Why is my led flashing?

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

  1. tbfunk

    Thread Starter Member

    Dec 9, 2010
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    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
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    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
    6,475
    1,282
    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
    4,269
    844
    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
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    814
    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
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    Last edited: Sep 9, 2016
  7. tbfunk

    Thread Starter Member

    Dec 9, 2010
    20
    0
    Sorry for such a late reply!
    Thank you everyone for all the good advice and reading material!

    Once damaged the LED's seemed to flash at varying rates from 2hz to well above 60hz even with more than adequate heat management.
    They just flash faster the more you over current/voltage them. I tried to burn one out and it seems they are quite resilient. At one point I put 12v with no current regulation on one and it just kept flashing rapidly but I couldn't get it to burn up like 20mA LED's do. Putting it back on the proper voltage the led was noticeably dimmer (probably have to force over-current to burn it?)

    I found the sweet spot for these LED's to be 0.7A @ roughly 4.3v for maximum brightness with a reasonable heat sink.
     
    Last edited: Jan 7, 2017
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