[Laser] Standard LED question.

[wa4qal at ix.netcom.com]

> Date: Thu, 14 Apr 2005 20:07:45 -0700
> From: "KD7JYK, 49H7KR" <kd7jyk at earthlink.net>
> Subject: [Laser] Standard LED question.
> To: <laser at mailman.qth.net>
> 
> Are all LEDs rated at a 20mA draw which is what I see in all the
> manufacturers specs?  

No, some are rated at more, and others are rated at less (Mostly the 
really tiny ones).  It really depends on the size of the chip, and how 
hard the manufacturers feel safe in pushing it.

> I have three dozen 3,000mCd red LEDs but no specs.  

20mA is probably safe, although you might be able to push them to
50mA if you watch the ambient temperature.  Maybe.  I've been able
to push some of the large 3,000mCd LEDs up to 50 mA, although your
mileage may vary.

> If
> I run them off of 12V with about 1200 Ohm resistance, I get a current draw
> of 15mA and an input voltage of about 1.8V.  I can lower the resistance to
> increase current to 20mA and thus the brightness, but I'd rather run them on
> the safe side.

15mA should be safe.  That's only a power level of 27 milliWatts, which the
chip should be able to safely dissipate (Don't forget that the optical power 
is part of the dissipation, too.).  

> What is the proper procedure for determining voltage\current of the average
> LED?

The voltage will be some complex function of the current flowing through the
LED.  Unfortunately, this may be somewhat difficult to predict, since it 
depends upon the individual chip, as well as things like the temperature of
the chip, and is, in general, some type of modified exponential function.

The best way to know how much current a particular LED can safely take is
to read the manufactuer's data sheet.  Failing that, the next best way is to
test one to destruction (Of course, if you only have one...).  Another approach
would be to gradually increase the current, while monitoring the voltage or 
optical output to watch for thermal effects, although this gets a little ambiguous.

It used to be that you could get slightly higher efficiency out of an LED by 
operating it on a pulsed current source.

> Kurt
> 
> Rosetta Proving Ground, Nevada
> Hell's Laboratory

Dave
WA4QAL