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I have many external USB disks like this one, that I would like to power from a single dedicated ATX PSU, instead of one 12V power adapter per disk. Because each power adapter would take one power outlet, and if we have say 10 disks, that's 10 power outlets required, and it begins to be ridiculous.

The PSU would be dedicated to power the disks, and started using the paperclip trick.

Each external disk is sold with a power adapter outputting 12V 1.5A 18W.

And the end of the 12V power cable looks like this: 12V plug

My idea would be to cut that cable (which itself includes 2 cables), and connect each of the 2 inner cables somewhere on the PSU connectors.

The questions I have:

  • How to connect this twin-cable cut from the power adapter, to the connectors provided by the PSU?
  • By reading the PSU power details (sticker on it), can I get an estimate of the maximum number of disks that can be powered this way?

This is the sticker with the power details: PSU power details

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  • \$\begingroup\$ Thing to consider: A single PSU failure might fry all of your HDDs at once. Are you ready for this situation? \$\endgroup\$
    – Eugene Sh.
    Nov 6, 2020 at 15:48
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    \$\begingroup\$ 46A/1.5A per disk is around 30 disks. But I would not push it that far, leave some margin. You might also need a staggered spinup system to prevent massive startup current from all drives at once to trigger OCP. \$\endgroup\$ Nov 6, 2020 at 16:06
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    \$\begingroup\$ Paperclip might slip out and power supply would turn off. Not great. Connecting all drives to same supply would create ground loops between PC, USB disks and the external ATX supply. If you have one loose ground connection between disk and ATX supply, the 1.5A could run via USB cables and other disk grounds. There's a reason the USB drives come with isolated supply. ATX supplies also are very unreliable when loading only the 12V. And when last 12V supply stops consuming current, voltage might shoot up and the ATX supply could hit OVP and turn off. ATX supplies are bad general purpose supplies. \$\endgroup\$
    – Justme
    Nov 6, 2020 at 17:11
  • \$\begingroup\$ @Justme in fact, I'm not using a paperclip to connect those 2 pins. But that's sort of how that connection trick is named. \$\endgroup\$
    – Blackbird
    Nov 6, 2020 at 22:31

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Using ATX supplies as +12V supplies is a well established technique among model airplane enthusiasts -- basically it's the cheapest way to get +12V for charging batteries.

So -- search on that. They'll have pinouts, and a workaround for the paperclip trick (whatever that is). If I recall correctly, the biggest non-obvious feature is that you'll need to put a load on the 3.3V and maybe the 5V supplies or the thing won't regulate correctly.

You can get cables that come terminated with barrel connectors, so you don't need to cut up your existing wall warts.

As mentioned in a comment, if the existing supplies are 1.5A, then you can take the 46A output rating of the supply and divide by 1.5A to get the number of drives you can run. What fraction of the 30 possible drives you actually run with one ATX supply is left to your judgement.

And -- burn you house down, burn your PC up, blah blah -- you've been warned, use your judgement, don't blame me if it happens.

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    \$\begingroup\$ The more deluxe ATX power supplies only generate 12V and then use DC-DC converters for the 5 and 3.3V and don't suffer from these regulation issues. \$\endgroup\$ Nov 6, 2020 at 18:00
  • \$\begingroup\$ @Unimportant that's good to know. Do you know how to tell? \$\endgroup\$
    – TimWescott
    Nov 6, 2020 at 20:12
  • \$\begingroup\$ Some manufacturers boast about it in the specs but mostly from reviews where they open them up. You can easily spot the separate DC-DC converters in the pictures. \$\endgroup\$ Nov 6, 2020 at 21:50
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    \$\begingroup\$ @Blackbird the wall wart manufacturers are free to decide themselves if the dotted line is positive or negative wire, so you can't count on that. What matters is that the pin and barrell end up having correct polarity regarding +12V and ground. You might want to verify if all hard drives have the same polarity as well, unless they are identical of course. Take measurements with a multimeter to get it right. \$\endgroup\$
    – Justme
    Nov 6, 2020 at 22:55
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    \$\begingroup\$ @Unimportant Yes that tells the plug polarity which is obvious. It just does not tell which wire connects to center or barrell. Blackbird, A cheap multimeter is handy to make sure everything is right so you don't blow up your expensive harddrives. And if they do blow up, the multimeter can be used to solve what was wrong. \$\endgroup\$
    – Justme
    Nov 6, 2020 at 23:47

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