RaPi power question

Hi forum
New member here, so thanks for being gentle.
I been using RaPi’s for small tasks for a while. Always been using a USB plugpack from 240v outlet.
(aka wall-warts)
Now I’m making a RaPBX phone server on a new RaPi 3 B+. RaPBX is Asterisk and FreePBX for RaPi.
Would like to power this kit from a motherboard USB-3 port of a regular computer with quality ATX power supply, and go without the wall-wart.
RaPi will also have a usb GSM dongle with sim card for cellphone calls.
Is a USB-3 socket from a computer motherboard likely to provide sufficient current and stability to keep the RaPi happy ?
TIA’s for any tips or clues.

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G’day David, my experience with the RPi 3B+ is that on its own it can use 2.5 amps. I was getting the yellow lightning bolt in the top-right of screen (insufficient power) when trying to run Netflix. When I purchased the genuine RPi 2.5A power pack that problem was solved.

I would have a look at the USB GSM dongle and find out what it’s maximum power consumption is. Add that to 2.5A max. for the Pi and have a look at your computer’s specifications to see what the maximum supply current is on each USB socket. I hope you’ll find one that can provide about 3A and that will be enough.

To be doubly sure it’s not expensive to add a USB power meter to ensure the voltage is not dropping too low and that the current draw isn’t exceeding the computer’s ratings.

Hope that helps! :wink:

  • Chris

One other thing if you’re trying to run this from say, a backup battery - Don’t try powering the R-Pi 3 from a 12V to 5V DC stepdown unless it can do at least 3A reliably. I have one such device and it can power both my R-Pi and my Windows 10-based MeeGoPad “mini PC”, but as soon as the MeeGoPad hit 100% CPU usage it would shut off because the voltage would drop suddenly.

The R-Pi 3B would run pretty comfortably but sometimes randomly pop the thunderbolt icon for a bit, until the stepdown convertor stopped being silly. It also would have slowdown when the thunderbolt icon appeared.

I also tested the Chinese brand (not AU certified, gasp!) 5V 3A MicroUSB charger/power supply that came with the MeeGoPad in the R-Pi and it happily runs on that. And that is the Pi + the NES Pi case’s internal electronics.

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The lightning bolt symbol appears when the 5V bus goes below 4.65V, which can happen under heavy loads or with cheap power supplies. The official power supply that Chris suggested is rated at 5.1V to try and stop this. If you’re trying to use a standard USB 3.0 port to power the Pi, my understanding is that they can only deliver 900mA, so you’d get the Pi to turn on but the voltage would almost always be too low to do any real work.

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Computer USB ports are only rated at 1Amp. They generally have a fuse, but you don’t want a fried MoBo.



Thanks for all the tips and clues above. Warnings too. I get the message.
There’s some relevant info at

IMHO wikipedia is a wonderful and usually reliable source.
Also, I won’t be having any lightning bolt symbol since I run my RaPi’s headless.

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