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I have 2 USB hard drives that combined take too much power to run through the pi USB port.

I have several 5V sources laying around that provide plenty of current for all of the hard drives and raspberry pi combined.

Could I wire up the USB any of these two ways to power the hard drives via the external source?:

schematic

simulate this circuit – Schematic created using CircuitLab

schematic

simulate this circuit

In the first case, the Hard drive power is in parallel with the USB power. In the second case, the pi power is disconnected and the external power is connected directly to the HDD.

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At best this would be unnecessary, and at worst destructive in some way.

Yes of course theoretically you could do this though... USB 2.0 ports can supply 500mA, so I assume you need 1A for the two drives. Allow 1.2A for the pi.

I'm not an electrical engineer, but I believe that if you wired a power supply to power two usb-powered drives and a raspberry pi in series, you would just end up with a power supply that didn't supply enough current to power any of them.

Remember Ohm's law, and at least simulate what you're trying to do with a circuit simulator - remember that circuit simulators aren't the real world though.

I enjoy DIY projects, but I just power the drives separately. My pi is connected to an ancient external drive that runs off an AC adapter, and they're both very happy.

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  • Powering two HDDs with 1A will not work for the vast majority of the drives. – Dmitry Grigoryev Mar 12 at 11:01
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Here is what you can do. An electronic circuit may have the tendency of pulling more current than it needs. So if you are using any kinds of power supply through USB make sure you are using a good quality power supply. If possible isolate the power of that hard disk.

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  • I simply don't understand what this answer is suggesting. – Dmitry Grigoryev Mar 12 at 10:50
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Assuming you have a common battery connection that can supply the amps you need for all three devices, here's an idea:

Why not just make two custom USB cables for the HDDs and snip the supply wire to the drives and connect them to your 5V supply separately? With the common ground between the three, I don't see any problem. Just make sure your 5V supply is truly clean and actually 5V so you don't have to worry about regulation. If it isn't, you might consider putting a TVS diode across the power supply to avoid overvoltage issues. I actually use the same one that's on PI boards to ensure I don't get unwanted voltage spikes.

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  • Why not just buy a powered USB hub then? Unless you do soldering for a living, building a custom equivalent won't be any safer, cheaper or faster. – Dmitry Grigoryev Mar 12 at 10:55

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