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I'm not sure if this is right question for this site, so I apologize if I shouldn't have asked this here.

I have two 400W (labels say that they are 400W, but I think they are much less because they weren't made by known brands) PSUs. I also have bought ATI Radeon HD4870 graphics card. I think that just one of these PSUs is too weak to power whole computer, so I thought why shouldn't I connect one PSU to power graphics card, and other to power everything else ?

In theory (or at least in my head) that should work :D Now I'm asking you if this is something that I should do? Could there be any complications/problems ?

If I can do this without killing every piece of hardware that I own, my next question is: how do I do this ?

This is what I think I must do. Please correct me if I'm wrong (actually I know I am wrong at something :))

  1. Connect ATX connector's pin 14 of first and second PSU together (that's power on signal).
  2. Do the same with pin 15 (ground) (do I need to connect ground together, and if I do should I connect all pins that say ground or just one ?).
  3. Somewhere I saw that I should also connect pins 8 together (that's power ok signal). Is this true ?
  4. Anything else ?
  5. What is best way to connect these wires (preferably without cutting them) ?

Pinout of ATX connector:

Computer specs: Intel CPU E8400 @3.0 GHz (wolfdale) 2x2GB DDR2 G-Skill 800MHz 1 SATA HDD 5400 RPM 2 IDE HDD 7200 RPM 2 PCI cards (SB Audiology and Edimax WLAN NIC) 4-5 USB peripherals DVD-RW drive ATI Radeon HD 4870

Thanks !!

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migrated from May 16 '12 at 13:32

This question came from our site for electronics and electrical engineering professionals, students, and enthusiasts.

This question fits best on superuser – clabacchio May 16 '12 at 11:47
@clabacchio: I don't think so ... I am concerned primarily with electrical problems that I can experience if I use both PSUs and I think that I can get the best answer on this site. – xx77aBs May 16 '12 at 12:49
This is more similar to PC modding than real electronic design, and I'm sure that it can fit there :) Anyway AFAIK the graphic card is powered through the motherboard, so you can try to stick them in parallel but I'm not sure that it can work. – clabacchio May 16 '12 at 13:00
OK, thanks :) But I'm concerned with consequences of connecting ground of both PSUs together, and the graphics card being powered by two PSUs (one from motherboard and another via 6 pin connector) – xx77aBs May 16 '12 at 16:28
Would it not be easier to just purchase a 800W power supply instead? – Ramhound May 16 '12 at 17:05
up vote 2 down vote accepted

This probably isn't worth it. The chance of messing something up and damaging your computer or yourself isn't worth saving the sixty dollars it'll cost to get a better PSU. Regular computers aren't designed for this (although some server motherboards support it). You'll also need to mod your case to fit another PSU, deal with the extra heat, deal with the extra power use (PSUs are more efficient at higher load percentages, one of these is going to be underloaded and 800w is a lot for that build).

Also, as HaydnWVN said, you might not need a bigger PSU, although 400 is pretty small.

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Server motherboards can use dual-PSUs for redundancy, rather than load-balancing. – paradroid May 17 '12 at 2:54
For a proof of concept and as an experience exercise it's doable, but definately not as a long term solution! Too many unknowns and the benefit doesn't outweigh any possible damage if this is an important system. – HaydnWVN May 17 '12 at 11:24

What makes you think that 400W isn't enough for your PC?

We'd need more specs of your system to check... Or you could use one of the many calculators to do it yourself.

Personally, i've used ATX PSU's in a similar way (shorting pin 14 to ground), but only for testing/temporary purposes.

The only long term additional PSU i've done was add an additional AT PSU (modded a second switch for it into case front) for running case fans along with a watercooling pump in a full tower 'home' server... Back when using an Athlon T'bird 1200 overclocked :)

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I've checked with PSU calculator, and recommended PSU wattage is about 400W. The problem is that PSU that I own is really old (approximately 3-5 years) and it wan't made by known brand, so even when it was bought maybe it wasn't really 400W PSU. – xx77aBs May 16 '12 at 16:25
@xx77aBs: I think the problem with cheap PSUs is not that they do not supply enough power in total - it's that not enough of current is supplied to the rails which get the heaviest load (is it the +12V rail?). – paradroid May 17 '12 at 2:59
@paradroid: Yeah, the problem is +12V rail ... I will get that card today and try it out with one PSU, so I'll see if it will work normally with one PSU. If not, I don't know yet what I will do. – xx77aBs May 17 '12 at 7:34
Test test and test it! It's quite unlikely the card will draw it's maximum wattage... I've got a 4870 at home and pretty sure i'm only using a 450W PSU with 4 hard drives and i've never had a problem! System is a Q6600 on an Asus P5Q with 8gig DDR3. Think my PSU is a Crucial though... Run a beefy game (BF3!) for your tests or 3DMark or something :) – HaydnWVN May 17 '12 at 11:22

I have found a simple plan to do this. Check both power supplies, the connectors that goes into the motherboard, one wire should be grey and the other wire should be green. (those are the only two colours that is not in multiple but single). Cut those two wires from the slave PSU (If i can call it that) and splice it with the same colour wire on the primary PSU. If you do that, you do not need to switch on the second power supply separately. Once you push the power button on the pc, it will switch on both power supplies at exactly the same time.

I have done this to use a 500W and a 460W PSU to power my iCore 7 with 12GB Ram (6 x 2GB in 3 Channel setup) and MSI N450GTS graphics card. With just the 500, the power supply is to small to carry the load of everything (I got a TV Tuner cards, WiFi card, 3 x SATA Drives, 1 x DVD RAM, 1 x Multi Memory card readers and 3 Fans (1 x 120mm & 2 x 90mm). That's quite allot to power. The only problem I have now is that the second powersuply is lying loose inside the case, that's why i want to mod my case to be a bit higher, one PSU at the bottom and one PSU at the top.

EDIT: you can check this link where you can buy connectors to do the same thing, it doesn't involve voiding of any warranties. Multi PSU Connector

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I like the idea to get both to turn on at the same time. Also, you could take two cases and stick them together. (and if you want to be super careful then a temperature monitor bay) – barlop May 10 at 8:27
That's my initial idea, so I am visiting the scrap yards quite often to look for cool things to add the the mod. I got already 2 frames, so this weekend I will cut the one and see best way to weld or screw them together. – Thylvin May 10 at 8:34

Putting aside whether it's efficient or whether you need it.

As to how to do it.

First see how to get a power supply turning on without plugging it into the motherboard. To do this, take a paperclip and stick it in the green and the black.

Like this

enter image description here

And hey presto. You do that for your second power supply, now plug it into whatever components. It is that easy.

I'm sure that at times when i've had a low power fanless computer and needed more power i've used a big power supply like this. I saw no problems.

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The attached picture looks preposterous for one simple reason. Sure, green-to-black connection makes some sense, electrically. However, if your paperclip is sticking out of the connector that much, then it will be in the way of being able to plug that connector into the other connector. – TOOGAM May 10 at 9:58
@TOOGAM You write "The attached picture looks preposterous......if your paperclip is sticking out of the connector that much, then it will be in the way of being able to plug that connector into the other connector." <----- What?! You don't plug that connector into any other connector! – barlop May 10 at 10:00
Well then, in that case, the picture might not be nearly as preposterous as it looks. Okay. – TOOGAM May 10 at 10:03

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