Tip: TFT backlight LED conversion / CCFL inverter alternative

[nctnico]

Every now and then I like to convert CRT / STN LCD screens in test equipment to TFT screens. One of the problems is that older screens are cheap but it is hard to source the right inverter (and they aren't cheap). Also some TFT screens turn out to be used instead of new and their CCFL lamp can be pretty much worn out (=more costs). Recently an EEVblog member pointed towards these LED strips to replace CCFL lamps:

This is the Ebay listing: http://www.ebay.com/itm/361624462481 It turns out these strips are dirt cheap at less then US $4.50 so I ordered 3 sets. If they work well they would make it much easier & cheaper to do TFT conversions. I was wondering (worrying is a better word) how they where going to make sure the fragile LED strips are going to survive handling by the postal system but the seller packed them in sturdy PVC tubes so they arrived intact.

Just for kicks I decided to use one set for this really tired TFT screen:


After some disassembly I got the panel out:


It has only one lamp and since the backlight set has two strips I guessed I needed to reduce the current the driver board pushes into the LEDs. The current sense resistors where quickly found. Origninally the board has a 0.68 Ohm and 1 Ohm parallel for a 0.4 Ohm resistance.


With this modification the current draw is 0.25A @12V (3W). With 8.5V across the LEDs and the number of LEDs this gives about 6mA per LED. 

The driver board is a single layer pressed paper board (it has been a long time since I've seen one of those boards) and for it to turn on the enable pin (the pin names are printed on the bottom) must be pulled high. Since I didn't had 3.3V or something like that I used a 10k Ohm resistor connected to 12V. That should be safe for any clamping diode in the circuit.

Removed the unused wires and tucked a 10k Ohm 0603 away between the enable and 12V pins:


Let's see if it works when connected to the system before going any further (yes it does work but didn't take a picture):


Time to take the TFT apart. In some TFT screens you can slide the CCFL out but this panel needed some more disassembly:


Much to my surprise the LED strip fits perfectly in the reflector although I needed to cut a bit of plastic away to make the wires fit:


Add some heatshrink tubing to prevent the connector from getting loose:


Time to fire it up!

  That doesn't look right! It is better but what is that yellow smudge???

Sigh... let's take the TFT panel apart again. Between the layers of the light spreader I found this crumbled piece of plastic with yellow stains which in a distant past probably served to diffuse the light:


It looked like tracing paper so I stuck in a bit of tracking paper to replace it and tested again. Much better although there still is some yellow-ish staining. I guess I should have cleaned the light spreader parts I didn't replace but then again I might have damaged those in the process.

[james_s]

Looks good.

If you want replacement light spreader material, look for a junk LCD TV with a cracked panel or other terminal fault. You'll get large sheets you can trim to size.

[Buriedcode]

Good call on the 'diffuser' material.  I bought a joblot of broken large laptop screens, and TFT monitors (anywhere from 15" to 23").  With just the panels, cracked, a couple of inches thick was enough to provide prismatic films, diffusers, polarising diffusers, high quality reflective material for back and edges.  Well worth the $20 US.