LED life.

About a couple of years ago I made up some edge lighting for a meter in a pal's small recording studio. To replace the tiny 24v tungsten which were forever blowing. Used 9 diffused 5mm white LEDs - 3 each in series and one series resistor per chain of three setting the current at something like 3 mA which was plenty bright enough - and the end result was very even illumination. Much better than the tungsten.

Very unlikely to be the PS as that is used for other indicators too - which were ok.

Checking things here on my own PS, it's the LEDs themselves which have all gone low output. Some more than others - but all well down on the spares I have from the same batch - bought off Ebay. And rather bluer than the new spares. The series resistors are all in perfect condition - no signs of ever even having got warm.

I've had the odd LED fail - but never anything like this. Any explanation?

Reply to
Dave Plowman (News)
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There's a large glut of LEDs on the market which have failed batch testing - they usually end up on Ebay, but they do also turn up in the reputable electronics supply chain, and get built into products with a much longer design life than the LEDs actually have (often low volume high value specialist products).

Your best bet to avoid this is to buy from reputable electronics supply chain, but that's still no guarantee.

It sounds like the ones you bought were produced extremely cheaply; poor quality LEDs and too little phosphor. At a couple of years old, they can't have done even 20000 hours, even if used 24/7. What is their forward current rating? What is their operating temperature likely to be?

Reply to
Andrew Gabriel

Which would suggest the fluorescent material has deteriorated and UV is escaping??

AJH

Reply to
news

Shit LED's via ebay, can't quite imagine LEDs running at 3ma I have some low current ones that aren't very bright they run at 2ma.

Reply to
whisky-dave

We had two GU10 LEDs that did the same, very much reduced output after a year or so of use. All the electronics is inside the 'bulb' but I still don't really understand how they managed to go so much dimmer without actually failing.

Reply to
Chris Green

Sounds like the phosphor isn't stable so that they are now leaking more blue light. Never seen a problem like that with any LEDs I have bought but I am careful not to buy them from untrusted sources on eBay.

If you were running them continuously at 30mA and Tj around 120C then I could believe that they might struggle after a couple of years but run cool at 3mA I can't see how they should fail apart from an intrinsic manufacturing defect. The odd one may fail taking out a chain but what you are describing sounds like a low quality unstable phosphor.

LED indicators I installed in the mid 70's when they were expensive and new are still going strong after all this time and ~5mA.

Even the ones for low power lighting I have running at 750mA on heatsinks have been fine for years of extended use.

Dodgy batch of unknown provenance. Dodgy capacitors are *much* worse.

Reply to
Martin Brown

partial film failure on the actual die is also an explanation have heard.

Possibly made from contaminated wafers way back in the chain, probably dropped through a few QC sieves before reaching ebay.

One possible weeding test is very low current performance, uA , duffers wont fire up until get a big enough kick, posh leds will light with virtually nothing.

If they didn`t spend the dosh on the dice , premium phosphors would have been a no as well.

Power density within an LED is very high.

Nichia can still command 60p an LED for a reason, best dice , best phosphors , tight binning and reliability have a cost.

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Reply to
Adam Aglionby

My understanding is that White LEDs are actually blue LEDs which also produce UV which causes a yellow phosphor to fluoresce. The 'white' light is a combination of the two.

So, perhaps in your case it is the phosphor which is under performing, thus the blue shift.

Reply to
Terry Casey

AIUI older "white" ones can be UV LED with phosphors to turn the UV into R+G+B, or newer ones are blue LED with phosphors to turn (some of the) blue into R+G.

Reply to
Andy Burns

They were certainly inexpensive. But I've bought lots from Ebay direct from China or whatever that seem to have lasted OK.

Oh dear. ;-)

I did ask, and the studio is powered down when not in use. So I doubt rums for much more than 50 hours a week on average.

Can't find the details but remember it as being a normal 20mA or so - and that they are run well below that as they were too bright. My brief was to give very even illumination hence using 9 of them on a 95mm wide meter.

Being an additional meter - a stereo PPM - mounted in a pod, room temperature plus their own heat. I doubt a moving coil meter generates much heat even when hammering the end stops. ;-)

Reply to
Dave Plowman (News)

Not really. The breakthrough for making white LEDs was getting a high efficiency blue emitter that was short wavelength enough to excite the yellow phosphor. UV leds have always had (much) worse efficiencies and have had to jump through more hoops to get to manufacture.

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AFAIK there was never a time when UV LEDs were used for white. They were too exotic, expensive and inefficient compared to shortwave blue.

Reply to
Martin Brown

They're mostly blue LEDs with phosphors to turn (some of the) blue into R+G. There's no UV from these.

There are UV ones - they are used where more control of the colour is required, and/or the light source for the R+G needs to be in exactly same place as the blue so they don't focus in different places. They're significantly more expensive to make (and possibly less efficient due to increased Stokes shift, although I haven't actually compared them).

Reply to
Andrew Gabriel

Well it was many years ago, when I bought some super bright red leds for power indicators on some home built devices. After about a year or so, they had dimmed. Its of course hard to say exactly what had happened but it seemed to me that although LEDs do run quite cool you can still feel heat on their bodies when in use, so maybe there is either a chemical change in the material, or the interface with the body material that changes over time due to either cycling or just prolonged heating.

Of course whit LEDs are more complex, being more than one colour in the package, and I'd not be surprised if one colour goes lower emission than others as it ages. I suppose we are still in relative early days with this form of lighting as light sources not just indicators still. Brian

Reply to
Brian Gaff

I have seen some pretty old indicator LEDs that are no longer in the first flush of youth. But then they were usually driven fairly hard. This is the first similar I've seen that gave problems after such a short time - especially since they were gently driven.

Reply to
Dave Plowman (News)

Have a cheapo NAS with a blue LED indicator on the front. A night it would be dazzleling and light up the entire room. I "got at it" and inserted a series resistor to drop the brightness. In the process I found it would glow with the leakage current through dry fingers lightly holding the wire. Eventually settled on 50 uA (micro amps) and it was still on the bright side.

Been playing with fairy lights and a Raspbery Pi as you do at this time of year. Raided from a commercial battery operated 240 LED set I've got 6 chains of 40 parallel connected bright white LEDs, each chain takes no more than 20 mA flat out = 0.5 mA/LED. The Pi provides programable, independant PWM control of each chain.

Reply to
Dave Liquorice

Dave Plowman (News) has brought this to us :

Tungsten is happy on AC, LED's are not. Any chance the 24v might be AC, or have an AC component?

Reply to
Harry Bloomfield

Yup. I changed the old red 5mm LED in my Quad 405 to a modern blue one, and was amazed how low I had to set the current to make it acceptable.

Reply to
Dave Plowman (News)

No. It's a fairly standard PS for that sort of thing - used to drive relays too. Definitely DC. And well smoothed.

Reply to
Dave Plowman (News)

Yes, I have a row of 6 diagnostic LEDs on my Raspberry Pi heating controller using a few different colours. There's one blue one (indicates boiler burner on, i.e. flame colour), and ISTR I ended up with 1/20th of the current through that one compared with the othees, and it's still much brighter.

It wouldn't surprise me if it was more efficient to make a red LED using one of the blue ones, a red phosphor, and a blue blocking filter, inspite of the Stokes shift and other losses.

Reply to
Andrew Gabriel

I think that must be done with some of the "clear straw hat" LEDs.

I have a multi-coloured R/G/B/Y set from ADSA, designed to run from 2xAA with no limiting resistors anywhere (unless buried within the LEDs themselves) so all colours run at the same ~3V, in fact I run them at 4V from a PSU, they've lasted 2 years of christmas without any burning out so far.

Reply to
Andy Burns

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