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repairing some led stage lights

Discussion in 'Troubleshooting and Repair' started by 180soundtech, Aug 30, 2013.

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  1. 180soundtech

    180soundtech

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    Aug 30, 2013
    first off, i'm a not a complete idiot when it comes to this stuff, but i'm pretty close; there are some pretty astonishing holes in my knowledge on electronics, so sorry in advance. not sure if i've got everything right, so stop me when i don't make sense.

    so, i'm trying to repair a bunch of led stage lights for my church. there are about 90 5mm through-hole leds in a light, all on one board (something like 40 red, 25 green and 25 blue). four wires come from the control board to the led board; +12 volts, red green and blue. mosfets on the control board connect each of the three colors to ground with, i guess pulse width modulation to dim each color.

    the red circuit is a number of 4-led-strings-in-series in parallel. each series string has a 201 ohm resistor between the last led and the mosfet, which connects to ground. in a few lights, ive got a string of 4 red leds that are out, and i've probed around, testing each led individually with 5 volts and a 330 ohm resister. i can get three of them to illuminate, but not the fourth. i can even quickly short the led's two pins and get the other three to turn on which i guess means that one has gone bad.

    so my questions are this: can i simply short over the one bad led and get the other three to work long term? i imagine that the lowered resistance on that string would cause something to go bad eventually, so do i need to replace the bad led? also, leds don't really burn out ... so any ideas why the led is bad? will another led go bad again because of some bigger problem?

    p.s. i'm asking because i'm having this problem in several of my lights, not just one
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,505
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    Jan 21, 2010
    If you don't replace the LED, you will need to change the value of the resistor.

    If you do neither, the remaining LEDs will be significantly brighter until the next one fails.

    Read this for more general information: https://www.electronicspoint.com/got-question-driving-leds-t256849.html

    You might want to concentrate on the sections about low power LEDs. Section 1 will give you some background, and Section 2 deals more closely with what you have.
     
  3. 180soundtech

    180soundtech

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    Aug 30, 2013
    okay, thanks steve!
    another question, if you don't mind, on one light, two strings of red have gone out, which kind of screamed that there was a bigger problem to me, so i looked closer, and found a resistor labeled 750 (which is what the green and blue series string used) between the 201 resistor and ground. the silkscreen on the board marked that spot as 0, which, everywhere else on the board is a 0 ohm jumper/resistor. comparing it to other lights makes me think it's a manufacturing defect.
    would that cause leds to burn out?
    and if i replace the bad leds and replace that resistor with a jumper, should that fix the problem?
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,505
    2,849
    Jan 21, 2010
    I would not reduce the value of resistance in the circuit. This may have been added to balance the light output or because LEDs with lower voltage drop were used.

    A nice sharp picture of the board and the LEDs would be great so we can see exactly what you;re dealing with.
     
  5. 180soundtech

    180soundtech

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    Aug 30, 2013
    sorry, i just cant get a picture with enough detail, (the traces are really hard to see even without my bad picture taking skills) but here is a circuit diagram i whipped up instead.the only difference is on every other light I've looked at, the 75 ohm resistor is a 0 ohm jumper type resistor.
     

    Attached Files:

  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,505
    2,849
    Jan 21, 2010
    Yeah, the advice in my previous post stands.
     
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