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Thread: need to operate a pulsed operated LED without modulation

  1. #11
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    I already have 3 LED drivers which utilize LM317s and more, just don't know what to do with the common ground pin and yet needing different voltages and currents for each LED diode.

  2. #12
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    ... are your LM317-drivers constant-current ones, or (more common) voltage regulators?

    This is the simplest schematics for three LED's with separate CC-drivers (select the three resistors for needed current):

    Click image for larger version. 

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    The supporting voltages can be the same - should only be around 3 (or more) Volts higher than needed for the LED ...

    Viktor

  3. #13
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    this is the drivers I have https://www.hellasdigital.gr/electro...-driver/?sl=en

    I was wrong, not quite LM317

  4. #14
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    ... they should do the job too, when I'm reading the specs correct:

    - High-power LED constant current drive
    - Module Properties: non-isolated constant current and voltage module
    - Input voltage: 7V-35V
    - Output current: adjustable maximum 3A

    Viktor

  5. #15
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    Okay, please bear with me as I dont understand your chematic drawing and explanation on using voltages.

    Do you say to use a separate LM317 together with each driver or is that just a label for the ICs you wrote in the schematic?

    Are the resistors there to prevent each LED from drawing more current then needed even though the current is set to constant value by the driver, because the diodes have a common ground?

    "The supporting voltages can be the same - should only be around 3 (or more) Volts higher than needed for the LED"

    Why?

  6. #16
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    ... you can use either your own drivers as current regulators OR three LM317T with a shunt/sense resistor for each, selected for the needed current.

    Look into the datasheet at the bottom right on page 5, where it's wired for constant current regulating (they are normally used as voltage regulators):

    http://pdf1.alldatasheet.com/datashe...CS/LM317T.html


    If your LED's needs 2.2V to 2.9V, then the LM317-circuit needs minimum 3 Volts more to have enough "working range" - so best support them with 6Volts or 9Volts (common PS values).

    The correct resistor for the individual current regulating is calculated with this formula:

    R (Ohm) = 1,25 / current (A) -> so for 400mA this will result to: 1,25/0,4 == 3,125 Ohms --> available are either 3 Ohm (=>417mA) or 3,3 Ohm (=>379mA) ... use resitors with 2 Watts or more ...

    Viktor

  7. #17
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    Thank you. So you're saying if I use 3 drivers with current regulation, then I don't have to worry about voltage/current issues when having a common GND between them and 3 LEDs?

  8. #18
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    ... yes.

    Look at the sketch with the three LM317, where the current regulators are 'inline' without other connections.

    With your drivers you can connect all GND's of the PS(es), drivers and LED's. The PS (+)output goes to the (+)input of the driver ... the (+)output of the driver then goes to the (+)pin of the LED ...

    Viktor

  9. #19
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    Neato. Thanks for guiding me through all this. Have a nice day.

  10. #20
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    doesnt seem to work. does the wiring look correct?


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