> 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.

I guess you've checked a few other simple things: make sure LED current not too 
high, resistor power rating suitable. Also check circuit can not inadvertently apply 
reverse voltage or more than a few volts. If all else OK might be bad LED's from 
manufacture or static damaged or heat damaged during soldering.

-- 
Brent Brown, Electronic Design Solutions
16 English Street, St Andrews,
Hamilton 3200, New Zealand
Ph: +64 7 849 0069
Fax: +64 7 849 0071
Cell: 027 433 4069
eMail:  brent.brown@clear.net.nz


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