I have some LEDs I would like to hook up but am not sure which resistors I need. Below is the info. I will just be running them off a 12V source. The lady at Radio Shack didn't really know what she was talking about and sold me .5 watt, 220k Ohm resistors and the LED just barely lit up. Also what resistor would I need if I hooked up the circuit to one resistor and then out to say four LEDs. I've already posted in the electronics from, but I know there are some guys who know about this stuff here too.

Features
SUPER LUMINOSITY BLUE LED(InGaN / Sapphire)
STANDARD T1 3/4 LED
I.C COMPATIBLE
LOW POWER CONSUMPTION
WATER CLEAR PACKAGE
Specification

Emitting Color: Blue

Luminous Intensity: 3000mcd@20mA TYP

Viewing Angles: +/- 15 degree ( See it from the pictures! 30 degree's 3000mcd = 20 degree's 4500mcd!! )

DC Forward Current: 30mA Max

DC Forward Voltage: 2.8-3.8V

Wavelength: 460-465nm

Reverse Voltage: 5V

Soldering Temperature: 260°C for 5 seconds


Thanks for your help.

 

you need at least a 500ohm resistor.

 

Is that to run just one LED?

 

That amount (500 ohms) will limit the voltage down from 12 volts so that you will not blow the LED. That is for one LED. How many do you have and how will you hook them up series or parallel?

 

yes 500ohm resister per led....but you should wire them parallel.

when they are wired in series, if one blows they all shut down. right?
i could be wrong.

 

Yes, in series, one blows out, the connection is broken and therefore, they all go out. It makes it harder to find the problem spot.

 

500ohm is not a standard value

I commonly use 470ohm for resisters though
stripe colors are yellow-purple-brown

if you need any www.jameco.com is a large component supplier that professors at my school recommended to us and have used couple times now.

exact link to 470ohm here 99cents for bag of 100

 

anything from 330 ohms to 3.3k will do fine, depending on how bright you want the LEDs to be.
a 330ohm resistor in series with it will draw 34mA:
(12v- 0.7V)/330ohms = 0.03434A

so that's more than you'll want to mess with.
since it specs 20mA basically, you'll want:
(12 - 0.7) / 0.020 = R = 565 ohms. pick the closest value you can find to that, which 560ohms is a common value.

one of the cheap sites I buy parts from is a place called Goldmine Electronics.
they've got all kinda of stuff (including your blue LEDs for cheap).. website is www.goldmine-elec.com . (their resistors are on page 73 of their PDF catalog. follow the links from the main page.)