We used an external power supply (DC plug) with 8V. After some time all the lights went off. We immediately removed the DC plug. Since then as soon as we plug in a 5V pin, the arduino shuts down.
Whats wrong?
What do you mean "plug in a 5v pin"? What did you connect to what?
Does it work when powered just off USB? If so, can you upload sketches?
What board are you working with? And official or clone?
I mean a 5V output, while powered by USB. I can upload sketches, but the 5V output doesnt work.
I work with an official arduino mega.
The 5v pin on the mega is tied to the power to the chip, so you definitly are getting something there. Measure voltage between the 5v pin and ground.
What are you connecting? How much current is it trying to draw? Could the thing you're connecting be shorted, or trying to draw more than USB can supply?
Actually, if you had something that was sucking down a lot of current when it shut off while running from 8v barrel jack, that would explain what you originally saw, with it working for a while and then turning off - you were putting a heavy load on it, and since the regulator is dropping 3v at heat, it overheated - typically they either foldback the output or turn it off entirely until it cools in that case, rather than the part failing)
you are right, i get 5V out of it but only if it isnt connected with the hardware (relay,sensor...)
The problem is that before I used the DC jack, everything was good and worked perfectly... So it is not possible that the currant is too high.
What is the device you have connected to the 5v rail? That way we can look up the datasheet and see how much current it is expected to draw. Could the connected device have failed? It's not uncommon for burned out devices to act like a near short, drawing much more current than they do when working correctly.
Also, you can get more current out of the 5v rail when powered off barrel jack than USB - USB is limited to 500mA typically, but you can get around twice that out of the regulator, for a short time (limited by heat dissipation).
Draw up a schematic and post it. There's no point playing 20 questions until you do at least that.
So it is not possible that the currant is too high.
Do not assume this. Things do not always fail catastrophically immediately after exceeding the maximum ratings, they can suffer degradation over a prolonged period of time.
You have mentioned nothing of any substance that would help identify the cause of failure. We need to know exactly what hardware you have hooked up in what way.