overheating resistors

P.S. with the Vr=5V , so if that is the maxi Voltage , i wont need a resistor, so is it? ..I cant even find the datasheet

Vr is reverse voltage max, and yes - you do need the resitor.

I will assume the voltage drop of the Ir's is about 2 volts, so that leaves 3 volts droped across the resistor, so 3v / 30r = 100ma, 100ma * 3v = 300mw. What size are your resistors? For this they will need to be a half watt (half  500 mw and 1/4 is 250 mw). So I can see why they may be over heating.

dirtdiver wrote:

 

And isnt that the resistors job here- to limit the current , how to limit the current going trough the resistor?

Yes, you are limiting the current the LED is allowed to draw.

dirtdiver wrote:

 

well if I use a 120ohm resistor the current going trought the LED will be about 20mA , right?

then .25mW resistors are perfect?

Give it a try and see

it seems to be rowking fine with .25W, 150 ohm resistors.Its not as bright as before but i hope it does the trick (im making  "freetrack helmet")

if it doesnt work i will try the 1W 30 ohm resistors and see if they preform better.But for now im happy about the fact that im powering the leds form a usb port and the computer is still on

Thanks for the help!

Right, the higher the resistance the less current is alowed to flow and the dimmer the LED. However it will reach a point to where it will just go out. Usually less than 15ma LEDs don't want to light, depending on the LED.

Also note that to truely dim LEDs you should use PWM as mentioned by someone else. You can only dim so far with resistors.

sorry about the post earlier dirtdiver after i read it it clearly didnt make any sense the way i worded it.    yes the resistor regulates current.  what i was trying to say is that you should up the wattage of your resistor since the mw calculated was in the ballpark of the max for the resistor to something on the line of 2 watts or bigger.   This would allow the heat being generated from removing the current to dissipate better but keep the ohms to what ever you calculate based on your current needs.  like you stated earlier upping the ohms will decrease the brightness.

It works ok with 150 ohm, 0.25 W resistors, it gets detected by a  web cam (i havent tested it with a visible light filter yet, but i think they are bright enough as it is).

I will post a video with the final project - A head tracking system for PC games-it allows you to look around with your head as a controller.

nice project dirtdiver let us know how it goes.  post a video when its done.

http://www.youtube.com/watch?v=qTUmQz5iWOQ here is the "beta" version , the camera doent have a IR-pass filter yet and its still good without direct light coming on it

Very cool. Good job.

very impressive!

thank you!

thank you!