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Failure mode of LEDs in overcurrent situation

Started by Andrew T. December 6, 2019
I'm curious about what happens to an LED when it is exposed to a
moderate overcurrent situation.

This question came about as I was troubleshooting some small white LED
lights I built using 3 5mm LEDs and a current limiting resistor to run
on 12Vdc (supplied by a lead-acid battery charged by solar).  After some
time, many of these strings would start to flicker or blink.

I was about to post a question about that, when I redid my resistor
calculation.  I had originally calculated that a 100 ohm resistor
would be sufficient to keep the current around 20mA (assumed 12.5V -
10.5V total LED drop).  But on recalculation, that would result in 30mA
(the absoulte maximum rating) at 13.5V, the low end of charging range.
Adding another 47 ohm resistor seems to have helped.

So now I'm curious what happens inside the LED when the current
exceeds the absolute max current by a modest amount (i.e., not the
obvious overcurrent of leaving out the resistor entirely).  The LEDs
I'm using seem to return to normal operation when the current is
reduced, and seem to produce a momentary open-circuit condition during
overcurrent, presumably from overheating.

Anyone know what's happening to the semiconductor material in this
situation?

Thanks,
--Andrew

-- 
Andrew Turnquist, Short Tract, New York, USA (USDA Zone 5)
(Remove numbers and .invalid for email address)
"Do what you can with what you have where you are." -T Roosevelt
On Friday, 6 December 2019 19:08:00 UTC, Andrew T.  wrote:
> I'm curious about what happens to an LED when it is exposed to a > moderate overcurrent situation. > > This question came about as I was troubleshooting some small white LED > lights I built using 3 5mm LEDs and a current limiting resistor to run > on 12Vdc (supplied by a lead-acid battery charged by solar). After some > time, many of these strings would start to flicker or blink. > > I was about to post a question about that, when I redid my resistor > calculation. I had originally calculated that a 100 ohm resistor > would be sufficient to keep the current around 20mA (assumed 12.5V - > 10.5V total LED drop). But on recalculation, that would result in 30mA > (the absoulte maximum rating) at 13.5V, the low end of charging range. > Adding another 47 ohm resistor seems to have helped. > > So now I'm curious what happens inside the LED when the current > exceeds the absolute max current by a modest amount (i.e., not the > obvious overcurrent of leaving out the resistor entirely). The LEDs > I'm using seem to return to normal operation when the current is > reduced, and seem to produce a momentary open-circuit condition during > overcurrent, presumably from overheating. > > Anyone know what's happening to the semiconductor material in this > situation? > > Thanks,
Flashing I'm told is caused by wire bonds detaching & moving due to thermal expansion. If so I suspect at some level of reduced i a flashing LED ought to work again, just at reduced output. NT
On 2019-12-06, tabbypurr@gmail.com <tabbypurr@gmail.com> wrote:
> > Flashing I'm told is caused by wire bonds detaching & moving due to >thermal expansion. If so I suspect at some level of reduced i a flashing >LED ought to work again, just at reduced output. > > > NT
Thanks. I'd never really considered the actual wire bonds themselves. That would make sense. Of course, now I wonder what the effect of a sudden physical shock would be, if that might knock the bond apart.... --Andrew -- Andrew Turnquist, Short Tract, New York, USA (USDA Zone 5) (Remove numbers and .invalid for email address) "Do what you can with what you have where you are." -T Roosevelt