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I am building a raid array in a separate enclosure. I will be putting an empty tower case next to an existing tower computer, and this second tower case will only hold hard drives.

There are many solutions for connecting the drives in the second case to the raid card in the first case (SFF-8088 and SFF-8087 cables).

But I prefer not to run power from the first case to the second case. Can I use a power supply in the first tower case and cause it to start the power supply in the second case based on an indication from power in the first tower case's power supply?

Maybe run a 12 volt cable from the first case to the power supply on the second case only for the purpose of initiating the second power supply.

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There's power strips out that already do that. Plug the main machine into the master socket and the drive array into one of the accessory sockets.

Agreed that running power across into another case is a bad idea. Last time I did that one of the power supplies blew up the first time the 2 cases bumped against each other. Apparently grounding? Not sure why there's no problem with signal cables though.

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It's very easy to switch on a PSU without a motherboard... All you'd need is some kinda electronic device to initiate a circuit when it detected power - a nice idea would be using an LED connector on the 'main' motherboard as the signal for it. Playing with ideas here ;) – HaydnWVN Nov 23 '11 at 16:58
@HaydnWVN this sounds like a different answer. Please post your answer and elaborate. – steampowered Nov 23 '11 at 17:06
@HaydnWVN The trick is that the "power on" for ATX is caused by grounding the Power-On pin, not by applying voltage to it. – techie007 Nov 23 '11 at 18:54
@techie007 - I was aware of this, but didn't know enough about grounding principles to recommend anything so described it as a 'kinda electronic device' able to detect the voltage to create the circuit/grounding! Exactly as your answer but you obviously know more about circuits than I do ;) – HaydnWVN Nov 24 '11 at 9:11
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Don't link them, at least not without some kind of isolation device.

You risk grounding issues, at least.

If you run a wire from pin 16 of the ATX connector (usually a green wire), to pin 17 (or one of the other ground pins/wires, usually black wire), the PSU will turn on when mains power is applied (from the wall).

If the PSU has a mains switch, you're done.

If the PSU doesn't have a mains switch, run a wire from pin 16 to a toggle switch on the chassis, and then back to pin 17. You can then turn the drives on and off with that toggle switch.

Put the PSU in the enclosure and hook it to the drives, and turn it on and off as needed, like any other external drive enclosure.

Another option would be to use only one PSU, physically bolt the two chassis together, cut access holes in between, and run (perhaps extended) power and data cables as needed.

If you really want to link them, then:

  • Get a SPST NO (single pole single throw, normally open) relay that's triggered by 5V, preferably of the photo-isolated variety (example). At 5V you could probably also get away with an opto-coupler, or even a transistor.
  • Tap the Power Good pin and a ground pin on the computer's ATX connector, run wires from those to the trigger poles of the relay.
  • Run wires from the Power On pin and a ground pin of the enclosure's PSU to the load-side poles of the relay.

When the Power Good line in the computer gets signal it will close the relay, causing the Power On in the enclosure's PSU to be grounded, turning on the PSU. Same in reverse Power Good goes away, Power On gets disconnected...

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