Why is my led flashing?

Discussion in 'General Electronics Chat' started by tbfunk, Sep 9, 2016.

  1. tbfunk

    Thread Starter Member

    Dec 9, 2010
    19
    0
    I bought some cheap 3 watt LED's from china and when I hook them up to a 625mA current source they eventually start flashing.
    I have never dealt with high power LED's before however I don't think they flash unless the power source is causing them to do so, right?
    Alternatively if for some reason I may have damaged an LED accidentally through over voltage/current could that cause this strange behavior?

    I am using an LM317 (also cheaply bought from china) to regulate current to 625mA, is this too low or high? I had trouble finding information on the ideal amount of current to use.
    LEDs: http://www.aliexpress.com/item/100P...2320568428.html?spm=2114.13010608.0.77.aeZpfA
     
  2. Kjeldgaard

    Member

    Apr 7, 2016
    73
    17
    With the little information, I can only guess at a thermal problem.

    How fast is the flashing?
    How high input voltage?
    What is the LED and LM317 temperatures? (Be careful not to burn your fingers)
     
  3. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,813
    1,105
    What is the voltage/current rating of the source supplying power to the 317? If it's being overloaded it may go into self-protect mode.
     
  4. dl324

    Distinguished Member

    Mar 30, 2015
    3,250
    626
    What is the voltage and current rating of your power source? What is the LM317 suffix? K, T, L??
    Use a variable current source to determine an optimal current experimentally. Increase current slowly until you don't notice a significant change in brightness, but make sure you don't exceed 3W of dissipation.

    Electronic components from AliExpress are of unknown source, quality, and reliability. I only buy from there when quality and genuine parts aren't required, e.g. for wires and such.
     
  5. DickCappels

    Moderator

    Aug 21, 2008
    2,664
    634
    The thermal shutdown on the LM317 oscillates some/all of the time. You probably need to reduce the input voltage or get a better heatsink. (You DO have a heatsink, don't you?)
     
  6. ci139

    Member

    Jul 11, 2016
    341
    38
    Last edited: Sep 9, 2016
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