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USB power

  1. Feb 6, 2012 #1

    This is probably a silly question. But it is something I haven't been able to find an answer too.

    When I connect something to my laptop through the USB, I know that there is 5V coming from the USB connection ...there are many pinout diagrams about that show this. But what is the power into the USB. Surely it is not just 5V's.

    Is there some sort of linear/switching regulator at work here?

    Like I said, sorry of this is a silly question, I have just being wondering, and I haven't been able to find a decent answer.


  2. jcsd
  3. Feb 6, 2012 #2


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    Sorry but this does not make much sense. Power is expressed in Watts not Volts. So it is nonsense to ask if the power is 5V. That is sort of like asking "what time is it ? and getting an answer of 5lbs"

    The usual spec for a computer USB port is 500ma @ 5V so the power available is 2.5W. The voltage is pretty well regulated and will not vary much unless you try to pull more then the 1/2 amp available. Then it may drop, and, likely, will be followed by smoke.
  4. Feb 6, 2012 #3
    Ah sorry. I meant voltage into the USB. I obviously didn't pay too much attention to what I was writing. Sorry.

    So, I take it there is a regulator on the input to the USB, which will take it down to the required spec?

  5. Feb 6, 2012 #4
    Correct, but bear in mind that it isn't going from 240V (or 110V) to 5V. There's a main transformer which takes it down from 240V on the way in and outputs it to a standard (I think it's 18V for my laptop) and then it is taken from there to 5V.
    Last edited: Feb 6, 2012
  6. Feb 6, 2012 #5
    That's great, thanks!!

    I figured it was something along those lines, but I was 100%.

    Many thanks.

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