[sdiy] LED Drops

[Roy J. Tellason]

On Monday 10 December 2007 19:43, Grant Richter wrote:
> You might look for a chip called a REF200 by Burr-Brown
>
> http://focus.ti.com/docs/prod/folders/print/ref200.html
>
> It contains a 100 microamp constant current source.
>
> If you put it in line with any diode, it should show the diode
> forward drop voltage (at 100 uAmp), but limit the current to a safe
> level.

My one digital meter (a Simpson 461) has a similar feature,  when one is using 
the resistance ranges.  Maximum current output on the two lowest ranges is 2 
mA,  and it goes down as you go to higher ranges,   200 uA for the next one 
and so on.  Maximun voltage is 200mV for the lowest range and 2V for all of 
the rest of them.  That's been pretty safe for everything I've used it on,  
and even shows a very dim illumination on LEDs,  excepting IR ones.  Even if 
you can't see it,  the indication on the meter in one direction and not the 
other is a pretty good indication of which wire is which.  :-)  Of course,  
this doesn't do much for those that need more voltage,  but so far it's 
worked on just about everything I've tried it on here.  What was particularly 
interesting was how intense this one blue one I'd salvaged turned out to be.  
So out of curiosity I took another meter and measured the current that the 
first meter put into it,  and it was under 1 mA!

How hard is it to make a constant-current source anyhow?  Can't be that 
complicated...

-- 
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space,  a critter that can
be killed but can't be tamed.  --Robert A. Heinlein, "The Puppet Masters"
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Information is more dangerous than cannon to a society ruled by lies. --James 
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