Can you reduce the power output of a usb port?

Posted:
in Current Mac Hardware edited January 2014
Ive bought a USB fan for my 17" powerbook - it runs fine but as its pretty fast its making a bit too much noise. I was wondering if you can reduce the power output from the usb port to the fan - which i think would slow down the fan?



Cheers!

Comments

  • Reply 1 of 5
    splinemodelsplinemodel Posts: 7,311member
    USB has a max bus current of, I believe 150mA. It may be 500mA, but I can't remember.



    The thing is, that's not important. Current is kinetic energy. it only exists when there's a change in potential energy (voltage). If unplugged, your USB ports are using no power, since the 5V DC output isn't completing a circuit.



    So what you'd want to do is change the voltage to something lower than 5V, so that less current (and less total power) go through the fan circuit. Unfortunately, you can't do this in software very easily, if at all. The easiest way to do it is to add a series resistance before or after the fan, which will reduce the voltage drop across the fan. This will reduce the power the fan uses, and also reduce the fans speed. Of course, the resistor will dissipate the leftover power as heat, but it is, far and away, the simplest solution.
  • Reply 2 of 5
    mark_lmark_l Posts: 39member
    Sounds a bit complicated to me and to be honest i dont want to mess with the fan itself - i was hoping there was some way of doing this via software! Its a nice looking fan (with chrome finish) and it does cool the laptop and my hand but it makes so much noise its annoying!



    oh well
  • Reply 3 of 5
    splinemodelsplinemodel Posts: 7,311member
    Quote:

    Originally posted by MARK_L

    Sounds a bit complicated to me . . .



    The other option, which may not even work, is to rapidly flicker the USB port off and on in software. This is actually much more complcated that putting a resistor in the fan.



    Just out of curiousity, what kind of fan is this?
  • Reply 4 of 5
    mark_lmark_l Posts: 39member
    here its is..



    http://www.wownpd.co.uk/giftology_products/usbfan.html



    realy nice bit of kit, looks good on the 17" powerbook, but like i say, its just a little bit too noisy for my liking!
  • Reply 5 of 5
    mynameheremynamehere Posts: 560member
    Quote:

    Originally posted by Splinemodel

    USB has a max bus current of, I believe 150mA. It may be 500mA, but I can't remember.



    The thing is, that's not important. Current is kinetic energy. it only exists when there's a change in potential energy (voltage). If unplugged, your USB ports are using no power, since the 5V DC output isn't completing a circuit.



    So what you'd want to do is change the voltage to something lower than 5V, so that less current (and less total power) go through the fan circuit. Unfortunately, you can't do this in software very easily, if at all. The easiest way to do it is to add a series resistance before or after the fan, which will reduce the voltage drop across the fan. This will reduce the power the fan uses, and also reduce the fans speed. Of course, the resistor will dissipate the leftover power as heat, but it is, far and away, the simplest solution.




    or you could just buy a regular fan with variable speed. I think that would be "far and away, the simplest solution."
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