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Charge internal batteries and auto switch from USB or 12V DC

Discussion in 'Power Electronics' started by kingdomoffurr, Feb 27, 2011.

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  1. kingdomoffurr

    kingdomoffurr

    9
    0
    Feb 27, 2011
    Hi, I have to build an aroma fan for a college project. I have 80mm computer fan that can run on a range of voltages.

    I want to put some re-chargable AAs in it, then charge them using a USB cable and auto switch over to the power and back to batteries when charged, a bit like a phone would.

    I am an electronics novice but I can solder.

    Could someone help me out with a circuit, or give me some guidance on this or a link to something that would help?


    Thanks
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,270
    2,718
    Jan 21, 2010
    Charging batteries is no trivial task. And automatic switching is even less trivial.

    The problem is that unless you do some (non-trivial) tricks, you won't be able to charge batteries above about 4.5V from 5V. And then (without resorting to tricks again) the batteries will be able to supply no more than about 4V to the fan.

    If that's still going to work, then you could firstly wire up diodes (and I would recommend schottky diodes) so that either the 5V from the USB or the 5V from the battery can power the fan. 2 diodes used like this will ensure the source with the highest voltage powers the fan.

    Then connect another diode and a series resistor so that the batteries charge from the USB supply. The resistor should be chosen so that the charge current is kept very low (possibly 1/30C -- so for 1500mAh batteries this is 50mA).

    That's the simples approach, and hopefully your batteries won't get too overcharged.
     
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,270
    2,718
    Jan 21, 2010
    Something like this

    [​IMG]

    And it's best to post your question in the thread than send it by PM.
     

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  4. kingdomoffurr

    kingdomoffurr

    9
    0
    Feb 27, 2011

    Ah I wasnt sure it would be noticed as its been so long and its such a little thread,

    thanks :)
     
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