i am new to leds so what i want to do is how do i work out what resistor i need when putting a single led on a 12v supply.And if i want to put more than one led in series would i do something different

 

"Led resistor calculator" here is one of the many links
returned by the search engine "Google"

http://ourworld.compuserve.com/homep...Bowden/led.htm

 

basically it depends on the color of the led. red leds work on about 2V, while blue or purple need about 4V. say you want to run a red led on 12V - you want the led to run on 2V (any more could blow it up!). so what you need is a resistor in series to drop the 12V down to 2V at 20mA (that's the average led current draw). so you want the resistor to have 10V across it, so there's 2V left over. So:

R=V/I

R=10/0.02

R=500 ohm, so use something close like a 470 ohm.

If you were to have 2 led's in series, they would need 2V each, so same current, double voltage. The datasheet will give you all the info - what voltage and current it should run on.

If you wanted to use a different battery (like a 9V), work out that the resistor would need a 7V drop.

Hope it's helpfull.

Josh.