Will a 550 watt PS power 24 scsi drives

[ZENARCHER]

Hello, I'm trying to setup a 24 drive server, but I would like to have all the drives in a separate
box. Is there anyway to start an atx ps without a motherboard, or use the ps in the server to
remotely start the ps in the drive box? I know this sounds crazy but I have all these drives
laying around collecting dust and it's just something I want to try. I will run the scsi cables
to the server mb and attach to a adaptec 3400 raid card. Any help would be great.

Thanks Zen Archer

[redwudz]

You would have to find how much power one of the drives takes first. I don't go much by PS wattage ratings, as they are for the whole supply, not the 12V and 5V buses that the drives would likely run off of. Find the amperage ratings of the two buses. I have a '480W' PS and it has a peak rating of 40A@+5VDC and 24A@+12VDC. That would provide 1.6A each and 1A each for the two busses if it were divided by 24. In theory, enough, but that is peak power.

Bottom line, give it a try. Without accurate PS ratings and HD drive current needs, really too hard to figure out.

As far as the power supply, that's easy enough. just jumper the sense leads and it will start up. To activate a ATX power supply, only PS_ON is needed. PS_ON can be activated by connecting ATX power output pin 14 (PS_ON) to pin 15 (ground).


A ATX PS has to have a load on it to regulate properly. But it's usually the 5V bus that needs loaded, usually about 1A. That should be no problem as long as the SCSI drives use the 5V bus. You could plug about half your drives in and see how it's doing, then add some more. If the PS overamp protection trips, then you need more power. A second PS could be added. I hope you have a really big case and a lot of fans.

[edDV]

Big servers need special controllers to phase start drives. Drive motors draw maximum current on startup. If all drives start at once, the power supply may overload. Sustaining power is much less.

[rallynavvie]

IIRC SCSI hardware starts up according to the SCSI ID assigned to it. The 6 drives I have in my server boot in pairs (because there are two channels and thus two similar IDs exist). You can distinctly hear those 15k drives wind up and the click that means they are up and running. This may be a function of the HBA too.

24 drives will need more than a single 550W PSU. Idle they aren't going to draw squat for power, but if you're planning to RAID them then you're going to have more than a single drive accessing at once. 24 drives is in the storage array category so you'll likely be spending some money to set them up properly. This thing will be loud so rather than have it sitting next to your computer run some cabling and power into a utility room or something (make sure there is ventilation, you can easily heat a room through winter with that thing).

What will you be using it for? That will decide the best way to set it up.

[ZENARCHER]

Redwudz

As far as the power supply, that's easy enough. just jumper the sense leads and it will start up. To activate a ATX power supply, only PS_ON is needed. PS_ON can be activated by connecting ATX power output pin 14 (PS_ON) to pin 15 (ground).

Hey Redwudz when I jumper those pins, does it have to be a constant on switch or just a
momentary contact switch?

Thanks Zen

[redwudz]

I have used a constant contact when I have tested power supplies with a jumper wire, so I'm not sure. You could try it, but I suspect it would need to be on all the time.

Even though the front panel power switch on a PC is momentary, it is hooked to a logic circuit on the MB that toggles the power supply off and on. Normally the motherboard would supply the ground to the sense lead and that would likely be on all the time.

You might also install a pilot light from one of the unused power feeds to indicate the PS is on. Monitor the PS air temps coming out from the exhaust fan. If they start getting hot, up above 100F, you may be overloading it. You can always add another PS if needed. I'm assuming some sort of custom enclosure for that many drives. It may be noisy, but make sure you have sufficient cooling for everything.

[InXess]

I recall checking HDD power consumption specs some time ago and 16-18W pops into my mind. 550-600 W power supply should be able to do it. See this:
http://www.pcguide.com/ref/hdd/perf/perf/spec/otherPower-c.html
IBM specs for a sample drive are listed as 11W. I'm pretty sure that it should be assumed a notch more then that when designing a system.
Power switch is momentary, you can test it by quick shorting MB pins (leading to a power switch).

[ZENARCHER]

I have used a constant contact when I have tested power supplies with a jumper wire, so I'm not sure. You could try it, but I suspect it would need to be on all the time.

Even though the front panel power switch on a PC is momentary, it is hooked to a logic circuit on the MB that toggles the power supply off and on. Normally the motherboard would supply the ground to the sense lead and that would likely be on all the time.

You might also install a pilot light from one of the unused power feeds to indicate the PS is on. Monitor the PS air temps coming out from the exhaust fan. If they start getting hot, up above 100F, you may be overloading it. You can always add another PS if needed. I'm assuming some sort of custom enclosure for that many drives. It may be noisy, but make sure you have sufficient cooling for everything.

Hello Redwudz, I figured I'd get experimental today and found something interesting. I took
the leads from my box with the MB and raid cards PS, soldered a wire from pins 14-15 and
ran them to my 550 remote PS that will power the 24 drives. To make a long story short it
didn't work the way I expected, but after some more experimenting it turns out a single wire attached to pin 14 (green wire) then to pin 14 on remote PS with 1 drive attached did the
trick. So in summation, a single wire from pin 14 to pin 14 with no other connection to the
to the PS in the system remotely will start both the system PS and the remote PS, and shut
them both down when computer is turned off. Does this sound like the answer to my problem,
or do you think it will shorten the life of my system or power supplies? I did hook up 12
drives this way and it started them all and I now have two 6 drive raid 5 arrays setup and
seem to be working good. As far as my drive case goes it is being custom built as I go.
Right now I have 12 drives stacked in a aluminum cage x2, cooling as of now consists of a
12 inch house fan and they stay pretty cool. Putting them in the case will be a different story.
But as I've said it's a work in progress. Thanks for all your help, I'm going for all 24 tomorrow.


Thanks Zen

[redwudz]

That sounds good so far. Your solution to turning on both PSs seems OK. I assume those are logic level signals to turn on the PSs, the same as the MB uses, so there is very little amperage there. If the PSs turn on, should work fine. Most PSs need some load, a drive or two or something or the regulation circuit goes crazy and the PS shuts down. As mentioned, if the PSs run at their normal temperature, great, they should be within their power range.

When you get it all together, post some photos, even if it looks a little ugly at present. Sounds like a fun project.