LED Driver Problem

Discussion in 'The Projects Forum' started by beon234, Jun 23, 2014.

  1. beon234

    Thread Starter New Member

    Jun 22, 2014
    2
    0
    Hey all,

    I have purchased the LDD 1000H LED driver. Here's the datasheet:
    http://www.meanwell.com/search/LDD-H/LDD-H-spec.pdf

    I am using a 48V, 950mA SMPS to supply the driver, and I'm using a function generator to generate a square wave as an input to the PWM pin. Its currently loaded with two 27ohm (power resistors. When reading the output using an oscilloscope, I find that the output voltage is not periodically driven to 0V with the PWM signal, and it stays constant at 48V. Am I doing something wrong here?

    Any help is appreciated :D
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,093
    3,030
    The OUTPUT isn't PWM, it's designed to be constant current. 48V across 54Ω is 890mA, which is within the likely precision of your resistors.

    You can turn off the driver using the remote OFF feature.
     
    beon234 likes this.
  3. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,769
    969
    It will work properly when you have an LED on there and not a fixed resistor load.
    But yes its constant current not PWM out.
    Also
    Adding a 10K pulldown resistor will allow it to dim all the way to 0
     
    beon234 likes this.
  4. beon234

    Thread Starter New Member

    Jun 22, 2014
    2
    0
    It ended up working after using the 10K pulldown. Thanks again for all the help guys :)
     
  5. BobTPH

    Active Member

    Jun 5, 2013
    782
    114
    I am not sure what you guys are saying. Do you mean the PWM input does not result in a PWM output, but actually adjusts the constant current according to the duty cycle of the PWM input?

    If so, I had no idea that they worked that way.

    Bob
     
  6. wayneh

    Expert

    Sep 9, 2010
    12,093
    3,030
    That was my interpretation - could be wrong.
     
  7. BobTPH

    Active Member

    Jun 5, 2013
    782
    114
    Ah, yes. There is a chart of Iout vs PWM duty cycle, so I think you are right. That is interesting. On the one hand, the LED will be more efficient using lower current than using PWM, but on the other hand, if linearity of light output was important, it would not be linear.

    Bob
     
  8. eetech00

    Active Member

    Jun 8, 2013
    648
    110
    Hi

    I don't understand.
    Where, in the datasheet, does it indicate it doesn't provide a PWM output?

    eT
     
  9. eetech00

    Active Member

    Jun 8, 2013
    648
    110
    Oh...see.

    It does provide PWM output but it depends on the level of dimming.

    eT
     
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