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

dc/dc converterpower supply

My PSU has connector for GPU. It has +12V, +3.3V and GND pins (according to volt meter). I need to connect cages with HDDs to this connector. HDD requires +12V and +5V. Because PSU doesn't provide +5V I'm going to convert either +12V or +3.3V to +5V. Usually PSU specifies limit of current for given line (say 20A for +5V or 40A for 12V). Specification for my PSU has no information about such limits for 3.3V or 12V – it only says that it's possible to connect 300 Watt GPU to aforementioned PSU connector. I think that +12V line allows more current then 3.3V line.

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

I believe that I need to connect +12V wire and GND wire from PSU connector to input of this converter and two output wires to SATA connector – please see schema below.
Will it work? Is this schema correct in general (my concern is the way how wires connected to converter. Whether it's correct to connect first pair GND/+12V to converter and another pair GND/+12V to SATA connector)? Is it safe? Sorry for naive question – I have no experience in electronics.

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Best Answer

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…

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