How to get 5V from 12V with server power supply unit?

My PSU has a connector for a GPU. It has +12 V, +3.3 V and GND pins (according to voltmeter).

I need to connect cages with HDDs to this connector. A HDD requires +12 V and +5V. Because the PSU doesn't provide +5V I'm going to convert either +12 V or +3.3 V to +5 V.

Usually a PSU specifies a maximum current for a given line (say 20 A for +5 V or 40 A for 12 V). The specifications for my PSU have no information about such limits for 3.3 V or 12 V; it only says that it's possible to connect a 300 W GPU to aforementioned PSU connector. I think that the +12bV line allows more current than the 3.3 V line.

I hope it's possible to use a DC-DC convertor to get +5 V from +12 V. +3.3 V is closer to 5 V but likely the +12 V allows more current. All HDDs consume about 30 A, so I think it's better to convert +12 V to +5 V. I'm going to use something like this DC DC converter.

I believe that I need to connect the +12 V wire and thr GND wire from the PSU connector to the input of this converter and two output wires to the SATA connector - please see schema below.

Will it work? Is this schema correct in general (my concern is the way how the wires connect to the converter. Is it correct to connect the first pair GND/+12 V to the converter and another pair GND/+12 V to the SATA connector)? Is it safe?

• It should be fine as long as your load's power consumption is lesser than the max output power of the PSU and DC-DC converter. Jun 21, 2021 at 5:21
• @ Prathik Prashanth Thank you!
Jun 21, 2021 at 8:08

There's no question what to convert here: the specs for the ATX Optional power supply specifies only 12 V power and sensing lines, from which you can't draw power. The HP-specific 10 pin version of that almost certainly doesn't provide much power over 3.3V over a long distance - that would be risky, as a 0.5V resistive drop due to cabling resistance is more sever at 3.3V than at 12V.

So, buck-convert your 12 V. (also, boosting 3.3V to 5V is usually technically more involved than going down in voltage).

Word of advise: 30A (at 5V: 150W!) is a lot, and these drives will all draw that at the same instant, namely on power on. make sure your power supply is potent enough to offer that amount of power!

I wouldn't even trust random amazon power supplies for private projects; for supplying hard drives in a server: certainly not. Get something from a name-brand power supply producer, through a reputable seller, like digikey, mouser, farnell, (tme), rs components…

• Thank you. got it - buck convert 12V. I'm searching among converters that promise > 500 Watt (with high voltage). I selected most expensive converter from what is available here. My current configuration is two PSU: embedded in server (4x1200W) and external ATX that provides 5V. They have no common GND wire - therefore I try to power everything from HP embedded PSU. ATX is 850W and is capable to run 6 cages with 4 HDDs each. Likely server can spin up drives not simultaneously - the led lights blink sequentially - cage after cage.