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I have a few 'old time' LEDs from the late 80s and early 90s. The red and green 5mm LEDs (amber was a rarity, blue was 'impossible' back then). Not being very smart, I used to test them with a 9V battery without any resistor and, surprisingly enough, they always outlived the experience.

Fast forward to the third millennium: I bought several dozens transparent 'high-efficiency' (or should I call them 'high brightness', hard to tell without a datasheet) LEDs on the Web. They are superbright, but the one time I tried to "Oh, here's a 9V battery: let's see what color this is" one of those, they almost instantly died after a faint flash that told me "I was blue, you #@@#!"

(Like this: https://www.youtube.com/watch?v=7IoyYj6BJlc )

Now, it is clear to me that a resistor is required to limit the current, but my question is about what exactly kills the LED, or put in another way: why do 'old technology' LEDs survive?

Is it related to the fact that 'old' LEDs exploited recombination between conduction and valence band of a sturdy PN junction, while 'new' LEDs are based on more exotic etherostructures that create quantum wells? Or is it because the 'old' manufacturing process used bigger dies, or thicker bonding wires, or materials that were so lossy that they provided enough series resistance by themselves?

I think I owe an answer to my two dead blue LEDs.

EDIT: just re-did the experience with an 'old' red LED: I can leave it on for seconds without a problem using the same battery that zapped the 'new' LED.

New EDIT : While I can let the LEDs light up for a few seconds, I managed to blow one up when trying to measure the current. So, they are harder to damage, but not immortal after all. Tried three - four more old LEDs and I can confirm that for at least one second they survive (appearently) unscathed. New LEDs die almost instantly. I will try later to measure the current in a more controlled setup, possibly with short pulses.

I love the smell of burning GaAs in the morning.

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    \$\begingroup\$ Maybe you have bought a newer 9V battery than the old nearly-dead one you had years ago... \$\endgroup\$ – Brian Drummond Jun 12 '16 at 16:24
  • \$\begingroup\$ You might have supplied the 9volt into those LEDs for microseconds back then...!! \$\endgroup\$ – soosai steven Jun 12 '16 at 20:43
  • \$\begingroup\$ Nope, just tested right now, with the same battery: red LED stays on for seconds without problems. \$\endgroup\$ – Sredni Vashtar Jun 13 '16 at 6:22
  • \$\begingroup\$ Led with built in resistor? Measure the current through the LED. \$\endgroup\$ – Passerby Jun 13 '16 at 7:16
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    \$\begingroup\$ Since you are still experimenting: If you did the I-V curve for both diodes you might find that the older ones have a higher series resistance, which perhaps lets them live longer. \$\endgroup\$ – George Herold Jun 13 '16 at 15:36
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Yes, newer LEDs are also static sensitive. I learned this the hard way when testing a batch of blue SOIC chips with (unknown to me) a soldering iron with defective ground which was later found to be floating at >30V. I can assure you that the LEDs did not work after this experience and it wasn't heat as a single touch to one side of a diode at even 100C ruined it. Some started flashing like demented strobes, some just died completely.

Incidentally newer LEDs based on quantum wells are also highly sensitive to ionizing radiation, I learned this by reading about folks venturing into the ruins at Tchernobyl and Fukushima. The white LEDs in their caving lamps would often begin to flicker and eventually fail, at survivable (for humans) radiation doses. Silicon carbide ones are less so but still eventually fail, rumor has it that Cold War era LED technology is still used today on the ISS Zvezda module and the Progress spacecraft. I did also find that some GaN based blue LEDs can be used as varicaps, in some cases with no effect from brightness loss. The mechanism can generate 100pF changes comparable with an expensive part.

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If you really want to know, you have to repeat the experiment with LEDs you got a datasheet for and with current measurement. Compare the measured current with the datasheet, especially with absolute maximum ratings. Search the datasheet for different duration of maximum current.

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There are many different reasons why they might fail more quickly. But it all comes down to overheating in the semiconductor junction in the device; Let's enumerate some of these:

1) The new devices are brighter, so that could mean the die is larger to give a bigger emissive area, a larger device will flow more current and thus will look like lower resistance device. When connected to 9V is will appear as a larger load dumping more heat into the same thermal structures. It's temperature rises more quickly and thus dies sooner.

2) The new devices flow more current due to a difference in process, higher current means more power consumed which means that ... well, you do the simple reasoning.

3) The higher efficiency LED is designed for lower current limits because ... well it doesn't need higher current to generate the same amount of light. That means that it will be more sensitive to over current.

Essentially there is nothing to be learned here. The "experiment" is not controlled, you're not examining the device before or after the "test" and more importantly you've only characterized these devices as "old" and "new". Which manufacturer is that?

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Modern 'high brightness' LEDs are designed to be strobed at high frequncy. This helps cool the junction as they are not on 100% of the time.

Usual control of modern LEDs as well as strobing is by current not voltage control. All (modern) LED driver circuits are current control not voltage control.

Due to this they are far less tolerant to over-voltage than old LED packages (some of which had broad voltage ranges across the diode junction), and far more volatile when a fixed DC voltage is applied across the junction without being strobed.

Check the data-sheet for max. gate current on modern LEDs they are quite low resistance and will try to draw more current than they can dissapate as heat and consequently fry themselves very quickly.

Check out these datasheets to see the V/mA levels - you will see quite clearly that luminosity is controlled by current. Give a modern LED unlimited current and it works too hard and as you saw after a brief flash is pffffttt!

http://docs-europe.electrocomponents.com/webdocs/0026/0900766b80026dcc.pdf

http://docs-europe.electrocomponents.com/webdocs/14a3/0900766b814a37c6.pdf

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    \$\begingroup\$ There are a few weird statements in this post, Geoff. LEDs have always been diodes and always run from current limited circuits. Strobing doesn't cool the junction. Strobing allows high peak currents while maintaining low average currents to give the same brightness in a multiplexed or PWM display while preventing the junction overheating. LEDs don't have gates. \$\endgroup\$ – Transistor Jun 13 '16 at 16:20

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