[Laser] Standard LED question.

[Chris L]

Dear Group,

Just on this maximum current thing with LED's... they do vary fairly widely.

In 1988 I purchased some 3000 mCd LEDs from the Stanley Electric Company 
Limited in Japan - deep red type - 660 nM. These typically take 50 mA of 
standing current and can be pulsed to 300 mA peak. The IR diodes on 880 nM 
(GaAlAs) and 930 nM (GaAs) are generally much sturdier, and can take 
typically 120 mA continuous. The new Luxeons really are phenomenal, though - 
385 mA continuous on their 1 watt red (630 nM) model; 1 Amp continuous on 
their white 3 Watt model. However the red Luxeon is faster and more linear 
in transfer characteristic.

I guess you could either try an unknown LED with a constant check on 
temperature rise, or destructively test one if you have a sufficient reserve 
of spares, taking note of the current at which they fail and then backing 
down by a safe percentage...

It's a bit like my late dad's story about how he learned to drive a Model 
'T' Ford in the 1920s - you drive them up to the speed where they rattle 
badly, then back off the speed until they're just 'below the rattle'!

Chris Long, Melbourne Australia.

>From: wa4qal at ix.netcom.com
>Reply-To: wa4qal at ix.netcom.com,Free Space LASER Communications 
<laser at mailman.qth.net>
>To: laser at mailman.qth.net
>Subject: [Laser] Standard LED question.
>Date: Fri, 15 Apr 2005 10:46:25 -0400 (GMT-04:00)
>
> > 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
>
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