Powering an LED strip - PWM or regular supply?

Discussion in 'The Projects Forum' started by jj_alukkas, Jun 5, 2012.

  1. jj_alukkas

    Thread Starter Well-Known Member

    Jan 8, 2009
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    I need to fix a metre long white LED strip which will run 24x7 from a transformer 12v power adapter. Previously I had individual white leds soldereed with resistors to run from a 5v supply @15ma each. They lasted only for about a year after which they became dim and dim month by month. So, for this new led strip, is there anything I can do to make it last longer, like a PWM power supply or running at lower current or even blinking it? I dont need the full brightness from them, its to be fixed near a photo frame, so no issues. The strip has several 3 leds in series with a 150ohm resistor just like normal chineese strip leds. They draw around 330mA per meter. Any help would be appreciated before I fix these. Thank You.
     
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    1 full year is 61,320 hours which is near the end of life for LEDs. For lighting apps life is typically spec'd at 80,000 hours. So unless you bought top quality LEDs you got what you paid for.

    You might get a little more life if you pulse them so the pulse makes a bright imprint on your eye yet the average power is lower then running continuous. (On time) x (temperature) is the variable you want to reduce to get more life.
     
  3. mcasale

    Member

    Jul 18, 2011
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    Running at lower current should make the LEDs last longer, but I'm surprised your LEDs got dim so soon.

    Check the current rating of the LEDs, if you know what they are. You can easily run at half of the rating.

    Doing a PWM will make no difference, except to reduce your overall efficiency.
     
  4. wayneh

    Expert

    Sep 9, 2010
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    PWM is not better, per se, than the same continuous average current, however it is often the most efficient way to achieve that limited average current.
     
  5. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    1 full year is 365.25 days x 24 hours = 8766 hours...
    close :p

    A properly setup/quality LED should last 5 years easily..

    If your crappynese strip is meant to be powered from 12V then it already includes the resistors or other current limiting method so just plug it in and hope you didn't get a pile of junk this time..
     
  6. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    My bad. Wonder how I got that number....
     
  7. jj_alukkas

    Thread Starter Well-Known Member

    Jan 8, 2009
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    5
    At 8700hrs, isnt that too short for led life span? I expected atleast 20k hrs out of it. If its just some 1000hrs or so that I get for reducing the average power, then I guess its fine to run it as is now. Also isnt white led life very much shorter than normal leds? Thank You.

    Thats why I used individual leds for the first setup avoiding strips and built it each with one resistor and 15mA, expected atleast 2 years before going dim. I think white leds last only maximum of 2 yrs 24x7 without dimming. Thank you for telling me that PWM is useless for this :)

    I strongly believe 5yrs lifespan is for all other colours except white. I've read in many places white wont last more than 2 yrs, what would you say?
    And the 'crappynese' strip you referred to, has 1 resistor per 3 leds in series and my setup used one resistor per led at 14-15mA, so I dont think I made a 'pile of junk' the way you say it. If you are blaming it on the way I built them, then maybe you should give it a try too and tell me what you find out after a year!

    Anyways thanks everyone for the suggestions! So no PWM for this, going the normal way this time too! :(
     
  8. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Post the exact datasheet for the LED's you used.. I'm not saying "you" made a pile of junk... I'm hinting at the LEDs you used were probably low/inferior quality. A white LED (from a good company) should last 50,000 hours with only a 30% reduction in brightness.

    I made a fixture with 12 Cree white XR-E LED's (and 12 royal blue) running at 700mA that run 12 hours a day and have done so for 3+ years now. That same fixture has a green LED thats run 24 hours a day too. No noticeable reduction in output yet..
     
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