Using multiple power supplies in one PC

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?

Baarn

Posted 2013-11-28T22:09:26.917

Reputation: 6 096

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. – None – 2013-11-28T22:45:20.263

2@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 – 2013-11-28T23:12:50.147

1Especially 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 – 2013-11-28T23:21:08.273

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 – 2013-11-29T00:03:43.993

@Baarn - The computer you purchased will not support multiple power supplies. If it did then you wouldn't need to ask us if it was psosible because you would already know it was possible. The case in the product video supports mounting multiple power supplies ( its basically a server case ). – Ramhound – 2013-11-29T01:10:52.710

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 – 2013-11-29T06:00:33.900

@Ramhound so you think it is only the case that determines the number of power supplies? How bad that I didn't even buy a case… – Baarn – 2013-11-29T12:23:57.230

@Barlop I use an old power supply this way for powering my lab equipment (circuit boards, etc). – Baarn – 2013-11-29T12:30:09.473

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 – 2013-11-29T13:42:40.000

maybe try on electronics.stackexchange particularly as here, the answer given didn't really address your idea of that addtopsu device that switches power supplies. – barlop – 2013-11-29T13:46:55.767

@Baarn - A case being able to support multiple power supplies is only one of the requirements. The other is a motherboard that will support it. – Ramhound – 2013-11-29T17:43:55.940

@Ramhound that is not true at all. the motherboard doesn't have to know there is a second power supply. e.g. suggestions for when power supplies are "isolated"(as even an answer to this question suggests). I haven't seen you make one technical critique of anything anybody has said, or the answer given that rightly contradicts you. And he never mentioned about purchasing a case anywhere(I checked his link, it doesn't come with a case), so I don't know what you are talking about even when you said that either. if the product said that the mbrd needs it then you'd have a point but it does not. – barlop – 2013-11-29T20:38:17.677

I am curious as to what hardware you are planning on running... just how wimpy is your PSU compared to the entire setup? – valsidalv – 2013-12-03T16:40:38.173

Answers

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.

Marcks Thomas

Posted 2013-11-28T22:09:26.917

Reputation: 5 749

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.

michael clyde

Posted 2013-11-28T22:09:26.917

Reputation: 81

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 – 2014-12-01T22:36:15.570

120+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 – 2017-06-26T20:06:19.977

@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 – 2017-07-05T19:20:58.677

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).

Curt P

Posted 2013-11-28T22:09:26.917

Reputation: 11

-1

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.

Stephan

Posted 2013-11-28T22:09:26.917

Reputation: 1