How to find voltage & current of Led

Discussion in 'General Electronics Chat' started by mrel, Oct 27, 2015.

  1. mrel

    Thread Starter Active Member

    Jan 20, 2009
    97
    1
    Hello
    I purchase a bag led at local swapmeet ,since led are not mark with voltage and current.
    How do i find out how much voltage and current these led can use?
    mrel
     
  2. DickCappels

    Moderator

    Aug 21, 2008
    2,651
    632
    You might want to find datsheets for LEDs with similar packaging to determine how much power in watts that your LEDs are designed to dissipate. After that run a few milliamps through it (note 1 below) to see what color it is and from this you can estimate the voltage (note 2 below). From that just divide the watts by the volts to determine the amps (miliamps most likely).

    Note 1: I often use a 9 volt transistor radio battery with a 1k resistor in series to judge the color of unknown LEDs or to verify the claimed color.

    Note 2: The chart below provides some guidance on voltage as a function of color. You can also measure the voltage across the reistor while checking the color but as you can see, the voltage varies considerably as a function of current with some LEDs.
    [​IMG]
    http://www.radio-electronics.com/info/data/semicond/leds-light-emitting-diodes/characteristics.php
     
  3. dl324

    Distinguished Member

    Mar 30, 2015
    3,242
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    You can build a constant current source/sink of 10 or 20mA and separate the LEDs by forward voltage. I wouldn't go higher than 20mA because most LEDs aren't spec'ed for higher constant current; since your LEDs are of unknown age (date of manufacture), 10mA is the safest current.

    Determining maximum current will be a guessing game. If you want to multiplex them, pick a sane peak current.

    I scanned an old (1979) HP Optoelectronics Manual and package size didn't seem to correlate to max power dissipation.
     
  4. DickCappels

    Moderator

    Aug 21, 2008
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    That was when LEDs were little more than indicator lights and the emitters in opto couplers.
     
  5. dl324

    Distinguished Member

    Mar 30, 2015
    3,242
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    Guess I'm giving away my age...

    LEDs operate at higher forward voltages and intensity these days, but power dissipation and peak currents haven't changed much. A T1 LED in the late 70's had a maximum power dissipation of around 100mW; about the same as a modern T1 LED.
     
  6. ScottWang

    Moderator

    Aug 23, 2012
    4,853
    767
    You should attach your LEDs.
     
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