EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: viperidae on March 09, 2016, 08:28:08 pm
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Does anyone know what would cause a cheap 12V LED bulb to flash the individual LED's randomly?
There's nothing flash in it, it's got a 68R series resistor followed by a 9 parallel strings of 3 white LED's in series.
Each string of 3 LED's is actually one 6 pin 5050 SMD LED.
I'm not trying to fix it, it cost me $2 shipped from China. I'm just curious.
My guess is it's because there is only a single current limiting resistor (which is easily dissipating 250mW, and started to turn brown from heat) which has overheated the LED's and changed the forward voltage drop of some of the slightly in the process. (it worked fine a year or so ago)
When these LED's start conducting, the voltage across the resistor increases due to more current flow and that causes another set of LED's to stop conducting, lowering the voltage drop across the resistor...
They should have had 9 resistors for the 9 series strings of LED's, but I suppose that would have cost $2.01 instead of $2.00.
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I have about a dozen GU5 LED assemblies I used to replace the reading lamps at my 12V camp. About four of them have taken to flickering which seems like a high percentage for lamps that aren't used much. Likely a wire bonding/thermal issue.
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I took them apart and when powered individually, the LED's behave themselves.
I've seen the same thing happen to cheap 240V GU10's, which were made up from series-parallel LED's as well.
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I've always assumed that leds flicking on/off was the bond-wire separating from the die as it heats up. Either manufacturing defect, or over-heated. In other words, it's a dying led.
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Just seems weird they don't flicker when they're powered separately and they do it from cold as well.
I'm not convinced with my theory, since the flashing is so slow compared to how fast an LED can turn on.
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If the LEDs themselves seem to work, then probably PCB has some intermittent connection.
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It's completely normal for Chinese ebay leds to flicker. They are just failing.
When LEDs are manufactured, they are binned according to brightness (i.e., efficiency). Then there is so-called reject bin, because unfortunately, the process yield is not perfect.
Random Ebay leds come from either the lower quality bins or the "trash" bin. Some are dead on arrival, some flicker. I'm not sure if the flicker is caused by wire bonding failures or on actual die, but in any case, they are dead and work somehow by pure luck.
Don't get LEDs from Ebay, or if you do, expect a significant percentage of failures.
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If the mystery remains you should post to bigclive of youtube, cheap china LEDs are his thing.
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Yes, I agree with @Siwastaja. You can just expect cheap Ebay LEDs to have considerable failure and flaky performance.
Watch some of BigClive's videos to see some dissections of cheap Chinese LED products and analysis of their failure modes.
https://www.youtube.com/channel/UCtM5z2gkrGRuWd0JQMx76qA (https://www.youtube.com/channel/UCtM5z2gkrGRuWd0JQMx76qA)
http://www.bigclive.com/ (http://www.bigclive.com/)
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I ran into these myself, they work fine when not wireing a whole bunch together. but after 5 or so, i get fliker..so i put a cap to smooth out the voltage to them. seem to help but not sure how well that will help after you start putting a whole bunch of them together.
But that is what you get when you get bulk from china, at quater the cost. Work for what i need it for so good enough.