USB Voltage Help.

Discussion in 'The Projects Forum' started by KitsuneFoxie, Aug 6, 2014.

  1. KitsuneFoxie

    Thread Starter New Member

    Aug 6, 2014
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    Hi everyone, First time poster.

    I am currently building a Laptop fan cooler and i wanted to run it from USB on my laptop with adjustable speed/voltage control.
    It is housed in MDF.

    I have not managed to find anything really with a diagram showing me how to make it. I'm a novice at electronics, but i know how to solder and such, just not the resistors/voltage changers/etc i would need to achieve it.

    Essentially i want to change 5vv to 12 and then from that to the adjuster, then to the fans.. thing is i don't know how/what i'd need to build the circuit and need a diagram of how to put it together.

    I've drawn up a picture of what i want to do so you guys can grasp the idea i have and possibly be able to provide help to my project :>
     
  2. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    You can make a simple adjustable power supply with an LM317..
     
  3. AlphaDesign888

    New Member

    Jul 27, 2014
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    Use PWM to control speed.
     
  4. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    from the few times I tried it with typical modern computer fans they don't like PWM.. It messed up the internal sensors.. and they just won't start/stall/hum..
    But others work just fine with it.
    I've found with 12V fans that you can usually go down to about 5V and get all the needed speed control without having to resort to PWM.
     
  5. KitsuneFoxie

    Thread Starter New Member

    Aug 6, 2014
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    Yeah, but how would i go about wiring it up? If you would'nt mind drawing it on the diagram i posted i'd be greatful. It's just helps me know where to position/wire up the LM317. And i use 3 pin or molex Fans.

    Thanks!
     
  6. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Oops.. missed the USB part for some reason.. That will require a step up DC/DC converter to go from 5V to 12V.. I'd think you could find an adjustable output one and then there is no need for the LM317.. For some reason I thought you might have a 12V power supply already when I wrote that.

    And you need to post the specs on the fan... we know 12V but how much current.. USB is really limited in how much current it can deliver anyways.
     
  7. MrChips

    Moderator

    Oct 2, 2009
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    Try to use 5V fans. That will make life a lot simpler.
     
  8. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    That would be WaaaaaaaaaaaY too easy.
     
  9. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    I have a laptop cooler. Its two fans have no speed control. You'll probably find you would run yours at full speed all the time :).
     
  10. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    Same, got mine from Big Lots for 6 USD and tax.
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    You really don't want to run the fans from the laptop computer's USB port; as there's only about 300mA available to begin with, which will be severely taxed when stepping up to 12v to run fans. Use a "wall wart" 12v adapter; they're available everywhere - you and/or your neighbors probably have several of them sitting around doing nothing.
     
  12. KitsuneFoxie

    Thread Starter New Member

    Aug 6, 2014
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    Well, i'm not always at the wall sockets, so i suppose a pair of 5v fas would work. But i want to control the speed, which is why theres a potentionmeter in the circuit.

    So what do you reckon i should use then to convert it? I don't know how to build a 5v - 12v converter, let alone where to put them in that circit. And i guess the fans would be 0.4A?
     
  13. BobTPH

    Active Member

    Jun 5, 2013
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    If the fans take 0.4A x 2 = 0.8A at 12V they will take 1.92A at 5V with no conversion loss, so many 2.2A. This is a non-starter. You USB port cannot deliver that power.

    Maybe you should consider getting a newer laptop, they run very cool and can go 9 or more hours on a charge.

    Bob
     
  14. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    Just try running 12 volt fans from USB (5 volts), see if they run, see if they run well enough.

    I have a few 12 volt pc fans, they are generally 0.1-0.15 A, so they should work ok with the amount of current that USB is able to provide.
     
    Last edited: Aug 6, 2014
  15. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    They are guessing that fans are 0.4 A. I would not trust that guess.
     
  16. KitsuneFoxie

    Thread Starter New Member

    Aug 6, 2014
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    The fans are probably much less, they are just basic 12v case fans with no leds. 0.4a was just a guess. I don't have any fans to hand at the moment but i know i've ran them from a usb before at a fair decent speed.

    They are standard 80mm fans. 150 - 500mA somewhere between these/
     
  17. BobTPH

    Active Member

    Jun 5, 2013
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    150mA * 2 = 300mA * 12 / 5 = 720mA, still out of spec for a USB port, with is a max of 500mA.

    Bob
     
  18. BobTPH

    Active Member

    Jun 5, 2013
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    150mA * 2 = 300mA * 12 / 5 = 720mA, still out of spec for a USB port, with is a max of 500mA.

    And 500mA is only allowed if the device is enumerated and request it. Without that, it is limited to 100mA.

    You cannot make this work without an additional power supply or fans that draw much less current.

    Bob
     
  19. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    Buried in the discussion of how you are going to power the fans, it seems that your plan to modify the fans speed with a potentiometer has been lost.

    This is not a good way to control the fans speed. The power rating of the pot will be high and finding a matching component at a reasonable cost will be difficult. Search the forum for PWM speed control circuits.
     
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