## Simple LED question... if you know how to do the math!

A few years ago, I found a plastic pumpkin with a "faux flame" set up inside of it. I liked the idea of it and am trying to replicate it to make a few faux flame torches around my backyard for halloween this year. The idea is that there is a small 12v fan near the top of a tube. There two or three leds glued around the top of the tube. The fan blows a flame shaped piece of fabric up and the leds light it up to look like fire. (possible future instructable?)
Unfortunately, I don't know much about circuits and the math of electricity. I found a small fan (12vdc .13a) and 2 red leds (2.6v). I hooked them up in parallel with a 12v 1amp wall wort coverter but nothing happens.
I have done a little reading online and wonder about needing a resistor in front of the leds.
I wanted to hook it up in parallel to get the fan to work at full speed to make the fabric really blow.
From all the circuits I have seen online, this one seems really simple, if I knew what I was doing. Can anyone draw me up a simple diagram? What am I goofing up on here?
Thanks!
Matt

active| newest | oldestNow, the two // branch will get 0.02*12 = 0.24W for the led, Imagine you have a fan like this one : http://www.globefan.com/6015new.htm , you see that it takes 0.15 to 0.25 mA. So the PSU will have to give 0.5 amp. Since it is rated for 1A, it will not over heat, and maybe (if it is a cheap poor quality one), it will give 11V instead of 12, but everyting should be working fine.

BUT, using a 12V fan (a 2 or 3W fan, the one used for cooling computer) will make a big lot of airflow and certainly a bit of noise. So you can either choose the lower speed/noise/airflow ones and/or add a resistor in serie with the fan. I will not give you the calculs for the resistor, but you can either use two fan in serie (12V each, great), or use a 100 Ohm potentiometer (which will have to be at least 1/2W)

Ok, now, you must promise me to write an instructable about this :D (if you don't, I may try to write one, I have the leds, fan, resistor, but I would need the silk tissue...)

take pics, show us :)

U=R.I

Where U is tension, in Volts, I is intensité, in Amperes, and R is resistance, in Ohm, so :

1 ohm = 1 V/A (Volet per Amper)

Your DC power supply gives12V. You know that the tension (voltage drop) over the leds are 2.6V, so with 2 leds in serie, it make 5.2 volts. since in the whole seri (including the resistor), then tension will be 12V, you want the tension over the resistor to be 12-5.2 = 6.8V.

the tension (U) over the resistor of value "r" depends on the intensité in the branch :

U=R.I

so : R = U/I

Then, if your leds are high brightness leds, which needs 20mA (0,02 amperes), it gives :

R = 6.8/0.02 = 340 ohms

The power lost in the resistor will be :

P=U.I= 0.136W, so 1/4Wat resistor is not enough, you'll want 1/2 watt.

Also, you can put three leds isntead of two, then you re-do the math and you'll see that more power is used in leds, fewer power is lost in the resistor, so 1/4W will be enough..