Operating a 12V fan from a USB port

Discussion in 'The Projects Forum' started by iONic, Sep 19, 2008.

  1. iONic

    Thread Starter AAC Fanatic!

    Nov 16, 2007
    1,420
    68
    I'm finding it difficult to locate the right size muffin fan (aprox 2 x 2 x 1/2) that runs from 5V, thus my quest to step up the voltage/current in order to operate a 12V fan. I don't need it to run at full speed as many 12V fans I have can run adequately near 9 - 10V.

    I was looking at a couple of circuits on Dave Cappels Website:

    http://cappels.org/dproj/PulseBoostLED/Pulse_Boost_White_LED.html

    ...thinking that they may be altered in some way to make the fan work for me.

    If you have any ideas, please let me know.
     
  2. Externet

    AAC Fanatic!

    Nov 29, 2005
    763
    57
    Can you expand the application please?
    Will the USB port be from a laptop, a desktop; will some other thing work on 12V too? What is the current needed ? What will the fan cooul down?
     
  3. iONic

    Thread Starter AAC Fanatic!

    Nov 16, 2007
    1,420
    68
    I want to add a permanently running fan to augment the single CPU fan in my laptop.
     
  4. Metalfan1185

    Active Member

    Sep 12, 2008
    146
    0
    just a question

    Do you know the voltages present from the AC PSU and the battery? there may be more than one voltage present.
     
  5. Externet

    AAC Fanatic!

    Nov 29, 2005
    763
    57
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    The USB port on a desktop/tower computer can supply about 500mA.
    The USB ports on laptops usually supplies far less than 500mA.

    I do not see an attempt to use a DC-DC converter to boost the voltage of a laptop's USB port to drive a fan as being a viable project.

    You would be better off to use an external 10v-12v supply to power the fan.
     
  7. Metalfan1185

    Active Member

    Sep 12, 2008
    146
    0
    oh, i missed the "from the USB port" part...

    my mistake
     
  8. iONic

    Thread Starter AAC Fanatic!

    Nov 16, 2007
    1,420
    68
    Maybe a better option would be to just tap off the Power Supply cord, which is 17 - 19V at 4.9A, and regulate this. I alread have too many USB devices.
     
  9. iONic

    Thread Starter AAC Fanatic!

    Nov 16, 2007
    1,420
    68
    Are you sure about these figures? I have a NiCd, NiMH battery charger that plugs into the USB port for charging. I will have to check to see what it's charging current is.

    The USB charger has two settings, 250mA for normal charge and 450mA for Fast charge.
     
    Last edited: Sep 19, 2008
  10. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Yep, I'm pretty sure. Your mileage will vary with laptop USB ports. My old Toshiba laptop had a hard time powering an old digital camera I had when I tried downloading photos to it.

    Charging NiCd/NiMH batteries is one thing. As long as you're feeding them small amounts of current, they'll be charging. With 1.2v batteries, you could charge three of them in series at 100mA and have power left over.

    However, you're talking about boosting the voltage from 5v to 9v or 10v.
    So let's say you get a DC-DC converter that's 90% efficient (which is very optimistic; you're more likely to see 85% efficiency). Let's also say you have 300mA available. So, you might get 9v@150mA out. That's not very much power.

    As far as tapping off your supply cable - well, replacements are expensive in case you manage to short something out. You also might "starve" your laptop for power if you don't set it up efficiently.
     
  11. nievesoliveras

    New Member

    Aug 3, 2008
    1
    0
    Hi!The attached file shows different step up circuits to get the voltage you need. Just connect the usb cable 5volts in place of the battery on the circuit.Jesus
     
  12. Papabravo

    Expert

    Feb 24, 2006
    10,163
    1,797
    You can only get 500 mA if you negotiate for it. RTFS!
    This is a misbegotten idea. There are any number of better ways, assuming you really are smarter than the laptop designers.
     
  13. iONic

    Thread Starter AAC Fanatic!

    Nov 16, 2007
    1,420
    68
    I suppose if the following circuit can be scaled down for 9V and 200+ mA out it might work.
     
  14. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    Why is the laptop running too hot? Are you over-clocking?
     
  15. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    A quick calculation of the power available from a robust USB port is in the neighborhood of 2.5 Watts. A quick check of the specs for a 12V brushless fan indicated a normal power consumption in excess of 5 Watts. The introduction of a boost-mode SMPS with an efficiency of 80% (a generous figure) would yield 2 Watts out. The speed of the fan would be so slow that the airflow would quite low.

    hgmjr
     
  16. DickCappels

    Moderator

    Aug 21, 2008
    2,664
    633
    The USB specification calls for hubs to be able to provide one of two amounts of current. Self powered hubs need to be able to provide up to 100 ma per port. Bus powered hubs only need to provide 100 ma per port. In my limited experience, all ports on PCs are self powered, and therefore can provide 500 ma. The output voltage may be as low as 4.75 volts.

    To boost the voltage, you might want to try something like the push-pull forward converter shown below. This was a quick benchtop experiment to see what I could get out of few 2N2222 transistors.

    The circuit is classic and a brute force approach. Its not regulated, but the fan should be able to work well with a wide range of voltages.

    Just adjust the value of the base resistors and the turns on the transformer to change to 5V input and 12V output. With the higher voltages, this circuit is bound to be much more efficient (a lot of the losses were in saturation voltage, diode drops, and and copper losses).


    [​IMG]

    (Please note that the 50 Ohm resistor across the otuput is just a dummy load for testing.)
     
    Last edited: Sep 20, 2008
  17. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    How nice of you to drop by, Dick! :) You should do so more often.

    However, the schematic you linked to really didn't help the OP at all, as they're dealing with DC from a USB port, and your schematic requires an AC input as a trigger, along with a bench supply for the rest of the circuit.

    While it's interesting, I feel that our OP would be better off with using a wall wart to power a fan.
     
  18. DickCappels

    Moderator

    Aug 21, 2008
    2,664
    633
    I heard someone mention my name, and thought I would see what the discussion was about.

    It may not be clear from the schematic, but the transform winding on the left is a winding on the same core as the primary and the power output. When you get the windings phased correctly, the circuit oscillates.

    The input from the bench supply would be replaced by +5V from USB. Forward converters are, in general, more power efficient than the flyback converters shown in previous examples.

    BTW, the 2N2222 I used in the experiment is hardly optimised as a power switch. There are much better choices out there. I was experimenting to see what could be done with transistors that are most widely available.

    And yes, I agree - a wall wart would be the best way to go. Why put that additional strain on the notebook's power supply. I can understand the portability argument.
     
  19. iONic

    Thread Starter AAC Fanatic!

    Nov 16, 2007
    1,420
    68
    OK lets not stress our brains out too much. I've found some 12V fans about 1.5 X 1.5 inches actually run from the USB port, but not really fast. I have decided to go with an adapter for the laptop power supply which is most capable of dishing out the power needed for a tiny fan. The adapter will allow me to plug in the Laptop supply, untouched and then have two outputs, one for the Laptop and the other for the fan, which will be permanently fixed to the underside of my laptop. The thought of carting around another wall adapter and even one extra cord is just too messy. The small adapter can also be fixed to the back side of the computer.
     
  20. hydra504

    Member

    Oct 18, 2007
    16
    0
    Op amp maybe? Just a guess.
     
Loading...