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IR led Doesnt work Plz help

  1. Jul 4, 2011 #1
    hi , every body here , I really seek a help , I have an Ir led from thorlaps it's wave lenght 1450 nm , I followed the specs
    Forward voltage at 20mA is = 1.2 v typical , 1.5 v Max
    Max DC forward current = 100mA
    Power dissipation is 120 mW

    I tried the following resistance 200 ,220 ,190,180 ,100 ,63 ohms the led doesnt work , i thought it was damaged i replaced it with another one same happens but when replaced it
    with 940 nm (have almost same specs ) it works

    so whats the problem
    here is the data sheet

    I noticed strange things when tring connecting 180 , 190 , 200 , 220 ohms the led have same voltage drop = 0.85 i keep lowering the resistance values until reached to 63 ohms then it gives 0.95 v and the led doesnt work under camera focus so what i shall do thanks
  2. jcsd
  3. Jul 4, 2011 #2


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    Your LED has a maximum forward current rating of 100 mA. Larger forward currents will cause the junction to fail; hence no IR light (how are you checking if there's an IR output?).

    Anyway, to calculate the correct current limiting resistance, you will need to pick the forward current you will be operating it at; say 20mA for example. Then you need to know your power supply voltage; say it's 5V for example. Note that @ 20mA=0.02A the LEDs forward voltage will be 1.2V. Now use this equation to determine resistance to use:



    For our above example, R = (5V-1.2V)/0.020A = 190Ω

    Not knowing what the power supply voltage you were using, I can't say for sure. But you may have over rated the forward current and blew the junction. Your LED can only dissipate 120mW before becoming damaged.
    Last edited by a moderator: Apr 26, 2017
  4. Jul 4, 2011 #3
    Iam using 5 v power supply , I have three of them when testing using 190 ohms or 200 or 220
    ohms the voltage drop across led is about 0.8 V , I tried to replace the led with anthor new one same happens thats led me to test it using low resistance
  5. Jul 4, 2011 #4


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    Okay, I download the only datasheet I could find (http://www.datasheetdir.com/LED1450-03+LEDs" [Broken]) and for a forward current of 50mA, the forward voltage has a Maximum value of 1.5V but Typically for any one part it's 1.0V. So if you apply the formula with the LED current=0.050A, the LED forward voltage drop=1.0V, and a supply voltage=5V,

    R = (5V-1V)/0.05A = 80Ω

    So my question to you is, how do you know it's NOT emitting IR. Your eye can't see this wavelength. What method are you using to see the IR?
    Last edited by a moderator: May 5, 2017
  6. Jul 4, 2011 #5
    thnx to your reply
    by using digital camera and also
    introducing it to a photodiode has cover the spectral range

    also datasheet from here
    http://www.thorlabs.com/thorProduct....umber=LED1450E [Broken]

    its from throlaps

    when i tried 63 the voltage drop increases to 0.9 v so I will go to buy 50 ohms and test it
    Last edited by a moderator: May 5, 2017
  7. Jul 5, 2011 #6


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    I wouldn't go much lower. At the maximum forward current rating of 100mA and with a typical forward voltage drop of 1V, the current limiting resistance of 40Ω would be the minimum value you should use.
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