Архив 28 февраля 2010

LED and CCFL LCD backlight

Оригинал на русском: http://shura.luberetsky.ru/2010/01/25/pro-podsvetku-zhk-monitorov/

Most modern LCD displays use the backlight technique – the semi-transparent image on the LCD panel is lighted from the back, and the visible image is formed by the light passing through the matrix and filters. A rather bright white light source should be used as a backlight, because the LCD matrix is quite dark – comparable to dark sunglasses.

Traditionally cold cathode fluorescent lamps (CCFL) were used for that. This lamps look as thin (2-3 mm in diameter) white glass tubes. The inner surface of the tube is covered with luminofore, and the tube is filled with mercury vapor. When an electric voltage is applied to gas, it is ionized, making the luminofore glow. For CCFL, you need to supply it with high frequency (40-50 kHz) alternating voltage of about 1500 V.

One of the most common failures of LCD panels is breakdown of backlight or invertor – a device which transforms DC voltage of about 12-18 V into AC foltage for the lamp. You can notice that either in partial darkening of the screen, usually near one of its sides, or in complete darkening – in this case you can see the dim image only when you light the screen with a bright lamp.

In authorised services this malfunction is repaired by changing the entire panel, especially in case of notebook screens. That is quite expensive, and the price can be compared to that of a brand new display. Luckily, there are not only authorised services, but a quite large number of “handymen” who are able to replace the backlight or inverter.

Replacing of a backlight lamp is quite easy, and is sometimes possible without full disassembling of a display. It is described in Igor Pichugin’s article at RadioCat website, as the article is in Russian, I’ll briefly retell it.

The lamps are mounted along a side of a display in special casing. To dismount the casing, you need to disassemble the panel – take off the metal case and the glass panel with some electronics. The electronics is a thin (0.5 – 1 mm) circuit board, connected to the glass panel with some flat ribbon cables. The cables are protected with sticky film, which you have to unglue carefully (never cut it! if you accidentally damage the ribbon cables, you will be unable to repair them).

For demonstrating of “classic” CCFL technology I use the LG Flatron L1970H display.


The disassemly begins with removal of a stand. You need to take off a plastic cover in the back, which hides the connectors and screws of the stand.


After removing the stand we can disassemble the plastic case. The front bezel is mounted on several clips and can easily be removed.


The LCD module is in its metal case. Through the holes you can see the transformers of the invertor with scary cautions.


Unscrew the case and remove the metal covers.


We can see the control board of the display and a separate inverter board.


The control board is connected with the decoder of the matrix, the latter is covered with protective film.


The decoder is connected to the matrix with thin flat ribbon cables. It you have to take off the matrix, you have to peel the protective film carefully – the damaged cables can not be restored, and any damage to them renders the matrix unusable.


The inverter, if broken, often can be changed to a compatible one. You need to know the supplied voltage and the number of the lamps. Also, the inverter is quite big, and often can be repaired with widespread tools.


The lamps are connected to the inverter by standart plugs.


In this display you can unmount the lamps casing without taking apart the whole panel. Undrive a screw…


…and pull the lamps casing.


The lamps are mounted in pairs. You can distinguish an old lamp by a dark ring across its end. When the lamp ages, the ring gets darker an darker.


I needed the lamps to test the inverter of a notebook PC Fujitsu-Siemens Amilo M7800, which had symptoms of backlight failure. The authorised service charged incredible money for replacing the panel, so I decided to use Igor’s article to revive this PC cheaply.


To access the panel, you should take away the besel. It is mounted on clips, but sometimes there may be screws fixing it.


In the lower part of the display one can see the inverter in protective case.


To test the inverter, you should connect a working lamp to it.


If the inverter of a notebook appears broken, it is considered unrepairable and is replaced by another one. If you can not find the original part, you can try to use “compatibles”, which can be found in many shops, including online, such as Dealextreme.


When replacing the inverter one should be sure about control signals of dimming and switching on/off. The cable of the inverter usually contains DIM and ENABLE signals, which change from 0 to about 3 V (always measure actual values!), 0 V means disabled/zero brightness, 3 V means enabled/full brightness. Usually their proper commection is not mandatory, but it allows to use power-saving features.

To replace the lamp we need to take off the LCD panel. Unscrew its mounting to the plastic cover and hinges and pull out.


Take off the metal guides on the sides of the panel.


In this panel you can get to the lamp without full dismounting. You only need to take off one side of the metal frame and open the plastic case.


The following seemed to be obvious – go to a spare parts shop, buy a lamp and put it on its place. But there were no shops with the lamps of the needed length in Moscow. At Mitino market I was advised to use a LED line designed specially for backlighting LCD panels.


A width of this LED line is about 3 mm. The diodes are grouped by three, with the length of each group about 15 mm, so you can cut the needed length.

Today, when ultra-bright white LEDs are easily available, LED backlight is installed in some LCD displays and TVs. Really, anyone can touch this breakthrougth technology, installing LED line backlight instead of a broken “lamp”. The 300 mm line which I bought costs about 8$ (250 roubles, which roughly equals the cost of a spare lamp).

The LED line perfectly fits on its place.


To test the LED backlight we need to connect it to any suitable power source (we need 12 V DC, observe polarity :) ). The unplugged screen should glow bright white.


Assemble in reverse order.

Instead of a non-needed inverter you can install a schematic like this:


The resistor values may vary in different notebooks, so I do not write them here – knowing DIM/ENABLE parameters and supply voltage, one can easily calculate them.

Finally, a few words on disadvantages of home-made LED backlight.

First of all, LED spectrum differs from CCFL spectrum. If you need accurate color reproduction, i. e. on a high-quality graphic workstation display, you should use only CCFL.

Also, there exist “smart” inverters which are controled not by analog voltages but by some digital bus, usually I2C, but more exotic variants exist. If such inverter is absent, the LCD may automatically switch off.

The third and main bad feature of this “do-it-yourself” backlight is non-uniform lighting near the lamp.


In the photo you can notice that the lower part of the display has some darkenings, and the lower right angle is completely dark as the LED line happened to be short a bit.

In any case, replacing a CCFL lamp with LEDs can be a cheap and easy way of restoring LCD displays. The drawbacks are not critical, and in case of non-standart lamp lengths it is quite a reasonable decision.