Any negative impact with setting `max_usb_current=1`?


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by geerlingguy » Sun Feb 15, 2015 3:51 am
I've been searching around for more information on the /boot/config.txt configuration directive max_usb_current, trying to find out exactly what happens when that is set to 1, but it's hard to find any official documentation.

I know the following:

  • Setting max_usb_current=1 sets the available current over USB to 1.2A (default is 600mA)
  • This can help if you have a decent power supply (2A, at least) and need to power something like a small external HDD or something that needs 300+ mA.
  • This feature is only available on the B+ and Pi model 2 (at the time of this writing).

But some of the people mentioning the setting warn against setting this value unless you absolutely need to... my question is, why is that? Is something else disabled or changed about the Pi that would increase a risk of damaging the Pi or anything powered over USB, or is it more a general warning to scare off people who don't know what they're doing and might not have a decent (2A+) power supply to the Pi?
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by DougieLawson » Sun Feb 15, 2015 10:35 am
geerlingguy wrote:
But some of the people mentioning the setting warn against setting this value unless you absolutely need to... my question is, why is that? Is something else disabled or changed about the Pi that would increase a risk of damaging the Pi or anything powered over USB, or is it more a general warning to scare off people who don't know what they're doing and might not have a decent (2A+) power supply to the Pi?


There's no risk as long as you have a genuine power supply that can handle the load. Cheap Chinese fake phone chargers have been known to explode when the switch mode circuit runs away.
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by geerlingguy » Sun Feb 15, 2015 9:42 pm
DougieLawson wrote:Cheap Chinese fake phone chargers have been known to explode when the switch mode circuit runs away.


Thanks for the clarification! I didn't know if there were any other things I needed to be worried about. I have a few good power supplies, and for the project I'm particularly asking about, I have a 6-port 50W power supply, and I'll be using two 2A supplies for two Raspberry Pis that will be powering external USB SSDs, which need about 350mA max. So it should still be safely within the limits of what the Pi can handle with the 1.2A bus current.

I also found the following from this thread, written by @mahjongg:

All that max_usb_current=1 does is to set GPIO38 high, which in turn turns on a FET, which connects a second 39K resistor in parallel to an existing one, on pin 5 of U13, the AP2553W6 USB power manager, lifting the current limit from 0.6A to double that (1.2A), see no possible scenario there why the PI resets because of that, except in case the gate of the FET Q4 is somehow shorted to GND. Which could be caused by a production fault. Inspect Q4, as look if there is solder shorting pins together. Also R6 (resistor mounted between gate of Q4 and GND) should be 100K not 0 Ohm. U13, Q4 and R6 should be near the USB ports.


So seems safe enough to me, as long as you are using a solid 2A+ power supply!
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