The Light that you see on the LCD screen of your monitor or the LCD screen of your Laptop or Note bookComputers is generated by Back light assembly. The LCD Back light consists of CCFT (Cold Cathode Flrorescent Tube) or EL (Electroluminescent light) source. The light which emanates from the display is interpreted as either being transparent or colored. Light that is absorbed by the energised liquid crystal material appears opaque.

How does the backlight function?

The LCD backlight consists of the CCFL or EL light source. In order to operate this light source an AC voltage of at least 500V to 600V is required. This kind of high AC voltage is, off course, not available in Battery operated LAptop or Notebook Computers. Hence,the available DC voltage has to be converted into high level AC voltage by means of an Inverter. This high level AC voltage is then used to operate the light source.

An Inverter is an electronic circuit which converts low voltage DC into high Voltage AC supply. The Inverter circuit consists of an oscillator which chops the DC voltage into a low-voltage pulsating DC output. This pulsating DC is applied to the input of a step up transformer, which converts the pulsating DC into high voltage AC supply. This high voltage AC supply is then used to operate the CCFL or EL light sources.

Trouble shooting the LCD power supplies.

The most common symptom with LCD screen is that the screen appears washed out or invisible. When this occurs, firstly make sure that the LCD has not timed out due to long period of inactivity. The LCD is made to automatically shut down after a certain timeout period so that the life span of the light source can be increased. You can increase the timeout period through the system setup software. Once you press any key or move the mouse, the problem should disappear.

However, if you are certain that the problem has not occured due to timeout period, you will have to troubleshoot further. Most probably the problem could be due to a faulty backlight assembly. Follow these simple steps and you might be able to get your LCD screenback on track again.

Steps for Troubleshooting the Backlight Assembly.

Step 1.

Open the front panel to get to the Back light Assembly.

Step 2.

Check for DC supply, with the help of a multimeter, at the input of the inverter. If there is no DC supply, there might be a loose cable connection. Check the cable connection of the backlight assembly to the motherboard. Tighten the cables and check whether your LCD screen is back to normal. 

Step 3.

If the DC supply checks ok in Step 2 you will have to check the AC output voltage from the inverter. Use good insulated test probes, while measuring the voltage with your multimeter, as the voltage may be as high as 600 VAC. If you find that there is no AC voltage at the output of the inverter, the fault might most probably be in the inverter circuit. Replace the inverter and check again.

Step 4.

If the AC voltage at the output of the inverter checks ok in step 3 but yet there is no LCD display, the problem could be with the light source itself. Replace the light source and check whether the LCD display is working fine.

Troubleshooting LCD problems is a simple Do-it-yourself job.