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I ordered a new computer and - as always - I think about upgrading it before it arrives.

I thought about adding another graphics card, which would probably push the current PSU beyond its limits. What measures do I have to take care of to combine the powered rails of the PSUs?

I searched for this question and found no conclusive answer, what I've found so far:

  • some people strongly advise against it but don't provide a reason "don't risk it, simply buy a more powerful one".
  • the power supplies will probably provide slightly different voltages, this will lead to the more powerful PSU providing most of the initial power until its voltage drops and the second one kicks in, too.
  • some people advise using high current diodes. I have no problem with soldering on the wires of my PSU, but I have not seen any reason why I should do it in the first place and high current diodes are not that cheap, especially because I would need one for each powered rail on each PSU (8-10 pieces)
  • there are some commercial solutions to this, like Add2PSU which is basically just a relay that switches the other power supplies.

So, can I assume it is safe to connect multiple power supplies to power my hardware?

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    @DanielRHicks I actually consider myself having the knowledge and wonder what all the fuzz is about that everyone advises against it without providing any argumentation why it should be dangerous.
    – Baarn
    Nov 28, 2013 at 23:12
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    Especially as there is the Add2PSU, which does not do anything else than switching further power supplies (using a slow relay - aka stone age electricity - instead of a FET).
    – Baarn
    Nov 28, 2013 at 23:21
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    I once thought of jumpering the green and black(to turn it on - try with a paperclip and see) and using it as a second psu. i never heard a reason why not.
    – barlop
    Nov 29, 2013 at 6:00
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    @Barlop I use an old power supply this way for powering my lab equipment (circuit boards, etc).
    – Baarn
    Nov 29, 2013 at 12:30
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    yeah Bam. The comments here are ranging from poorly informed, to FUD and stupidity. The answer doesn't seem bad, but you're probably a better judge than I on this. It is quite a specialist question, you could try electronics.stackexchange Note the answer given here is ok with the idea of two power supplies isolated. I have tested PSUs with a multimeter and seen fluctuation (though i've heard that apparently even if you don't see it on a multimeter, they fluctutate just too quick to be picked up by a multimeter).
    – barlop
    Nov 29, 2013 at 13:42

4 Answers 4

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Support for multiple power supply units is often found in server machines, but its purpose is to keep the server running if one of the units fails, not to increase the maximum total power output. Second PSUs can be used to distribute the load if a single unit cannot cope by itself and it is safe to do so as long as any rail is powered by at most one supply. E.g., you can use the second PSU to power the hard drives, but the hard drives must be disconnected from the first PSU.

Indeed there are good reasons for not using multiple power sources in parallel. PSUs are normally switching; they operate by frequently toggling between full-on and full-off. The output voltage is regulated by spending more or less time in the on-state. If load increases, the voltage drops and the feedback controller reacts accordingly.

When two units are installed in parallel, two controllers are hard at work to maintain a stable voltage, but they are unaware of one another. As a result, they may enter a hysteresis loop by consistently overshooting their target. Alternatively, one unit might try to lower the output voltage but fail, because his partner is pushing the output in the other direction.

With a few tweaks, these problems can be avoided. The second PSU can be hooked up to the controller of the first to ensure the output remain stable and diodes are required to avoid one unit from pushing current into the other when both aren't working equally hard. I highly recommend experimenting with this for the sake of learning about electronics (remember, smoke is bad). If you just want more juice for your computer, I suggest you keep both power supplies isolated, or even simpler, get a more powerful unit.

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  • ...and I have never seen a server that actually NEEDED two PSUs, they only came in handy if one were to fail, that is about it---oh and to keep the status indicator from flashing. Fun fact: every server we ever threw out due to end of life-and its a lot-none ever actually needed that second PSU. If a company is going through PSUs like candy then it is more likely that the devices aren't properly ventilated or cleaned, or both. In the old mammoth days we used stagger the chassis so there was one empty slot between each server, worked like a charm.
    – osirisgothra
    Jun 22, 2023 at 16:39
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I've been using 2 power supplies on more than one computer for over 4 years. . One runs fans, optical drives and half or more of my HDD's, the other runs mobo and graphics. 20+4 connectors have a jumper so the both start @ the same time. Neither one ever breaks a sweat. I run a 400 watt oem with a cooler master 460 on top and a 450 watt oem with another c/m 460 again on top. Both c/m's were bought on sale @ newegg for less than $30 each.

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  • Welcome to SuperUser! As worded, your post is more of a comment than an answer. Please edit it to directly address the OPs question. You can of course also refer to your own experience as you have done.
    – I say Reinstate Monica
    Dec 1, 2014 at 22:36
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    20+4 connectors have a jumper so the both start @ the same time – it would be really nice if you could provide a link to more information on this.
    – Piotr Dobrogost
    Jun 26, 2017 at 20:06
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    @Piotr there is one line "think it s the green" which should be connected together on both PSU's so one the one connected to the MB starts the other does too
    – Chebhou
    Jul 5, 2017 at 19:20
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An additional issue that may arise:

Multiple power supplies will mean that there are two ground connections. This may increase the risk of creating ground loops in various circuits of the power supply units. Ground loops could also become a problem if you power different parts of the PC with different power supplies, as these circuits would need a common ground or voltage rail in order to talk (although, not if optical transfer or DC-DC isolating converters are used).

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its real ez. just hook the sense wire (typically Green) to the the since on PSU 1. Find a comfortable spot to safely mount it if possible. Run your extra power to the devices you want extra power for, hook up your power to both PSU's and turn on PC. Just built this for extra power to a quadro 4000. Works flawlessly. Using it to type this message.

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  • Some of your components might take damage due to getting a slightly higher voltage than they are rated for - while other components might randomly not work because of getting a lower voltage than rated - depending on the quality of the PSUs. Measure the voltage between the same ground rails on the different PSUs. The easiest way to fix this is to use the same PSUs and to put the exact same load on them - which is easy to do if you have fans or LED's with regulators - you can just tweak the regulators until the voltage between the same ground rails measures 0.
    – Dagelf
    Feb 9, 2021 at 15:09
  • Also NOTHING should be connected to both PSUs at the same time. (positive OR negative)
    – Dagelf
    Feb 9, 2021 at 15:09
  • Oh and most important: PLUG THEM ALL INTO THE SAME WALL OUTLET!
    – Dagelf
    Feb 9, 2021 at 15:19
  • Yeah the only thing the power supplies should share is their ground. They should also run off the same circuit, because electrical splits the breaker box in half, the one phase on the one side. You won't know for certain which phase your running from, so having it plugged into the same outlet is a safe bet unless you want to trace each one to the breaker box and ensure the phase, or have the tools to check that. You might want to have the sense wire protected so if the worst were to happen both supplies shut down. And of course never combine rails!
    – osirisgothra
    Jun 22, 2023 at 16:44

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