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LED's unknown forward voltage & current measurements

Discussion in 'LEDs and Optoelectronics' started by rajrocks, Jan 21, 2014.

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  1. rajrocks

    rajrocks

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    Jan 21, 2014
    Hello everyone

    I want to know the practical method of finding forward voltage & current for a given unknown LED . . please help me . .

    Thanks :eek:
     
  2. BobK

    BobK

    7,682
    1,686
    Jan 5, 2010
    Without knowing one of the two, you really can't. The only way to know you have gone too high is when they blow out, and that is likely way above the current at which they will have a long lifetime.

    Bob
     
  3. mahone

    mahone

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    Dec 21, 2013
    I can only offer you the method I use for salvaged LEDs.

    I am assuming that you have no datasheet or any clue of the specs and that all the LEDs are the same spec (not obviously mixed)

    what I do is to take a sample of for example 3 LEDs, stick them on a breadboard one at a time (in series with a pot) and hook 2 dmms (one for volts, one for current). increase the power very gradually, and write down where they fail. take an average of the fail points, when I re use the LEDs, I take 80% of the fail to be my max spec.

    if you can afford to burn up more in test, you will get a more accurate result, if you can only afford to burn one you will at least have some idea.

    that's how I do it, I'm sure there are better methods but it has yet to fail me :)
     
  4. rajrocks

    rajrocks

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    Jan 21, 2014
    hehe nice idea
    ya sir i don't have any specification as i purchased 2 or 4 as per use & grt it from the present lot available in any electronic shop . . They are not dealer so don't have any details about any electronics device in my area . .
    Here as you explained i can afford to burn up more & more in practice but if i get LEDs in future from diffrent lot (available in shop) then i ll have to repeat same method again & again which is a lil. bit cumbersome . .
    So any idea other than these ?
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    If it's a small 3mm or 5mm LED, 20mA (and then measure the voltage at 20mA). FOr smaller LEDs, maybe 5 to 10mA, although many are fine for 20mA.

    For high power LEDs (i.e. they are designed to operate on a heatsink), measure the voltage at 20mA then use this voltage to determine the current for 1W dissipation (Assuming it's a 1W or larger LED -- there's not too many that are smaller).

    How do you determine the power rating of a high power LED -- that can be hard.

    Remember never to power the LED from a voltage source. The voltage across the LED is only useful for calculating the value of a series resistor for low power LEDs, or to calculate the minimum voltage required for the drive circuit.
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    I'm not so sure of the 80% of the failure rating.

    If you have a look at the life of LEDs vs. temperature (which is pretty closely related to current at a given ambient temperature) this method is likely to result in a much shortened lifetime.

    Also, with a LED designed for a heatsink, do you do this with it attached to a heatsink? If so, how large?

    You might decide that the maximum current you use gives you a Tj of 80C (for example) but this is pretty hard to measure.
     
  7. rajrocks

    rajrocks

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    Jan 21, 2014




    :) woww sir its nice to know much about these hidden matters about LEDs
    well i don't know much but i have
    3 mm orange colour (round type)
    5 mm white,red,blue,green (round type)
    10 mm white (straw hat type)
    &
    small power LEDs (as you said,so i identified one)

    below here are images of each type (from net)
     

    Attached Files:

  8. rajrocks

    rajrocks

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    Jan 21, 2014


    no sir heat sinks are not provided with power LEDs
    Sir too you told above "measure the voltage at 20mA"
    So here how to apply 20 mA ?
    i mean would you please tell me practical step in short ?
    actually i didn't get that as i am begining with very basic
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    To apply 20mA, I would place a 220 ohm resistor in series with the LED and connect it up to a variable power supply, initially set to whatever minimum voltage it has (less than 3V preferably)

    I would then wind the voltage up slowly until one of the following occurs:

    1) you measure 20mA
    2) the voltage hits 5V and there is no measurable current

    In the former case, measure the voltage across the LED.

    In the latter case, you have the LED around the wrong way (turn the voltage back down, reconnect it and try again).

    If you don't have a power supply with a variable output voltage then you can get a 6V battery pack and several resistors. 330, 270, 220, 180, 150, 120, and 100 ohms. Start with the 330 ohm resistor. If you have no current, turn the LED around. Once you see current, keep trying resistors until you get close to 20mA. Then measure the voltage across the LED.

    You'll need a multimeter to do this, two would be even easier (unless your power supply can display current accurately)
     
  10. rajrocks

    rajrocks

    7
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    Jan 21, 2014


    hmm ya thats look like a preety cool idea what i want to know for proceeding :)

    thanks sir & ya i have regulated variable supply starting from 1.5 volts to 12 volts max & have a digital multimeter with bread boards etc etc , just i ve to follow your suggestions above very carefully

    Sir one more thing i want to ask that , suppose if i want to connect several LEDs in parallel of diffrent type (as shown above) , then how to apply resistors in that case ?
    Example : assume for round head LEDs i am getting 20 mA & for straw head i am getting 30 mA & for very tine LEDs i am getting it as 10 mA . . then how to choose resistors for limiting respective currents ?
     
  11. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
  12. rajrocks

    rajrocks

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    Jan 21, 2014


    aah awsome sir hats off to you , it solved my 50% problem :)
    Sorry for late reply as i was busy in my shows . . thanks for the support & let me go through it fully :eek: thanks once again
     
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