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USB voltage

  1. Aug 13, 2015 #1

    Gaz

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    Hi I know little about this but I am wanting to wire up my 1mw green laser to my usb port to use it as a power supply it is 3 volts I'm wondering is there a program you can use to change the 5 volt output to 3 volts or would I need to use a resister in the wiring ?
     
  2. jcsd
  3. Aug 13, 2015 #2
    do you know ohms law?
     
  4. Aug 13, 2015 #3
    I've never heard software which can modify USB voltage, not that it would be impossible, just no demand for it really, and most likely it would have to drive a specially made hardware interface which is not present in most standard computers.
    USB devices are designed to operate at 5v (+/- .5v)because that's part of the USB specification.
    Yes you could make a very simple resistor based device to step down the voltage,
    A slightly more complicated but more efficient voltage reduction device could use a specialised transistor-like device and a few other components,
    This for example.
    https://en.wikipedia.org/wiki/LM317

    Here's another one more specifically intended for the 3vdc output you want.
    http://ie.farnell.com/stmicroelectronics/l78l33acz/ic-v-reg-3-3v/dp/1467768

    Edit: scratch that above one, it needs 8V minimum input , but devices like that are common and cheap.
    A bit of googling should be able find one best suitable for your particular project.
     
    Last edited: Aug 13, 2015
  5. Aug 13, 2015 #4

    BvU

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    Sure. Why not simply two zener diodes in series ?
     
  6. Aug 13, 2015 #5
    :eek:yes all you need is a resistor

    USB 1.x/2.0 standard pinout
    Pin Name Wire color Description
    1 VBUS Red (or orange) +5 V
    2 D− White (or gold) Data−
    3 D+ Green Data+
    4 GND Black (or blue) Ground

    Single-LED-Resistor-Circuit.jpg

    you'll need a 900 ohm resistor, a quick look at the available resistors closest to this are:
    http://web.mit.edu/2.744/www/Project/Assignments/conceptRefinement/herdd/dmass/images/resistor_code.jpg
    910 ohm, which is fine.

    edit: or 91 ohm, someone check this, seems i'm borderline retarded with math now for some reason:eek:
     
    Last edited: Aug 13, 2015
  7. Aug 13, 2015 #6

    BvU

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    Something like 2 V / 1/3 mA = 6 kΩ :rolleyes: ?

    The 1/3 mA from (1 mW / 3 V )
     
  8. Aug 13, 2015 #7
    ohms-watts_law.jpg
    V squared / watts = R
    to double check
    R / 3v = 30.333ma
    30.333ma * 3v = 91ohms
    for laser power .030333amps(remember si units) * 3v = watts = 0.090999

    now I feel stupid, I misplacing 0 or i'm not subtracting the 2 from 5 but I know from doing this many times that 30ma is the right current from experience. boy do I feel dumb:mad: something is wrong right now with my brain as it is missing placeholders so I did it on paper with the voltage drop and req current and got 66 ohms. no worrys it's only temporaryo_O but still this should be second nature to me.

    btw, I got the whole town trying to steal and none knows or can do ohms law!:biggrin:
     
  9. Aug 14, 2015 #8

    BvU

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    Wheel of fortune is way too complicated. Stick to V = I * R and P = V * I. All the others follow with simple math. :smile:
     
  10. Aug 16, 2015 #9

    Gaz

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    Thanks guys
     
  11. Aug 16, 2015 #10

    meBigGuy

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    Not sure about all that was posted above.

    USB supply is not adjustable.

    USB supply is 5V @ 100ma, but it might not work of you don't enumerate. Hubs will generally work, but some pc ports will shut down if you draw current without enumerating. Also, the max allowable draw is 100ma unless you enumerate and request 500ma and the host gives you permission.

    When you say your laser runs off 3v, are you saying a 3V battery? Or, 2 1.5V batteries? If that is the case you need a regulator to drop the voltage to 3V, but 100ma may not be enough. Or is it something like this?
    http://thelaserguy.stores.yahoo.net/er651mwreddi.html which is a 1mW red laser that requires 60ma @ 3V
    http://www.z-bolt.com/greenlasermodules.com/diodespecs/DPSS-1M.pdf is a green one that requires "<300ma" @ 3V

    Do you have a part number?
     
  12. Aug 23, 2015 #11

    Gaz

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    I had a old mouse just chopped the wire of it and wired it straight to the laser and its working.

    No idea how long it will last before it burns out but I read that lasers have a built in power regulator so might last just as long as it would with batteries

    Now I can crack on with my double slit experiments without having to change batteries every 2 mins =)
     
    Last edited: Aug 23, 2015
  13. Aug 23, 2015 #12

    Gaz

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    My usb ports say there 500ma each port either way it works =)
     
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