Scavenge LED strips from laptop (Backlight Repair)

[lv128]

I searched around the forums but didn't find anything that was exactly like what I was looking for.

Basically, I scavenged about 5 different LED backlight strips from a slew of old LED backlit Laptop LCDs that have come through my shop. Connection wise, they all seem to have the roughly the same layout in common which is approx 54 LEDs, 3 pins combined into ground (or I think it's ground judging by the layout) and 3 pins that travel the length of the board, however after about 2 hours probing with a dmm, I'm still no closer to figuring out the layout and how I can power them without a complicated driver board.

Has anyone actually worked with these scavenged strips and/or repaired a completely different CCFL LCD using these as a replacement?
If so I would greatly appreciate a glimpse of your knowledge on how to wire these with a 12v source so I can repair a fairly expensive ELO touchscreen.

I've attached some pictures of what I'm working with. I'd love to figure out how these work, since I come into about 10-20 of these a week and I really want to use these strips for other projects including cabinet and desk lighting.

Last picture is of the driver? circuitry.
http://www.flickr.com/photos/6857478...ream/lightbox/
http://www.flickr.com/photos/6857478...ream/lightbox/
http://www.flickr.com/photos/6857478...ream/lightbox/
http://www.flickr.com/photos/6857478...ream/lightbox/
http://www.flickr.com/photos/6857478...ream/lightbox/
http://www.flickr.com/photos/6857478...ream/lightbox/
http://www.flickr.com/photos/6857478...ream/lightbox/

(Tried to do image links, but flickr didn't like that)

Optional Information:
I have a bench power supply and I was able to get each LED to light individually and some "zones" (groups of 5-6) before I burned a couple out with over voltage I assume, I tried at 9,12,and 24V, I assume that all these should run off 12V since that would be a common voltage in the laptops they came from. Current wise since there's no markings on the LEDs, I did some guessing, but 500ma seems about right to light an entire "zone"

The problem I'm running into is, that if I put a - probe to the 3 combined ground pins and then touch the + to the other pins, nothing happens at all, but if I keep put the + to the combined pins and then individually touch one side of the LEDs, I was able to get them to light up individually and then in banks up to a point where I have 4 that are super bright, then I touch the 5th one in series and they all go very dim.
I'm stumped.

[lv128]

Re: Scavenge LED strips from laptop (Backlight Repair)

*bump*

No ideas on this?

[PlainBill]

Re: Scavenge LED strips from laptop (Backlight Repair)

For several reasons pictures should be attached to the thread using 'Manage Attachments' - below the text entry area. That way they become a permanent part of the thread, not subject to the whims of some hosting service.

After several bad incidents with pictures hosted offsite I have adopted the practice of ignoring those threads.

PlainBill

[lv128]

Re: Scavenge LED strips from laptop (Backlight Repair)

I was actually abiding by someone else's post/suggestion that I shouldn't use in-line pictures because they cause problems on some people's browsers.
Either way, this should satisfy everyone if I leave the links above (which I can't edit anymore for some reason) and these pictures below,
I appreciate the input!

[PlainBill]

Re: Scavenge LED strips from laptop (Backlight Repair)

I was actually abiding by someone else's post/suggestion that I shouldn't use in-line pictures because they cause problems on some people's browsers.
Either way, this should satisfy everyone if I leave the links above (which I can't edit anymore for some reason) and these pictures below,
I appreciate the input!

The terminology may not be clear. Using the 'Insert Image' button (in the header of the test entry area) posts the picture inline. Using the 'Manage Attachments' button (below the text entry area) adds a thumbnail to the end of the message. That thumbnail links to the full size image.

I'd say that trying to avoid the 'complicated driver board' is a bad idea, If I remember my lessons, the eye tends to 'remember' peak brightness. By pulsing the LEDs with a low duty cycle, high current drive you can achieve a higher apparent brightness at a lower net current. By adjusting the current or duty cycle you can also vary the apparent brightness.

PlainBill

[lv128]

Re: Scavenge LED strips from laptop (Backlight Repair)

I was testing these just trying to use a current regulated bench power supply, and while I'm familiar with most basic electronics, for some reason I can't seem to trace how these are wired. I saw a schematic somewhere where some LED strips are grouped in groups of 4-5 and every 4-5 are reversed in polarity. But what appears to be a ground doesn't seem to trace to ground when it's actually in-circuit in the LCD panel driver board, and connected to the computer.

[lv128]

Re: Scavenge LED strips from laptop (Backlight Repair)

Figured out the layout for that strip, it seems some strips go in banks and some go in alternating, the banked ones are the easiest to work with, also figured out 36v @ 100ma is about their max brightness. Strangely enough going with 18v @ higher current didn't work, although I'm a bit of a noob at electrical theory, this was the voltage amount that was being sent by the controller board that I probed but it wasn't sufficient to light a full bank of LEDs, let alone the whole strip.
The higher the voltage the brighter they are, I always thought the current was what controller brightness on LEDs.

[PlainBill]

Re: Scavenge LED strips from laptop (Backlight Repair)

Figured out the layout for that strip, it seems some strips go in banks and some go in alternating, the banked ones are the easiest to work with, also figured out 36v @ 100ma is about their max brightness. Strangely enough going with 18v @ higher current didn't work, although I'm a bit of a noob at electrical theory, this was the voltage amount that was being sent by the controller board that I probed but it wasn't sufficient to light a full bank of LEDs, let alone the whole strip.
The higher the voltage the brighter they are, I always thought the current was what controller brightness on LEDs.

The only idea I came up with is they included a current limiting resistor in each group, which doesn't make a lot of sense.

PlainBill

[lv128]

Re: Scavenge LED strips from laptop (Backlight Repair)

So far as I can tell, there's no other components on the LED strip... I destroyed about 5 of them while I was testing. some of them have groups that reverse polarity and then I ended up blowing a bunch of them,
I wish it was possible to figure out which is which before applying power, but the boards are coated with a heavy epoxy that hides the trace lines on some of them and even a scratch brush takes too long to remove them.
It's probably not worth my time to try to use their circuit board, so I'll probably just make my own board/strip and save the LEDs from the panels. Least that way I know how they're wired.

It is safe to start at a low voltage with an LED and keep increasing it until it's lit to a specific level, for example with my bench power supply I can turn a pot to increase voltage by the mV while it's connected to a circuit.

[FIXITNOW]

Re: Scavenge LED strips from laptop (Backlight Repair)

try looking at this controller chip may beof some use
The LT3760 is an 8-channel LED driver with a step-up DC/DC controller capable of driving up to 45V of LEDs

[essam123]

Re: Scavenge LED strips from laptop (Backlight Repair)

try looking at this controller chip may beof some use
The LT3760 is an 8-channel LED driver with a step-up DC/DC controller capable of driving up to 45V of LEDs

trying to do the same , very much interested to know how did you do it, please share the information

[stj]

Re: Scavenge LED strips from laptop (Backlight Repair)

the driver is on the lcd pcb, why not cut the pcb down so you just have the driver?
i'v done it twice - it's easy.

[stefos]

Re: Scavenge LED strips from laptop (Backlight Repair)

the driver is on the lcd pcb, why not cut the pcb down so you just have the driver?
i'v done it twice - it's easy.

Can you please add a schematic how to power on the leds using the default pcb driver? I have a strip with 9 leds in series x 4 paraller = 36 total leds). Thanks !!

[budm]

Re: Scavenge LED strips from laptop (Backlight Repair)

Can you please add a schematic how to power on the leds using the default pcb driver? I have a strip with 9 leds in series x 4 paraller = 36 total leds). Thanks !!

Do you mean the ANODE of four LED srtrings are tied together to the LED supply output and the Cathode returns of each string goes to each individual LED sink (more likely going to GDN through the LED driver IC) ? I doubt that they will parallel all 4 strings together, that will be very bad design.

[stefos]

Re: Scavenge LED strips from laptop (Backlight Repair)

That's right (Similar to figure 1 of your attached pdf file).

[budm]

Re: Scavenge LED strips from laptop (Backlight Repair)

That's right (Similar to figure 1 of your attached pdf file).

So they are not connected in parallel as I thought, that would have been bad/cheap design.