Re-pinning an ATX PSU for Supermicro mobo?

I'm on the quest to build an expandable, robust NAS for home backup/media library curation/encrypted stores/etc and have been looking at the Supermicro A1SA7-2750f octo-core Avoton board (not because I necessarily need eight cores, but it is cost competitive with a marvell controlled board + 16 port HBA and has the capability to expand to 33 drives with a 16-port HBA).

At any rate, it has a rather peculiar power requirement: 1x 8-pin connector similar to ATX12/EPS (i.e. 4 pins of 12VDC and 4 pins of ground) and 1x 4-pin connector with 2 grounds, 5VSB and the PS_ON wire.

Now, Supermicro (of course) offers a proprietary psu for this system, with what I imagine is a quite noisy 1U fan. Not exactly an ideal solution (for my application), so what I would like to do is re-pin the 24-pin ATX plug from a 400W Seasonic fanless platinum unit to provide what the motherboard needs. My question is: do you think the PSU will throw any fits about the remainder of the pins not being plugged in (they will be terminated)? There will obviously be other 5V draw on the psu through the hdd array (staggered spin up, though), but no 3.3V.

The DC-DC nature of modern PSUs make it significantly less likely that it will have issues when there's no load on the minor rails.

That said, I highly doubt that anyone can give you an exact answer. You'll just have to try it and see what happens. Just make sure you have enough 5v power for your hard drives.

Good to know about the minor rail separation, thanks!

Based upon the HDD datasheet (1.0A startup, 0.46A during write, 0.4A during seek, and 0.17A idle, IIRC), with staggered spin up, the seasonic fanless series (which all provide 20A on the 5VDC rail, regardless of model) should have enough even if 32 drives are writing and the final drive is spinning up (still under 16A, or 80% of the rail max). In this regard, it's actually helpful that the motherboard doesn't want any 3.3VDC or 5VDC (other than the separate standby line), lol.

I plan to build the drive pools by sixes for cost reasons (hopefully, by the time I need the full capacity 16GB udimm modules will be reasonably priced), so this will serve as a good integration ramp for power checks / etc.

you'll be fine, I have a supermicro X8DTT that only uses +12vdc and 5VSB running off a seasonic power supply without issue

Excellent -- thanks!

and if you were to run short of 5v just throw some TO-220 case 7805s off a 12v rail, easy peasey

I'd avoid 7805's and similar devices for 12V-5V at any significant current. More than half of the power will be dissipated in the regulator and that is both wasteful and means you have a lot of unwanted heat to deal with.

jojo69 said:

and if you were to run short of 5v just throw some TO-220 case 7805s off a 12v rail, easy peasey

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actually not sure about the OPs board but my X8DTT has a molex output with 12v and 5v from the boards VRMs for drives

plugwash said:

I'd avoid 7805's and similar devices for 12V-5V at any significant current. More than half of the power will be dissipated in the regulator and that is both wasteful and means you have a lot of unwanted heat to deal with.

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while you are correct the servers the X8DTTs I have came out of were actually using a 7805 in a TO220 for the 5v standby, was pretty surprised to see a 7805 in a modern box!!

jojo69 said:

and if you were to run short of 5v just throw some TO-220 case 7805s off a 12v rail, easy peasey

Click to expand...

Actually going with a slightly different setup, so I've got a 10A buck stepdown that I'll hang off the 12v rail to support the 5v rail on the PSU. Once I test the conversion, I just need to monitor wire temps at max load for a bit (going 16 or 18awg on the front plane 5vdc wire).

Machupo said:

Actually going with a slightly different setup, so I've got a 10A buck stepdown that I'll hang off the 12v rail to support the 5v rail on the PSU. Once I test the conversion, I just need to monitor wire temps at max load for a bit (going 16 or 18awg on the front plane 5vdc wire).

Click to expand...

some plug and play switching module? what part did you find?

They're a couple of your garden-variety variable stepdowns (something akin to this) -- they will definitely bear watching as they're not an ideal solution and will get plenty of stress before *actual* components are added

I just had them in a drawer in the garage, lol.

Alternately, this entire setup could be tossed if they don't work or if they don't fit in the baseline enclosure (which will be modded) with some decent cooling.

FWIW, decided to go the c2550 route instead and that 20 drives were enough (2x z2 vdevs of 10 each), going to try and power it with a pico if I can get enough 12v converted to 5v to keep the drives (Samsung 2TB M9T's) happy. If the pico won't work, I'll just go back to the full-sized PSU and pick up a different enclosure.