rogersmithiii
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PI 3b power to USB and peripherals. Dependent on PI or power supply?

Wed Apr 17, 2019 10:04 pm

If I'm hooking things up to my Pi3B via USB, the TOP HAT, and such, is the amount of power that I can supply to those devices dependent on the power supply or the PI or both?

Meaning, if I hook a 5V DC supply that can deliver 10 amps to the PI, can I deliver 10 amps to my peripherals, or does the Pi itself limit how many amps of 5 volts can be supplied to things that are attached to the Pi? I know that some of that 10 amps has to go to power the PI, but is there a limit on how many amps of 5V DC that can be made available to attached hardware?

Thanks

Andyroo
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Re: PI 3b power to USB and peripherals. Dependent on PI or power supply?

Wed Apr 17, 2019 11:23 pm

The USB spec and components actually limit that - power details for the standard GPIO and USB ports are here

Note the zero boards are odd - you only have one USB port and I've not drawn much through it at all.
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Imperf3kt
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Re: PI 3b power to USB and peripherals. Dependent on PI or power supply?

Thu Apr 18, 2019 12:19 am

Also note that if you supply power via the micro USB port, you will be limited to a budget of ~2.5A as there is a polyfuse which will limit the current the Pi can draw.
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Re: PI 3b power to USB and peripherals. Dependent on PI or power supply?

Thu Apr 18, 2019 3:50 am

The combined total of all 4 USB ports is limited to 1.2A by a USB power controller circuit.
As mentioned the total current allowed into the micro USB power port is limited to 2.6A by the 'poly fuse'
So current available to the 5V GPIO pins 2 & 4 is = 2.6A - (whatever the RPi uses + USB + camera + Display)
If you attempt to draw more than that then the fuse will shut off power to the RPi but will self heal when allowed to cool for a few minutes..... preferably overnight.

If you power the RPi via GPIO pin 2 or 4 then you are limited to whatever the copper traces in the board can handle. You will know it's too much when you let the magic smoke out of the board.
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rogersmithiii
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Re: PI 3b power to USB and peripherals. Dependent on PI or power supply?

Fri Apr 19, 2019 1:46 pm

Can't I buy a can of Magic Smoke to recharge the board?

http://www3.telus.net/bc_triumph_registry/smoke.htm

It works on British automotive electronics, so why not PI?

Thanks for the info.

Which pins on the GPIO are + and - 5 Volts? Also, I have a ham radio hot spot board plugged into this GPIO. Is there something that I can put between the PI and the ham radio board to allow me to connect the power supply to the PI?

And please, nothing from Adafruit. They're horrible to deal with.

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rpdom
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Re: PI 3b power to USB and peripherals. Dependent on PI or power supply?

Fri Apr 19, 2019 4:30 pm

rogersmithiii wrote:
Fri Apr 19, 2019 1:46 pm
Which pins on the GPIO are + and - 5 Volts? Also, I have a ham radio hot spot board plugged into this GPIO. Is there something that I can put between the PI and the ham radio board to allow me to connect the power supply to the PI?
+5 Volts is on Pins 2 and 4 of the GPIO connector.

There is no "-5 Volts" connection on the Pi. If you mean 0 volts, then it is the Ground connecions you want, which are pins 6, 9, 14, 20, 25, 30, 34 or 39.

See https://pinout.xyz/# for a handy picture.
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hippy
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Re: PI 3b power to USB and peripherals. Dependent on PI or power supply?

Fri Apr 19, 2019 5:17 pm

klricks wrote:
Thu Apr 18, 2019 3:50 am
The combined total of all 4 USB ports is limited to 1.2A by a USB power controller circuit.

If you power the RPi via GPIO pin 2 or 4 then you are limited to whatever the copper traces in the board can handle.
It's probably worth clarifying that, other than on the Zeroes and A's, even if one connects the GPIO power pins to a supply which can deliver however many amps, the board will still limit USB to drawing 1.2A total.

Unless one adds jumper wires to connect the USB Vbus pins direct to the GPIO 5V pin. That used to work with older boards but I have no idea if it works with the Pi 3B+ and its PMIC. If that bricks itself the Pi is pretty much dead.

Unfortunately we don't have full schematics and without access to those it's not easy to predict what the consequences of bypassing the USB current limiting would be.

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