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I have a bunch of questions. I have searched LOTS but I can't seem to figure this out. I hope an expert can answer this for me.

I read that a Raspberry Pi can only output 0.6A for those 4 USB ports in total. That is, 0.15A per USB port.(Yes, I know you can tweak it to 1, but I haven't done that.)

How am I able to power an external HDD of 0.75A, a USB 3.0 of 0.91A, and a USB 2.0 of 0.06A all at once? That's a total of 2.2A! And then the Raspberry Pi requires an additional 1,1A, which is a total of 3,3A. The power supply that I am using only has 2.5A available (5V). Can someone explain how this is possible?

Shouldn't the Pi shut off whenever the 2.5A limit has been breached?

Also, if I were to buy a USB-hub with a separate power supply and plug all my USBs into that and the hub in the Pi, would that mean that the Pi would output 0A to the hub and the USBs since it's all being powered by the hub? Freeing 2,2A?

My HDD sticker says that it needs 0,7A. The port of the Pi can only give 0,15A to that port that my HDD is connected to. How does this thing ever power on? That's a big difference...

Or do my USBs operate at a total of 0.25A? I did read about unit load too. Still doesn't explain the HDD part then to me, unless that thing can run on 0,15A...

Can someone please clarify I've been studying electricity for 2 days, but I can't understand this. All I've read is going against me.

May
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2 Answers2

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The Pi3B and Pi3B+ USB can supply 1.2A without modification (subject to an adequate PSU).

Your question has a lot of incorrect assumptions about USB - this is all explained in Raspberry Pi Power Limitations

You also appear to be confused by the ratings of devices; the current is usually the maximum required, and HDD particularly consume much less most of the time, so adding maximums does not determine the actual current required.

Having said that, I recommend a powered hub with HDD; despite the fact that I have confirmed my Pi can deliver 1.2A at close to 5V, I often have problems with HDD (probably due to transients) so use a hub when reliability is required.

Milliways
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Powering the Raspberry Pi can be challenging for high current draws: 1) use a powered USB hub (but do you really need all those USB ports?) 2) use a wall wart that provides the current you need (but why are these so expensive and hard to find in the current you need?) 3) use a DC-DC buck converter to convert a higher voltage (e.g., 12V) to 5.2V and feed that in thru GPIO 5V/GND. (lots of these cheap on Amazon).

I've done all three and settled on #3 because it lets you do things like hook your Pi up to a car battery and solar.

OyaMist
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