Lots of failing LEDs - Page 1
Forum section: electronics / projects

author: FireBird
author_rank: Regular Contributor
timestamp: January 16, 2015, 09:33:15 pm
content: I’ve bought more than 2000 ofthese 5mm bright, green LEDsand even though they are driven with around 8mA, about 10% fail. They flicker and then short themselves out. It’s a typical exponential curve, the majority dies within 10 minutes.What experience do you have? Is this just bad luck with a bad batch? Or usual figures for a missing quality check?For a beta test, I’ve bought several hundreds of the white ones from the same dealer and even though they were used in the 15mA region, no single LED failed.Cheers,Alex

author: Skimask
author_rank: Super Contributor
timestamp: January 16, 2015, 09:39:42 pm
content: Quote from: FireBird on January 16, 2015, 09:33:15 pmIf they short themselves out, what does that do to your power supply? If, in fact, theySHORTthemselves out.Let's see a schematic...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 16, 2015, 09:47:56 pm
content: I would think for cheap LEDs, a bad bond would be more likely, also with the flickering. Thermal failure then it just goes completely loose..Tim

author: FireBird
author_rank: Regular Contributor
timestamp: January 16, 2015, 09:48:36 pm
content: There’s no real need for a schematic: 3 LEDs and a 330E resistor in series, powered by 12V. In this configuration, I’m measuring about 2,8V across each LED. If one fails, the other 2 get a bit brighter.

author: Zero999
author_rank: Super Contributor
timestamp: January 16, 2015, 09:53:32 pm
content: Are you sure the power supply really is 12V?If so then you've bought some junk LEDs.

author: Skimask
author_rank: Super Contributor
timestamp: January 16, 2015, 09:55:15 pm
content: Quote from: FireBird on January 16, 2015, 09:48:36 pmIf they SHORTed out, your power supply would be SHORTed out, and the other 2 LEDs should go OUT after your power supply smokes.Since you say that 2 get brighter when one fails, that tells me you might be using a cheapo wall wart, with 12v showing the side of it...when it's loaded down. But actually putting out much more voltage under the light load of 3 LEDs.

author: FireBird
author_rank: Regular Contributor
timestamp: January 16, 2015, 09:57:53 pm
content: It's the 12V rail of a PC power supply. My Fluke is confirming that. The only thing I didn't check is if there are any spikes.

author: FireBird
author_rank: Regular Contributor
timestamp: January 16, 2015, 09:59:37 pm
content: Quote from: Skimask on January 16, 2015, 09:55:15 pm3 shorted LEDs would be 12V / 330E = 36mA. Where should the smoke come from?

author: Zero999
author_rank: Super Contributor
timestamp: January 16, 2015, 10:00:51 pm
content: Quote from: Skimask on January 16, 2015, 09:55:15 pmHe said they're 3 in series with a 330R resistor. This would give 11mA with a voltage drop of 2.8V per LED. If one of them shorts out the current will increase to 19mA which is near the maximum rating of 20mA. If another LED shorts out, the current will increase to 28mA which is above the maximum rating.

author: Skimask
author_rank: Super Contributor
timestamp: January 16, 2015, 10:03:04 pm
content: Regardless, according to that datasheet (however useless it may actually be), you're driving them closer to their peak than their nominal. +12V rail has a 10% tolerance on it as well, not to mention all the HF noise you can't see on a Fluke.12v + 10% = 13.2v + the HF noise, - 3.6v (Vf max for green according to the suspect datasheet) = 9.6v. 9.6v / 330 = 29mA. Assuming the resistor is actually 330ohm, which is probably a 5%, so, -5% on the resistor gives you 313.5ohms. 9.6v / 313.5 ohms = 30.6mA, just slightly above the peak.No, I wouldn't expect a mass of LEDs to die at slightly above peak forward current either, but these are also cheap eBay LEDs.'nuff said...

author: Skimask
author_rank: Super Contributor
timestamp: January 16, 2015, 10:03:30 pm
content: Quote from: FireBird on January 16, 2015, 09:59:37 pmDOH...no kidding right?

author: Skimask
author_rank: Super Contributor
timestamp: January 16, 2015, 10:05:09 pm
content: Wait are these actually in series, like Resistor-LED-Resistor-LED-Resistor-LED, or in series/parallel like:-Resistor-LED-12v in <-Resistor-LED--Resistor-LED-

author: FireBird
author_rank: Regular Contributor
timestamp: January 16, 2015, 10:06:21 pm
content: 12V - 330R - LED - LED - LED - GNDA quick conclusion is that it is not normal - even for cheap LEDs.

author: sleemanj
author_rank: Super Contributor
timestamp: January 16, 2015, 10:07:39 pm
content: If you have a scope, have a look at the 12v line, but it would have to be really bad I think.At least measure across that 330R and see what the V and thus I is (and of course measure the 330R to see if it really is 330R) and that you havn't fudged a decimal or something.More likely is you've got some reject-bin LEDs I think.

author: wraper
author_rank: Unknown
timestamp: January 16, 2015, 10:09:40 pm
content: Quote from: Skimask on January 16, 2015, 10:03:30 pmSkimask, seems that you got good weed today

author: FireBird
author_rank: Regular Contributor
timestamp: January 16, 2015, 10:13:01 pm
content: Quote from: sleemanj on January 16, 2015, 10:07:39 pmI've used two meters at the same time and also the measured current confirmed the calculated value – at least the average. I’ll do a check with my scope tomorrow to be on the safe side.QuoteIt seems so.

author: Skimask
author_rank: Super Contributor
timestamp: January 16, 2015, 10:16:43 pm
content: Quote from: wraper on January 16, 2015, 10:09:40 pm...just a little something extra in the solder smoke...

author: Skimask
author_rank: Super Contributor
timestamp: January 16, 2015, 10:18:07 pm
content: Can you hook up a few, apply some "percussive troubleshooting" to a few LEDs and see if you can get them to fail at will?

author: FireBird
author_rank: Regular Contributor
timestamp: January 16, 2015, 10:22:56 pm
content: So far I haven't seen one that seemed to have responsed to some more or less gentle troubleshooting.

author: lewis
author_rank: Frequent Contributor
timestamp: January 16, 2015, 10:27:32 pm
content: Cheap, crap LEDs I'm afraid.Your numbers are tight, 330R in series with 12V and 3 LEDs works out to about 18mA @2V per LED (which can happen with green LEDs), bit close to the mark there assuming the LED spec is correct (which of course it isn't).Try 470R.Or get some proper LEDs from Farnell/RS/Mouser/Digikey and try again. Don't moan they're more expensive, at the moment the ebay ones areinfinitelyexpensive because they don't work!

author: amyk
author_rank: Super Contributor
timestamp: January 17, 2015, 12:25:49 am
content: Or drive them with a real current-regulated source.In fact if you bought "more than 2000" you probably have enough to do some interesting analysis; they might actually work indefinitely at far lower currents in the 1-2mA range.

author: Psi
author_rank: Super Contributor
timestamp: January 17, 2015, 05:24:39 am
content: Quote from: FireBird on January 16, 2015, 09:33:15 pmMost likely the ebay seller is selling off the contents of the "reject" bin from quality control.And yes, im serious.

author: IconicPCB
author_rank: Super Contributor
timestamp: January 17, 2015, 05:44:04 am
content: Over the last few years I have bought close to 100 000 LEDs typically infant mortality has been in the order of about 50 pieces in total.I drive them from a 15V supply through 220 ohm resistor with red, infrared and blue in series at 20% duty cycle lowest frequency 12Hz.

author: sleemanj
author_rank: Super Contributor
timestamp: January 17, 2015, 10:46:29 am
content: Quote from: IconicPCB on January 17, 2015, 05:44:04 amThat sounds interesting, what's the application?

author: geppa.dee
author_rank: Contributor
timestamp: January 17, 2015, 11:07:16 am
content: Quote from: sleemanj on January 17, 2015, 10:46:29 amLiving organisms growth. The chlorophyll using kind. I'd guess.http://en.wikipedia.org/wiki/Chlorophyll#mediaviewer/File:Chlorophyll_ab_spectra-en.svg

author: digsys
author_rank: Unknown
timestamp: January 17, 2015, 11:14:19 am
content: He's only had ~2 negatives in the last 6 months related to quality out of 37,000 sales !! We'd have to assume he's not a shonky dealer.There's a very small possibility that you DID get a bad batch. What I would try is - wind the P/Supply down to say 6V, then wind up the voltage(watching current as well) and get an idea of their light<>current characteristics .. ie does the brightness slow down after a certain voltageetc etc. Take the voltage all the way up until you DO blow one or two or all !!! It will cost you 3 leds, but you'll find out how close to rating they are.

author: paulie
author_rank: Frequent Contributor
timestamp: January 17, 2015, 01:10:36 pm
content: It's surprising how little experience and knowledge demonstrated here regarding Chinese LEDs. Over the last few years I've purchased well over a million of these for a sign customer including a few hundred thousand from that same seller. The first thing to realize is those max current specs are copied from expensive Japanese devices and not applicable here. Pure fiction. At the very least cut in half.Another thing is the non-linear relation to current vs light output. Cutting current in half or even quarter the light output will be hardly affected to human eye. One thing about Chinese LEDs is they are very efficient if not durable.Last but not least is the infant mortality issue just mentioned. It's wise to "burn in" for a few hours at higher current (not too high). Bottom line is for about 1/20th the cost a tiny fraction of a percent failure is tolerable. Specially to those who are concerned with profit margins. It's unfortunate western hobbyists and weekend engineers expect Lamborghini quality and service for Nissan prices.

author: dannyf
author_rank: Super Contributor
timestamp: January 17, 2015, 02:07:05 pm
content: QuoteThere is something seriously wrong with that kind of fail rate. I have used thousands of leds and I would say maybe one or twice I have ran into a failure like that.So maybe the leds are sub-par quality or they are driven incorrectly (high rms current, high peak current, with an ac source, etc.)

author: madires
author_rank: Super Contributor
timestamp: January 17, 2015, 02:22:17 pm
content: Quote from: FireBird on January 16, 2015, 09:59:37 pmPoor little resistor12V * 36mA = 0.436W

author: Zero999
author_rank: Super Contributor
timestamp: January 17, 2015, 02:26:30 pm
content: Quote from: lewis on January 16, 2015, 10:27:32 pmThe forward drop is specified as 2.8V.VFcan vary between 2V and 3.5V for green LEDs, depending on the type of semiconductor used.

author: voja
author_rank: Contributor
timestamp: January 17, 2015, 03:06:05 pm
content: I bought more than 15K white LEDs from China, made some special purpose display and traveled to Kazakhstan to install it. After a month, more than 50% were burnt out (current was limited to 15 mA). Later the supplier attached this info:"Every chip is "burning" in time when working, with LEDs that means LED is losing brightness slowly. Generally first 1,000 hours of using is most critical - the burning is quickiest and from this burning can know quality of a LED. XW7C burns 30% in first 1K hours, MW7C burns 20% in first 1K hours, PWC burns 0% in first 1K hours and VW6C increases it's brightness after first 1K hours 10%. Further burning (5K hours, 10K hours, 20K hours etc.) is at much slower pace."Of course, the worst one, XW7C, was "my" type. Yes, 30%, my a**. More than 50% were burnt out.Bismarck said "Only a fool learns from his own mistakes. The wise man learns from the mistakes of others." So please don't be the fool like me and avoid this supplier:http://ledz.com

author: amyk
author_rank: Super Contributor
timestamp: January 17, 2015, 04:29:46 pm
content: Those are talking about the phosphor degradation ("losing brightness"), not absolute failures.It would be interesting to know what the actual failure mode of these is - are the dies too small to handle the current, the wirebonds badly done, or something else?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 17, 2015, 06:54:16 pm
content: Losing brightness is due to a number of atomic or molecular failures in the semiconductor and phosphor, aggravated by high current density, light flux and high temperature. Dislocations in the semiconductor cause recombination to increase, effectively short-circuiting the desired emission mechanism (which has to do with direct vs. indirect semiconductors; I forget how all that works exactly). Dislocations in the phosphor can have the same effect, though the "short-circuiting" allows non-emissive decay paths for the excited states.Spectrum also changes, for example if the phosphor is a bulk mixture of different colors, or a crystalline or polycrystalline blend of different color centers; different colors age at different rates.Tim

author: FireBird
author_rank: Regular Contributor
timestamp: January 17, 2015, 07:10:38 pm
content: Today I did some burn-in testing of the remaining LEDs of the last package I’ve received. No single LED failed. I think that I can conclude that at least one of the older 1000 pcs bags contained really nasty stuff.

author: David_AVD
author_rank: Super Contributor
timestamp: January 17, 2015, 11:36:07 pm
content: A few questions: How are the LEDs mounted? On a PCB? How tightly packed? Sealed enclosure? Ambient temperature? Soldered by hand?

author: FireBird
author_rank: Regular Contributor
timestamp: January 18, 2015, 08:48:59 am
content: You can reproduce it on a simple breadboard at room temperature.

author: dannyf
author_rank: Super Contributor
timestamp: January 18, 2015, 11:37:23 am
content: QuoteA draw of bad luck, I guess?

author: SeanB
author_rank: Super Contributor
timestamp: January 18, 2015, 12:08:58 pm
content: I bought some cheap UV led's and tried them at 20mA for a 6 month period, on 24/7. At the end of 6 months all were very degraded, compared to the one that was deliberately unused. They had started off with a white LED in the series string, and it was still as bright as original. The cheapies were barely visible.