LM317 need help

Discussion in 'General Electronics Chat' started by raffter, Mar 14, 2009.

  1. raffter

    Thread Starter Active Member

    Feb 28, 2008
    113
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    Hi all..

    I wish to power up my green dpss laser pointer (<50mW) from an external power source...

    Input is 12VDC (regulated)
    Output: im expecting 3V @ 240mA

    I looked up the datasheet for LM317T.. wired it UP as a current source (5R = 240mA).. I want the output voltage to be stepped down to 3V.. is my configuration below correct? if so, what possible value of R2 should I use?

    [​IMG]

    -Ralph
     
  2. hgmjr

    Moderator

    Jan 28, 2005
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    You should take a look at using the LM317 in the constant current mode as shown in this link.

    hgmjr
     
    Last edited: Mar 14, 2009
  3. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Don't apply a constant current to your laser because you will burn it. Rewire the LM317 as a voltage regulator and adjust the voltage to 3V, the laser will draw as much it needs to operate.

    Note that the LM317 needs to supply at least 10mA for proper regulation of the output voltage. Thus connect a 270 ohm resistor between the output and ground to ensure proper regulation.
     
  4. raffter

    Thread Starter Active Member

    Feb 28, 2008
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    ohh is it? I thought laser diodes need constant current(or so I have read from laser forums)

    I have measured 220mA current drain from a AAAbattery... so thats why I resorted to current regulated ... My initial plan was 12V supply -> 7805 ->LM317(current source) -> laser pointer

    I wanted a voltage-controlled current source REGULATOR (3V@240mA)
     
  5. leftyretro

    Active Member

    Nov 25, 2008
    394
    2
    Most laser diode modules have an internal current regulator and you just are required to supply a constant voltage.

    A simple plain laser diode with no support circuity does require a constant current rather then a constant voltage. So it really depends on what you have. Do you have a spec sheet for it?

    Lefty
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    For a diode laser without a current regulation supply, you are dead wrong - and so would be the laser.

    Many lasers do have internal current regulation, but this one sounds rather high-powered (on the order of 500-700mW); it is likely that it doesn't have an intrinsic (built-in) regulator.

    Laser diodes are extremely sensitive to overcurrents; they will die within nanoseconds. Power supplies for such laser diodes are engineered for very low response times and high linearity over temperature variations to reduce the possibilities of such disasters. That's why they're so expensive.

    An LM317 with a resistor just isn't accurate enough for such a supply over temperature.

    But you're going to try it anyway, right? :rolleyes:

    With an LM317 in current regulation mode, you specify current only. You don't get to specify both current and voltage.

    The regulator will attempt to maintain Vref at a nominal value; somewhere between 1.2v and 1.3v inclusive. At room temperature, it's quite stable. At temperature extremes, it's not so good.

    Vref is the difference of potential between the Vout and the ADJ terminals. When Iout (output current) is between 10mA and 1.5A, the regulator will try to keep Vref constant over the entire temperature range.

    Note that Vref can vary from 1.2v to 1.3v right from the factory, and still be acceptable. Such variations are acceptable for "normal" electronics, but not for lasers.

    You can wire an LM317 regulator for constant current mode by connecting a resistor (R1) from Vout to the ADJ pin, and taking current from the ADJ pin.

    Iout = Vref/R1.
    Conversely:
    R1 = Vref/Iout

    Make certain that the resistor is an accurate one of high wattage. It's much better to err on the side of caution here.
     
  7. mik3

    Senior Member

    Feb 4, 2008
    4,846
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    Another device came in mind, that is why I said to the OP to use it as a voltage regulator.
     
  8. hgmjr

    Moderator

    Jan 28, 2005
    9,030
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    Code ( (Unknown Language)):
    1.  
    2. Best circuit I've found:
    3.  
    4.  
    5.              In +-------+ Out   18 ohm*
    6.   (+) o-----+---| LM317 |-------/\/\/\----+-----+------o LD anode
    7.             |   +-------+                 |     |    
    8.            _|_      | Adjust              |    _|_       __|__
    9.      22 uF ---      +---------------------+    --- 1 uF  _\_/_
    10.             |                                   |          |
    11.             |                                   |          
    12.   (-) o-----+-----------------------------------+------o LD cathode
    13.  
    14.  
    15. * Note: Resistor value depends on your specific laser diode current
    16.   requirements.  Discussion below assumes a laser diode with a 72 to
    17.   100 mA drive range --- sam.
    18.  
    19.  
    Here is a constant current circuit that I found on the internet.

    hgmjr
     
  9. raffter

    Thread Starter Active Member

    Feb 28, 2008
    113
    0
    Hi,

    Im well aware of the laser pointer having an internal regulator, but did *some* more research on this, I think (this) has NO feedback, just plain IR diode(maybe >200mW)...its a DPSS laser... IR diode ->some crystals->GREEN

    so I will make this clear IM NOT gonna mess with the internal regulator, just tapping in on the battery supply line ONLY..

    I have done a bit of research and found out that green dpss lasers (>5mW) require 3-4.5V and currents in the order of 200-300mA...

    my solution : use LM317T as CC source
    my PROBLEM: is it OK to "connect" it to a 12V REGULATED supply?

    say I have SET the LM317 to output 220mA, HOW would I SET the OUTPUT voltage to 3V?? Im doing this just to BE SAFE...

    I hope im clear here :)

    DPSS lasers are NOT cheap!
     
  10. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Why don't you source power from a lower voltage supply?

    Or are you trying to supply it from an automotive battery?
     
  11. raffter

    Thread Starter Active Member

    Feb 28, 2008
    113
    0
    well, main supply is 12Vdc because (this) is part of a laser show circuit... I need 12v to power up the motors,galvos,etc..for the controller(PIC), its another 7805 reg from 12V..


    no

    I guess the CC source is for current only.. maybe I DO need a pre-regulator prior to the LM317.. so 7805(to limit voltage to constant current source) would do it?
     
  12. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    I don't see why using a 317 circuit set for 3V to replace batteries would work, it's pretty straightforward.

    Just curious, how expensive is this laser?
     
  13. raffter

    Thread Starter Active Member

    Feb 28, 2008
    113
    0
    I want the LM317 set as current source...

    this laser pointer cost $31.25 <-- I know this is NOT that much, but im cheap!! ;D

    and I dont want to ruin my 1st green dpss laser...been wanting these for almost 4 years since it was comercially out
     
  14. Wendy

    Moderator

    Mar 24, 2008
    20,766
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    Unless you're replacing the circuit along with the batteries, you'd be better leaving it (the laser circuit) in place and using the LM317 as a voltage source. Less stress on the circuit, less work for you, and less chance of an opps moment. Just my opinion.
     
  15. raffter

    Thread Starter Active Member

    Feb 28, 2008
    113
    0
    Hi bill,

    I "peeked" inside my laser pointer and I see a trimpot in there...so there is INDEED a regulator there... some sort of Roithner EU38 laser diode driver(maybe).. but I think most generic green dpss lasers use this... mine was made in china.. and I have read from many sources IT(circuit) regulates current drive...

    with this setup, then I should just discard the LM317(set as CC source) and use it as a 3V VOLTAGE regulator instead?? since the internal laser driver takes "care" of regulating current?

    man, Im really confused..
     
  16. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    I was thinking of using the LM317 as a 3V regulator. The chip is a variable voltage regulator, after all. It just also happens to make a good current regulator too.

    [​IMG]

    R2 should be around 168Ω, if it were 180Ω the output would be 3.125V, and if it were 150Ω the output would be 2.813V. A variable resistor might be called for, or a 150Ω and a 18Ω in series.

    The other thing to work out in advance is wattage. If the circuit draws 20ma (0.02A) and the input voltage is 12V then the LM317 drops 9V. Since power is voltage times current the chip will disapate 0.18W, which means it will get warm but not hot.

    It's not shown in the diagram, but you definately need some 0.1µF capacitors on the input and output, to prevent oscillations.
     
    Last edited: Mar 15, 2009
  17. Von

    Active Member

    Oct 29, 2008
    65
    0
    There is no need for confusion... as others have said if you are just going to "eliminate the batteries" then the LM317 should be used as a "battery eliminator" i.e. constant voltage.

    If you are designing your own semiconductor laser from a discrete diode then you will need more design work.
     
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