Power for a Hard Drive

[MrPresident]

I have a Raspberry PI 2 running with a HiFiBerry DAC + Light, PiMusicBox and a good micro USB power supply. I am connected to the connected to the network wired. Everything is working great. I want to connect a laptop hard drive to it. I have a USB Y cable with the data/power plugged into the hard drive and the RPI and the power only leg of the cable plugged into my desktop USB for additional power. At the hard drive I am using a non-powered case made to convert USB to SATA. The USB Y cable is the one that came with the case.

Plugged it all in and everything seemed to be working fine. I was able to play MP3’s on the hard drive. No issues. Then for some reason I wanted to turn the PI off so I shutdown the PiMusicbox and unplugged power supply. That’s when I discovered that the PI was being powered through the USB cable from the hard drive.

I am assuming that this could be a problem because the USB power will not be protected. In any case I want the power separated from the PI. I Gesearched for data only USB cables but no luck. I have dissected a spare USB cable and determined which wires to cut out for data only but it is shielded and I don’t know how big of a problem that is. Can I just wrap aluminum foil around the splice to maintain shield integrity?

I am building a 2 rail linear power supply which ultimately will power both the PI and the HD. Anyone been down this road? Suggestions? Comments?

[elatllat]

Just use some strategicly placed paper and tape.

[MrPresident]

Thanks for the help. I assume you mean to cover the power contact in the USB connector. Sometimes the simpler solution is the best (and least obvious). Would I need to cover only the positive (pin 1) or should I also cover the ground (pin 4)?

[rpdom]

Just cover the positive pin. You need to keep the ground as a return for the data signals.

[mosespi]

I am fairly certain there was a thread recently about shutting off the power to the USB ports via software.. specifically on the Pi 2. I can't seem to find it myself, maybe someone else knows. What happens to this 'setting' on reboot and how that will effect a setup like yours, I'm not sure. Maybe worth looking into.

Some more ramblings..

- Have you tried powering up the drive by the Pi itself? Not sure if this would work well, but worth a try if you are not scared of some SD card corruption. Check out the max_usb_current=1 options too.
- You could go the other way and power up your Pi from the hard drive supply. I'm sure there is 'some' protection for overload on your hard drive USB supply.. how much is a question. If you don't have too much experimental hardware that might short on your Pi.. this may be good enough for a single supply solution.
- I'm sure some extra foil and tape wouldn't hurt to wrap your splice. But I don't think you will have trouble with an inch or so of missing or somewhat damaged shielding. I would recommend a small splice in the cable, pull the wires out, cut them and stuff them back in best you can.
- On that note.. if you can reach the pin a few coats of nail polish usually works good to isolate pins!
- Linear supplies generally need heatsinks. I would recommend a DC-DC module. There are plenty available fairly cheaply ($2-5 each). Search for "lm2956 module" or "DC-DC module". You can use one for the HD and one for the Pi if you want 2 power rails.

Regards,
-Moses