This is one of my most visited posts, so i decided to make an updated version.(2011.02.18)
DISCLAIMER, this is a “quick and dirty” fix that works, and might not be recommended by an electrician, and might be against new electricity safety guidelines. I am not an electrician, I just know this works, but you are doing it on your own risk. That said, here we go:
Are you having problems with a flickering external monitor on your laptop?
Does the flicker go away if you remove the laptop charger?
If you answered yes to the above questions, you probably have a ground loop or ground between the laptop and the monitor.
This is because in the best situation we would like to have only one ground point in any system. Since your laptop is grounded and your monitor is grounded, and we connect these two with a cable, we are essentially creating a loop antenna. This “antenna” picks up noise from the electric system and this is what affects the analog signal on the monitor. This problem can also create an audio hum in your soundcard.
What you need to do is completely disable the ground for one of these devices. For example the monitor. If you are using a European 240V connector just put some scotch tape over the metal strips on the sides of the connector in the socket, that is the ground. Check that the tape does not break when the connector is inserted. You might have to put several layers of tape on the connector. Your monitor will still be grounded, as long as it is connected to the laptop with a VGA or DVI cable.
If you have other connector types you can take a spare power cable and remove the ground prong for the monitor, or you could just use an adapter that only has 2 prongs but accepts three, like this one:
After this the flickering should go away.
BEWARE your monitor is now ungrounded by itself, so don’t use it in the bath-tub.
P.S. If anyone really likes this fix and wants to buy me a coffee or a beer as a thank you can do so by clicking here, totally optional of course but it would make my day! -I love beer :)