# How to make two computer power supplies work together

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#### jimc52

##### New Member
Hello everyone. I sought out this forum for specific purposes. I see this forum has a lot of electrical and electronics genious' and I thought I would turn to all of you for your advice.

I have been building stand alone computers (desktops) for years, for friends, family, businesses and my self. I frequent various forums and assist other people in building their own computers as well, help them troubleshoot problems, suggest mods and upgrades, or just building a computer from scratch - working up a list of parts, helping to sort out questions and make decisions. Through all of this, I have learned of needs that typical enthusiasts have that are not being addressed (to my knowledge) by commercial applications. As computing advances, the need for more power in personal computers is rising fast, especially for gamers who are building dual, triple, even quad video card gamer computers. Of course, each of these video cards consumes electricity, and in response to that need, various manufacturers have started to build 1 Kw and 1.2 Kw power supplies. However, these power supplies are very bulky and very expensive. A typical 1 Kw switching power supply, can cost $350. Since most enthusiasts have a rather tight budget, this makes it difficult to build a multi-video card (what is called, an SLI'd or Crossfire driven) gamer computer. Enthusiasts, of course, want to spend their money on the fastest processor, the most RAM they can stuff on a motherboard, and the most video cards they can afford. Power supplies, which as you know, are key to stability and longevity of the computer platform, is usually the LAST and most underconsidered feature in any new computer build. The cost of a 650 Watt PSU can run as low as$70. So logically, the question has arisen, more often than you would think - Could we put two $70 psu's into a computer (for a total of$140 cost) and get a 1.3 Kw output rather then buy just one psu with a 1 Kw output which costs double that amount?

I would like to hear from any of you about how this might be achieved. Imagine having two psu's ganged together electrically, providing double the output for half the cost of a single higher output power supply. What would be a practical way of doing this? Would a single psu providing main power to the motherboard be interrupted by using a second power supply which is mainly used to feed video cards? Would there be a chance of burning out the components due to uncontrolled flux between the two power supplies? How could we put two of the exact same psu's together and make them work uniformly without causing variances, interruptions, etc. in the other components such as the microprocessor, RAM, motherboard, hard disks, ROM devices?

I would like to hear the concerns and suggestions of any of you who might have suggestions on what needs to be considered in ganging two psu's together to provide power to a single computer platform.

Thanks bunches for your help!

#### Grossel

##### Well-Known Member
You need to know how to jumpstart a atx psu. This is done by connecting the green cable to ground, and the power supply stays on as long the green cable is connected to ground. You can simply use one transistor and one resistor to make a second power supply start together with the main power supply.

#### blueroomelectronics

Seems like a lot of work for something that could do more harm than good for a savings of $210 on an otherwise expensive system. Plus not many cases have a spot for two supplies. #### Grossel ##### Well-Known Member Seems like a lot of work for something that could do more harm than good for a savings of$210 on an otherwise expensive system. Plus not many cases have a spot for two supplies.
If you have a better idea, why don't share it.

In case it isn't obvious - Any modification onto a computers hardware involves a risk for failure.

In this spesific case I presuppose that treadstarter does know the transistor basic.

#### bokbaard

##### New Member
I find a common occurrence on electro-tech, where responses are 'condemning', without being descriptive of why, or even suggesting something else. 3+ such posts a day.

I think this is a cool project, and the OP seems quite aware of the risks and costs. I would like to know for interest sakes if this can be done, in practice, not only theory.

#### flat5

##### Member
I don't see a real problem. Tie the grounds together and try it with old stuff you don't care about.

#### notauser

##### New Member
I don't see a problem with the method posted.. They'd alnost positively be plugged into common ground, and besides, each psu would be powering different components..

#### ericgibbs

##### Well-Known Member
I don't see a problem with the method posted.. They'd alnost positively be plugged into common ground, and besides, each psu would be powering different components..

hi notauser, Welcome.

I have a nagging feeling the OP is going to effectively link the two power supplies together in order to supply a single PC motherboard.

That is +5V to +5V etc, which IMO dosnt sound a good idea.

#### blueroomelectronics

##### Well-Known Member
If you have a better idea, why don't share it.

In case it isn't obvious - Any modification onto a computers hardware involves a risk for failure.

In this spesific case I presuppose that treadstarter does know the transistor basic.

My better idea was to cough up the dough for a proper supply. It's like running your $100k sports car on Costo tires. #### arhi ##### Member Using 2 PSU's work perfectly (in more then one of my computers at home) if you ground them together and use them to power different devices (one PSU power the motherboard other one to power the storage bank for example). If one decide to connect +X of PSU1 and +X of PSU2 the troubles will begin and not only that it is not a good idea (as Eric noted) but it is a huge load on both PSU's as they will not produce "identical voltage" on +X pin, so there will be a constant current flow from one PSU to another resulting, most often, in both PSU's shutting down and rarely just in "loaded" PSU's. In any case, that will NOT produce the "desired" effect of "double the available current". So if one need extra juice, one can easily power the storage+optic bank using separate PSU. It is good to either make a delay circuit to turn on the MB powering PSU few ms later or enable "wait for HDD" option in bios if possible ... but in 99.9999% cases it will work without any of those. PC PSU is fairly usable thing, and it is very cheep ... donno why ppl do not like them but they can be used also as great power source for development (no, not as those sites show to use -5 and +5 to get 10V etc ... you add some additional electronics and many capacitors and ...) #### blueroomelectronics ##### Well-Known Member How did you mount the second PSU in your case? #### arhi ##### Member My better idea was to cough up the dough for a proper supply. It's like running your$100k sports car on Costo tires.

I must say I do not agree ... I have few boxes here under the table with 2x300E PSU's .. 1PSU power's the MB, GPU, SPU and second PSU powers the storage bank (6x1T SATA disks) ... each of those PSU's can run the whole box but that's on the ~80% of the load (and 95% of the load on 12V line) so it is much better to split it...

Now, ok, it is much better to use 1 PSU of 50E then 2 PSU's of 10E, much safer etc... but 1PSU is not always better then 2PSU's

#### arhi

##### Member
How did you mount the second PSU in your case?

easy

2 cases are some intel server 5RU cases that have space for 2 regular PSU's

1 case is some regular desktop case (chiefteck or something like that) but the PSU's are "half width" so they can fit in place of one PSU. (they are actually intended to be used in some thin cases but )

1 case have normal PSU + one PSU mounted "externally" to the case.

These 4 boxes are all with lot of hdd's that take loads of current (first 2 have 8xSCSI 12KRPM drives ... they use loooooooooooads of current other two have SATA drives that are not that power hungry but same SCSI drives used to be inside them too)

EDIT: bill, do not forget, I'm consultant for mysql cluster... big systems, db mining etc .. the elco is just a hobby

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#### notauser

##### New Member
I agree you don't want to tie the dc outputs together to power anything sensitive..
There are pc-soundcard based oscilloscope apps that you could use to see how clean your power would be whatever you choose to do.. Be careful you don't fry your SC though.. Most sites that offer those apps explain risks/protections..

#### arhi

##### Member
I agree you don't want to tie the dc outputs together to power anything sensitive..

you do not want to tie them together ever. As the output is never "identical" the current will flow from the PSU with higher voltage to PSU with lower voltage, so the PSU with 5.01V will try to "charge" the PSU with 4.99V and you do not want that

In any case if you need to power a single device with more current, get a PSU that can supply it.

as for how clean the output of PC PSU is .. well, check out the number of electrolytes on the MB

#### arhi

##### Member
ah, almost forgot, there is actually a "device" you can purchase that takes power from two ATX psu's and have a single output. I have seen it only inside a computer, never had a chance to open it (was not mine) and it is used not to double the power but to double the security (if one PSU dies the other one continues to power the computer), I recon it has few diodes and few capacitors and 2 transistors but ... never opened it so ...

#### arhi

##### Member
The server cases are designed for dual redundant hot-swap power supplies not a pair of standard supplies. And the OP indicated it was simply to save \$210. Heck good SLI video cards cost more than that.

uh ... don't get me going, you can find "anything" ... the real good ones are fairly modular ...

in any case, 2xcheap psu = bad solution, everything else is another matter... today you can find psu with few separate 5v and 12v lines, before those came up, the multiple psu's were only choice for "overloaded boxes" as those 10krpm scsi discs slurp large amount of current from time to time. In newer boxes that's not a prob, sata and iscsi discs are using 10 times less power and have 10 times more capacity, and over the top, the decent 1KW psu is not expensive at all.

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