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

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



Well the first test went fairly well. I got everything running. Except that the fan wouldn't push the cloth. I ended up using a 1/4 watt 270 ohm resister on the two leds. That was all they had. The problem, I believe, is that the 40mm fan just can't push enough of a volume of air to move the "flame". I really wanted to see if I could make a little torch, so I could use it as an accent light in my halloween set. But, I think the physics are against me. Plan B is to move up to a bigger size fan. And, see if I can actually find the thin silk a friend of mine recommended. Anyone have any internet sources for small amounts of silk?

So, I should put 2 (or 3) leds together with 1/4w resister and then hook the fan up in parallel with the led circuit? Will that keep maximum power into the fan? or should that one be hooked up with a resistor as well? Thanks for your great explanation! Matt

To make it simple : If the fan and leds + resistor are parallel, they with get the same tension, which is the one output by the PSU, which is 12V.

Now, 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 :)

Okay, I will put it to the test this thursday or friday, after a run to the local resistor shop. And, if it works, we will get up my first instructable. Please check my numbers here: I need to hook up 1 .24v resistor to the two leds that are in series to make up one circuit. The fan, which is rated at 12v .09a, is the second circuit, which is hooked up in parallel to the first circuit and both are powered by the 12v 1a ac/dc voltage adapter. The fan should be very fast and the lights will be bright and the silk will flap in the breeze! I hope. Thanks again for your help. Pics soon!

>I need to hook up 1 .24v resistor to the two leds you need one (anything standard close to) 340 ohm 1/4 W resistor, which will be in serie with the two leds If the fan in not strong enough, you need to find lighter cloth (remember that you are looking for something light and translucent), but you also can make a fanduct (make a cone with some paper, so the output diameter is smaller than the fan diameter, this will accelerate the air (same airflow in smaller area mean higher speed).

Oh, ho! I was wondering when the ohms thing was going to come in. So, the 10 ohm one I picked up needs to go back. Not to be begging for a basic intro to electronics but... How do ohms figure into the equation?

you can certainly find good basic electronic course online (ohm's law would be first or second lesson), but ohm law is :


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)

The wall wart is 12v at 1 amp.


9 years ago

You put a resistor of value "r" in serie ith the leds. The only probleme is how to calculate "r".

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 :

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..

Whats the output of the wall wart in Voltage and Amps?