Wish to use a 600W PSU to drive 19 3.5" HDDs

Thread Starter

Daniel Buus

Joined Jan 21, 2017
2
I hope that subject is descriptive enough :)

EDIT: This is a ranty post, I'm sorry. I noticed after a good long time, and I've put my ill-formed questions under a TLDR heading farther down :)

Hi! This is my first post here, and I fully admit to being mostly ignorant about the subject matter of electronics on the circuit-level (if one can even put it like that?). I suspect I might be performing a faux pas or two here, and if so, I apologise. Please ask me to sod off if so, and I'll gladly comply.

I've searched the forums, unfortunately to no avail. I suspect my ignorance is at fault. I find it hard to formulate exactly what it is I'm in doubt about, and I find it hard to decipher many of the results that turn up when I try to do so. If you don't mind, I'd like to just jovially try to explain what it is I'm trying to do, and what it is I'm in doubt about.

Full disclaimer: I do own a lot of tools, including soldering irons, but I've only ever used them for soldering together wires with other wires or with buttons or sockets and stuff like that. I like to tinker, but this level of tinkering is new to me. I only today started looking for more geeky insight to electronics because, a) I have a cold and am bed-ridden, and b) I have a particular project I'd like to make. So I've been YouTubing (I know, I'm sorry!) from bed and looking at stuff on AliExpress all day.

So, the project. I have this hobby server of mine, an old quad-core Intel system inside a gigantic aluminium Lian Li case. What makes it special is that I have inside it 19 3.5" SATA drives (also two 2.5" SSDs, but those aren't relevant to what I'm trying to achieve here). It's a terrible mess of DIY wiring and (ignorant, let's-google-that) hacking including four PSUs of which three are dedicated to the SATA hard drives and some fans. The reason being that while running this many drives was possibly with the single PSU I had, booting them all at once was impossible — each drive draws around 2 amps each while spinning up. And, I had a bunch of old computers in the attic with PSUs I could use. Plus, it was fun to do :D

And then the other day I found this Mac Pro G5 Dual on sale for 200 Danish kroner. That's $29 US. I just had to buy it. Didn't know exactly what for, but it's just a thing of (engineering) beauty. And then, I started to play around with the idea of using the case for my butt-ugly server, and make it more "flashy". And also, just get a good deal of fun out of tinkering with something geeky again. I noticed that the case is so wide that I can actually fit an ITX or Mini-ATX motherboard sideways in the top compartment, with room for SSDs, and an extra PSU! That means the entire main compartment could be used 100% for HDDs. I can fit three columns of seven drives safely in there. I have to do this. But, I can't do it if I have to cram extra PSUs in there as well. So, that's what I've been looking at, that existing PSU at the bottom of the case, and other ways of getting enough (temporary) juice to spin up 19 drives that'll suck about 2 Amps each until they're booted.

I admit, I'm at the level where I've been (re-)learning about Ohm's law, resistors, and capacitors all day. I mean, I knew already sort-of what a capacitor was — some big "magic pixie buffer" that's good for evening out currents and giving great hi-fi sound (you know, pick the one heavy with the big silos inside and the low wattage rating instead of the one with the puny silos and the "1000W" sticker on front). With regards to capacitors, I thought for a moment it might be useful; that is, somehow add enough capacitors similar to the way an amp works, to allow temporary "boosts" of juice higher than what the power supply can sustainably deliver, to let those drives spin up once in a while.

I don't know if that's feasible, but I'm feeling like it's not. I mean, we're talking about at least 19 * 12V * 2A = 456W here. And, if I paid enough attention to YouTube today, that's 456 Joules of energy each second. And I'm thinking they take about 3-5 seconds to spin up? So that's easily 1.5 kW or more of energy needed over a few seconds. Granted, the specs on the PSU show (I think) that I should be able to draw 432 W on the 12V rails, but it's a 13-year-old PSU, so... I don't know. If you disagree and think I might achieve this with capacitors (if I'm not already completely mistaken about the approach in the first place), please let me know :)

Okay, I'm sorry, I'm ranting. Here are the PSU specs:


First of all, I'm not sure if all of those outputs are available at the same time. I'm pretty sure that the top two (the "stb" — I assume standby?) are not available when powered on, because if I add all those numbers together, I get 761.9W in total, and it clearly states that maximum continuous power is 600 watts. And, if I deduct the two standby numbers, I end up at 620.9W, which fits much better with the max continuous output rating. By the way, are these different specifications describing what people refer to as "rails"? Can I consider each of these outputs a power supply of its own with that rating?

I did a bit of math, and if I were to just use the PSU as-is, I'd suspect I should able to spin up 16 drives on the 12V lines as they are. Six on "+12V1" and "+12V3" each, and four on "+12V2". Then there's the 3.3V line which I'd be using for nothing. And the 5V line which is grossly overpowered for running the drives. 5V is not used at all while spinning up, and once running, the drives consume less than half an amp on the 5V line. So that's less than 9.5 A needed. But I have 22 Amps there.

This led me to think that I could do voltage conversion on one of these supplies to get another 12V line. If I converted the 3.3V line to 12V ("step up", right?), I'd get 6.325 Amps of 12V juice, if I converted the 5V, I'd get 9.167A. Stepping up the 3.3V would seem to be the logical choice, because I need 5V anyway, but then the theoretical 6.325 Amps are just so very close to what I know I'd be drawing, and I assume the step-up circuitry consumes power on its own, and again, this is a decade-old PSU... Maybe there wouldn't be enough juice? So I starting thinking that it would be better to "step up" both the 5V to 12V and the 3.3V to 5V, which would leave me a lot of headroom to "waste" on both those lines?

But then, I looked around on AliExpress to find converters to do this, and this doesn't seem to be something people do a lot. All the converters support quite low amp draws. And this is where my second question comes in. Actually, I should probably create a TLDR section now and reference it in the header, because this rant has been going on for ages :D Sorry. So, the questions:

TLDR;
1) Is it feasible and non-retarded (and price-competitive to the alternatives) to try to use capacitors to somehow make a PSU capable of producing the extra perhaps 100W required to spin up HDDs for three to five seconds on a 12V line?
2) If I need to "step up" a "rail" (?) on a PSU from 3.3V to 5.5V and another rail from 5.5V to 12V, does it have to be done with one "step up circuit" which supports a lot of amps, or can I attach several circuits in parallel that each provide lower amps? That is, if I have 5V@22A, can I connect four "step-up"pers to get 12V at ~2.3A each? Or will they mess with each other?

Again, I'm sorry that my questions are obviously ignorant. I hope I can still get some help :)

Thanks a lot in advance!

Daniel
 

Dodgydave

Joined Jun 22, 2012
11,284
Don't waste your time using step up converters, use the 12V rails as per the current outputs on the psu, to drive the Hdds, as long as the total current per output doesn't exceed its limit, it will be balanced..

So if you need 19hdds @ 2amps thats 38amps total looking at your psu its maximum is 36amps on the 12v rails.
 

Thread Starter

Daniel Buus

Joined Jan 21, 2017
2
Don't waste your time using step up converters, use the 12V rails as per the current outputs on the psu, to drive the Hdds, as long as the total current per output doesn't exceed its limit, it will be balanced..

So if you need 19hdds @ 2amps thats 38amps total looking at your psu its maximum is 36amps on the 12v rails.
Thanks, Dave :) As it turns out, I just disassembled the machine and it turns out it actually has a 450W Samsung PSU. Which seems weird, as all I could find on Google for this dual-CPU setup was that it ought to have a 600W PSU.

This model has only two 12V rails at 16A each. I'm gonna try to see if they can drive sixteen of the drives. Then, I'll add a second PSU at the top of the case for the motherboard and the remaining drives. I did a lot more digging into step-up/buck converters in the meantime, and the ones that are affordable seem like they might be pretty crappy quality. And I can easily fit a second PSU in this case, so it's probably the best option :)

Thanks again!
 

tcmtech

Joined Nov 4, 2013
2,867
Since almost all of your load is 12 volt based I would just put a dedicated 12 volt 40 amp power supply in and use that to power everything HDD related and not bother with trying to cheat a computer power supply to do that sort of work.

By doing it that way pretty much everything else in the server could be powered by a far smaller dedicated power supply and be largely isolated from the numerous HDD units power demands and loading effects.
 
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