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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.

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2 Answers 2

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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.

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  • Is there a formula to calculate a 'minimum current' of a device if I know the max current of it? Or do I just have to plug it in and see if it operates? I guess the current required depends on how you use the device. Yeah doing max currents makes no sense, but I can't use it's full potential then I guess. One more question, let's assume my Pi gives 1.2A then, does it give 300 per port OR can it give let's say 500 to port 1 and then 100 to port 2? Like scale the power over to another port as long as it stays within 1.2A?
    – May
    Commented May 31, 2018 at 10:29
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    @May There is no relation between max and min; for a resistor it is 1; depends on circuit. The other question is answered in the link - basically the Pi only limits total current.
    – Milliways
    Commented May 31, 2018 at 13:13
  • All right I see, that's good to know. Thanks a lot! That answered everything. The resistor part is still a bit hard to follow but in 1 week I'll understand that. Need to read more online. Thanks again!
    – May
    Commented May 31, 2018 at 13:25
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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.

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  • I was thinking about option 2 before posting, as you said it's really hard to find. Option 3 is really cool I just read it decreases volts and increases current. You said go for 5.2V BUT I read somewhere uhh, that voltages need to be below a 5% tolerance of volts? I just did calculate it and you'd end up with 0,05% too much volt. Yes I need all 3 USB slots I use 3 for on my LAN Network and WAN as storage. However I saw a 7 slot USB 3.0 hub with 5V each 10 amps which perfectly fits my expectations but I don't know if my Pi would output 0A to the hub. Sorry, I don't know much about electricity.
    – May
    Commented May 30, 2018 at 19:57
  • 5% tolerance is +/- 0.25V. So 5.2V is within tolerance. Voltage is like water pressure--the more you have, the better the system responds to random things like toilets flushing when you're in the shower. So we try to nudge it up to give us "that little edge". Powered USB hubs such as the ones by Plugable have been used happily with the Pi. but you will need to google for your specific hub and how to connect it with the PI.
    – OyaMist
    Commented May 30, 2018 at 20:10
  • Oh yeah I'm dumb my calculation was also 0,25V but I for some reason I said the 0,05V was outside the 5%, my bad. Okay I'll just buy a hub that's the simplest solution for me. BUT I still don't know if the hub will draw current from the Pi (expect the hub to have it's own power supply.) My question is how my HDD and USB sticks can even power on. I have no issues at all just that I want to know how this is possible. My HDD needs 0,7A and my Pi USB port can only supply 0,15A how does it start and work just fine?
    – May
    Commented May 30, 2018 at 20:45
  • Plug your power hungry devices into the USB hub and let the Pi sip what it needs.
    – OyaMist
    Commented May 30, 2018 at 21:16
  • (btw, if this was helpful to you, feel free to accept the answer so I can help others. :D
    – OyaMist
    Commented May 30, 2018 at 21:18

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