Author Topic: LED failure. LED is blinking  (Read 2333 times)

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Offline StefanTopic starter

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LED failure. LED is blinking
« on: January 04, 2016, 02:17:32 am »
I have a cheep LED flashlight. It consists of five white LEDs, one of which has started to blink. I'd guess about 10-20 times a sec or so, with a constant frequency, as far as I can tell. The odd thing is that the LED's are all wired in parallel and the others are working fine. The little PCB is in rather poor condition, but I don't think it is a slack joint, as the effect is constant no matter what I do to it.

What kind of failure mode can cause that? What causes the oscillation? The cirquit is as simple as it gets. Two AA batteries in series connected to the LEDs. There isn't even a proper switch. The flashlight is turned on and off by losening or tighening the two parts of the housing that are screwed together (which causes the positive pole of the battery to push against a pad on the pcb).
 

Offline djQUAN

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Re: LED failure. LED is blinking
« Reply #1 on: January 04, 2016, 02:31:31 am »
The LED failed internally with the bond wires causing an intermittent connection.

When cold, the bond wires make contact, when the die heats up, it opens and then the die cools down. Then repeat as long as power is on.
 

Offline alsetalokin4017

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Re: LED failure. LED is blinking
« Reply #2 on: January 04, 2016, 02:36:52 am »
"If thine own right eye offends thee, pluck it out."
The easiest person to fool is yourself. -- Richard Feynman
 

Offline SeanB

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Re: LED failure. LED is blinking
« Reply #3 on: January 04, 2016, 05:56:12 am »
Root cause is stress on the leads when soldering, likely the insertion into holes a little out of the tolerance, causing stress on the leadframe inside the case which then softened with the soldering, and started the bondwire cracking. Then with time and thermal cycling it cracked through to make the thermal oscillator you have. Get another LED the same colour and casing, or replace them all if you cannot get matching colour( pretty hard, they are generally using the reject units that failed colour binning there in any case)
 

Offline StefanTopic starter

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Re: LED failure. LED is blinking
« Reply #4 on: January 04, 2016, 08:53:24 pm »
Thanks, that makes perfect sense. A thermal-bond-wire-oscillator-switch. I'm gonna patent this and demand money everytime an LED is failing :)

Being short of white leds, I've followed Matthew and soldered the offending led out and casted it from me, I woudn't want the hole flashlight to go to hell, after all.

 

Online Kleinstein

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Re: LED failure. LED is blinking
« Reply #5 on: January 04, 2016, 09:11:56 pm »
I had a similar, though slower (e.g. 0.5 Hz) blinking in an IR LED used in wireless headphones. Here there where several LEDs in series and all turnd of due to partially failing LED.
 

Offline Audioguru

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Re: LED failure. LED is blinking
« Reply #6 on: January 05, 2016, 01:03:45 am »
I am lucky. I have a cheap Chinese flashlight with 24 LEDs in parallel and after replacing its Super Heavy Duty very weak battery cells with powerful name-brand alkaline cells it still works fine except its cheap on-off switch is intermittent.
 


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