Date:         Fri, 26 Jul 1996 13:45:13 -0700
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         natasha!kombi@adobe.mv.us.adobe.com
Subject:      Re: basic electronics question

> jeez, why didn't i pay attention more back in high school?
> 
> question:
> 
> i have a super bright , jumbo LED that reqires 2.6v @ 50ma
> 
> how do i figure the resistance of the dropping resistor for a 12v 
> source?
> 
> frustratedly yours,
> -dan
> 
V = IR (?)

You actually are running about 14v in the car, and normal LED's have
about that 2.5v drop across them, so you want a resistor that will pass
50ma at 11.5v.  If you round to 12v (after all, resistors are commonly
only 10% accurate), this works out to 240 ohms -- 12 = (.050)x(240).
Notice that by doing this though, you mostly heat up the resistor rather
that produce light.  You might want to series a bunch of these (like 5)
which will give five times the light for the same current draw.

malcolm