LED Backlight Re-purpose

Discussion in 'The Projects Forum' started by Jermi4, Jul 1, 2015.

  1. Jermi4

    Thread Starter New Member

    Jul 1, 2015
    Recently scrapped a 46" LCD with LED backlighting. This yielded 6 2" LED strips on aluminum board each which each contain 54 LED's. When powered off the PSU of the television each strip measures 170v 97.5 mA for 16.4W. All six of them, plus the board when measured with a Kilawatt meter at the plug draw ~120w. There are no resisters on the LED strips and they appear to be wired fully in series.

    My wife and I do a lot of seedling starts indoors for our garden and also grow mictogreens for salads etc. These LED's have a very harsh white glow and I think would be excellent for using as grow lights.

    Do you guys have any suggestions of how to design a power supply for these lights which would be more energy efficient than running them off the original television PSU which seems to draw around 26w itself?

    Photo of one of the strips:
  2. JohnInTX


    Jun 26, 2012
    I'd power it off the original supply. If your measurements are correct, the PS is supplying the high voltage needed for 56packages * 2 LEDdies/package * 1.5Vf LED ~=170V that you are observing and providing current limit as well. Not a bad tradeoff for 26W overhead.

    How these would work with your plants depends on the spectrum emitted. 'White' LEDs are blue->UV LEDs with a phosphor coating. Different constructions have more or less blue in them and few if any emit pure white light but your idea has merit. See if the seedlings like it.

    Good post with the measurements and photos et. al. and welcome to AAC.
  3. Jermi4

    Thread Starter New Member

    Jul 1, 2015
    If I'm going to use the original board I'd like to understand a bit more about the control circuits. Here is the front and back of the board that contains the circuits controlling the 6 LED strips. Anybody have an idea of what circuit design these are?


    2015-07-01 19.12.35 (Custom).jpg
  4. AnalogKid

    Distinguished Member

    Aug 1, 2013
    It would help to know what is printed on the chips, but here's a guess. One of the ways LCD tv's get high contrast ratios is by turning the backlights up and down based on the brightness of the video in different areas of the screen. Couple that with the fact that the brightness of a string of LEDs is more easily and accurately controlled by modulating the current through the string than the voltage across it, and we get to six programmable switching constant current regulators. And possibly boost regulators rather than working from a bulk 170 VDC.

  5. ian field

    Distinguished Member

    Oct 27, 2012
    There is a way that some of the cheaper makes of LED replacement bulbs derive that kind of voltage level at (more or less) constant current - but because of safety concerns, it not permitted to discuss it on this forum.