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I have an external SATA hard drive enclosure that has its own power input. I would like to be able to run a Raspberry Pi from the same power input.

The enclosure takes a 12VDC input (currently I'm using a 1.5A power supply but I'm sure I have a 2A one lying around somewhere) and converts it to a SATA power input for the drive. The hard drive says it wants 0.65A from the 5V on the SATA power, and the RPi draws 1A from its 5V input at most. The intended setup is for a small NAS so the Pi will be using its ethernet port on the usb hub chip but no peripherals after the original setup (which can be done when powered normally).

So, is it possible/sensible to split the 5V cable on the SATA power and hook it up to a µUSB to power the Pi, bearing in mind the variable power consumption of the drive and Pi?
If not, why?

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It might work, but I wouldn't recommend it. Purely because the components in the power system for the HDD enclosure will be sized for running a hard drive. The controller board will include a 5V regulator, and a 3.3V regulator. That 5V regulator may not be able to reliably provide both the power for the HDD and the Pi.

Instead I would recommend adding a second power board that takes power direct from the 12V input and converts it to a separate 5V supply just for the Pi. I'd recommend a switching power supply, and the simplest form would be a UBEC (Universal Battery Elimination Circuit) used in many RC vehicles, and available from most RC stores both online and otherwise.

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It is not sensible, without know how much current the onboard dc/dc converter on the enclosure can provide, not how much power it can safely dissipate. If you had pictures of that, with the IC names maybe the datasheet can help you, but it would be best to use a separate 12V to 5V step down converter and tap into the 12V power line, which you already plan on beefing up.

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