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# Using Edge-lit LCD's LED array Answered

Hi everyone!

Recently, a friend of mine managed to crack the LCD of his laptop. While he got a replacement, I took his old LCD screen apart and salvaged the LED array which edge-lit the screen.

Now my question is, is there a way I can power this on? There's no information printed on the LED array about its power requirements and I'm looking for any hints / clues I can get.

I've attached a few closeups of the LED array, hope to get a lead on this. Thanks in advance!

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## Discussions

And here was me thinking my cracked LCD monitor was useless. Thank you :P

My first thought was to look for a datasheet, but the couple I found weren't very helpful. The LED used in the ones I found has a forward voltage of 3.6V @ 20mA which is a good place to start. Next let's look at the array you have.

First of all, how many LEDs are in the array?

Next we need to figure out how the array is wired. I can't see the traces in the picture, but you should be able to figure out a wiring diagram by following the traces. The markings on the strip seem to indicate that the side toward the plus sign is the anode and likewise the side toward the negative sign is the cathode.

My initial guess for the connection was all LEDs in a series string, but this would lead to a very high operating voltage. A more likely scenario is that the LEDs are grouped in to smaller strings of series LED. Looking at the ribbon cable there seems to be six narrow traces and one wide one. This could be six power connections with the wide trace being a ground rail. You can follow the traces to see if this is actually the case.

Assuming that the strip is wired in groups of series connected LEDs, the voltage each group will drop should be 3.6V*(# of LEDs per group).

So, now how to power it? First of all note that there is no current limiting resistor in the strip.  These backlights are usually powered by constant current sources. This means that you you need to limit the current going to the strip.  Once you know the total forward voltage of each group (forward voltage of one LED * # of LEDs) you need to apply at least that voltage while limiting the current.  The current should be the current each LED draws * the number of LEDs.  (The current in each LED should probably 20mA or less.)  If there are in fact six positive connections and one ground, you could apply power to each line individually or connect all of the power lines together, you just have to take into consideration where your resistor(s) are.

Hopefully this is helpful.  I'm trying to give some insight on how I went about working through the problem.  If you post the total number of LEDs and a wiring diagram I can give you some less open-ended suggestions.

Hi advanceMischief! I counted the leds, there are 48 of them. You are right about the grouping. I can't draw a circuit diagram, as I don't know how to. Is there a tool for Windows that would help me do that? Thanks again, I will report back any findings from my experiments. Cheers! Jag

Have you got a multimeter?

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