Laptop Backlight Test - Testing A CCFL LCD Backlight Tube

Computer Repair with Diagnostic Flowcharts

The printable eBook version of The Laptop Repair Workbook is now available for download for $13.95 anywhere in the world.

Warning: Laptop inverters put out high voltage, usually between 500V and 700V, so don't play try these tests unless you know your way around electricity. I'd also strongly advise against my sloppy approach of twisting wires and not even taping them.Since I recently did a page on testing a laptop inverter with a multimeter, I thought I'd go over some of the testing options for a backlight. Laptops employ CCFL tubes for backlight, due to their bright white light at relatively low power. Like all fluorescent lamps, CCFL tubes require a high voltage, high frequency input to strike a plasma and cause the tube coating to fluoresce. Someday not too far into the future, white LEDs should be available to do the backlight job with even less power and higher reliability. But in the meantime, a pairing an inverter with a CCFL is the way to light up a laptop screen. CCFL modding lamp lit by battery powered inverter

Laptop LCD backlight lit with inverter from CCFL hobby lamp The picture above shows a 4" blue CCFL tube from a PC modding kit that cost around $3 by mail order. In fact, I think the shipping cost more than the whole kit, which included a dual inverter and the tube. Instead of using the PC connector provided with the kit, I cut the leads off and powered the inverter with 4 AA batteries in an 8 cell Radio Shack holder that cost $1.89. Since it was an 8 cell holder and I only wanted around 5V, I had to jumper the across the top, which I did by just sticking a piece of wire in the connectors. I also cut the output connector of the inverter since it was different from the connector laptop backlight I wanted to test. Then I remembered I hadn't tested the original modding backlight first to prove the inverter was functioning, which explains the first twist together job.

The picture above shows the modding inverter hooked up to the LCD backlight. I've folded up the white flap that covers backlight to help keep the light in the LCD assembly. The inverter did fire up the plasma, but not completely, which left me wondering if the CCFL tube was bad after all. However, it turned out that the inverter output was essentially linear with the input, as my neighbor with a variable linear power supply was able to demonstrate. I came back and put 8 cells in my battery holder and it fired the whole tube up. Unfortunately, I got the other four batteries by taking them out of my camera, so a picture was out of the question:-) Next I decided to try to fire up the backlight with the original inverter from the laptop. This inverter looked pretty standard and I guessed it would be happy on 5V, I know some laptop inverters take a 12 V input but I figured the lower voltage wouldn't hurt. Applying the plus 5 Volts to the fuse on the inverter board

Laptop backlight with original inverter run on battery pack I'm simply touching my somewhere around 5V DC to the in-board side of the fuse, and you can see to the right that the backlight fired up at full brilliancy, even with the camera flash. I finally zoomed out so you can see that with the correct inverter, the whole backlight is lit up, and the light is in fact spread through the LCD substrate and visible at all the edges, as it should be. I had to take the metal back off the LCD to expose the backlight this way, which means that all the intermediate filters and layers wanted to fall out of the LCD if it was tilted. But it's much easier to tell what's going on from the back than the front, because in the "off" state, an LCD doesn't transmit much light, all the little crystal cells remain twisted closed. If you move the whole screen to a shaded area and fire up the backlight, the screen will visibly light a little, a sort of dull grey that's hard to capture with a digital camera.

To the right you see the LCD with the backlight lit full on, shot in the dark. You'll notice that there's actually more light leaking out the back of the LCD than transmitting through the screen. The lighting gradient you see is an artifact of the camera angle, the screen was even lit with a dull grey grid. Keep in mind that an LCD displays white by turning on the red, green and blue subpixels, which mix the light together for white. But the fact that some light leaks through when the LCD doesn't have any video input implies that for real black, the liquid crystals need to be twisted hard into an opaque state, their unpowered "off" mode allows some light to leak through. Backlit LCD without video input

The printable eBook version of The Laptop Repair Workbook is now available for download for $13.95 anywhere in the world.