Dual USB power

Discussion in 'Mac Accessories' started by skir0987, Feb 11, 2010.

  1. skir0987 macrumors regular

    Oct 14, 2007
    I'm not sure if this is in the right thread (or in the right forum, for that matter), but I was just wondering if anyone knew. I'm trying to power a fan using USB, but the fan does not run fast enough (it uses about 10V) using one USB port, so I tried connecting it to 2 by making a Y-like splitter that merges the power from 2 USB ports. I can make the fan run using either plug plugged into the computer (a MacBook Pro, if it matters), but when both are plugged in it just runs at the same speed. I have an external HD that uses the same kind of configuration (Dual USB), so I was wondering if there was anything special I need to do to make this work?

    This is the kind of thing I'm trying to make, just without the mini-usb on the end: http://www.lindy.co.uk/1m-usb-2-dual-power-cable-2-x-type-a-50cm-apart-to-mini-b/31782.html

  2. MetalPhreak macrumors newbie

    Feb 11, 2010
    You can't get 10v using two USB ports.

    Each USB port can provide 500mA of current at 5v. You can use one of those double adaptors to get more current, ie 1000mA at 5v, but you CANNOT wire the usb ports in series to get 10v. Trying to do so will most likely cause serious damage (as you effectively short circuit the 5V power supply behind the 500mA current limiters). The fan still spins at the same speed because its still only getting 5v (and using less than 500mA)

    Ditch the 12v fan, and buy a 5v one instead. Make sure it's rated less than 0.5A
  3. nanofrog macrumors G4

    May 6, 2008
    USB's voltage output = +5V DC (+/- 5%) @ 100mA (v. 2.0 spec)

    The cable you linked attaches the power signals in parallel, which adds the current outputs, but the voltage stays the same. So you still get 5V DC, just additional current (200mA).
  4. skir0987 thread starter macrumors regular

    Oct 14, 2007

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