Dual Power Supplies

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Asgorath

[H]ard|Gawd
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Jul 12, 2004
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I just bought a Cooler Master Stacker case and it came with an interesting connector. First, let me say that this case can fit two power supplies. Now the connector. Basically it's a 24 pin ATX pass through but with an extra green and black wire tapped off of it. This green and black wire go to another 24 pin ATX female connector.

I can only assume that this is to power a second power supply. If this solution is safe, I would very much appreciate this option! I can split the load of the computer onto two power supplies if needed! In the days of 80+W graphics cards that are in SLI, giant stacks of hard drives, and gobs of power needed, a single supply to handle it all gets very expensive. The question is, is this solution safe?

For instance, if I power my hard drives from a different supply than my motherboard, is that bad? Is the logic voltage in the IDE/SATA cable going to matter that its slightly different between the supplies?

What are the ramification and precautions one must take to use two supplies?

Discuss.

Thanks,

Greg
 
-=Antimatter=- said:
One good supply is safer and easier. Dual supplies are possible but can have issues.
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That is true...but I was hoping to discuss the specifics of this topic. I have been looking around the internet and cannot find much talking about this scenario.

I'll start.

Terminology: Primary Supply - One that powers motherboard
Secondary Supply - One that does not power the motherboard
Pass Through Adapter - An set of cables with a matching male and female side to it.

Precautions when using Dual Power Supplies.

The Primary Supply must supply power to EVERYTHING on the motherboard. This may include: Primary ATX Connector, Secondary ATX Connector, Auxiliary Video Card Power, Etc

Hard Drives, CD-Roms, Card Readers, Lighting, Fans, Fan Buses, and similar can be run on the secondary supply.

The two supplies cannot be directly linked together. This means for instance that a 12V rail on the primary supply cannot be electrically connected to the 12V rail on the Secondary Supply. If you do so your other rails may swing wildly or your supply may malfunction and destroy itself.

If you power a fan with the secondary supply, it is ok to connect the RPM Sense wire to the motherboard for monitoring.

You cannot connect the secondary supply to the auxiliary power adapter on your video card. The motherboard is also providing power to the video card and by doing so, you would disobey the rule of never electrically connecting the two power supplies.

-------------

In short, it seems like a secondary supply would only be useful if you had ALOT of hard drives. Using two supplies to power your killer SLI rig would not work well.

If anyone would like to counter anything I've said here please do. I have never done this and am just hypothesizing on what would be good practice if you are running two supplies.

Cheers,

Greg
 
Sounds like wiring both PSUs so the motherboard is actually signaling both PSUs to power on/off. I wonder what would happen if one PSU did not give the "power good" signal and the other one did.

It's quite odd because normally you'd have 2 power supplies for redundancy. Alot of server mobos have dual power connectors and can draw from either, or a hot-swap redundant PSU cage will switch as necessary (providing single connectors to components).

But in your case, you're just getting a small amount of extra current "headroom". So unless you've got an overtaxed primary power supply (which I'm guessing you don't).... it's not clear why they added that "feature".
 
venm11 said:
It's quite odd because normally you'd have 2 power supplies for redundancy. Alot of server mobos have dual power connectors and can draw from either, or a hot-swap redundant PSU cage will switch as necessary (providing single connectors to components).
Click to expand...

Not really. There's generally only one power supply connection on a given board. There's one old p3 board I know of that does have dual 20-pin connectors, but for the most part these days you buy a single redundant power supply which connects via one 20/24 pin connector and has two or more power supplies inside it. So the motherboard doesn't need any switching circuitry, it's all a power supply thing.
 
I tried to get a dual power supply system setup and couldn't get it right. I purchased a Lian-Li PC-G70B case, it's a tower with room for 2 power supplies and includes a wiring harness which allows both units to be turned on/off at the same time using the normal case button (the harness connects the #14 - Green Power Supply On wire for both harnesses to #15 Black Common Ground; it does not connect the #8 Grey Power Ok wire, that is still done by the harness that connects to the motherboard).

Both of the power supplies are PC Power & Cooling Silencer 750's, one Quad PCI, one Dual PCI. After getting everything installed I tried to run my SATA harddrives and my IDE and CD/DVD which both have SATA adapters (really like not using the old-styles cables) off of the Dual PCI with the Quad PCI hooked-up to the motherboard and the computer would not bootup, giving me a missing diskdrive error. The only stuff that will run for me are case fans, I tried hooking-up my pair of 8800GTX's and got the loud screech warning that no power was connected.

I assumed that if the Fortron BoosterX 3 (which is sold to only add extra power to the PCI connections on a video card) would work then any 2nd power supply would work as well but not in my case. I've read you need to put a 5v load on the 2nd PSU because they are switching power supplies but no combination worked for me.

I wanted to run the 2 PC Power Silencers so that each would run at below half of its rating and keep the variable fan noise down to a minimum, the 750 can get really loud when using SLI mode and gaming.

I'm hoping this thread develops as the OP wanted, Cliff
 
I remember asking this same question last year when I got my Stacker case. I got numerous responses of it lowering efficiency and just being plain dumb. I decided not to do it and gave the wire to a friend.

Now a year later I see that it is needed at times. If your running a TEC, High end wcing, or big SLI cards I would say go for it. As long as you have an actual need for it, it will be benifical. Just make sure you use a good quality PSU that meets the need of the extra components and balance the load properly.
 
I did it in my Stacker STC-T01 using the supplied dual PSU wiring harness. I had two Antec Neo HE 550 PSUs running my dual Opteron rig, then later my dual 8800GTX Core 2 Duo rig.
 
I'm running dual in my PC-343B and I used to run dual in two stackers... it works out great, especially if you are trying to aim for silence but want to carry a lot of hardware. It's also very good if you really want to load your machines with hard drives.
 
Ockie said:
I'm running dual in my PC-343B and I used to run dual in two stackers... it works out great, especially if you are trying to aim for silence but want to carry a lot of hardware. It's also very good if you really want to load your machines with hard drives.
Click to expand...

I was running 8 hard drives in mine at the time. 4 of which were Ultra SCSI drives. Add a dual Opteron 254 system to the mix, and SLI'ed 7900GTX's your power requirements end up quite high.
 
Dan_D said:
I was running 8 hard drives in mine at the time. 4 of which were Ultra SCSI drives. Add a dual Opteron 254 system to the mix, and SLI'ed 7900GTX's your power requirements end up quite high.
Click to expand...

Holy crap batman.
 
cawalton said:
I tried to get a dual power supply system setup and couldn't get it right. I purchased a Lian-Li PC-G70B case, it's a tower with room for 2 power supplies and includes a wiring harness which allows both units to be turned on/off at the same time using the normal case button (the harness connects the #14 - Green Power Supply On wire for both harnesses to #15 Black Common Ground; it does not connect the #8 Grey Power Ok wire, that is still done by the harness that connects to the motherboard).

Both of the power supplies are PC Power & Cooling Silencer 750's, one Quad PCI, one Dual PCI. After getting everything installed I tried to run my SATA harddrives and my IDE and CD/DVD which both have SATA adapters (really like not using the old-styles cables) off of the Dual PCI with the Quad PCI hooked-up to the motherboard and the computer would not bootup, giving me a missing diskdrive error. The only stuff that will run for me are case fans, I tried hooking-up my pair of 8800GTX's and got the loud screech warning that no power was connected.

I assumed that if the Fortron BoosterX 3 (which is sold to only add extra power to the PCI connections on a video card) would work then any 2nd power supply would work as well but not in my case. I've read you need to put a 5v load on the 2nd PSU because they are switching power supplies but no combination worked for me.

I wanted to run the 2 PC Power Silencers so that each would run at below half of its rating and keep the variable fan noise down to a minimum, the 750 can get really loud when using SLI mode and gaming.

I'm hoping this thread develops as the OP wanted, Cliff
Click to expand...

Wouldn't 2 750's draw more than the available 15 amps that is standard on house wiring? They would likely peak at startup, so maybe that's why they won't turn on.
 
Dan_D said:
Well my power requirements on my current rig are less than those of my last machine, but they are still pretty high thanks to G80 SLI.
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Just out of curiosity, how high *are* your requirements? Do you have a kill-a-watt or similar meter?
 
Kil4Thril said:
Wouldn't 2 750's draw more than the available 15 amps that is standard on house wiring? They would likely peak at startup, so maybe that's why they won't turn on.
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Only if they need to both be outputing 750w's would they pull anywhere near that number which is exceedingly unlikely startup or not.
 
unhappy_mage said:
Just out of curiosity, how high *are* your requirements? Do you have a kill-a-watt or similar meter?
Click to expand...

No I don't. My power requirements are purely theoretical at this time. You figure at max load each 8800GTX can grab 150-185Watts. You multiply that by 2. From what I understand each memory module is 5 watts. So you add that to the list. The Core 2 Duo's have a TDP of 65watts, add that to the mix and then add another 10 or 15 watts for good measure. Add each hard drive which is what? 30watts or something? Multiply that by 2. Add an optical drive which won't always be pulling any significant current, but on startup it should require 25 or 30 watts too. I have no idea how many watts the motherboard needs, but I'll say 30 or 40. Add another 20 to 40 for my misc. fans. I haven't a clue how much power the Nautilus requires.

You end up with around 600-630 or so. (Give or take.) Indeed a fully loaded basic 8800GTX SLI Core 2 Duo system can pull around 500 watts at full load so my numbers don't seem unreasonable.
 
The PC Power & Cooling Silencer 750s have a max 12 amp AC rating, so I would say someone could have unexpected problems if anyone was really running these hard on one typical household 15 amp outlet.

In my case I had an electrician add 2 extra outlets (4 outlet box each) in my computer room, each on their own 15 amp circuit. I run a Tripp-Lite 20 amp Line conditioner/automatic voltage regulator from each outlet and then plug all my computer power needs from there. I also use the regular room outlets for odds & ends. With lcds' and printers/copiers, power needs can really pile up. I dont use a UPS since I'm more interested in clean power and can't recall the last time power was lost in my neighborhood (Central Massachusetts).

I ordered two 5 volt 1 amp load resistors from PC Power & Cooling and will put one on each power supply, that should allow enough load on the switching power supplies to put out power on all the rails (I'm playing it safe on the 2nd load resistor since I doubt the power supply that gets plugged into the motherboard would need any artificial 5 volt load).

Since I'm making the move to water cooling (Swiftech H20-220-APEX-GT plus a 2nd pump and 2nd 120mm radiator for the cpu, 2 gpu's and NB) I really hope to get the 2nd power supply working as a slave. I'm trying to find one of those slave wiring harnesses that uses a 12v relay instead of using the Green wire/Common ground approach. From everything I've been able to read, the electrical isolation aspect is the most important to prevent premature electrical failures.
 
Spectre said:
Only if they need to both be outputing 750w's would they pull anywhere near that number which is exceedingly unlikely startup or not.
Click to expand...

That's assuming there are no lamps/lights/monitors/anything else in the room.
 
Kil4Thril said:
That's assuming there are no lamps/lights/monitors/anything else in the room.
Click to expand...

You asked about the draw of two power supplies not anything else on the circuit. I answered your question correctly, you will not draw 1500w total (2x750w) at startup with the systems being discussed.
 
Here is a snippet from www.xbitlabs.com when they were evaluating the Fortron/FSP VGA-only power supply last May 2006;

"Every high-consumption component is now powered from the +12V source and the +12V power rail is split in several PSU outputs. As a result, the graphics card and processor have acquired their own external power connectors that are not electrically connected to each other (the CPU connector is located on the mainboard – it’s the well-known 4-pin ATX12V).

In old times when the ATX12V connector already existed, but PSUs had only one +12V rail, there was no reason why this connector shouldn’t be connected to the appropriate contact (the +12V yellow wire) of the mainboard’s main 20-pin connector. More contacts minimize resistance and power loss. However, this meant that if you tried to power the CPU from a separate power supply whose output voltage differed from the main PSU’s voltage (and they are never going to be exactly the same, of course), you got a short circuit through the mainboard with all the consequences.

The connectors had to be separated on the mainboard after the division of the PSU’s output lines had been made: it would be illogical to divide anything in the PSU only to join everything back together on the mainboard. So it has come to be that the 12-volt CPU power connector is not electrically connected to the 12-volt contacts of the mainboard itself. And you can apply different voltage to them, from different PSUs, without fearing any problems.

......The same is true for graphics cards after they have acquired dedicated power connectors – first of the 4-pin HDD or floppy variety and then special 6-pin ones (and the power consumption of graphics cards has grown dramatically, too, up to 100 watts and higher)."

Seems like splitting the 12v load between 2 PSU's really has no drawbacks but I would guess that for vga cards with 2 pci-e power connections that both connections would have to be from the same PSU, not one from each PSU, to avoid that voltage mixing problem.
 
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