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Circuits with fans, should be pretty easy.

  1. Jun 26, 2006 #1
    I'm not an engineering major but I've got a fair amount of knowledge about circuits. I have a few questions about the diagrams below.

    I am connecting two fans to 2 computer USB Ports. A single USB port puts out 5v at 500 milliamps. The fans are unfortunatly 12v each.:grumpy: Because of this I have decided to use two usb ports. Giving twice the voltage and twice the amps.

    I am not sure if I should put the fans in parallel or series. I understand the resistance difference between parallel/series but i'm not sure what that's actually going to do to to the fans. (ie: heat them up, shorten their life) The powersource is very sensitive (a computer lol) and I don't want to mess it up.

    The other question is.. I want to make sure the power is smooth to these fans so I figure i'll put a capacitor in the circuit. I'm not sure where to put it or what size to use. My voltage is pretty low so it will probably need an average sized one.

    (Sorry if that post was confusing lol)
     

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    Last edited: Jun 26, 2006
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  3. Jun 26, 2006 #2

    Danger

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    I know that there must be some reason that you chose to do it this way, but might I suggest that you just buy a couple of USB fans? They don't cost much, and they're designed to run directly from the ports.
     
  4. Jun 26, 2006 #3
    ah yes, the easy way out... :wink: :wink: lol.

    I am an industrial design major and I am interested in developing computer peripherals.

    The current project I am building calls for the circuits displayed above. Standard USB fans are meant for size and portability, not what I need them for.

    I am opting for USB power because i've never done it before and it's pretty easy to do.. just.. limited, and I like a challenge.:biggrin:
     
  5. Jun 26, 2006 #4

    NoTime

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    You are not going to get 12 volts out of a USB connector.
    Unless you use a DC to DC converter.

    And connecting the + of one USB port to the - of a second port
    as shown in your diagram will likely damage your computer.

    All the "-" pins are common ground.
     
  6. Jun 26, 2006 #5

    Danger

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    You're out of my league with this stuff. Sorry... I was just going for what appeared to be the most practical approach. Next to it would be running 5VDC primary relays to switch 12VDC power from a wall-wart to the fans.
     
  7. Jun 27, 2006 #6

    berkeman

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    NoTime is correct. You will only be able to get 5V (unless you stack floating laptop USB ports or something). You can boost 5V to 12V, but the current ratios with the inverse of the voltage for a DC-DC converter (actually it's worse than that because they are not 100% efficient). So the 500mA that you say is available at 5V will net you around 200mA at 12V. Is that enough for your fans? Or maybe you should look a little harder for 5V fans.
     
  8. Jun 27, 2006 #7
    Power
    I put both + USB together and - USB together, they're not in series. I did this becuase the ports aren't going to act like batteries. If I put a 5v and a 5v in parallel I won't get 10v?

    DC-DC
    I'm not sure what a DC-DC converter is but i'll look around online. I'm not sure how many amps the fans will need.(how can I find that) The fans are just basic computer fans.

    Fans
    About the fans, I need fan and motor to rise no higher than 3/4". Standard motors that I found at radio shack and online are 3/4" to 1" not including the fan. I chose computer fans becuase they are very low profile. For example a 40mm fan has a rise of 1/2" which is perfect.
     
  9. Jun 27, 2006 #8

    Danger

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    No, you'll get 5V; putting them in series would give you 10. I don't know if you can even do that without slagging something inside the computer.
     
  10. Jun 27, 2006 #9

    chroot

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    You cannot connect two USB ports "in series" to get 10V. By doing so, you are connecting the +5V pin of one port to the ground pin of another, thus completing a circuit between +5V and ground. They are not like batteries, and share a common ground. Your USB hardware almost certainly has fail-safe power electronics that will instantly disable both ports (and, if it doesn't, you'll damage the computer).

    - Warren
     
    Last edited: Jun 27, 2006
  11. Jun 27, 2006 #10
    Okay, looks like the 12v fan idea has been squashed. I found a good fan with these specs: 3V DC & 230 Amp. I don't think this is going to work either becuase the USB puts out 500milliamp which is .5amp.


    If I take a 9v battery, how can I drop that down to 6v? And, how can I figure out how long that battery is going to last?

    The batteries I found give out 9v @ 150mAh.
     
    Last edited: Jun 27, 2006
  12. Jun 27, 2006 #11

    chroot

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    Two hundred and thirty amps? I think you must have read that incorrectly.

    And the USB port does not "put out" 500mA. The USB port permits a maximum of 500mA to be drawn from it. You don't have to use it all!

    - Warren
     
  13. Jun 27, 2006 #12
    Okay, looks like the 12v fan idea has been squashed. I found a good fan with these specs: 3V DC & 230 Amp @ .7v (This doesn't make much since becuase .7w/3v = 0.23333A NOT 230A as stated by the website or i'm doing something wrong)


    If I take a 9v battery, how can I drop that down to 6v? And, how can I figure out how long that battery is going to last?

    The batteries I found give out 9v @ 150mAh.
    ==================================
    EDIT:

    Watt/Volt = Amps
    BattAmps/Amps = battery life..

    So.. first convert all the units..

    Battery
    9v @ .15A

    Motor
    3V @ .230A @ .7w

    -------
    And plugin the numbers

    .7w/3v = .233333 Amp (not 230Amp on the website)
    .15A/.23333A =.64Hours

    So if I hook up this 9v battery to the motor, it will die in .64 hours? That's pretty crappy. I found (http://www.gizmology.net/batteries.htm) to calculate battery life and that says 0.7hr if I plugged numbers in correctly.
     
    Last edited: Jun 27, 2006
  14. Jun 27, 2006 #13

    chroot

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    The 9V battery is not going to be able to produce enough current to satisfy the motor.

    I thought you were going to use your PC's USB ports? I'm very confused as to what the application is here.

    If you can so easily switch from using the computer's USB ports to using a 9V battery, why can't you just use a standard wall transformer to produce 12V for the fans you originally selected?

    - Warren
     
  15. Jun 27, 2006 #14
    I came to the conclusion that usb ports do not put out enough power for a motor. period.

    The application of this power source is a computer peripheral that is mobile and does not require power from a wall socket. It can be battery, usb, or anything that is small and lightweight. Basically something easily portable in an average/small laptop bag.

    This is the motor I am getting specifications from:http://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=1524
     
    Last edited: Jun 27, 2006
  16. Jun 27, 2006 #15

    chroot

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    The computer's USB ports provide more than enough power to run the 3V 230mA fan you just listed.

    A laptop's battery is usually around 1000 mA-h @ 12V (or sometimes 16V), for a total energy capacity of 43kJ. If your fan consumed 0.7W, or 0.7 J/s, the laptop's battery would run it for about 17 hours. (Of course, the laptop itself consumes much more power.)

    - Warren
     
  17. Jun 27, 2006 #16
    Alright then, the device i'm working on needs two of these motors. Can you suggest a wiring scheme? Two ports are fine if another is needed.
     
  18. Jun 27, 2006 #17

    chroot

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    You could probably put two of the fans in series and run them off 5V. Each fan will see only 2.5V rather than 3V, but I doubt this would be a serious problem.

    - Warren
     
  19. Jun 27, 2006 #18
    Sounds good, I'll order them and see how things go. If it doesn't work for some reason $10, boohoo.:wink:

    Thanks for the help!
     
  20. Jun 27, 2006 #19

    berkeman

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    Kirro, to be a good industrial designer in the computer industry (or elsewhere for that matter), you sometimes need to step back and ask if there are other ways to accomplish what you are trying to do. For example, having a fan in a product is usually a disadvantage (power, noise, reliability problems). You only use a fan if there are no other cooling possibilities.

    So, is it a requirement of your assignment that you include a fan, or could you take care of the cooling chores in another way. With a good overall design, you can remove heat from the internals of a product and just sink it to the ambient air. You haven't really told us what you are trying to do, so I don't know what-all might be possible. Just think a little outside the box every now and then, and you'll be surprised at what a little creativity can result in.
     
  21. Jun 27, 2006 #20
    I wouldn't have expected such a design concious person to respond in an electrical thread. :biggrin: :biggrin:

    I am honestly not sure if the fans are needed however I do know the following about the project:

    Right now I have a product that produces 135-155F of temperature. It holds this temperature for about 5-7 hours continuously. If it reaches and holds this temperature the life will be significantly shortened. Right now it gets that hot and stays that hot which is REALLY bad.

    A fan may not be required for the application however I would like to have one just in case the heatsink itself gets too hot.
     
    Last edited: Jun 27, 2006
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