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I am planning to wire a 4U DIY JBOD top loader.

Looking at the power output of several power supplies they have a a maximum rated 5v rail of 100-125w. By looking at the spec sheet of my chosen HDD I'll require more than double for all drives. There is plenty of current available on the 12v rail and I'm wondering if it's reasonable practice to use a 12v to 5v DC-DC converter?

Single drive active: 12v 0.56a = 6.72w | 5v 0.65a = 3.25w | total = 9.97w
64 drive active:     12v 35.84a = 430w | 5v 41.6a = 208w  | total = 638w

The spin-up power is well over 1600w so I'll be using staggered spin-up.

With 8 dedicated SATA power cables from the PSU I can power 8 drives on each one. The cable has 18AWG wires which should handle 8 drives over a distance of less than 300mm. There I could put an inline DC-DC converter.

Here is the proposed wiring diagram. SATA wiring diagram I will not be using the 5v provided by the ATX PSU but the one from the DC-DCs. Is this a good approach, potential issues or any other thoughts?

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  • \$\begingroup\$ With something this large I wouldn't even look at a ATX PSU. I'd consider multiple PSUs at each voltage that can be slaved together. \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Commented Apr 12, 2018 at 17:11
  • \$\begingroup\$ @IgnacioVazquez-Abrams, ATX12V PSUs are cheap. Anything else would cost 10X. \$\endgroup\$
    – Ale..chenski
    Commented Apr 12, 2018 at 17:39
  • \$\begingroup\$ Potential issue will be cost. You can find a server-class (blade format) PSU with 12V at 100 A (say, Murata D1U54P-W-1200-12-HA4PC) and use a bunch of 12-5V converters. And then add the cost of wiring/mounting/cooling. \$\endgroup\$
    – Ale..chenski
    Commented Apr 12, 2018 at 17:43

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I'm not sure what hard drives you are using, but I'd seriously doubt that you would exceed the +5V rating of any reasonable power supply.

Here is a list of Corsair power supplies with +5V rating of 125-180W

A reasonable Seagate Barracuda range drive draws less than 300mA from the +5V line (1.5W) and 2A peak from the +12V line.

It seems you would not ever exceed the +5V line capability since for the drive above you could support at least 80 drives.

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  • \$\begingroup\$ On the Seagate datasheet you linked, page 10 table 3: Sequential read 64K queue depth 16 has a 5v current draw of 0.65 which would exceed 200w total. As a worst case scenario. Or should I just take the random read write power figures? \$\endgroup\$
    – James Martin
    Commented Apr 13, 2018 at 9:58
  • \$\begingroup\$ Of course you have to work out how many of your drives would SIMULTANEOUSLY write 4k blocks. You have a limited number of controllers supporting the drives so will have a limited number of drives that can write at the same time. Worst case is 4k16Q @ 1.11A, but if you have only 8 or 16 drives drawing this current and the rest at idle (0.26A) you won't have a problem. \$\endgroup\$
    – Jack Creasey
    Commented Apr 13, 2018 at 14:21

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