-=> On 09-06-96  08:26, Roy J. Tellason said to Wayne Ray,<=-
-=>"About LEDS (Help...,"<=-
 
Hi, Roy;
 
 WR> LED's come in all sorts of voltage ratings, with different 
 WR> current ratings too. 
 
All that I remember seeing are in the 1.5-1.8 V range also, even the
high-brightness ones found at Radio Shack.
 
 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. 
 
You ALWAYS need to use a series resistor if they don't have internal
current limiting!  For a single typical LED, I find that 1000 ohms in
series from a 12-volt DC source works just fine.  Lower values would be
needed if several are connected in series, depending on the applied
voltage and current limitations of the LED's.
 
 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!
 
That's why you need a series resistor.  To figure the values required,
use the formula R=E/I for the resistance, then P=E*I for the power to be
dissipated in the resistor.  P = Power in Watts, E = Volts, I = Current in
Amperes and R = Resistance in Ohms.  Note that in almost all cases for
these figures, P and I will be decimal fractions.  Round to 3 decimal
places.  (In most cases, a 1/4 Watt resister will work just fine.)
 
Catch you later...
   Ivy
 
 
 
... Quick!  Hand me that solar-powered flashlight!   :-}
 
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