need some help with a mini little tiny project

minnieman · Post by **minnieman** » Mon Sep 19, 2011 4:09 pm

i just need to power 2 3v leds and i am using 4 AA batteries. how should i aline them to power the 2 leds? i have them in a straight line right now but no power is getting to the leds. any help?

jdmlight · Post by **jdmlight** » Mon Sep 19, 2011 6:53 pm

minnieman wrote:i just need to power 2 3v leds and i am using 4 AA batteries. how should i aline them to power the 2 leds? i have them in a straight line right now but no power is getting to the leds. any help?

You need a resistor with LEDs, always. LEDs aren't so much concerned about the voltage, but rather the current. (to a point, you have to have at least the minimum voltage specified and there is a maximum) Typically you'd aim for around 20mA current. (.02A)

To calculate the resistor you need, take the voltage you have (4xAA = 6v), subtract the voltage that the LEDs take (6v-3v=3v), and divide that by the current. (3v/.02A = 150 ohm) You can pick a higher value resistor, but lower will probably burn out the LEDs. (higher value resistor = dimmer LED but also = less current drawn, which may be important for a portable) This isn't magic, it's just the V=I*R formula. (V=volts, I=current in amps, R=resistance in ohms)

In this case, you can't put the LEDs in series (straight line) because each takes 3v, so if you have two of them, you're trying to calculate the resistance necessary based on 0v, which ends up being infinity. Which is impossible.

tl;dr: Go here.

minnieman · Post by **minnieman** » Mon Sep 19, 2011 8:46 pm

my friend there was to tl;dring here. i didnt know you needed a resistor with leds, know i know! MANY THANKS!

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need some help with a mini little tiny project

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