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I have an LED in a home project. It's in a simple loop with a resistor and the GND and V+ ends connected to appropriate pins on a microcontroller. The LED works properly for extended periods of time - including cycles of both on and off at appropriate times according to the software on the microcontroller - but then eventually stops working, meaning that it stays off even in cycles when it should be on.

The LED is physically connected to two lead wires, soldered with "pigtails" between wire and lead of the LED. I originally thought that physical connection was broken, but I've opened it and there was no signs of that being loose. I re-soldered it anyway, with no change in behavior. I also had the full thing apart at a time when it was working, and I could tug and twist any segment of the wiring without causing it to fail, so it seems that nothing is loose.

Now the really strange thing, in my opinion, is that the LED will always start working again when I try to measure voltage across the leads. Apparently even the slightest touch of the multimeter probe will "fix" the problem and the LED goes on (provided, of course, that I run the test during an "on" cycle). I only need to touch one side to observe this, which I believe is always the V+ side. There is voltage when I take the measurement, although that's redundant by that point since the LED is on, which also indicates a voltage.

So, it seems that there is voltage across the LED when there should be, that the LED is not burned out because it can be made to work again, that the physical connection is apparently solid. I'm beginning to suspect some sort of failure of the LED itself, but I have not idea what that would be. I could cut it out and replace it, but that's a bit of work that I'm not sure will solve the problem.

Is there some sort of internal failure mode of an LED that would explain this type of behavior?

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  • \$\begingroup\$ What happens if you touch another LED in parallel with the first? \$\endgroup\$ – Transistor Apr 14 at 14:59
  • \$\begingroup\$ It's currently working, so if I do it right now I feel very sure that it will keep working and both lights will run. I don't know how to make it fail, so I will have to wait to do the experiment suggested by @Transistor. \$\endgroup\$ – Brick Apr 14 at 22:11
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Sure, the leadwire bond could be broken inside the LED (either at the LED or the leadframe end). Or the die attach could have fractured (the bond between the semiconductor die and the metal leadframe).

If so, the same thing should happen when you wiggle the leads or prod them with something non-conductive. Image from Wikipedia:

enter image description here

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  • \$\begingroup\$ As I tried to note originally, I can wiggle and twist the leads at the LED and the wires anywhere else that I've tried. I cannot make it fail when it is working. That seems to eliminate your answer as I understand it and according to your own second paragraph. \$\endgroup\$ – Brick Apr 14 at 22:14
  • \$\begingroup\$ You've already resoldered it. You can try soldering leads to the LED and monitoring the voltage. If you ever observe it exceeding the normal Vf of a few volts (tops) then the problem is inside the LED. Otherwise there's a small chance it's something weird with the presence of the probe. \$\endgroup\$ – Spehro Pefhany Apr 14 at 22:47
  • \$\begingroup\$ I see, that was not clear. I did the twist-and-tug test before resoldering and repeated it after resoldering. I did the resolder in this case even though it seemed that everything was secure because I was running out of ideas. \$\endgroup\$ – Brick Apr 15 at 12:19
  • \$\begingroup\$ Did you understand my suggestion to ‘instrument’ it by adding leads to the DMM so the voltage is continuously monitored? \$\endgroup\$ – Spehro Pefhany Apr 15 at 12:22

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