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I'm building essentially a giant NAS box for personal experimentation, as a learning experience, and to help run some things. To accomplish this I have roughly 9 IDE hard drives that I need to power. This is all going into an old spare computer.

An issue though I'm running into is power. The power supply only has 4 molex connectors. While I have a spare power supply, I'm not going to be using my universities unlimited power forever, so I'm trying to make it at least somewhat power efficient by using only one power supply instead of two

My question is how many times can I plug a Y adapter into a molex before I ask too much from the power supply? I believe the PS is in the 300-400 watt range.

Bonus points if you can say how you can figure out if your power supply can handle that many hard drives.

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With a 400 watt PSU, you should be able to handle about 5 HDDs, a DVD RW, one processor, 2-3 DIMMS of DDR2/3 RAM and a low end video device. You might be able to handle 6 HDDs, but 9 would definitely be pushing that PSU over the edge. –  MaQleod Oct 14 '11 at 21:07
    
@MaQleod But even if a PSU supports 400 watts, does it work in such a way that most of it can be pulled through random connectors? I just thought PSU's where made in a way that you can't pull almost the full power of the PSU though a handful of connectors –  TheLQ Oct 14 '11 at 21:11

1 Answer 1

up vote 6 down vote accepted

Picking a hard drive at random and finding a datasheet for it, the Seagate Baracuda appears to have a "working" power draw of approximately 6.5 watts, multiply that by 9 and you get an average power draw of about 63.5 watts. Not much at all.

I would be surprised if that stresses any power supply particularly much.

The thing that will kill your power supply if you're not careful is the startup current of the drives. You see, the problem with spinning platter drives is that they use motors, and motors take a huge amount of current to get them moving compared to just keeping them moving. The Barracuda has a startup current of 2 amps.

That's a nice 18 amps of current required before your drives can spin up to do anything.

Unless you have an intelligent BIOS that spins the drives up one by one you are going to need to make sure that your power supply can put out some rich chunky amps down both the 5 and 12 volt lines (most probably the 12 volt line will get hammered the heaviest) and that's assuming that you don't have something else, like say a processor or graphics card or something trying to use that power at the same time. Assuming the motors get their power exclusively from the 12V line, 18 amps at 12V gives a power draw of 216Watts, a very significant percentage of the full power output of your power supply.

Basically your power supply should have a sticker on it that, along with the "300W" information, should tell you what current (in amps) it can put out on each voltage line. The 5V line is probably going to be something like 5-10 amps, but if the 12V line cannot put out at least 20 amps, then I think you're very quickly going to end up with a power supply that's going to put its feet in the air and do a pretty good Dodo impression.

I'm also ignoring the fact that putting 20 amps down a single set of thin copper wires may not be a good idea in the first place, unless they're pretty chunky wires, and you can actually end up with the wires heating up quite impressively and potentially causing a host of other problems completely unrelated to the matter of just how much power your power supply can put out.

Another thing we're also ignoring here is that the figures that get printed on power supplies are laughable and are (being completely honest) only vaguely related to reality. Check the first part of this Dans Data article on how inflated manufacturer power supply figures are... Chances are the wattage listed on the power supply is a momentary peak wattage and I would not expect the power supply to last very long having to regularly deal with spinning up all those drives.

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This doesn't seem like something that's easy to test simply because if your wrong, you fry the power supply and possibly everything that's connected to it. –  TheLQ Oct 14 '11 at 21:36
    
I admit I may be overstating a couple of points, but chances are you'd be fine in the short term (it may well work perfectly well) and if you only rarely power the machine down then back up you're probably not even going to notice a problem. I apologise for my "enthusiasm" about the matter but I just wanted to get across the fact that it's the initial startup that would cause problems. With a reasonable PSU chances are that you'd get it to boot, but I'd have concerns. –  Mokubai Oct 14 '11 at 21:43
    
For explanation purposes, this is my PSU: geeks.com/details.asp?invtid=ATX-300-12E-BULK&cat=PWR . I think the relevant specs are "+12V, 15A" and "+5V & +12V & +3.3V, 288W Max". I'm starting to doubt the feasibility of this... –  TheLQ Oct 14 '11 at 22:17
    
To put this in perspective with a real-world storage model, a single Isilon storage node (IQ6000x) runs dual power supplies (power consumption is approx 660W max with a shared load). This is used to power 4GB Ram, one quad core 2.33 ghz processor, minimal video processing, 4 NICs, 3 case fans and 12 HDDs at 7200RM 500GB (2.5"). –  MaQleod Oct 14 '11 at 23:06
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The main issue is you need a bios that can do this, and somehow I doubt that most consumer stuff will. –  Pharaun Oct 15 '11 at 0:26

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