How To Test And Repair A LCD CCFL Inverter

How To Test And Repair A LCD CCFL Inverter

Screen Dark .. And Now?

I often work on laptops whose screen is ‘off’ but not completely; basically, if you pay attention and you look at it with a strong backlight, you notice that the display actually is working normally displays because your PC’s but it seems as if the light is turned off .. In short, lacking the brightness (or better backlight) but one can see the pictures below .. so at least we are sure that the PC is running and there are far more serious problems of video card.

I Turned Off The Light Display!

The problem is this: in the LCD display of a few years ago is a neon lamp called the call or technically backlight lamp CCFL: cold cathode fluorescent lamp.

The lamp of an LCD can be turned off for two reasons:

the lamp is out: it is possible that the problem of ignition failure is the neon same course that has run out: but in the cases I have met with neon exhausted, there was a definite symptom arose long before, the screen It becomes pinkish and does not go out suddenly ;

the lamp is not powered: the electronic device, called inverter, which has the purpose to supply the lamp does not work and therefore not turn on. It often happens that the screen turns off after a few minutes: this is because the inverter overheats and crashes.

Inverter: This Unknown

The problem of non-lighting of the lamp is only on LCD displays with CCFL lamps used until a few years ago on the Notebook and not on the latest LED (not fitted nor any neon inverter). For some reason the device that powers this that, in effect,is a miniature of the classic neon lapada used for the common illuninazione, stops working.

The inverter and is an electronic card usually positioned inside the body of the display, more precisely below the screen.

L ‘inverter has the purpose of generating the voltage – high voltage by turning on the 700-800 volt alternating current – by supplying to the lamp neon display.

To better understand the role that has the drive in a laptop, we can observe the following schedule:


In the lower part of the diagram we find the motherboard: it is present in the LID that the switch that signals the motherboard if the display is closed or open; according to this switch (usually magnetic or represented by a switch) is given an ENABLE signal to the inverter ON / .

From the motherboard part the connecting cable to the display (1) that carries the video signal real; in laptops then this cable branches off the cable that carries sOFFignals and voltages to the inverter (2). On some models this cable is completely separate from the video cable. The inverter then converts the input power in a voltage ‘digested’ by the lamp CCFL (4).

Below are some examples of inverter models for very common laptop PC:

examples of inverters for LCD screens for laptop

In the next picture you see one of the classic places where the inverter is located on laptops Windows: under the screen, inside the body.

The green arrows indicate: the left incoming power from the motherboard of the PC, usually within the 19 Volt; one on the right hand shows the output power to the lamp almost always 700-800 volts AC.



The inverters also exist with two outputs: the case of some display, normally of size greater than or equal to 17 inches, which feature two lamps.

In this case, the display has two outputs, as in the photo below which is an inverter c

8920G dual lamp inverters

How To Test An Inverter?

How do we know if the inverter is defective? It’s not easy. Meanwhile, the output voltage, a 800 Volt AC is not fatal but can give a good jolt. Test it with a voltmeter is not ideal.

In the vast majority of cases repairers use a neon test taken from another display of result.

test backlight lamp LCD display inverter

In fact the connector’s output, at least the ones mounted on laptops, have almost all the same connector and it is easy to hook a neon just to make a test run. The connector of the CCFL lamps more common on laptops it is like the photo below: wire has two-tone pink / white and one direction of insertion.


inverter connector

By connecting a working lamp inverter (a pc off I recommend) and cycling the computer we can check the correct functioning of the inverter. If the light comes on obviously means that the inverter operates and then the problem is elsewhere (lamp display empty / faulty).

test lamp LCD Inverter

We can of course test the inverter with a multimeter / voltmeter.

To test the input power on laptops that typically range from 12 to 19Volt:

we can use the tester with scale up to 20 volts (or higher) and test some points from the motherboard, as in the following photos to see if there is input voltage.

Test inverter lcd ccfl

The pin input inverter is standard: often the pins are in input 5 or 6 or 7. Some pictures as in the previous pin and signals are replicated (pictured above the first two and the last two legs) but often the pin is as follows:


Pin 1 : port signal V-in , input voltage supply. E ‘of 12 -19 Volt and is addressed to the MOSFET;
Pin 2: signal is V-logic , power supply for the logic chip. E ‘of 3.3 / 5 Volt
Pin 3: signal Enable . E ‘signal which acts as a switch according to the closure screen, signaled by the LID. And ‘3.3 Volt / 5 Volt.
Pin 4 : Signal ADJ (adjust). It allows adjustment of the display brightness. E ‘of 0 / 3.5 Volt.
Pin 5 : ground signal GND .

To test the power output that can be up to 800 Volt AC:

to do this we must prepare the tester in AC with a full scale of at least 1000 volts as the operation voltage is between 700 and 800 Volt alternating currents with low, in the order of 5 – 6 mA.

Replace Or Repair The Inverter?

Usually it’s quicker to replace the defective inverter with a new replacement.

Now they can easily find on the market and also cost (relatively) little. But it can happen to work on a model can not be found then groped a repair.

Normally when a drive dies, the first thing that jumps is the fuse that almost all inverters possess upstream, near the connector for the cable from the motherboard (input voltage).

Below here is of such major components comprises generally an inverter laptops:

as did a CCFL inverter

Among the main elements of an inverter for single lamp we find, among connector for power supply for input and output: fuse, converter from direct current to alternating current (DC / AC), mosfets, power supply.

Under some wiring diagrams instead of as its inverter, with more lamps.


scheme inverter

Scheme inverter LCD CCFL dual lamp

Then, try first to test the fuse: if we do a bypass is open with the pond; usually this is enough. If this is not the cause of the problem we can proceed to test the transformer.

Turn off the computer and, in the absence of power, with a tester check the continuity of the coils on the points indicated with P1 – P2 (primary) and S1 – S2 (secondary). The transformer coils may have a different number of pins, are some common examples:

CCFL LCD inverter transformer

CCFL LCD inverter transformer 5 pin

transformer 6-pin LED LCD CCFL inverter

If the coils are ok then it is some other component of the inverter to give problems.

If you have problems with your LCD or with your laptop, please contact me!

Credits:   Thanks Magur of for the inspiration, information and the schemes.

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