both power on at the same time.

There exists a professional product that will do this for you, called the Add2PSU, available for $12.95, which I would consider ``inexpensive", if it were not a glorified jumper cable.

 

Personally I would stick to my guns and just jumper the damn thing, but you do you.

Edited by illeatmyhat
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@illeatmyhat

The strange thing is...it actually works, or at least appear to.

Tried to power the pc also and I was able to use the GPU, install new drivers and even run some benchmarks such as Cinebench and Indigo Renderer pure GPU test scene.

GPU-Z state the GPU was drawing about 20-30w under heavy load at 99% utilization.

I suppose this is wrong since the card TDP is 200w, and GPU-Z can read only the power supplied from PCIe slot probably.

I just tried to power the PC disconnecting the 2x6pin coming from PE4C and got two horrible sound warning at start and no start screen, I immediately shutdown the whole. I suppose is meaning the internal PSU is in overload and can't power properly all the components.

It remain for me a mystery how the power balance is defined btw internal and external one.

How can I sure I'm not overloading internal PSU, even with the external power source working?

Edited by davide445
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It's just math. You add up the power consumption of every component used by the first power supply.

The PCI-e slot provides 75W of power by itself. Each 6-pin connector provides 75W of power. Each 8-pin connector provides 150W of power.

Check this number against the rating of the first power supply. You can get this from the manufacturer's manual. Typically prebuilt power supplies usually have a very low maximum watt rating (~200-300W), but are built to supply power at a rate very close to the limit. Having said that, power supplies generally perform their best at 50% load.

 

You also want to be careful of providing too much power from a single rail. Check your second power supply to see how much power the 12V rail can provide. If that number is lower than the card requires (add it up based on the connectors used), then you will need to use multiple rails. This means either using a second cable if your power supply has multiple rails, or even a third power supply if the lone 12V rail can't deliver enough power by itself.

Edited by illeatmyhat
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I'm not using an external ATX PSU but the Dell power brick sold with PE4C, so that BPlus adapter is powering the GPU. 

IMG_20160524_223224.jpg

You can see PE4C below the GPU inside PC case. GPU is connected to PC mobo PCIe and so receive power from internal 320w PSU, and also to PE4C receiving power from the  Dell brick.

The PC it's an old HP  Compaq 8100 Elite Convertible MiniTower, with i7-860 cpu, 14GB Ram and one SSD.

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Then it's only one rail. 12V, at 18A. You could have figured that out by looking at the label on the Dell adapter.

 

 Incidentally you're using two 6-pin connectors, which means your card tops out at 150W (theoretical max, as opposed to actual usage) from the second power supply.

 

But really, does this even matter? We already know that the Dell adapter can deliver enough power to the graphics card (because it doesn't cause your PC to crash)
We already know that the internal PSU can deliver enough power to the entire system excluding the GPU (because it was designed to do so given an unmodified system.)

 

What's the point of this again?

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Simply not sure why is working, since it's not the standard set up where the PSU can receive info from mobo about needed power.

But more simply I'm ignorant about the topic and afraid to find some component broken or worst in flames. Anyway thanks for your feedback, really informative. 

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The additional 2-pin segment of any 8-pin PCIe power connector MUST be connected. To ground. Otherwise, the GPU can also sense that it is not connected, and refuse to operate.

 

You CANNOT have a 6-pin connector plugged into an 8-pin slot with the 2 remaining pins floating. They must be grounded.

 

Source: Read PCIe 8-pin pinouts.

Edited by Arbystrider
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On ‎5‎/‎25‎/‎2016 at 3:21 AM, davide445 said:

Simply not sure why is working, since it's not the standard set up where the PSU can receive info from mobo about needed power.

That's not how power supplies work.

They're very much passive devices. You stick a load on, and it sources as much power as it needs (or fail spectacularly, if your power supply is insufficient. I would say this results in "undefined behavior".)

Electricity is about as automatic as it gets. I suggest looking up Ohm's Law.

To get into detail, semiconductor devices (a graphics card) have a variable load. For example, a transistor's resistance depends on whether it's on or off. An oversimplification would be that the collective resistance of these transistors is directly related to the load of the device.

 

So long as the power supply can source as much power as the maximum load, you won't hit that undefined behavior condition. Manufacturers provide some protections against this, but they're not perfect. No matter what, it ends in the computer crashing.

Edited by illeatmyhat
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