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We have setup, where 16-24, 2.5" Drives will be arranged in mobile racks of( 8 or 12 each).

We are selecting ATX SMPS to satisfy these needs.

We need to power these drives with a ATX Power Supply.

As these drives only takes current from 5V rail only. 12V and other rails were just not much required (12V required upto 100W).

I have seen specifications suggest maximum current from 5V rail close to 20A for 500W/600W ATX SMPS, and mainly(largely) power is mentioned on 12V rails.

If we are using only 5V on ATX SMPS of 500W, whether complete 500W or maximum portion of power can be taken from 5V? if no, How to find what is the peak current (for short duration) can be taken from 5V rail?

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  • \$\begingroup\$ Look at the side of the PSU, it commonly lists the current limit break down per rail. \$\endgroup\$
    – Dan D.
    Nov 21, 2014 at 7:35
  • \$\begingroup\$ Take a look at the edited version, may it give you some reference. \$\endgroup\$
    – diverger
    Nov 21, 2014 at 9:52

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You should rely on the specifications of your PSU.

The ATX standard has several versions. The power allocation between 3.3 V, 5 V and 12 V rails are very different between older and newer ATX PSU designs.

The original ATX (1995)

The power distribution specification defined that most of the PSU's power should be provided on 5 V and 3.3 V rails, because most of the electronic components (CPU, RAM, chipset, PCI, AGP and ISA cards) used 5 V or 3.3 V for power supply. The 12 V rail was only used by fans and motors of peripheral devices (HDD, FDD, CD-ROM, etc.)

In ATX12V 2.0:

Most power is now provided on 12 V rails. The standard specifies that two independent 12 V rails (12 V2 for the 4 pin connector and 12 V1 for everything else) with independent overcurrent protection are needed to meet the power requirements safely (some very high power PSUs have more than two rails, recommendations for such large PSUs are not given by the standard).

The power on 3.3 V and 5 V rails was significantly reduced.

Update:

In "ATX12V Power Supply Design Guide, v2.31"(It's published in 2008, a little old, but you may take it as a reference), it gives the power distribution of 3.3V/5V and 12V rails from 180W to 450W, and their related max. current limit. In page 69, it gives: for a ATX 450W PSU, the typical 5V+3.3V power < 120W, and 5V rail has a max. curren 15A. .

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  • \$\begingroup\$ Thanks Diverger.....for wost case 24 drives each with 5V@ 1A -> 5W ----> 24*5 = 120W + 100W(12V rail) --> normally 450W SMPS is too much. But, we need to see intially when Power is applied to drives, they will take a transient current during initial spinning or in between. I need to see this current can be handled by ATX 5V rail... In addition to it, if more current is supplied from 5V rail, it is very advantageous to us... \$\endgroup\$
    – user19579
    Nov 21, 2014 at 8:03
  • \$\begingroup\$ As the main power is shifted to 12V rails, the current on 5V rail may much less than the 12V rail. You can install the ATX and monitor the voltage output, look if the power rail collapse when your drive start. \$\endgroup\$
    – diverger
    Nov 21, 2014 at 8:17
  • \$\begingroup\$ Take into account that most RAID controllers know how to sequence drive start ups, to minimize transients. \$\endgroup\$
    – Lior Bilia
    Nov 21, 2014 at 9:30
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Note that many low-end or older PSUs use group regulation. If you have one rail loaded much more heavily than the others, this can cause them to go out of spec.

Look for a supply with independant regulation.

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