?5v regulator instead of lm317

Discussion in 'The Projects Forum' started by Gadersd, Jul 10, 2013.

  1. Gadersd

    Thread Starter Member

    Dec 8, 2012
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    I am going to make a laser driver. Most of the schematics I see use a lm317 regulator. Would it be fine if I used a 5v regulator that outputs 1.5 amps instead?
     
  2. crutschow

    Expert

    Mar 14, 2008
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    Depends upon whether they are using the LM317 as a voltage regulator or a current regulator. Lasers typically require a current regulator.
     
  3. Gadersd

    Thread Starter Member

    Dec 8, 2012
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    It is used as a current regulator. Couldn't I just use resistors to keep the current at the right value?
     
  4. Shagas

    Active Member

    May 13, 2013
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    Are you planning to drive a laser diode or a laser module??
     
  5. Markd77

    Senior Member

    Sep 7, 2009
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    You can use a 5V regulator as a current source:
    [​IMG]
     
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  6. Gadersd

    Thread Starter Member

    Dec 8, 2012
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    Could you explain to me how that works? How would connecting the ground pin to the vout pin regulate the current?
     
  7. MrChips

    Moderator

    Oct 2, 2009
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    The voltage regulator tries to maintain a constant voltage across R1.
    Using Ohm's Law you can calculate the current through R1. This is your constant current.
     
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  8. Gadersd

    Thread Starter Member

    Dec 8, 2012
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    Why is the ground pin not connected to ground?
     
  9. MrChips

    Moderator

    Oct 2, 2009
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    The GND pin is actually a reference pin. The constant voltage regulator tries to maintain a constant voltage between the output pin and this reference pin.

    You can offset this reference pin to a voltage different from 0V.
     
  10. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Shhh. If you don't let the chip know it isn't connected to a signal named ground it will keep on working anyway.
     
  11. Gadersd

    Thread Starter Member

    Dec 8, 2012
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    So if the reference pin is 2v, then the output pin will output 7v?
     
    Last edited: Jul 10, 2013
  12. wayneh

    Expert

    Sep 9, 2010
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    Yes, but notice that you need a supply voltage high enough to reach 7 at the output plus the drop across the regulator.
     
  13. Gadersd

    Thread Starter Member

    Dec 8, 2012
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    How far can I push a laser diode if its max current is 60ma? Could I go to 100ma?
     
  14. MrChips

    Moderator

    Oct 2, 2009
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    Push it harder than the specified max and you risk shortening its life.
     
  15. Gadersd

    Thread Starter Member

    Dec 8, 2012
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    It looks like I have to use a lm317. I just read somewhere that a laser diode's voltage drop will increase as it gets hotter, so I guess I am going to have to use a lm317 to keep the current constant.
     
  16. crutschow

    Expert

    Mar 14, 2008
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    The disadvantage of using the 5V regulator as a current source is that it drops 5V across the sense resistor R1 whereas the LM317 only drops 1.2V. The resistor must thus dissipate 5/1.2 = 4.16 times more power for a given current output.
     
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