-=> On 09-09-96  19:15, Roy J. Tellason said to Ivy Iverson,<=-
-=>"About LEDS (Help...,"<=-
 -=> On 09-06-96  08:26, Roy J. Tellason said to Wayne Ray,<=-
 
Hi again, Roy;
 
[snip]
 
 II> All that I remember seeing are in the 1.5-1.8 V range also,
 II> even the high-brightness ones found at Radio Shack.
 RJT> 
 RJT> Some of the ones you end up with in their assortments (at least at one
 RJT> time) would show oddball readings across the pins and would take up to
 RJT> maybe five volts across them before you saw any light coming out.  I
 RJT> had one that wouldn't light up until there was twelve volts across it.
 
True... I wouldn't get one of those "bargan packs"... IMO, they are
probably the rejects that didn't meet any of the standard specs... You are
likely to get red, green, yellow, IR, (two different wavelengths), and if
you are lucky, maybe even a bipolar red/green.  I's just as soon leave
those toys to the experimenters who will play with anything they get.  I'd
MUCH rather get them in packages of 1 or 2.  That way you know what you
are getting.
 
 RJT> Eh?  Most of what I've seen is spec'd in the 1.6 - 1.8 volt range,
 RJT> with the exception of the stuff that's designed with a built-in current
 RJT> limiter,  but that's not the same thing at all.
 
 WR> If wired in series (N of one hooked to P of the next, and so 
 WR> on) 3 volt LED's can have four across a 12 volt circuit. 
 
 II> You ALWAYS need to use a series resistor if they don't have 
 II> internal current limiting!
 RJT> My feelings on this exactly.
 
 II> For a single typical LED, I find that 1000 ohms in series from 
 II> a 12-volt DC source works just fine.  Lower values would be 
 II> needed if several are connected in series, depending on the 
 II> applied voltage and current limitations of the LED's.
 RJT> 
 RJT> Hmm,  that's only around 10 mA,  not very much.  You can make that
 RJT> work but I prefer to go a little higher,  maybe around 20 - 25 mA as an
 RJT> upper limit.  For a 12 volt supply I'd use 470 or 510 ohms.
 
True, but then I don't try for maximum brightness in in "indicator"
application, unless it's somewhere that may be difficult to see, such as
where direct sunlight may be striking it.
 
 RJT> Where do you find 3 volt LEDs?  And what's going to limit the current
 RJT> in that setup?  Have you ever seen the voltage vs. current curve of one
 RJT> of those parts? It's damn near a vertical line!
 
 II> That's why you need a series resistor.
 RJT> I know that.  I have a curve tracer in the other room and know how to
 RJT> use it. 
 
 II> To figure the values required, use the formula R=E/I for the
 II> resistance, then P=E*I for the power to be dissipated in the 
 II> resistor.  P = Power in Watts, E = Volts, I = Current in 
 II> Amperes and R = Resistance in Ohms.  Note that in almost all 
 II> cases for these figures, P and I will be decimal fractions.  
 II> Round to 3 decimal places.  (In most cases, a 1/4 Watt resister 
 II> will work just fine.)  
 II> Catch you later...
 II>    Ivy
 RJT> 
 RJT> Dunno why you are writing this to me instead of to him...
 
Sorry, actually, I was writing it to anyone who may have use for the
information.  :-}
 
 II> ... Quick!  Hand me that solar-powered flashlight!   :-}
 RJT> Was it dark when you were looking at the from: and to: lines?  :-)
 
Musta been.   :->
 
Catch you later...
   Ivy
 
 
 
... A flashlight is a case for storing dead batteries.
 
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