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One of my older computers has a 365-watt power supply that, for various reasons, cannot be replaced with a new power supply. I would like to upgrade the graphics card in that computer, but all the acceptable cards I can find demand the extra 6-pin PCI Express power connector. After using a few power supply calculators, I've come to the conclusion that the additional 6-pin power connector would push my little power supply over the edge (not to mention my PSU lacks a 6-pin connector anyway).

So, would it be possible to create my own 6-pin PCIe connector that plugs into a regular wall outlet? I assume it would be possible with some horrid freakshow of power adapters tangled together, but I don't know where to start.

Some Wikipedia background info about the power connector I'm referring to:

PCI Express cards are allowed a maximum power consumption of 25W (×1: 10W for power-up). Low profile cards are limited to 10W (×16 to 25W). PCI Express Graphics 1.0 (PEG) cards may increase power (from slot) to 75W after configuration (3.3V/3A + 12V/5.5A). PCI Express 2.1 increased the power output from an x16 slot to 150W so that some high-performance graphics cards can be run from the slot power alone. Optional connectors add 75W (6-pin) or 150W (8-pin) power for up to 300W total.

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As mentioned below, I've decided to simply purchase a low-end slot-powered (PCIe 2.0) graphics card instead. Since I can't upgrade the power supply, this is probably the path of least resistance. Thanks again for all the answers. –  Jourgen Jun 6 '12 at 17:43
    
If this as bad as everyone claims then visiontek.com/products/components/power-supplies/… how can this exist / work? –  chx Jan 20 at 2:37
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4 Answers

up vote 2 down vote accepted

So far the three answers are: Yes, No, and Maybe. I feel like I need to chime in here and give a more pragmatic answer. But before I do that, let me give you my credentials so you know who this is coming from.

I design PCB's for a living. Large and complex PCB's. Recently I have designed a custom motherboard based on an Intel CPU as well as several PCI Express boards. One of those PCI Express boards is powered by a supply that is separate from the ATX supply that powers the motherboard. These are embedded systems, which means that I can get away with things that you couldn't in a standard PC.

On to my answer:

Don't do it! If you value your sanity, don't bother trying. It might work, but probably not. The problem is, if it doesn't work then what are you going to do? Do you have the tools and knowledge to debug it? I'm guessing that you do not have a PCI Bus Analyzer or a copy of the PCI Express specification so the answer to that would be no.

Some of the things that might not work correctly:

Some PCIe cards might connect the +12v from the MoBo to the +12v from the PCIe Power connector. This is fine if both come from the same power supply, but not if there is a second supply. In my opinion, this is a bad design but that doesn't matter. You should check the card before trying anything.

There are tight specs for the time from when the power supply comes up until the PCIe card has to respond to PCIe activity from the motherboard. If the two power supplies take different amounts of time to come up then this spec could be violated.

Feeding a PCIe card with 100-200 watts of power is still non-trivial. The supplies themselves are expensive, too. It turns out that the cheapest power supplies available for this are ATX supplies!

There could be some power sequencing issues, where certain PCIe signals come up before the receiver is ready. Although unlikely, this could result in a damaged motherboard and/or PCIe card. If something is damaged, it is likely due to a bad design-- but bad design are out there.

In my opinion, you will be time, headache, and possibly money ahead to replace your current PC and simply get a new one that can handle the PCIe card you want to use. Otherwise, you would likely spend a lot of money on a new PCIe card and a power supply to power it only to end up with something that doesn't actually work. Or worse, you might damage your current PC.

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Thanks for the answer. You've done an excellent job of explaining the possible issues one would encounter. It sounds like creating this power connector is not an option for the average person. After reading the responses here, I've decided to simply purchase a low-end graphics card that does not require additional power. –  Jourgen Jun 6 '12 at 16:59
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I don't see why not. You can build a +12V PSU and wire it to a 6-pin PCIe connector (the yellow wires coming out of a standard connector are +12V, and the black are COM).

Keep in mind ATX specifications (namely, +12V should have +-5% tolerance, max of 120mV p-p ripple, etc), which can be found here: http://www.formfactors.org/developer%5Cspecs%5CPSU_DG_rev_1_1.pdf. Also keep in mind PCIe specifications, and how much current you'll need your supply to deliver (6.25A for a 6-pin connector, 12.5 for 8-pin connector).

You should be able to purchase connectors and pins online if you do some searching (I can't think of any distributors off the top of my head).

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Thanks for the answer. I'm no pro at this, so having basic info like that will really help me get started. I was also wondering about the fact that the card might still be half-powered when the rest of the computer is turned off. Do you think that might cause a problem? Will I have to synchronize it with the on/off of the ATX power supply? –  Jourgen Jun 4 '12 at 5:47
    
I figured that would be assumed when I answered, but yes, that is a very good question. I would definitely synchronize the PSUs. You can get relays to do the trick, so once you turn on your main PC PSU, the auxiliary PSU will turn on with it. –  Shamtam Jun 4 '12 at 6:27
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To make your life easier, I would go ahead and buy another small ATX PSU that has a PCIe connector that will work without a significant load (some PSUs will not work correctly unless they have a reasonable load), and then just have that turn on with your main PSU instead. This can be done by putting a 5V or 12V relay that is switched by the main PSU, and connects the green PWR-ON wire of the ATX24 connector of the auxiliary PSU to COM, which will jump-start it and turn it on. (See here: overclock.net/t/96712/…) –  Shamtam Jun 4 '12 at 6:30
    
Thanks again for your response. I was considering having two PSUs at once. However, I've decided to simply purchase a low-end slot-powered graphics card instead. It might be a compromise, but I think I would regret investing hours/days of tinkering into this old computer. –  Jourgen Jun 6 '12 at 17:34
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Don't do it. May chips have voltage sequencing requirements. If you use an external adapter you can't be sure the voltage will come on at the same time as the ATX PCIe power connector.

However if you insist, @Shamtam has a good suggestion above.

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Thanks for the warning. Your response saved me from wasting my money on a solution that wouldn't end up working. However, Kessner's answer is more complete, so I have to select that one as the accepted answer. –  Jourgen Jun 6 '12 at 17:05
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Have you considered plugging your system into a "Power Mate" or "Kill-a-Watt" or something similar to see how much power (in watts) is actually consumed? Every time I've plugged a computer into such a device, it's always shown far less power consumed than anyone around here expected.

If I were in your place, I would start with running stress test software to put the system under a high load to see how the system currently works. During the test I'd check that the voltage droop and see how much power I'm really using using a "Power Mate" or "Kill-a-Watt" or something similar. Then I'd try the simplest thing that could possibly work : A simple adapter to Y off from some 12 V and GND connector already on your power supply to a 6-pin PCI Express power adapter. The 6-pin PCI Express power connector only has 12 V and GND signals on it (a) . Such a Y adapter is much simpler than trying to synchronize two power supplies. Then I'd run the test again to see if the voltage droop is within specs, the power drawn is within spects, and maybe I've lucked out and Mr. Murphy didn't show up today.

If, hypothetically, you actually do have so much stuff crammed into your machine that your current power supply can't handle a full-load stress test, I'd still try to get away with just one power supply. Completely disconnect the original power supply, and power everything off a new power supply. Leave the old power supply in place if it's too much hassle to pull out. Then you have just one switch to turn on everything and you don't have to deal with synchronization.

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Make sure you crank up the monitor brightness, too. –  tyblu Jun 5 '12 at 5:35
    
Thanks for the answer. I did not physically test my system, but the PSU calculators have left me with the impression that my PSU is already at its max. One calculator that seemed very precise recommended 363 watts minimum for the current graphics card. Since my PSU is 365 watts, it doesn't seem likely that it could power a graphics card that requires the 6-pin extra power. Also, AMD recommended 450 watts minimum for the specific card I was looking at. –  Jourgen Jun 6 '12 at 17:20
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