How do you rack 50 Mac Minis and use only one power cord?

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As mentioned during the Monday meetings, we’ll be deploying a farm of Mac Minis to augment our Firefox performance cluster.

Here’s the dilemma – 50 power bricks converting AC to DC is a waste of heat and space (and my time managing Apple’s 6′ power cables) and makes me having to find a PDU with 50 outlets.

Can someone make me some electrical system  that takes one 110V/30A circuit and powers 25-50 Mac Minis over DC?

Categories: Mozilla

4 responses

  1. Thomas K wrote on :

    Assuming first that your idea of a separate power supply is a good one, looking at a mac mini power supply, those put out 18.5V at 6A for a total of ~110W. Your circuit of 110V(whether DC or ACrms) at 30A is only going to give 3300W, which is just enough to cover the output for 30 mac minis pushing their power supplies flat out, never mind any losses in the power supplies.

    Say you would like to run 25 reliably. You need that 18.5V, at 6*25A = 150A. That is one _beast_ of a power supply, I’m telling you now. And they just don’t make those for buying off-the-shelf.

    Regulating your incoming voltage down to lower DC is never going to be super-efficient. Your better power supplies might be getting up around the 90% mark, but you’re not going to beat the heat problem by putting it all in one power supply. It just puts it all in one place, which likely doesn’t help.

    The amount of time you spend running power boards around and plugging in 6′ lengths of cable is always going to be less than the time it takes to chop a power cable, strip it back, poke some heatshrink on, solder the wires up to your power supply line and apply the heatshrink.

    So really, you’re best off just using the power supplies which come with the macs. Take careful note of how much power you’re drawing from your supply, because otherwise you’ll throw circuit breakers or start fires or cause other interesting hapennings.

    Also, 30A seems quite a high supply current — in Australia our usual wiring will only do 10A at 240VAC. The “usual” might be a higher current where you are, but make sure that your power boards or whatever you use to distribute the power can handle it.

    Good luck. :) I’m sure Apple will appreciate the business.

  2. Johnathan Nightingale wrote on :

    So it took a while, but I’ve decided you don’t want to do this. :)

    110v @ 30A = 3300W
    3300W/50Minis = 66W/mini

    Apple says 23W idle, 110W max, which puts you into dangerous territory, though it does seem that two circuits would have you sitting pretty. So on each of two 110V circuits you’ll want (say) 2500W of DC power.

    Mini power supplies are rated 18.5V at a little less than 5A, IIRC, so you’ll need an 18.5V supply delivering 2500W. That’s 135A.

    I’m not an EE, but according to PocketRef that means that you’re gonna want at least 00 gauge on your trunk lines, and that’s assuming you can keep the raceway under 60C. 000 gauge would not be unreasonable.

    Of course, Mac is Mac, so their power supplies aren’t simple things either – the minis won’t power on without a key signal on one of their pins. I hear tell that a 6.8k resistor inline on the signal pin does the trick, but that means splitting and splicing 50 cables.

    As for the conversion itself, a couple of these might do the trick:

    http://www.astecpower.com/Products/Display.asp?SeriesID=48

    But it’s not immediately clear to me whether that 75%-82% efficiency figure is much better than the mini PSUs. On the other hand, you could run a spot-welder off the leftover outputs.

    You could probably do it with a load of car batteries, some step up DC-DC converters and a lot of hookup wire, but now you’ve got double conversion losses, and hydrogen off-gassing.

    I know (I hope) you were kidding, but damn.

    Practically speaking, you probably *could* rig 10-in-one power adapters to simplify cord management, splicing the AC side of several lines parallel into one giant cord of doom, but that almost certainly voids the warranty.

  3. mawrya wrote on :

    How many amps (or watts) does a mini pull on average and peak? I’ve read 85 Watts is the max but it typically will pull no more than 40, best to confirm, though, I’m assuming you have nothing hanging off your firewire ports, etc. The 30A circuit will handle this if the 40W average is held: 40W*50Minis/110V = 18A

    You could buy a couple of 1000W industrial DC power supply(s), some DIN rail and a bunch of terminal blocks to create a DC bus to connect your minis directly too. How much were you imagining spending? I’m guessing you will come out around $1000 USD or more. Nevermind all the work of cutting DC leads on your existing power supply units. It will be a lot cheaper and quicker to find several high-density PDUs, but not nearly as much fun!

  4. ohr wrote on :

    I think The amount of time you spend running power boards around and plugging in 6′ lengths of cable is always going to be less than the time it takes to chop a power cable, strip it back, poke some heatshrink on, solder the wires up to your power supply line and apply the heatshrink.