Do dongles or hub draw power..??

Discussion in 'MacBook Pro' started by Username Here, Jan 14, 2017.

  1. Username Here macrumors member

    Joined:
    Aug 19, 2012
    #1
    This maybe a silly question but if you have three USB's plug into a Thunderbolt-c hub does it need to be or should be powered from an outside source..?? (The wall !!) Will the power coming thru it remain constant just from the MBP (2016)..?
     
  2. bartvk macrumors 6502

    bartvk

    Joined:
    Dec 29, 2016
    Location:
    The Netherlands
    #2
    A hub that doesn't have a power supply, is called a bus-powered hub.

    If you hook up a bus-powered hub to your MBP, then all gadgets connected to the hub will draw power from the MBP.

    Now say you connect three USB sticks to the hub, and each uses about 150 mA while in use, and the hub also uses 50 mA, then at 5V, they use 5*(150+150+150+50) = 2500 mW = 2,5W

    But that power is not drawn constantly, because when the USB sticks aren't in use, they would probably drop to less than 30 mA instead of the 150 mA when reading/writing.
     
  3. Username Here thread starter macrumors member

    Joined:
    Aug 19, 2012
    #3

    Thank-You.....great explanation for me....
     
  4. bartvk macrumors 6502

    bartvk

    Joined:
    Dec 29, 2016
    Location:
    The Netherlands
    #4
    It's quite interesting because the internal battery is 76 Wh. So because 76Wh / 2.5W = 3-point-something hours, it means that if you would constantly read and write three USB sticks over a hub, it would drain your battery in three hours (ignoring the normal power usage of the machine) :)
     

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