-=> On 09-06-96  08:26, Roy J. Tellason said to Wayne Ray,<=-
-=>"About LEDS (Help...,"<=-
 II> Hi, Roy;
 WR> LED's come in all sorts of voltage ratings, with different 
 WR> current ratings too. 
 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.
Some of the ones you end up with in their assortments (at least at one time) 
would show oddball readings across the pins and would take up to maybe five 
volts across them before you saw any light coming out.  I had one that 
wouldn't light up until there was twelve volts across it.
 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!
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.
Hmm,  that's only around 10 mA,  not very much.  You can make that work but I 
prefer to go a little higher,  maybe around 20 - 25 mA as an upper limit.  
For a 12 volt supply I'd use 470 or 510 ohms.
 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.
I know that.  I have a curve tracer in the other room and know how to 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
Dunno why you are writing this to me instead of to him...
 II> ... Quick!  Hand me that solar-powered flashlight!   :-}
Was it dark when you were looking at the from: and to: lines?  :-)
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