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Easy Way of Testing a Laptop Backlight(CCFL) and Inverter


We will be concentrating on LCD Inverter and CCFL bulb.
Laptop with black screen problem is commonly caused by:

1. a defective backlight, which is the cold cathode fluorescent lamp(CCFL) bulb
2. defective laptop’s inverter board
3. no video output going to the LCD Screen

It is not an easy job repairing laptops specially if we don’t have parts that we can use to replace to the parts we are suspecting to be the culprit. So now, we will going to try to make our own tools for testing a bad CCFL backlight of a laptop.

repair tools for laptop repair

Tools Needed For this Project:

1. a 9volts battery

2. a 9v battery connector

3. a known good CCFL inverter from any laptop or LCD monitor.

Once you have the tools that we needed, we can continue to the next procedure.

About The Video Output:

If the laptop is equipped with an external monitor connector, we can easily check if the problem of having a black screen is caused by a faulty video card or GPU within the laptop or maybe the problem resides inside the LCD which is the inverter and CCFL bulb.

Laptop External VGA Connector

VGA Port on a Laptop

Connect an external monitor to the VGA Port of the laptop and check if there is a video output to the monitor.

There is some model of laptop that requires you to press a FN key plus a key with a picture of a monitor to switch between the LCD laptop to external monitor.

Then now, if you get a video on your external monitor then you now know there is a LCD screen, CCFL bulb and inverter board problem. If the external monitor doesn’t get any video at all, then suspect a video (GPU) or video cable problem.

Checking the CCFL Bulb:

On checking whether your CCFL bulb if it is still functional, we can use our own known good inverter to test the CCFL bulb is still working or in good condition.

Known Good Inverter

Known Good Inverter

Using the Known Good Inverter:

Now, with our known good inverter, connect the CCFL bulb. And next, supplies a power to it using our 9v battery. A laptop inverter is usually powered by 12v and up but with our 9v battery, it is just enough to start the circuit of an inverter.

Test Probe on Inverter

Connect the black probe from our battery source to the yellow wire, which is usually the 12v supply coming from laptop circuits, but to find more accurately the connection of the power source of an inverter, all you have to do is to find where is the connection of the SMD fuse in the circuit of the inverter. And the black probe to the ground screw hole of the inverter board.If our CCFL bulb lights up like in the picture below, then we now know that our CCFL bulb is good and working and now we can suspect that our inverter board is bad.CCFL Bulb Lights Up

Checking Inverter:

Since that our known good inverter have able to light up the LCD Screen, and then connect the inverter of the laptop back to its position. Then we are going to conduct a test to it using the procedure we did to our known good inverter. And if it won’t lights up the CCFL bulb, then we can suspect that our inverter is defective and needs to be replaced to a new one or simply you can check it if the fuse is open. If it is open, try to make a jumper through the fuse using a thin wire to serves as our fuse, then redo the testing again with same procedure as we use on our known good inverter.

Checking the Power Supply Line:

If our known good inverter light up our CCFL bulb and the Laptop’s inverter also lights up using the 9v battery as its power source, then we are going to suspect the 12 volts power supply line from the laptop’s motherboard.

Checking Power Supply Line

Let us get our multitester/multimeter and set it to DC Volts.

Plugged our AC Adapter and turn on our Laptop.

Put the black probe to the ground screw hole on the motherboard and the red probe to the yellow or the supply line and see if you have a voltage reading on that connection.

If you are can’t read any voltages from the connector then let us suspect a problem on the motherboard itself and this needs a lot of knowledge and experience on repairing Laptop.

At this point, we will have to bring the unit to a technician who have the knowledge of repairing a complicated problem of a Laptop.



Posted by Ronelex 14 Responses

14 Responses to “Easy Way of Testing a Laptop Backlight(CCFL) and Inverter”

  1. I tried to fix my old thinkpad backlight by following your post.
    Well, it didn’t go smoothly and took me almost 1 hour to make it work!
    Thanks for your guide!

  2. mounir says:

    est ce que il ya d autres méthodes pour tester l’inverter

  3. Peter Donoghue says:

    First off.. I would like to thank Ronelex and the authors at for sharing there knowledge with us.. A lot of time and effort goes into producing these simple step-by-step guides. And I’m sure the Authors have spent years gaining their education.

    I would like to ask a Question. I know this is an old post..but if somebody out their know the answer i would be grateful

    I purchase a 14.1″ WXGA LCD CCFL Screen to replace my broken one. The 30 Pin Video Connector is Fine! – But unfortunately the backlight connector Plug wont fit into my Inverter!!

    Can i replace the Plug on my “new” Screen with the one from my Old Screen –
    I’m not going to strip down doth Screen to swap the CCFL’s as i think the CCFL is broken in my old screen!!

    I hope somebody reads this :)

    Thanks in anticipation

    Peter Donoghue

  4. danwat1234 says:

    Is there a way to swap the inverter from one laptop and put it on another with the goal of slightly over-volting the CCFL so you get more screen brightness, though shorter bulb life?

    • ronelex says:

      That thing is a big question, I don’t think it is a good idea to over-volting you said the CCFL cause it can decrease the lifespan of the bulb

    • stoat says:

      Overvolting (actually overcurrenting) won’t achievse much except shortening the device lifespan.

      If the ccfl is old then it’s probably faded (they fade out over time just like florescent tubes do.

      If you _really_ want brightness, consider a LED backlight conversion.

  5. Paul says:

    I was trying to get a touchscreen working on a laptop. I got mad, and was unplugging / reconnecting the ribbon cable (power and data) and the screen lit up very brightly and started buzzing.

    Same happened to another screen. Then it went black unless I changed the resolution.
    Then it stopped powering on the back light altogether. Data still visible if I hold a flashlight up to the screen – or in direct sunlight.
    The screens are fine and work on a different laptop, but they don’t light up on this one.

    I don’t want to spend $200 to replace the mainboard. Do you think I can fix components on the mainboard? Did I blow a fuse?

    I’ve searched pretty hard, and you seem to know what you’re talkin’ about. Thanks!

    • ronelex says:

      I think the backlight (CCFL Bulb) on your Laptop is busted, it can be fix by replacing the Backlight but check first if it is still ok or maybe the inverter is the fault.

  6. David Rosamond says:

    Without a Display Inverter I’m gonna have to try a little trial & error. I have used my TV as a monitor so I know I’m getting action. Where is the ccfl bulb located and what is the part # ?
    I have a part # for the Display Inverter. Before the screen went totally black there were different color lines flashing across the screen at a very rapid pace, then it would go back to the image I was viewing… This happened several times before it died all together.

  7. themuse says:

    can someone please tell me how to power an inverter without a laptop or without the monitors motherboard? i have some inverters and ccfl tubes i salvaged from old laptops and monitors
    i have the 12v DC out ac adapters too, but i can’t find out how to power the damn inverters on their own with just the 12 dc output , is there some simple circuit i can build to convert the 12v DC into something i can use to power the inverter on its own , so i can use the ccfls to light something else?

    i saw some people do it on youtube but i didnt understand how they did it, one guy said something about a string of resistors on the enable wire, also i found a site that is supposed to have all the pin config info for different models of inverters of course i cant find the info for the inverters i have, they sell a device that is supposed to power inverters for testing them but its big and it’s 36$ , i want something small i can hook up between the DC out of the ac adapter and the inverter board ,there must be a way there are ccfl case mod kits that come with small self contained inverters that power from the molex ,why cant i build something to power up inverters?
    i want something

  8. themuse says:

    also the guy on youtube said he had used ” a string of resistors” on the control wire to make it max brightness to simulate the motherboard on the laptop , but he didnt even say how many k ohm they were, and i have also heard there are positive and negative type of control wires depending on the inverter , i have watched the video on the inverter tester box but i still dont understand it and without knowing whats in the box, which is shown being powered with a 12v AC to DC adapter i can’t tell how to power them , the box is powered from 12 v DC and has 4 screw terminals and 3 buttons one for power one for enable and one for dim that they don’t even get into or demonstrate in their video though, it is supposed to be a tester for powering any inverter ,they say you have to go to their website and enter the model number of your inverter to get the pin out details , but when you do half of them have no info for the pin outs , so kinda a scam if you ask me they sell your their tester and tell you you can find all the pin info on their website to use it to test any inverter and then half of the models don’t even have and info listed on pin outs and one of the inverters i have is not even listed at all

  9. David says:

    Hello, I have a HP Envy 23-d034 TouchSmart desktop that turns on but has black screen. It seems that there is no power going to it. Do you think you can post the +12vdc wiring locations for screen HP Converter Board 681680-001 so I and others can try to isolate the problem component? Your posts are very informative. I just have to find the wiring diagrams for my HP envy converter bd. Any info will be greatly appreciated.


    David R.


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