White LED problems

[piglet 14]

Do white LEDs die fairly quickly?

 

I've had some running in my word clock for some months now, and the ones which are always on are becoming more yellow, and there is more variation in colour in some of the other than I'd like.

 

One is starting to flicker occasionally, when others parallel in the same circuit are not.

 

The LEDs were these. I'm running them @ 5v with 120 ohm resistors which should be more than enough.

[enl 227]

White LEDs, as all components, vary in quality. Most white LEDs use a phosphor to produce a yellow that mixes with the blue emit by the LED forming whte. Thre are a number of different phosphors used, depending on the exact blue produced by the LED, and the cheaper ones tend to be single phosphor types. Over time, the phosphors may change properties due to heat or just age, even in high quality devices. Running the device cooler (lower peak current and lower average current) will reduce this. Read the data sheet carefully, and be conservative with design to reduce the risk of early failure.

 

Also, it is generally not good to parallel LEDs. It is done, but is bad practice due to the nonlinear V-I curve. Slight unequal sharing of current (which is inevitable) leads to the LED getting a littlemore current heating more, which shifts the curve in the direction that makes it worse. Same mechanism as thermal runaway for an IC, which is where the second breakpoint comes from on the safe operating region graph for most power IC's. LED brightness is determined primarily by current, so 'identical' units in series should be the same brightness, and the limit resistor provides a little negative feedback to keep things stable. Or, use a constant current source for each chain of LEDs, and the drop in the V-I curve will tend to reduce power dissipation and self heating as the devices get warmer.

 

Anothee possibility, if only one is going, it may have been borderline to begin with. A lot of the components that pop up on ebay, even from the more reliable supliers, and sometimes from reputable suppliers like digikey or Newark, are relabled seconds or are from less reliable manufacturers. The supplier may not know this, because substitutions can happen up the chain several steps.

 

Recently, there was an artical in IEEE Spectrum looking at this problem (specifically relabeled devices pulled from scrap and counterfiet devices), especially in the military supply chain. It happens a lot more in the commercial supply chain.

[piglet 14]

 

Also, it is generally not good to parallel LEDs.

 

Hmmm - I don't know how else I can do it! I'm very new to this. I thought msp430->shift register->darlington array as sink was an OK way to be able to turn on or off sets of 3 of 4 leds

 

 

I'm not surprised that ebay is a dumping ground for sub-par components. I'd have thought a seller specialising in LEDs wouldn't want the bother of the -ve feedback on duff things though.

[enl 227]

I see. Schematic helps.

 

You don't have the LED's paralleled with a single limit resistor. You have the LED/limit resistor pairs paralleled.

 

What you did is correct.

 

ANY component will degrade with time/heat/use.... White LEDs degrade fairly quickly due to phosphor aging/burning, like the phosphors on an old CRT-type display, but higher quality ones hold up better.