Determining the correct resistor for LED's ?

I am trying to do some LED projects but have no clue as how to determin what size resistor I need for maximum MCD output?

Does anyyone have a cheat sheet for 12vdc-18vdc or an easy formula?

 

R = E/I

Resistance (R) = Voltage (E) divided by Current (I)

I will make some assumptions in my example.
.025amp LED.
(1) LED 3.6V drop across the LED
12V supply

12V - 3.6V =8.4V of drop you need.
8.4V/.025A = 336 Ohms

Resistors are rated in Watts.
Voltage X Amperes = Watts
12V X .025A = 0.3Watts

You'd need a 336 Ohm 0.3Watt resistor (10% ?)

*edited to add:
Corrected my current draw 'given'

3.6V drop would be a white LED - much less for
red, green, or yellow

336 Ohms is not (I don't think) a standard resistor size. You'll want to go with the next LARGER standard resistor.

[ March 26, 2006, 10:35 AM: Message edited by: FiveFlat ]

 

Wow I never realized that it's basic Ohms law thanks.

what is a typical amp draw for a single led?

 

Most are 20mA. But every LED should have specifications when you purchase it.

 

Yeah should?????????

But like a dummy I bought the Radio shack multi pack whitch has absolutly no specs on the packaging (didn't realize it untill everything was solderd and it was time for the resistor) Oops. Thinking about cutting my losses and buying the right stuff.

although 750ohm resistors work just not at full potential

 

The resistor value isn't all that touchy.
With the voltage you are working with, 12-18vdc anything in the 750-1000 ohm range will work just fine.

It is best to use the higher resistance as this will insure a long life.

The difference in percived brightness is very small using a resistor that has a value less than 1000 ohms.

 

I always use resistors in the 750-1000 range.


Stay cool and run steam....

 

This is helping out alot.
I'm actually going this on one of my Williams U33C.

Does anyone know how to make them flash and occolate

 

True a 1000Ohm resistor will work fine, you can tell the difference in brightness, but only if they are next to each other probably.

You can make it occilate with a simple modified RC circuit. You can probably google it and get a good schematic to work off of.

 

Jim at Richmond Controls can help you out. They have many different circuit boards to help you.

Stay cool and run steam....

 

I just tried a simple google query and it resulted in pages of answers.

Try this link:
http://www.qsl.net/yo5ofh/hobby%20circuits/led_circuits.htm