question about wiring 12V fan to USB

Discussion in 'Mac Accessories' started by rawdawg, Dec 9, 2013.

  1. rawdawg macrumors 6502

    Joined:
    Jan 7, 2009
    Location:
    Brooklyn
    #1
    Hi, I recently bought this PCI slot fan for a cabinet I keep my MacMini and HDD docks in. My HDDs have very important media on them and I have a good backup strategy. Still, out of abundance of caution, I wanted to use that fan to vent the cabinet which gets pretty warm (hot?) - I haven't used a thermometer but it's probably in the 90's at times, definitely below max temp for HDDs but thought it could be helpful to make them live longer.

    I happen to have a 4-pin 12V power supply that came with my Newer Tech Universal adapter, but the power supply is one of those giant wall wort monstrosities.

    I would like to wire it to be powered off USB (like my other Thermaltake USB fan), but I know nothing about electrical engineering and if going from 12V to 5V power supply is a bad thing.

    I rigged it using wires to test it and it "seems" to be pushing out the same amount of air. Does anyone know if this is safe or if this effect performance? thanks
     
  2. mfram macrumors 65816

    Joined:
    Jan 23, 2010
    Location:
    San Diego, CA USA
    #2
    If the fan actually turns, then that tackles one obstacle: that 5V wouldn't be enough to push the fan. The other risk is that at 5V the fan will draw too much current out of the USB port.

    The only way to know would be to hook an ammeter in series to look at the current draw. The USB standard is max current 500 mA. Any more than that then either the O.S. could shut down the port or it's possible you could blow out the port. You might have to add a resistor to keep the current down.
     
  3. rawdawg thread starter macrumors 6502

    Joined:
    Jan 7, 2009
    Location:
    Brooklyn
    #3
    Thanks for this info! I cannot find exact specifications for this slot fan, but another from a different manufacturer is only 0.16A at 6VDC (though through 4-pin Molex, not USB).

    My other fan that is a USB powered fan I feel would draw more power (it pushes more air, is larger, sounds louder) has the following specs:

    Operating Range 3.5 V ~ 5.5 V DC
    Input Power 1.75 ~ 2.00 W
    Start Voltage 5V DC
    Rated Voltage 5V DC
    Rated Current 0.35 ~ 0.40 A

    I hear you that I do not know the amperage of this PCI slot fan and giving it a USB connector runs that risk. I assume it operates properly since your other concern was whether or not it would start. Not being electrically inclined I do not know if it's as simple as plugging something in to lower voltage raises the amperage proportionately. The fan is small and extremely quiet, I did not know it was on which was very surprising considering I clearly hear my other USB fan.
     
  4. mfram macrumors 65816

    Joined:
    Jan 23, 2010
    Location:
    San Diego, CA USA
    #4
    I'm not much of a EE either. I'm just a software guy who plays a EE on TV. But I do know the basics.

    The fan is going to use about the same amount of power to do it's thing. In simplified terms the power is proportional to the voltage ("push" of the electrons) times the current (number of electrons). So if you decrease the voltage, the current has to go up for the fan to do the same amount of blowing. That's the idea.

    Given the specs you stated, it looks like you are well within the safe margin with the USB port.
     
  5. drsox macrumors 65816

    drsox

    Joined:
    Apr 29, 2011
    Location:
    Xhystos
    #5
    Read this : http://www.instructables.com/id/How-To-Make-A-Cheap-USB-Powered-Fan/
    or buy one of these : http://www.amazon.com/Best-Sellers-Electronics-USB-Fans/zgbs/electronics/3015416011
     
  6. rawdawg thread starter macrumors 6502

    Joined:
    Jan 7, 2009
    Location:
    Brooklyn
    #6
    Just want others to know I hooked up my fan using an old USB cable but it is not very powerful.

    Turns out this needs the 12V to run full power.
     

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