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I would like to add more disks than my main PSU can handle, so I plugged them on another PSU (Corsair VS450) but it does not power them as CPU and motherboard connectors are not used.

How can I force the second PSU to provide power to my hard drives?

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    "more disks than my main PSU can handle" - this is quite surprising, how many disks are we talking about? Do you have other large power draws? (e.g: big CPU, big graphics card) – Attie Jul 9 at 16:49
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    It is for connecting 20 sas hard drives on a server. CPU is quite big but could not be and no graphic card. Any of the two PSUs have enough molex and sata ports. – Gaël Barbin Jul 9 at 16:56
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    @GaëlBarbin - understood... fair enough – Attie Jul 9 at 17:11
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    You should make sure the grounds of the two power supplies are well connected. If you don't then any imbalance in the return currents could pass through the SAS cables. – Alex Hajnal Jul 10 at 3:18
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    @AlexHajnal Yes, that will royally mess up your day. A shared ground is essential. – Mast Jul 10 at 6:39
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You can turn an ATX power supply on by using the "paperclip technique".

Find the 24-pin connector. If the wires are colored, then you'll have a slightly easier time, if they are all one color (e.g: black), then be careful about which pins you use.

Orient the connector so that you are looking "in" to the holes (i.e: as if you were the socket, with the wires coming out of the back), and with the clip on the top.

Starting at the top left pin, and counting right, you want to connect (aka "short") the third and fourth pins (i.e: "Ground" and "Power On").

This will cause the power supply to enable its main outputs (instead of just the standby 5v supply). The power supply's fan may spin, and any connected peripherals should power up.

20/24-pin ATX connector


As mentioned in the comments, if this is going to be a permanent installation, then you would be much better off using an adapter as shown below that uses the motherboard's signal to control both power supplies.

If you're comfortable with it, then you can make one of these yourself - depending on which route you take (crimping, housings, etc... vs. cutting, splicing and soldering) it may require permanent modifications to one of the power supplies - make sure you label it as such.

In any case, ensure that the signals are properly isolated, and you don't leave any bare wires exposed.

The product pictured below is ThermalTake's AC-005-CNONAN-P1.

AC-005-CNONAN-P1


Regarding Grounding - in PCs, the "Ground" / 0v is typically bonded with Mains Earth and the Chassis, meaning that the two supplies should be at the same potential. If there is any doubt, or if this is a permanent installation, then it would also be highly advisable to connect a number of the ground / 0v signals from each power supply together.

If you are using two separate wall sockets to power each supply, then you may introduce a ground loop, as each socket may take its ground from different points... If possible, use the same or neighbouring wall sockets for both supplies.

If you do not, and the supplies are not at the same potential, then current will flow through equipment (e.g: hard disks) via the power and data cables.

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    The Reserved slot, as it is "empty" is a good reference to determine the correct orientation. – Gaël Barbin Jul 9 at 17:02
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    Great, no problem - I've revised the pinout for clarity – Attie Jul 9 at 17:08
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    It would be better if you could wire the "power on" (green) wires from both PSUs in parallel. That way the HDDs will turn on and off with the host. (edit: NB: do not wire the power lines in parallel! Grounds are ok to join, but probably unnecessary) – abb Jul 10 at 8:00
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    @Abb GROUNDS MUST BE CONNECTED. Otherwise you can run a ground-loop through the drives, which may fry them. – Tonny Jul 10 at 8:17
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    There are adapters that can start both PSUs at the same time. An example is this: thermaltake.com/dual-psu-24pin-adapter-cable.html (It does the same as the paper-clip does, but uses the same motherboard signal for it.) – Ismael Miguel Jul 10 at 9:13
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You can get 1 of those PSU 24 pin jumpers if you don't want to use the paper clip method.

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    Thank you, I have used a wire with dupont connector, it is well plugged. – Gaël Barbin Jul 9 at 17:05
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    Personally I wouldn't trust a wire for a server application. A proper connector (as suggested by @user3992) would be far preferable. – Alex Hajnal Jul 10 at 3:20
  • @AlexHajnal This is a Frankenserver anyway... I doubt the asker is worried about "Proper". – Tonny Jul 10 at 8:19
  • Good solution - but maybe even better: If you google for "dual psu adapter" you'll also find cables which use the same pins as these pin jumpers to link both PSUs, so that both units start on a powerpress. Used one of those once and it worked like a charm. – s1lv3r Jul 10 at 8:27
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In your situation, I'd avoid the "paperclip jumper" and simply cut both appropriate lines, and join them properly with solder and heat shrink. This is less likely to fall out.

Do please label it well.


I did something similar years ago when time was cheap and parts were not. Ended up with twelve 3.2 GB IDE hard drives in a large AT case, which had an AC power switch. It was used as a cache disk for a squid server.

It worked fine if the drives were turned on first, or at the same time as the board, and up to ~5 seconds after the board. Any more than that and the host was booting past that part of the POST.

Since the host was ATX based (Pentium2 from memory) I planned on using a small relay running off the Power LED connector, which would switch mains voltage on to the other unit.

However I was dissuaded from this by management, who pointed out that mains electrical work was outside my job description and that I was not licensed for such work. In the end I just made sure to power on the drive rack before the host. I left in ~2006 and that host ran on for another 5 years before being destroyed by an earthquake. I must have some something right.

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    These days you could put the relay (or MOSFET) on the low voltage side: A small automotive relay (automotive because spade connectors rather than PCB mount, 12V coil, built in flyback diode), powered by the 12V line of the primary PSU, could be used to pull down the green "power on" line of the secondary. All crimped connectors. No need to touch mains (though I'm using a setup with an SSR switched by my PC to power accessories at home). – Chris H Jul 10 at 8:59
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    I would use an optoisolator chip to switch the second PSU on, enabled by the 5V line of the first PSU. You'll probably need a small resistor on the LED side of it to limit the current correctly. I would then tie the PSU's ground lines together, and use a IEC Y lead to power both PSUs – CSM Jul 10 at 11:32
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    @ChrisH If you like building electronics then maybe. Otherwise buy the adapter from Thermaltake. – user253751 Jul 10 at 18:15
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    Or you did something wrong for so many years that you caused an earthquake. – Prometheus Jul 11 at 0:03
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    @user253751 My boxes of electronics junk probably provide 3 or 4 different ways of doing this; the slowest part would be finding what I need but I could still make it work today compared to waiting several days for an order to arrive (slower than normal at the moment). For a home/development machine I'd DIY. For production, I'd probably buy unless it was incredibly urgent, but that's about future maintainers more than anything else. But you're right, if you don't do electronics, don't start with this – Chris H Jul 11 at 10:55
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You could use a part like the Supermicro JBOD Power Board - CSE-PTJBOD-CB2 or the newer, and more expensive, CSE-PTJBOD-CB3 (with IPMI controller and other flashy features). The Serve the Home website has detailed write-ups on using these boards.

They have the advantage of not voiding warranties (probably), not requiring a "botch" job, and saving paperclips. The newer board offers sophisticated management features.

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  • Good idea, but to be fair, that is a different cost bracket to a paperclip and some elechicken's tape. – Criggie Jul 12 at 1:15
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    My job security is worth more than a paperclip (though not more than electrical tape). A botch job could also cost a $10K server. The boards run $50 and $150 very roughly. The added IPMI and automatic fan control tip the scale for me in favor of the latter. – newyork10023 Jul 12 at 1:22
  • "serve the home" has definitely moved away from home-grade budgets. OP is working with home-level budgets. This is a good answer for a business-level version of the same question, though they're more likely to uprate with replacement PSUs inside a chassis for the additional load. OP's doing this on a shoestring. – Criggie Jul 12 at 1:36
  • OP wrote, "It is for connecting 20 sas hard drives on a server. CPU is quite big but could not be and no graphic card. Any of the two PSUs have enough molex and sata ports." A wee better setup than my home rig. – newyork10023 Jul 12 at 1:57

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