So I have a problem with my old laptop. Internal monitor is not working. External is working. So I tried to understand what's the problem.

I tried to check if inverter is broken. It get's a good 19V DC voltage on VCC, but BackLight pin has 0V. That's the inverter input pins. So I believe the monitor controller doesn't enable backlight. So there could be issue with the main monitor circuitry.

enter image description here

Then I tried to see what's going on with monitor data pins as I don't even get dim image which basically says that the issue could not be in the inverter. I have noticed that monitor circuitry doesn't get VCC3.3V from the motherboard. It does get EDID voltage, bet not VCC.

It's getting more difficult to track where is the issue as the track goes back to motherboard. I believe that one of MOSFET switches could be broken near the lcd socket therefore not providing power.

Long story short, I want to try to feed 3.3V into monitor circuitry VCC pin from external power supply. Is it safe? Any comments on that?


I am talking about two different pinsets. Again.

With inverter I get VCC, and I dont get the rest BKL and ADJ.

With video data pinout I get EDID but not VCC.

I don't have datasheets. So far I traced the problem with multimeter and pin names near sockets.

  • 1
    \$\begingroup\$ Data sheet? Otherwise we can't help you. What is EDID? Why do you believe a MOSFET is the problem. \$\endgroup\$ Commented Aug 2, 2012 at 0:40
  • \$\begingroup\$ No datasheet. If I had it, it would be much easier. There are pin names near the socket. \$\endgroup\$ Commented Aug 2, 2012 at 7:50
  • \$\begingroup\$ I would fix the issue by myself if I had a datasheet. \$\endgroup\$ Commented Aug 2, 2012 at 7:52

3 Answers 3


BackLight pin has 0V

It is highly likely that this is an active low input, which means 0V = Backlight ON.

Even with the backlight off (or br0ken) the LCD will display the picture internally. This can be made visible with a light source (LED flashlight), provided the picture has high contrast. Then you know that the LCD surface still works.


It's easy to know if the backlight is the problem: put the display in sunlight or a well-lit room. You should still see a faint image, even if the backlight is failed.

Otherwise, no one can tell you if it's safe, correct, or a good idea to connect 3.3V to your monitor circuitry without a datasheet, schematic, service manual, or known-good identical component to compare to.


I've never heard of any circuit being burned by 3.3V. As long as it isn't reversed polarity go for it. Use a current-limited bench supply and raise the voltage slowly.

  • \$\begingroup\$ I am not saying that it was burned by 3.3V. There may be different causes of the damage. What I am saying is that it doesn't provide 3.3V anymore. \$\endgroup\$ Commented Aug 1, 2012 at 22:42
  • 1
    \$\begingroup\$ Did you even read your own question? Please re-read your last paragraph then my answer with that in mind. \$\endgroup\$
    – Kristoffon
    Commented Aug 1, 2012 at 22:47
  • \$\begingroup\$ Oh. I apologise. Misunderstood your answer. Long day :) \$\endgroup\$ Commented Aug 1, 2012 at 22:48
  • 8
    \$\begingroup\$ @Kristoffon: Circuits that run on ~1.8 V or less can be damaged by 3.3 V. \$\endgroup\$ Commented Aug 2, 2012 at 0:40
  • \$\begingroup\$ One way to answer the question "Is it [3.3V] safe [in this case]?" would be to look up the datasheet of the chip(s) around which the inverter is built. So, what are the part numbers of the chips on the inverter board? \$\endgroup\$ Commented Aug 2, 2012 at 2:01

Not the answer you're looking for? Browse other questions tagged or ask your own question.