The Black Art of Dual PSUs in Your Enthusiast PC @ [H]

FrgMstr

FrgMstr

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The Black Art of Dual PSUs in Your Enthusiast PC - Putting two powers supplies in your computer has been a recurring subject in our forums for years. While the physical process of making that happen is not exactly rocket science, it still can be daunting for some users. Today we show you a few products that make it easy for anyone to double up on the power in their computer should your wattage needs increase.
 
I've been using a lian li type connection for years. Works great. I think I bought it for alot less than 14.99 however
 
Used dual psu's once when the one I had didn't have enough connectors.
 
Yea, I'll admit that I'm about 78 watts short in my system with everthing overclocked.
 
For most people, upgrading their psu to a better one to support xfire/sli, 12 hdd's + 20 fans etc, makes the most sense with a single unit. To me, where this niche is, are people that dont have the $ to be able to afford (even on sale) say a good 750 watt psu, and they have a say, 300 watt inside their case, with an updated mobo/cpu/gpu. For an extra ~$50 all - in (and please dont talk about where I can get a good 650w psu online for $50) :D...

they can get that extra 250 - 350 watts of power they need without breaking the bank.


That said, I would never buy one, I would rather save up the say 200$ and buy a 1000w unit if my 750 wasnt enough. Thats just me. I interpret this is for the application, the cheapest solution wins.
 
what about a dual pci-e powered card taking half and half from seperate power supplys, i cant seem to get an answer about that one
 
Ideally, you should be running one PSU per pc, but while we are here though, it just seems to be one of the very few pratical uses for dual PSU's cause HDDs are not that power hungry, and if your CPU / RAM / MOBO need more power your doing it wrong to begin with
 
Acoustique said:
Ideally, you should be running one PSU per pc
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Drewis said:
For most people, upgrading their psu to a better one to support xfire/sli, 12 hdd's + 20 fans etc, makes the most sense with a single unit. To me, where this niche is, are people that dont have the $ to be able to afford (even on sale) say a good 750 watt psu, and they have a say, 300 watt inside their case, with an updated mobo/cpu/gpu. For an extra ~$50 all - in (and please dont talk about where I can get a good 650w psu online for $50) :D...

they can get that extra 250 - 350 watts of power they need without breaking the bank.


That said, I would never buy one, I would rather save up the say 200$ and buy a 1000w unit if my 750 wasnt enough. Thats just me. I interpret this is for the application, the cheapest solution wins.
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Tell that to those with Quad-SLI...
 
btw the two concerns about the Add2PSU thing could be rectified by 1) Mounting with two zip ties instead of the sticky tape 2) Use a molex y-splitter to keep your net molex connector count the same - of course you're going to need a long y-splitter cable unless you mount the Add2PSU module close to another piece of equipment.

Also, aren't Molex connectors becoming more scarce on newer PSU's? I'm sure they still have some molex connectors but does anyone see a time in the nearish future where they might forgo 4-pin molex completely in favor of SATA connectors? I'm not currently aware of any PSU's that solely have SATA power since there is a lot of legacy kit out there, but I'm sure someone makes one.
 
brshoemak said:
btw the two concerns about the Add2PSU thing could be rectified by 1) Mounting with two zip ties instead of the sticky tape 2) Use a molex y-splitter to keep your net molex connector count the same - of course you're going to need a long y-splitter cable unless you mount the Add2PSU module close to another piece of equipment.

Also, aren't Molex connectors becoming more scarce on newer PSU's?
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The mounting and the fact that most people don't use all of their Molex connectors anyway were stated in the article as I gave both the issue, and the solution/reason why it is not as important as it may seem.
 
EvilWays said:
Tell that to those with Quad-SLI...
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There is also the issue about being able to put each power supply on a different breaker. You aren't going to be able to run a 20A line to your computer for anywhere near $200, even if you DIY (those living in the basement with a direct line to the breaker box need not point out the exception. Even just those parts will be painful, though).

This isn't to say you need more than a 15A line for 1000W, but you are starting to cut it close. A couple of 600~700W beasts start to make a whole lot of sense, and it makes more as you go over 1000W.

Finally, every article on power supplies should start out with "you don't need that many Watts". You want reliability. The fan is much more important that the number of Watts.
 
I started using dual PSUs even before the ATX spec existed. I got "serious" about it when I got a geforce 5900 and my 300w PSU couldn't handle it. Adding a 200w PSU took care of it.

Since then I've been using them in a variety of setups but mostly one for mobo/gpu and a cheap generic for HDDs/optical storage.
 
There's always a slight danger of running different power supplies. If one puts out a slightly different voltage (say 5V vs 5.1V) That's a .1V different. Usually such a small voltage difference won't cause an issue. But it could cause a reverse current flow. And that could be dangerous to certain components.
 
Like I said last time this was brought up......
I built a switch about 12-15 years ago that cost me all of $5 in parts and a half hour in time that would automatically turn on the second psu when the first was powered on. (molex power from the first would close the circuit and bridge the 2 pins on the second)

D.
 
Did not know people made retail set parts for these ventures, I had dual psu set up while back older comp days. Had 8 drives 2 cd, the mobo for 1 and 1 PSU to run the way over powered sucking watts dual SLI cards nivida used to sell( we all remember those right lol).

Was way cheaper 2 600w psu was 50 bucks at the time a 1000w+ psu at the time could not be had for less then 260. So saved 160 bucks rigging the dual psu system.

I saw 630w psu with rebate recently goin for 35 bucks. 2x of those = 1260x for 70 bucks, cant get that price in a single psu for watts anyplace, this kind of thing makes good sense and saves a few bucks in the wallet along the way.
 
I wouldn't recommend very many $35 630w PSU. The products in the review should not be used as an excuse to get two bad PSU vs one good PSU.
 
My case came with a two into one 24 pin cable similar to the lian li one but sleeved.
 
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Interesting, what about one large Power Supply powering multiple computers......
 
the 2x cheep 600w psu's i used in the build years ago, now in diff systems are still running fine. So cheep PSU's doesn't mean its not good or stable product. Cost has no real bearing on quality, as ive also owned so called expensive single 1000+ PSU the corsair highly rated 1000HW series model, 2 of those, neither lasted 2 months. The 2 off brand cheeper psu's bought years ago are still running perfectly. So basing quality Psu, the cost has nothing to do with it. cheap does not mean its not a good psu.

There are actually alot of good PSU's out their of decent quality that dont cost and arm and a leg, just have to know which ones. Read some [H] psu;' reviews they can show U a few good or some of the turds they have tested for us readers over the years.
 
Why not do a full review of these option. It would be much more valuable. Is there noise, crosstalk, How well do they operate under a load?
 
KOOLTIME said:
the 2x cheep 600w psu's i used in the build years ago, now in diff systems are still running fine. So cheep PSU's doesn't mean its not good or stable product.
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Wrong. The reason you think those PSUs are good is because you lack the means to properly load them or measure their performance. If you were able to do so, you'd be in for a rude awakening.
 
That's too bad that there isn't a product that allows you to use two separate PSUs as a redundant solution with seamless failover. That 800w dual psu is awesome, but I just wish there was something else.
 
P5-133XL said:
Why not do a full review of these option. It would be much more valuable. Is there noise, crosstalk, How well do they operate under a load?
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If you had read the article you would have seen many many times that we are doing full reviews of the ones that actually have reviewable material (FSP Booster X5 and Athena Atlas as well as the Zalman's). For the other two options there is nothing to really review as there is no difference in the electrical noise and there is no real load across either of the products except for the primary unit connected to the Lian Li cable.
 
Now what I am waiting for is someone to make a 2000+W PSU than draws as much current as my heater...

Must suck that the US only has 115V hence the 1150W 10A limit of normal household circuit vs 2400W 10A limit of 240V AC.
 
ljbade said:
Now what I am waiting for is someone to make a 2000+W PSU than draws as much current as my heater...

Must suck that the US only has 115V hence the 1150W 10A limit of normal household circuit vs 2400W 10A limit of 240V AC.
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You do realize that they make 15, 20, and 50 amp breakers? You can always have a high-current breaker installed and run low-gauge wire to a dedicated outlet. As long as it doesn't violate any building codes and the current on the dedicated line doesn't push the main breaker over limit it can be done. Just don't go fooling around with an AC power box if you don't know what you're doing since they're always electrically hot and touching the wrong thing WILL kill you.
 
DigitalGriffin said:
There's always a slight danger of running different power supplies. If one puts out a slightly different voltage (say 5V vs 5.1V) That's a .1V different. Usually such a small voltage difference won't cause an issue. But it could cause a reverse current flow. And that could be dangerous to certain components.
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As an electrical engineer I'm pretty disappointed this article was posted without any cautions about the potential problems of running two supplies to one system such as is described in this article. DigitalGriffin hinted at some of the potential problems; every power supply outputs a slightly different voltage, and every supply uses feedback to maintain its output close to that set voltage level. If you hook two supplies together (and parallel their output rails directly) then the slightly higher voltage supply will attempt to drive current into the lower voltage supply to drag its output voltage up to match. The lower voltage supply will attempt to counter this, resulting in a lot of current running in reverse into its output rails through the very low resistance circuits used to drive output voltage.

This is not to say you can't use two supplies in a system, but you need to be careful about what you hook up to each supply; hooking 6 or 8 pin power connectors from two separate supplies to one graphics card (regardless of if it's dual-gpu or not as the power pins are probably tied together) or the 24-pin and 4/8-pin 12V connector on a mobo is a big no-no, and if you can manage it you want to power the motherboard and graphics cards all from the a single supply. Since the motherboard does provide some power to the PCI-E slots and some graphics cards use that there is a power connection between the two, although it tends to be a somewhat higher resistance path such that at normal load levels it's not too much of a problem to have graphics cards powered from a separate supply.

Things like optical drives and hard drives (as well as fans, lights, etc..) are all great candidates for powering from a second supply; the only other thing to keep in mind here is that certain power supplies have crossloading requirements that you need to be aware of. This means that the supply is only designed to output near its rated maximum on one rail (say 12V) so long as other related rails (3.3v or 5v) also have at least some kind of load applied to them. Most high-end supplies handle heavily imbalanced loads quite well, but this is a big one to watch out for on cheap supplies. If you're worried about this, you can either build a small resistor pack to apply some load to 3.3v and 5v, or just make sure you have some load applied to the requisite rails by hooking the right things up to them (hard drives, for example, always draw some 5v load, whereas graphics cards draw 12v exclusively).

As a real-world example, I have a very powerful CUDA system I built for a client that has 4 GTX590 graphics cards in it; it has a 1250W main supply and a 1000W secondary supply. I power the motherboard and top two graphics cards from the main supply and the bottom two cards and the peripheral devices (HDDs, opticals, fans, watercooling pumps) from the secondary supply. This works great, although the system is run from two separate 15-amp house circuits to provide enough power for it ;)
 
Excellent. I will buy two 1350watt PSU's. That should be enough for what I have LMAO. Anyway I only need 850watts maximum (thats calculating for 50% capacitor aging which will give me lots of years of use)
 
siberx said:
(hard drives, for example, always draw some 5v load, whereas graphics cards draw 12v exclusively)
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Nope, the VGA memory is fed by the minor rails.
steve30x said:
(thats calculating for 50% capacitor aging which will give me lots of years of use)
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:rolleyes:

ps. cool article.. would like to see a Zippy review though:D
 
debaucher said:
Like I said last time this was brought up......
I built a switch about 12-15 years ago that cost me all of $5 in parts and a half hour in time that would automatically turn on the second psu when the first was powered on. (molex power from the first would close the circuit and bridge the 2 pins on the second)
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That's exactly what this 'add2psu' thing does. Except for charging $20 for about £3 of parts. Use an optiso instead of a relay, and you could cut the cost even further by integrating it into the cable rather than using a PCB (and not have to worry about a mechanical relay failing).
 
You don't need anything as fancy as an optoisolator since you will be tying the two PSUs grounds together anyway. A cheap jellybean transistor such as a 2n3904 and an appropriate base resistor (10K or so will be fine) is all you need.

The connectors will probablly cost more than the transistor and resistor.
 
siberx said:
As an electrical engineer I'm pretty disappointed this article was posted without any cautions about the potential problems of running two supplies to one system such as is described in this article. DigitalGriffin hinted at some of the potential problems; every power supply outputs a slightly different voltage, and every supply uses feedback to maintain its output close to that set voltage level. If you hook two supplies together (and parallel their output rails directly) then the slightly higher voltage supply will attempt to drive current into the lower voltage supply to drag its output voltage up to match. The lower voltage supply will attempt to counter this, resulting in a lot of current running in reverse into its output rails through the very low resistance circuits used to drive output voltage.

This is not to say you can't use two supplies in a system, but you need to be careful about what you hook up to each supply; hooking 6 or 8 pin power connectors from two separate supplies to one graphics card (regardless of if it's dual-gpu or not as the power pins are probably tied together) or the 24-pin and 4/8-pin 12V connector on a mobo is a big no-no, and if you can manage it you want to power the motherboard and graphics cards all from the a single supply. Since the motherboard does provide some power to the PCI-E slots and some graphics cards use that there is a power connection between the two, although it tends to be a somewhat higher resistance path such that at normal load levels it's not too much of a problem to have graphics cards powered from a separate supply.

Things like optical drives and hard drives (as well as fans, lights, etc..) are all great candidates for powering from a second supply; the only other thing to keep in mind here is that certain power supplies have crossloading requirements that you need to be aware of. This means that the supply is only designed to output near its rated maximum on one rail (say 12V) so long as other related rails (3.3v or 5v) also have at least some kind of load applied to them. Most high-end supplies handle heavily imbalanced loads quite well, but this is a big one to watch out for on cheap supplies. If you're worried about this, you can either build a small resistor pack to apply some load to 3.3v and 5v, or just make sure you have some load applied to the requisite rails by hooking the right things up to them (hard drives, for example, always draw some 5v load, whereas graphics cards draw 12v exclusively).

As a real-world example, I have a very powerful CUDA system I built for a client that has 4 GTX590 graphics cards in it; it has a 1250W main supply and a 1000W secondary supply. I power the motherboard and top two graphics cards from the main supply and the bottom two cards and the peripheral devices (HDDs, opticals, fans, watercooling pumps) from the secondary supply. This works great, although the system is run from two separate 15-amp house circuits to provide enough power for it ;)
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That is all good an all, but on any given powersupply, even the rails voltages can fluctuate quite a bit based on wire lengths, connector resistances, load, wire quality, etc between them. Understanding your background, but over 26 years of PC experience/engineering and enthusiast rig construction talking, I dont see much risk in what the article has talked about. I think you are over selling the risk. The tolerances of the average PC build just isnt that high and minor fluctuations among quality built powersupplies is not going to sufficiently deplete the lifespan of the average PC. I say quality, because any moron putting a lacklustre power supply in a build is already hedging his construction.

I have used multiple power supplies in some of the builds over the years mostly to power secondary devices like fans and hard drives, and although I do agree that is how I would do it (primary PS on PC etc), do not believe the article faulted in not mentioning it, but could have. I believe what would work best is up to the builder to use a bit of research to learn what best to do. Again not saying the article was wise to ignore it, just not agreeing that the problem would be as prevalent as you are leading others to believe. To each their own.

In a quad SLI rig where voltage is creeping to a realm that can not be dealt with, I would consider splitting the duties among the two or three power supplies.

In reality of course, it is the very few this will even matter and it is of my opinion that the vastly majority of those are already aware of the minor risks you mention.
 
ljbade said:
Now what I am waiting for is someone to make a 2000+W PSU than draws as much current as my heater...

Must suck that the US only has 115V hence the 1150W 10A limit of normal household circuit vs 2400W 10A limit of 240V AC.
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OMG, that is just some outdated information though. The average home in the US (depending on region) built in the last few years will have at the very least 15A circuits. And just about any home made in the last few years in many area is required to be wired with 30A wiring regardless. Most homes only have 10A or 15A circuits because they install a 250A Main and allow for future growth, but that doesnt mean you cannot tap into. Have an electrician (if you are not capable) check your wiring and configuration to see if you have growth.

I had a certified electrician (didnt need to but insurance required it - make sure you check) come out to the fairly new house one weekend and for $600 dollars and about 5 hours of work had 90amps added to my upstairs Computer Office and Media room. Why, because I was hosting 25-30 man LAN's in the house every other month. Good times...
 
I have the following:
HX650, 2x AMD 6950, 1x AMD 965 125w, 4PCI-E mobo, SSD, 6fans.

I want to put just the GPU's on the HX650 and I want to buy a cheap Corsair 400w for the rest and then OC everything good.
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Good idea?
 
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