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I have a Raspberry Pi 4B 4GB RAM variant that will be running 24/7 with the following attachments:

  • an ethernet cable leading to a router for internet connectivity
  • a 2.5" external HDD attached through USB 3.0
  • 30mm cooling fan connected to GPIO 5V + GND pins
  • built-in Bluetooth ON
  • built-in Wi-Fi ON

I want to power this Pi using a YU0508 PSU rated for 8A @ 5V. In case the HDD demands more current will my Pi be able to draw that extra juice from the PSU and supply it to the HDD through USB 3.0 ? if yes, is there some hard limit of how many Amps the Pi will supply over USB 3.0 ? and is there some risk/hazard in doing this ?

PS: A powered usb hub is currently not an option.

Thanks !

3 Answers 3

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The original unmodified Pi will not draw more than 3A, no matter how many your PSU can provide: the USB current is limited to 1.2A, shared between all 4 ports. The USB 3.0 connector is rated for 3A anyway, if memory serves, so a more powerful PSU only makes sense if you also make a custom power cable.

Of course, it won't hurt to have an 8A PSU, it will not be unsafe or anything, just not very useful.

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In case the HDD demands more current will my Pi be able to draw that extra juice from the PSU and supply it to the HDD through USB 3.0 ?

Your RPi will never draw that much current, and so the over-capacity of an 8A-rated PS won't, in and of itself, provide any advantage. Here's a list of the "official" power requirements., and this note seems to address your question specifically:

"If you need to connect a USB device that will take the power requirements above the values specified in the table above, then you must connect it using an externally-powered USB hub."

Which is to say, if your USB requirements exceed the limit in the table, you'll need to have a powered hub.

if yes, is there some hard limit of how many Amps the Pi will supply over USB 3.0 ? and is there some risk/hazard in doing this ?

The limits to current flow into the RPi are based somewhat loosely on USB specifications, but more definitively by the RPi's minimum voltage requirement (4.63V), and the voltage drop imposed by the wiring used to deliver the power to the RPi's input.

Re the risk/hazard: The only potential downside to using the supply you've chosen is in its ability to deliver the required voltage across the range of loads your RPi will draw. Power supplies produced by responsible manufacturers will include a set of specifications that will give you this information; it may be called load regulation, and it simply reflects how much the voltage may vary across a range of loads. It's not unusual to see power supplies that do not regulate their output voltage very well at very low output currents.

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The Pi4 is limited to supply 1.2A total from USB ports. It is not compliant to any of the USB power specifications.

Even if the current draw is less than this the Pi has a poor record of powering HDD.

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