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    L.E.D forward current measurment

    Hello sir

    If incase we don't have any idea about the forward current ratings for an L.E.D while purchasing or getting it from somebody then how to calculate the maximum required forward current rating ???

    •   Alt3rd June 2013, 21:44

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    Re: L.E.D forward current measurment

    I dont know my friend, I see you stuck with these LEDs. :smile:

    Sometimes is easier to buy new from catalog with known manufacturer, model ID and good datasheet then to experiment and destroy LEDs on test.

    LEDs what you show in previous thread 3mm 5mm or 8mm THT LEDs usually have current ratings 25mA 35mA, not more. Monitor heating of LEDs body and legs on short and long time.

    If LEDs are white, supply 3,4V to LED and measure current.
    “Let the future tell the truth, and evaluate each one according to his work and accomplishments. The present is theirs; the future, for which I have really worked, is mine.” Nikola Tesla



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    Re: L.E.D forward current measurment

    Quote Originally Posted by tpetar View Post
    I dont know my friend, I see you stuck with these LEDs. :smile:

    Sometimes is easier to buy new from catalog with known manufacturer, model ID and good datasheet then to experiment and destroy LEDs on test.

    LEDs what you show in previous thread 3mm 5mm or 8mm THT LEDs usually have current ratings 25mA 35mA, not more. Monitor heating of LEDs body and legs on short and long time.

    If LEDs are white, supply 3,4V to LED and measure current.


    hmm seems no other idea we can apply to get results easily . . ok as you say

    Well as you told about 3.4 volts earlier so may i apply 3.6 v inplace of it ? or i ll hv to make 3.4 v exactly ???????



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    Re: L.E.D forward current measurment

    You skip first step in electronics to get or make as first project nice small lab power supply. :smile:

    If you dont have it, use 6V or 12V battery and voltage regulator LM317 or similar and adjust voltage to your needs.

    For short you can try Li-Po or Li-Ion battery which is half empty and shows 3,4V - 3,6V. Even small battery from phone can help you.
    “Let the future tell the truth, and evaluate each one according to his work and accomplishments. The present is theirs; the future, for which I have really worked, is mine.” Nikola Tesla



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    Re: L.E.D forward current measurment

    Quote Originally Posted by tpetar View Post
    You skip first step in electronics to get or make as first project nice small lab power supply. :smile:

    If you dont have it, use 6V or 12V battery and voltage regulator LM317 or similar and adjust voltage to your needs.

    For short you can try Li-Po or Li-Ion battery which is half empty and shows 3,4V - 3,6V. Even small battery from phone can help you.


    okk what about it ???

    Click image for larger version. 

Name:	3.6v bat..jpg 
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    i ve this one as a compact hand held



    •   Alt3rd June 2013, 22:30

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    Re: L.E.D forward current measurment

    You can try, but for testing current you need more current reserve. This small tiny battery maybe is old and weak to supply needed current and this can give wrong reading in your test.
    “Let the future tell the truth, and evaluate each one according to his work and accomplishments. The present is theirs; the future, for which I have really worked, is mine.” Nikola Tesla



    •   Alt3rd June 2013, 22:35

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    Re: L.E.D forward current measurment

    Quote Originally Posted by tpetar View Post
    You can try, but for testing current you need more current reserve. This small tiny battery maybe is old and weak to supply needed current and this can give wrong reading in your test.



    okk sure

    so for LM 317 i GOT R1 as 150 ohms & R2 as 258 ohms from the oline calculator

    to get this required 3.4 volts as you said

    so here is the diagram


    Click image for larger version. 

Name:	lm317t-voltage-regulation-circuit.gif 
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    so is it ok or any changes we need here ?



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    Re: L.E.D forward current measurment

    Better use potentiometer that you can adjust various voltage.

    Easy Adjustable Supply
    http://www.ladyada.net/library/equipt/diypsupp.html

    “Let the future tell the truth, and evaluate each one according to his work and accomplishments. The present is theirs; the future, for which I have really worked, is mine.” Nikola Tesla



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    Re: L.E.D forward current measurment

    hmm ohh okk

    thats a preety cool circuit for me

    now let see then how much i can proceed to get all results . . well thanks



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    Re: L.E.D forward current measurment

    Input voltage can be higher then 9V, such as 15V-20V.

    If you use transformer with Graetz bridge use higher capacitor values, plus on output add 100nF.
    “Let the future tell the truth, and evaluate each one according to his work and accomplishments. The present is theirs; the future, for which I have really worked, is mine.” Nikola Tesla



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    Re: L.E.D forward current measurment

    Quote Originally Posted by tpetar View Post
    Input voltage can be higher then 9V, such as 15V-20V.

    If you use transformer with Graetz use higher capacitor values, plus on output add 100nF.





    okk sure i ll take care of it

    well sir a 9 volts battery is shown here so is it a rechargable high current rating one or simple one ??

    is it possible to drive LM 317 with normal batteries ?



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    Re: L.E.D forward current measurment

    As LED's are current sensitive devices, testing them with a direct variable power supply or 3.6 volt battery is never recomended. Try searching a constant current source to test led's. Connecting the X led to the tester and measuring the voltage across will give you the 99% perfect forward voltage of a unknown LED. make a small handly circuit shown below to test almost any through hole LED's



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    Re: L.E.D forward current measurment

    Quote Originally Posted by rajaram04 View Post
    okk sure i ll take care of it

    well sir a 9 volts battery is shown here so is it a rechargable high current rating one or simple one ??

    is it possible to drive LM 317 with normal batteries ?

    Can be alkaline or standard, no problem.
    “Let the future tell the truth, and evaluate each one according to his work and accomplishments. The present is theirs; the future, for which I have really worked, is mine.” Nikola Tesla



    •   Alt5th June 2013, 15:07

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    Re: L.E.D forward current measurment

    Quote Originally Posted by pranam77 View Post
    As LED's are current sensitive devices, testing them with a direct variable power supply or 3.6 volt battery is never recomended. Try searching a constant current source to test led's. Connecting the X led to the tester and measuring the voltage across will give you the 99% perfect forward voltage of a unknown LED. make a small handly circuit shown below to test almost any through hole LED's



    ohhhhhh i see gr8 work yaar :smile:

    nice nice ok i ll proceed as per the diagram

    so here X LED means ?

    can we connect 5 LEDs parallely at a time as per figure ??????????



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    Re: L.E.D forward current measurment

    Quote Originally Posted by pranam77 View Post
    As LED's are current sensitive devices, testing them with a direct variable power supply or 3.6 volt battery is never recomended. Try searching a constant current source to test led's. Connecting the X led to the tester and measuring the voltage across will give you the 99% perfect forward voltage of a unknown LED. make a small handly circuit shown below to test almost any through hole LED's




    this circuit is much better option thanks

    do you have any kind of it for current measurement ???

    please have me a diagram



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