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USB powered desk fan

Discussion in 'Electronic Design' started by [email protected], Apr 2, 2007.

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  1. Guest

    It is hot in my office and for some reason I don't have any available
    power outlets near me to power a desk fan so I want to make a fan that
    is powered by USB. I bought a cheap handheld fan that is powered by 2
    AA batteries (3V) and I want to convert it to run off a 5V USB
    computer port. I need to step the voltage down from 5V to 3V. What
    type of resistor would I need to do this? It would be nice to make
    the fan have adjustable speeds so would I use a potentiometer to do
    that? What type and rating? I am using this article as a reference
    for my design...

    I greatly appreciate your thoughts.
  2. Si Ballenger

    Si Ballenger Guest
  3. a standard USB port will only delivery 100mA before shutting down, you
    have to talk to the USB port to tell it you need more power, up to a
    maximum of 500mA. Measure your fan current, and see if it is less than
    100mA, if not you need to put a micro inside your fan to talk to the
    port, not a trivial task

  4. Guest

    Perhaps this would help you:
    Although i think that dc-dc converters are a little expensive.
  5. I have a 'USB powered fan', it crashed my PC USB port repeatedly before I
    soldered a AC adapter plug to it.
    Forget it, it sucks.
    USB is about 600mA max, and you likely have otehr stuff on it that uses
    some power.
    New mobo is expensive too... reboot is annoying.
    30 Euro buys a huge Chinese fan with 3 speeds....
  6. Why don't you just open the door of the fridge ?

  7. Joel Kolstad

    Joel Kolstad Guest

    Although the USB implementers would prefer it that way, many computers
    (probably even the majority of them) don't have such fancy current limiting on
    their USB ports and *will* pass a full 500mA (or more) to *any* device, such
    as a 100% "dumb" fan. Relying on this behavior obviously doesn't make for a
    robust design, but many ceahp USB "novelties" make use of that fact
  8. AZ Nomad

    AZ Nomad Guest

    I prefer using 110V when I in the mood for destroying the circuitry on my
    computer's motherboard. You can reach so many more subsystems that way.
  9. I had suspected that, but I thought that I should try and point out
    the "correct" approach.

    I'm not going to stick anything onto my laptop USB ports that is not
    "proper", but maybe with cheap USB cards for a desktop PC, who cares

  10. Guest

    Oh that's mean, Rene.
  11. Guest

    Two words: extension cords.
  12. Guest

    wow. lots of bird walking going on here. i just want to know if i can
    put a resistor in series to reduce 5v to 3v and what the value of the
    resistor would be. if anyone has the ability to answer this question
    please help. otherwise please leave your 2 cents in your pocket.
  13. That would be a ( 2 V / I_fan ) ohm resistor, rated at least ( 2 V *
    I_fan ) Watts.
  14. If you are such an idiot as not to listen to good advice WTF should anyone
    help you anyways.
    Now go an f*ck off.
  15. Nico Coesel

    Nico Coesel Guest

    I fit a USB to serial converter on almost anything with a
    microcontroller (especially when it is a prototype). No need for a
    bench power supply.
  16. Joel Kolstad

    Joel Kolstad Guest

    Aw, c'mon, you haven't lived until you've shorted a kilowatt or so transformer
    and set the wire's insulation on fire! :)

    I take your point though... the other day I was thinking of making a little
    test board with a Max232-variant on it, when it occurred to me... hey... I
    might as well just use an FT232R, and that way I wouldn't have to come up with
    power from somewhere...
  17. Nico Coesel

    Nico Coesel Guest

    Many years ago I asked where a test lead had gone. A co-worker
    answered, "well it is right there" while pointing at a black stripe
    inside a transformer casing. They had been testing the transformer
    while I wasn't there and they shorted it using the test lead.
    I have had very good results with the CP2102 from Silabs so far. Never
    quits, driver never hangs and it has an internal 5V to 3.3V regulator
    (100mA max).
  18. Joel Kolstad

    Joel Kolstad Guest

    Ah, thanks for the pointer. I took a look at the data sheet over lunch, and
    while the CP2102 doesn't seem to have quite as many bells and whistles as the
    FT232R, I do like the fact that it has substantially bigger internal buffers
    (640/576 bytes for Tx/Rx vs. FT-232's 128/256), and it's nice that they
    provide various utility programs for customizing the INF files rather than
    making you hand-edit them as FTDI does!
  19. Guest

    you can run the 3V fan with 5Volts, it will just run faster
  20. jasen

    jasen Guest

    noone can tell from here. how much current does the fan use?
    This is not something that can be determined (practically)
    without measuring it.

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