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LED Characteristic

  1. Oct 29, 2016 #1
    1. The problem statement, all variables and given/known data
    I have a LED
    Given: u-i-characteristic curve -> thus the wavelengh

    It's Part of a curcuit.
    Also (After the LED) There is a resistance of R (known)
    An a capacitor, (known)




    2. Relevant equations

    If i have a Voltage of U, how can i find the current Running through the LED and the voltage Inside?
    3. The attempt at a solution
    I tried Kirchhoff law for meshes but it wont work... Anybody have another idea?
     
  2. jcsd
  3. Oct 29, 2016 #2

    Merlin3189

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    Gold Member

    If the LED is in series with the resistance, then the pd across the LED + the pd across the R add up to the supply voltage U.
    If you have the u-I graph for the LED, you can plot the u-I graph for the R (which is just a straight line, U=IR) backwards starting from u=U. Where the two graphs cross, the current is the same in both elements (series circuit) and the voltages across them add up to U. This is the operating point. Look up "load line" for more info.
    Unless U or I is varying, which it normally isn't for R-LED circuit, then the capacitor is irrelevant. It is probably there to smooth ripple on U.
     
  4. Oct 29, 2016 #3
    But i don't have a function, only the Graph. Should i try to find one for the LED?
     
  5. Oct 29, 2016 #4

    gneill

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    Staff: Mentor

    Draw the resistor load line on a copy of the LED's graph.

    Can you post an image of what you've been given?
     
  6. Nov 1, 2016 #5
    Well. Now i Know, and im sry, i couldnt copy what i'm given.

    But i have another question, how can i determine the series resistance of My LED? Only with the Characteristic curve ?
     
  7. Nov 1, 2016 #6

    NascentOxygen

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    Staff: Mentor

    Yes, use the LED’s characteristic curve. Its equivalent resistance changes with the current.

    Are you sure that there is a need for you to know the LED‘s resistance?

    Sometimes when "determining the resistance" of a non-linear device you first need to establish should you be determining its large-signal equivalent resistance OR should you be determining its small-signal equivalent resistance.
     
  8. Nov 1, 2016 #7
    Oh i mean the Series resistance i need for My LED. i don't Know the maximum current
     
  9. Nov 1, 2016 #8

    NascentOxygen

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    Staff: Mentor

    You can't safely power a device until you know its current capability! Surely its graph indicates its working current and voltage values?

    To determine the series R use the fact that the current in that R is the same as the current through the LED, and you can make use of Ohm’s Law to calculate the current in R.
     
  10. Nov 1, 2016 #9
    Okay Thats nice but is there a Way to Know the Working current ?
     
  11. Nov 1, 2016 #10

    NascentOxygen

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    Staff: Mentor

    If you are designing a new circuit then you consult the LED manufacturer's spec sheet; if you are repairing a circuit board, then the local supply voltage and the LED’s color and its series resistor allow you to estimate the current it has been using.
     
  12. Nov 1, 2016 #11
    Okay i have a voltage supply of 5v and a Blue LED. How can i make estimations?
     
  13. Nov 1, 2016 #12

    gneill

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    Staff: Mentor

    Is this a construction project and you have actual parts on hand, or a theoretical exercise?
     
  14. Nov 1, 2016 #13

    NascentOxygen

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    Staff: Mentor

    Earlier I thought you said that you do have its characteristic graph?

    Without specs on the device, the next best approach is to google for the specs on a few similar LEDs, in particular you need to find out a typical value for the voltage across it when conducting, and a ballpark figure for its working current. If you don't need it to glow especially brightly, then err on the side of caution when setting current.
     
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