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Testing LED's

Discussion in 'LEDs and Optoelectronics' started by spike1947, Feb 15, 2022.

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  1. spike1947

    spike1947

    56
    2
    Feb 4, 2016
    Hi
    I am trying to replace some dead LED's on a couple of strips from a LED TV, tested a row of 4 using 12.2 v dc and it shows 2 working, now I am trying test a single LED using 3.23 v dc ( 2xAAA Bats ) , but even the good ones are not lighting with that voltage, what am I doing wrong here !.
    attached pics showing 2 LED's lit with a supply to the strip of 12.2 v dc.
     

    Attached Files:

  2. Harald Kapp

    Harald Kapp Moderator Moderator

    12,508
    3,000
    Nov 17, 2011
    You need a series resistor to limit the current through the LEDs. Without resistor the current can become too high for the LEDs and they will burn. You may have damaged the good LEDs if you haven't used a resistor.
     
    spike1947 likes this.
  3. spike1947

    spike1947

    56
    2
    Feb 4, 2016
    Hi
    I wouldn't know what size resistor to use !
    cheers
    Spike
     
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    12,508
    3,000
    Nov 17, 2011
    Depends on the voltage source, the LED and the current.
    Use 3 × 1.5 V batteries = 4.5 V. Assume ~ 3 V for the white LED and 10 mA then R = (4.5 V - 3 V) / 0.01 A = 150 Ω.
    The exact value is not critical. anything between 100 Ω and 220 Ω will work, even 300 Ω should work.
    2 × 1.5 V batteries may not be enough to power the LEDs. White LEDs can have > 3 V pass voltage (the good point here is that you will not have destroyed these LEDs with the 3 V from the batteries :)).

    You also need to observe the polarity. LEDs light up only with anode to '+' and cathode to '-'. But of course you are aware of this, aren't you?
     
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