Fading LEDs?

I think you have hit the nail on the head. Sounds like poor quality 5mm white led of no name brand being driven hard. By the way what current are you putting through the string?

Thanks Epke and txmedic 3338.

Can any one suggest me a good 5mm white LED manufacturer - tried and tested ?

thanks,

Madhav

Have you calculated the size and rating of your electrolytic properly? Would you be better off with a slightly more sophisticated filter?

You'd probably be better off moving most of your series resistance to limit the current in the LEDs to the bridge side of the electrolytic.

?? ~240VAC / x-capacitor + resistor / bridge rectifier / elecrolytic capacitor

you likely want to redesign the chain something ...

/ bridge rectifier ± resistor:2 / N(εZ) x (ceramic cap / ± small resistor) / elecrolytic capacitor -- i'm not shure

but the not transformed direct rectification kills semiconductors at the DC side faster than transformed 1

you dont want to chain too many LEDs in series coz they have not matching parameters as below ! simulation !

LTspice simulation (/!\ not veryfied that everything is safe down here /!\)

R-C - Chain / Line - Noise
/!\ i don't design such systems but i've done misc. stat. signal proccessing /!\

"Real Fact"#947 on my Diet Snapple bottle cap states "4,000,000 individual LEDs light up the America's Got Talent stage". I guess someone knows how to do it. As previously stated by others, your problem could be related to poor quality LEDs, but my money is on the probability that you are not effectively removing the heat they dissipate. The cooler they run, the longer they last. Try mounting some of them on a thermally conductive heat sink with a thermally conductive adhesive and see if they last longer.

LEDs are always fed with constant DC current source to maintain LED junction temperature constant otherwise it will thermally runaway. If AC supply voltage fluctuates that is fed to DC constant current source, the constant current source will absorbed the AC voltage fluctuations and maintains output as constant DC current to LEDs. So LEDs will emit same lumens. LEDs always need to be fed with DC current source only.

This is because whenever a LED fails in the string it fails as a shot but not as a open( That is the reason they are connected in series). Once one LED fails the total voltage across the LED string decreases by 3Volt DC but DC constant current source will reduce its output DC voltage by 3.2Volts and maintains constant current (20mAmp).

Your answer is precisely correct-however, you missed my point. Even with a properly constructed constant current source, the temperature at which the LED operates is determined by the thermal resistance from its junction to ambient temperature. As this thermal resistance is reduced, the device operates at a lower temperature and its life is extended.