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Thread: need to operate a pulsed operated LED without modulation

  1. #1
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    Default need to operate a pulsed operated LED without modulation

    Due to a limitation of a controller IC (slow modulation speeds) for this LED I need it to be constantly on. https://www.osram.com/os/ecat/OSRAM%...evice_2190826/ The datsheet can be found here. https://dammedia.osram.info/media/re...LCG%20H9RM.pdf The datasheet states that the typical forvard voltage of 2.97V is, "The forward voltage is measured during a current pulse of typically 8 ms, with an internal reproducibility of 0.05 V and an expanded uncertainty of 0.1 V (acc. to GUM with a coverage factor of k = 3)." If I want to run it with a constant power source, not modulating, should I try going lower than the 2.97V figure or does "pulsed" actually generate more heat? Never worked with specialized LEDs such as this before, figured someone here might have since they have so similar characteristics to laser diodes.

  2. #2
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    ... the "essential" parameter is not the voltage, but the current through the diode.

    It's specified from 100mA to 500mA in CW mode (ON/OFF) or up to 1A when pulsed (50:50 percents ON/OFF-ratio).

    So best is to use a constant current regulator set to 350mA (rated) to max. 500mA current and something like 6 to 9V supporting voltage - there are some preconfigured, I'm using LM317 in constant-current-configuration (as shown in the datasheet of the LM317T) ... then the current is defined by the shunt resistor with the formula: current (I) = 1.25 (V) / Res (Ohm) -- or Res (Ohm) = 1.25V/0.35A => 3,6 Ohms for a continuous constant current of 350 Millamperes ...

    Viktor

  3. #3
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    I see, so kind of like laser diodes / laser diode drivers? As you increase the current trimpot the voltage increases as a result rather than set separately? I have adjustable constant current laser and LED drivers I could use.

  4. #4
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    ... yes -- I'm running high power LED's and laserdiodes with the same drivers with set or adjustable currents, CW or pulsed, form some ten mA to more than 20 Amps

    Viktor

  5. #5
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    Gotcha. Forgot to mention the LEDs in the unit I have actually have a single power cable with 3 positive pins for each color and common GND. Since I am going to use 3 separate drivers for each color, should I connect the GND to the GND of one of the drivers only?

  6. #6
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    ... "common GND" is common - but avoid loops and extra long connections ...

    Viktor

  7. #7
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    But can you have different voltages and currents for each of the three leds but a common gnd cable? This is the first time Ive seen such a setup.

  8. #8
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    ... can you sketch your setup? - it's no problem to drive three diodes from three different drivers with connected GND's.

    I'm regularly using electronics an signals with different voltage levels of 5V, 10V, 12V or 24V by connecting the grounds and adjusting the voltage differences by resistor bridges, transistors or optocouplers (sometimes too with connected grounds to support it with the same PS).

    Viktor

  9. #9
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    It's very simple, the green diode needs 2.97V, blue 2.7V, red 2.2V (100-500mA)

    I think I get now why the stock PCB has a common ground pin for them, because they are meant to be modulated in sequential order (when one is on other two are off). So the original PCB probably readjust voltage on the fly. Makes sense right?

  10. #10
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    ... don't rely on Voltage! - it will vary with temperature, so the (much more important) current won't be constant.

    Set three LM317 (with the shunt resistors selected for the needed current) before every LED and switch/modulate the voltage to the LM's accordingly ...

    Viktor

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