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Electronics question regarding power supplies

  1. Aug 24, 2013 #1
    Running a 70mW laser off a 4V 200mA power adapter?

    Hello All,

    I have a question regarding what voltage and current I need to supply for a load. I got a laser that's rated at 3-4V and 70mW and I have a power adapter that's providing 4V and 200mA.

    My question is: How do I make sure I don't burn out the laser? For example, if my power supply were 4V and 800mA, I would have to place a resistor in series, wouldn't I?

    To find out if I need a resistor with the 200mA adapter, do I need to use P=IV for the laser, then sub it in into V=IR for power adapter and find what resistance I need to balance out the equation?

    Thank you.


    Regards,
    Trafiq
     
    Last edited: Aug 24, 2013
  2. jcsd
  3. Aug 24, 2013 #2

    davenn

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    lasers are a little different to standard LEDs
    they require a current driver ( so do LED's but for them it can be as easy as a resistor to limit current)
    doing a google search for laser driver circuit should supply a few answers :smile:

    cheers
    Dave
     
  4. Aug 24, 2013 #3
    I think the driver's already there (it's a laser module) operated by 2x1.5V AAA batteries in series, but I just wasn't sure what were to happen if I were to hook it up to a DC power supply. That's why I'm asking.

    The batteries apparently provide 1500mAh (Energizer lithium) which comes up to over 0.41mA. And the diode has no trouble with running off the batteries, while my power adapter is max 200mA.

    Thoughts?
     
  5. Aug 24, 2013 #4

    davenn

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    as long as you are really sure it has an assoc driver
    then it will only draw what current it needs

    Dave
     
  6. Aug 24, 2013 #5
    Great! I just connected it and it works fine. Gonna have a nice 70mW green scattered light source in my room now!
     
  7. Aug 24, 2013 #6

    davenn

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    awesome ... have fun but be extremely careful 70mW is a quite powerful laser .... 65mW above that which is mormally available for public use.
    you could easily totally zap your eyesight

    Dave
     
  8. Aug 24, 2013 #7
    Thanks for the heads-up Dave. I am aware it is a powerful laser and as such, I am quite careful where I point it.

    One issue that I noticed is that the module seems to make the diode drop in intensity after about a minute or so. I think it has to do with heat dissipation and its inability to do so efficiently (the module's currently covered in plastic), so I'm wondering what would be the fix for this if I leave the module how it is.

    Could I possibly lower the current to the module to reduce the power it has to dissipate?
     
  9. Aug 24, 2013 #8

    jim hardy

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    In my day higher powered lasers usually had a third wire for temperature sensing. They expect a power supply that adjusts current to keep the laser within some temperature limit and that third wire provides temperature measurement of the laser element..

    Modern ones might have that built in, I don't know.

    Since its performance changes after a 'decent interval', I would investigate further. Maybe it has a part number someplace.

    Does it get warm after that minute? Try the two finger test - we are more sensitive to temperature difference between two things than to one by itself.

    And seconding Dave's advice, that thing should be considered eye-dangerous for hundreds of yards. Don't ever point it in direction of people.
     
  10. Aug 24, 2013 #9
    Yes, it does get a bit warm, jim hardy. A rough, subjective, personal approximation puts the temperature at about 30+-5 deg Celsius after 2-3 minutes of turning it on. And that's through about 1mm of plastic. I can confirm that there is no third wire. There are the two positive-negative electrode wires and a spring - also negative electrode - at the end of the circuit board in case I were to decide to make it battery-operated, which is not my intention. I just wanna point it at the ceiling so that the reflection off the matte surface can light up my room. Goes well with a black light!

    Perhaps I should look into making some sort of heat sink for it? The diode is already housed in a copper tube to fulfill that purpose, but apparently it's not enough.
     
  11. Aug 24, 2013 #10

    davenn

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    yes definately... it should be heatsunk :smile:
    and heed what Jim said about temperature sensing -- current limiting
    if you dont have a datasheet for the diode, I would suggest doing a google search


    Dave
     
    Last edited: Aug 24, 2013
  12. Aug 24, 2013 #11

    jim hardy

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    Hmmm

    I suppose you verified with your ohm-meter that there is zero ohms between spring and 'negative' ?

    If you read one diode drop.. well,

    laserschematics.png

    here's the link
    Laser Diode Driver Basics
     
  13. Aug 24, 2013 #12

    davenn

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    nice one Jim, Thanks ... have saved that page

    Dave
     
  14. Aug 24, 2013 #13

    jim hardy

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    Thanks !

    here's a fun little kit ,
    It wobbles mirrors to deflect a laser pointer beam onto a wall or ceiling.

    Thought OP might find it interesting

    LLS1 - Laser Light Show - Ramsey Electronics

    [Broken]

    If I recall it'll demonstrate lissajous patterns nicely.
     
    Last edited by a moderator: May 6, 2017
  15. Aug 25, 2013 #14
    I don't understand what you're trying to say, Jim. Please explain

    Edit: Forgot to mention that I did check the resistance between spring and negative and it wasn't 0 ohms, it was more like 4 or 5 ohms.
     
  16. Aug 25, 2013 #15

    jim hardy

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    I'm suggesting that there's something in that package besides just a laser diode. I'd have guessed the spring is a power connection and the third wire is for monitoring temperature or power output..

    That link describes typical driving schemes .

    Since you have a meter, measure how much current it's drawing.
    Not knowing anything about the device I'd limit current to ~100 ma by adding resistors.

    If you're interested in lasers, here is a hobbyist page about tinkering with them.
    This is lifted from just one of his many pages.
    http://repairfaq.ece.drexel.edu/sam/laserdio.htm#diodlm4

    Or you could just go with what you have and see how long it lasts.
     
  17. Aug 25, 2013 #16
    The diode's got a circuit board attached to it and I can see an op amp-like component, as well as a few resistors or capacitors, so it does have a driver. I guess I'll try to limit the current to 100mA and see how I go (the laser still drops in intensity after a few minutes if I have it connected to two AAA lithium batteries, so I'm pretty sure it's the driver that does this. Which sucks :< )
     
  18. Aug 26, 2013 #17

    jim hardy

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    With a magnifying glass you should be able to see the part numbers and figure out what the board was designed to do and whether it is still doing it.
    One starts by looking up the part numbers and getting the datasheets.
    Then try to trace out the circuit from studying the board and draw it neatly on a clean sheet of paper.. Usually it'll be close to one of the suggestions in manufacturer's "Application Hints" section.

    When you search for datasheets, always choose the offerings from the manufacturer's website.
    There's quite a few really annoying 'middleman' sites that barge in and promise you a datasheet but you have to put up with a lot of advertisements to find even a second rate copy. You'll figure that out quickly.

    You just might become the neighborhood "Laser Driver Expert" !

    good luck and have fun !
     
  19. Aug 27, 2013 #18
    Thanks for the info, Jim. I'll have to keep it in mind for next time however, because it looks like the diode's borked. I took the focusing lens off to see what's going on. When I first turn the laser on, everything is coherent and there is one large blurry green circle under the laser. But about 30sec-1min in, as it warms up, it looks like the circle gets separated in a number of dimmer circles with the same diameter as the center one. It looks like the whole beam becomes out of focus as the cicles are not centered anymore which means the lens can't focus all the circles that are not incident on it, hence the drop in intensity.

    Here are some pictures to show you what I mean.

    The module:
    layz_hurr3.jpg

    Before:
    layz_hurr1.jpg

    30s-1min after laser on
    layz_hurr2.jpg


    Thoughts?
     
  20. Aug 27, 2013 #19

    jim hardy

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    Are you saying it does okay until it gets warm?

    That sounds like it needs its controller board, or something similar, to limit input power.

    Picture of the module doesn't show (windows, ie10)
     
  21. Aug 27, 2013 #20
    Yes. It seems that it doesn't matter if I run it off batteries or adapter, it still does it after a while.

    I tried with an 100 ohm resistor in series, but then it wouldn't light up. Would you say that's too much? I stuck an ammeter in series and it was telling me about 125mA was going through, but then the laser wouldn't light up either.

    Code (Text):
    [PLAIN]http://s7.postimg.org/oc2wkg8dm/layz_hurr3.jpg[/PLAIN] [Broken]
    http://s23.postimg.org/m31nj8ah7/layz_hurr1.jpg
    http://s23.postimg.org/hg4s8y1vv/layz_hurr2.jpg
     
    Last edited by a moderator: May 6, 2017
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