Help with a CMR issue

Discussion in 'Homework Help' started by eladta, Apr 21, 2013.

  1. eladta

    Thread Starter Member

    Apr 20, 2013
    41
    0
    Hi all,
    As u can see in the attached circuit there is a resistor in the cathode side and one in the anode side.
    The question is why not to put one equivalent resistor equals to the series sum of them (when the led is on lets assume it's a short) ?
    I know the answer has something to do with CMR but i didn't get it.
    Thanks.
     
  2. WBahn

    Moderator

    Mar 31, 2012
    17,725
    4,788
    It's impossible to tell without seeing more of hte circuit that this is a part of.
     
  3. eladta

    Thread Starter Member

    Apr 20, 2013
    41
    0
    this is 99% of the circuit. there is just a connector connected to the resistors from the left
     
  4. WBahn

    Moderator

    Mar 31, 2012
    17,725
    4,788
    The the circuit doesn't do anything because there is not current flowing through the diode. This thing is intended to be part of a larger circuit.

    But as it stands, I can see no reason for putting the resistors on either side of the diode versus both on one side. I certainly don't see any CMR benefit.
     
  5. eladta

    Thread Starter Member

    Apr 20, 2013
    41
    0
    maybe this can help u help me :)
    refer to pages 12, 13
     
  6. WBahn

    Moderator

    Mar 31, 2012
    17,725
    4,788
    I don't have the bandwidth to download large files. Perhaps later when I am in the city I can take a look at it. Or, you could extract the relevant pages and post just those.
     
  7. eladta

    Thread Starter Member

    Apr 20, 2013
    41
    0
    sorry but i can't do that since it's a PDF protected document.
    waiting for your reply when you get back to the city..
     
  8. WBahn

    Moderator

    Mar 31, 2012
    17,725
    4,788
    It's not "back" to the city. I'm at home right now! :D
     
  9. eladta

    Thread Starter Member

    Apr 20, 2013
    41
    0
    :) my bad..
     
  10. t06afre

    AAC Fanatic!

    May 11, 2009
    5,939
    1,222
    Hope I got it correct
     
  11. eladta

    Thread Starter Member

    Apr 20, 2013
    41
    0
    nice how did u do that ?
     
  12. eladta

    Thread Starter Member

    Apr 20, 2013
    41
    0
    nice. how did u do that ?
     
  13. t06afre

    AAC Fanatic!

    May 11, 2009
    5,939
    1,222
    According to my tool it was not protected. So I just extracted the pages
     
  14. WBahn

    Moderator

    Mar 31, 2012
    17,725
    4,788
    Sure looks like it. And thanks for including page 14, which is the key.

    They are looking at a more complicated model that included the effect of capacitive coupling between the LED's anode and cathode to the output side ground. Thus, you no longer have a strictly series circuit. The goal is to pick resistor values that will equalize these impedances. Read the section very carefully and try to walk through the analysis and the math alongside them as you go.
     
  15. eladta

    Thread Starter Member

    Apr 20, 2013
    41
    0
    can you elaborate on that ?
     
  16. WBahn

    Moderator

    Mar 31, 2012
    17,725
    4,788
    Have you carefully read what the datasheet has to say on those pages? If not, please do. Then ask specific questions about the points that are tripping you up and we will try to provide specific answers, if we can.
     
Loading...