Powering Raspberry Pi inside a External USB Hard Drive Enclosure

Question

I suspect this may have been covered someplace already and if it has, please comment with a link to the project. I have done some searching and maybe I am just not hitting it right.

I have an older Fantom Drives USB to SATA Hard drive enclosure (Model: GF500EU) that I'd like to make use of to power both a Raspberry Pi 3 and a SATA drive inside the enclosure. The enclosure has a 12V/2A input. It contains a MS-ME198407 board that handles the USB to SATA drive connection.

Q: I am trying to identify the best way to steal power from the enclosure. I suspect that a 12v to 5v DC step-down will be the best way to do this. I also wanted to see if anyone has any alternate suggestions or can offer some pros and cons. Can you suggest a way to steal power from the enclosure?

Thanks for your feedback.

Gough Lui · Accepted Answer

I don't think this the best idea - while it is possible, one must consider the power budget and also the associated cost to the lifetime and stability of the power supply and hard drive.

A 12V-2A power supply provides, at most, 24W of power. A 3.5" hard drive that is inside the enclosure, assuming a "green" model, will require about 1.75A on 12V to spin-up and consistently consume up to 4.5W reading and writing once up to speed. If the enclosure sleeps/spins-up the drive (as some energy saving ones will), there is very little power margin to ensure the drive will start while not overloading the power supply, although some supplies are more forgiving of short-term overloads. Another thing to consider is that the 4.5W read/write power is computed by the datasheet on a specific I/O pattern - others may consume slightly more but it will be a mix of 12V (direct) and 5V consumption (via an integrated power converter on the PCB). Anything consumed from the 5V line internally will be slightly higher due to efficiency losses of the 12V to 5V converter.

That being said, in theory, if the drive is running you might have about 18W of power available to run your Pi. That's probably enough without any major downstream peripheral loads, as the Pi will consume (of its own, under load) about ~4-5W which still leaves margin after using a buck converter. In which case, you can attach a buck converter module to the 12V input (e.g. tack solder some wires to the solder points at the back of the jack). If you want it to be switched, then you might want to tack the wires to either the points for the power to the drive or physical power switch if your enclosure has one.

Just be sure that the module gets enough cooling - as some modules won't be happy under claimed load continuously unless they receive additional cooling. More than that, you might expect the power supply to live a slightly shorter life as it will be under increased load which will increase the temperature which shortens the lifetime of key items such as the electrolytic capacitors within most switching supplies. This is one reason why even with a 12V/2A supply, they will not use it for a device that absolutely requires "all" of the 2A it can deliver (i.e. derating for reliability).

- Gough

Gough Lui · Answer

If going for a 2.5" drive, they normally only use 5V and use a lot less power by comparison, so it should be safe to do as you had planned. I would only think about this if I wasn't intending to use the drive with the Pi ... because ... why bother with all this if you could just use the Pi off a beefy 5V supply (perhaps jumper out the polyfuse if you must or power via GPIO) and hook a bus-powered 2.5" enclosure with your SSD on one of the downstream USB ports? After all, 2.5" SSD and Pi both want 5V ...

- Gough