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Discussion in 'LEDs and Optoelectronics' started by vinod chandran, Aug 8, 2012.

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  1. vinod chandran

    vinod chandran

    Jun 21, 2011
    Hi all,
    I need to know the forward current and voltage of an unknown white LED. One salesman told me that this is an 1W LED. Another salesman told me that this is a 0.5W LED. Anyway i just tried to know the forward current and voltage. I am attaching an image of my experiments with LED. Anybody please tell me that how to find the current and voltage of my LED. All this is because my LED resistor calculator wizard asks me to enter forward current and voltage. But How ?.

    Attached Files:

  2. BobK


    Jan 5, 2010
    If you don't know any of the forward voltage, current or power, I see no way to determine these parameters. If you know any one of them, you can find the other two.

    Edited to add: The basic problem is that the LED will operate at above the rated power form some period of time, then fail. If you don't know any of the limits you can only keep increasing them until failure, and then that only gives you an upper limit on the parameters.

    Last edited: Aug 8, 2012
  3. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    BobK is right, you can't determine the LED's power rating through measurement. You could measure the temperature rise and use that as a guide.

    You're measuring the parameters the right way. You need to understand that LEDs are primarily current-driven devices. The voltage you measure across the LED doesn't vary much over a wide range of currents. The power dissipation in the LED is equal to the current flowing in your circuit multiplied by the voltage you measure across the LED at that current.

    If you're measuring 3.1V at a forward current of 170 mA, you can assume that the LED's forward voltage is ABOUT 3.1V and calculate the current you need based on the wattage. P = V I (power = voltage times current), so I = P / V. If you assume P is 0.5W and you know V is around 3.1V you can calculate I (current) to give that much dissipation. P / V is 0.5 / 3.1 which is about 160 mA as in your drawing. So you are running the LED at 0.5W already.

    Now I would measure the temperature of the heatsink (does it have one?) and if this seems low (say under 50 degrees C), increase the current. But without knowing the specification of the LED you can't be sure you're within its limits for the long term.

    Find some data sheets for similar LEDs and see what they say about operating temperature to get some idea of how hot they're supposed to run.

    That's all I can suggest. Good luck!
  4. vinod chandran

    vinod chandran

    Jun 21, 2011
    Hi BobK and KrisBlueNZ,
    Thanks for the reply.
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