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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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In general don't do it. If you understood the implications and had the knowledge to deal with them it would be a different matter, but clearly you don't. –  Daniel R Hicks Nov 28 '13 at 22:28
    
Don't do that. Some chassis and motherboards allow for multiple power supplies to be added, but what you're talking about sounds extremely dangerous. If you kill yourself or burn down your house, the insurance won't pay out. –  user3463 Nov 28 '13 at 22:45
    
@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 '13 at 23:12
    
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 '13 at 23:21
    
Like I said, if you understood the implications it would be one thing... (Not that you're likely to burn down the house.) Unless the supplies are properly matched and other precautions are taken the voltage regulators are apt to "fight" each other, and the crowbars may trip randomly. –  Daniel R Hicks Nov 29 '13 at 0:03
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1 Answer 1

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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