Hi, does anyone know how the whole "extra current" thing works on the USB ports on a 2012 Macbook Pro? I'm trying to get the additional current draw to power a device that normally runs off a separate 12v line (but needs minimal current - likely ~ 8-12w). Is it available by default or is there some sort of signal sent by other apple devices? Am I going to short out my mac trying to run the device through a step-up converter?
Hi Fidelity,
Thanks for visiting Apple Support Communities.
See this article for some information about high power USB ports on a Mac:
Apple Computers and Displays: Powering peripherals through USB
http://support.apple.com/kb/HT4049
Best,
Jeremy
"needs minimal current - likely ~ 8-12w"
Watts = amps x volts.
So to draw 8 to 12 watts of power from a 12 volt supply means a current of 2/3 to 1 amp. That's 667 to 1000 milliamps.
You should confirm that your MBP can provide that current. I believe that built-in USB2 ports peak at 500 milliamps.
The output of the USB ports is 5Volts and 500-1000mAmps (max 5W at 5 volt), a device that needs 12 Volts and then 8-12 Watt should not be connected: it is much too heavy for the USB port and it will not work because the USB port is only 5V.
My suggestion would be to see if you can find a powered USB hub that is capable of delivering that amount of power.
I am skeptical of you finding it because I believe the 5V and 500 mA is in the USB spec.
No matter what the Mac can't do it.
The only other alternative I can think of is to talk to the vendor of the device to see if they can supply with something that provides that amount of power.
Allan
Extra current from USB on 2012 15" Macbook Pro
