Nearly all European TV sets were built like that until well into the
80's. The problem: Knobs in the olden days had these little set screws. When someone touched a knob with wet hands or when li'l Joey tried to poke the small hole in the knob with a metal piece ... *BZZZZT*
Some Philips Lumileds specifications describe the protector blob thats on top of their surface mount LEDs. I've seen it on the inside of some non-name surface mount indicator LEDs while inspecting the soldering.
You can get a cheap ultra high efficiency green or blue LED and test it. Reverse breakdown might be only 4v to 10v. Give it a very low forward current, say 100 microamps, after reverse current and you'll see it flicker or short itself out. It will seem fine at high currents but it won't last long. I've made a few variable power bicycle/hiking lights and I've found that flickering at low currents predicts rapid aging. I also found that ESD carelessness on my part predicted the flickering and aging, even for LEDs with internal protectors :P (Not sure if I toasted the LED or the MOV. Either way it's dim after two years.)
Interesting, thanks. LED forward voltage exhibits a lot of 1/f noise when you drive them too hard, which seems to be caused by moving dislocations and interstitial atoms, i.e. the LED disassembling itself.
Cheers
Phil Hobbs
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