Well...turns out the answer was as simple as a bad batch of LEDs from the vendor. Guess its nice they admit it...but it isnt the first time according to the client. Time for a new LED source me thinks..... Thanks to everyone that offered advice and opinions Howard Winter wrote: Alan, On Mon, 9 Apr 2007 16:06:14 -0700 (PDT), alan smith wrote: > Have a board with several LEDs on it....ok....about 100. Getting some strange failures, in that some of them will dim out...but not die. Its the standard configuration of the current limit resistor hooked to +V, and a NPN switching ground. I had them short the collector to ground to see if that fixed it (ie...transistor not turning on hard enough) no effect. I think...need to verify, that there was no change in the voltage drop across the current limit resistor. They put a higher voltage across the LEDs and no change (that I saw on an earlier failure). These have been running for better part of a week now, and all of a sudden they do this. Granted its one or two out of 100....and they are chinese manufactured (I did say to them....the quality can be questioned...but usually its brightness between batches) and they have used them in the past without issues on a different product. > > Just wondering....what might make them....sorta fail dim like this? I believe LEDs do "age" with use - and raising the current through them accelerates this. I have played with them by overdriving them - some will dim almost immediately (a second or two) if you approach their absolute maximum current (presumably overheating) and they come back again after they're disconnected and cooled. I haven't tried measuring if there's any long-term dimming, but I have an Electronic Bath Control (controls water flow to tap or shower, temperature and amount) and it has green 7-segment LEDs showing temperature and volume. It's 16 years old, and some of the segments started dimming after about 5 years, and they're now so uneven that it's virtually unreadable. I did wonder about dismantling it and seeing if I could replace the displays, but the front panel containing all the electronics is riveted together and I don't want to end up destroying it! But it just goes to show that LEDs dim with age, and in your case it may be premature ageing caused by overdriving and/or poor quality of manufacture. Cheers, Howard Winter St.Albans, England -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist --------------------------------- Ahhh...imagining that irresistible "new car" smell? Check outnew cars at Yahoo! Autos. -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist