overheating resistors

You need to ask yourself "Do I need to drive with 100ma or not?". 100ma is the maximum non pulsed current allowed. As you already know, then the resistors need to dissipate about 500 mW. If the answer is "Yes", then buy ceramic resistors of the value required, but with at least a 2 watt rating.....they will still get warm though and may need some cooling if enclosed.

The Arduino should be able to pulse the output pin and use a PWM method to control the brightness and to actually reduce the power needed to go through the LED.

To learm more look here:- http://arduino.cc/it/Tutorial/PWM

This may or not be appropiate for your application of course, only you know that.

regards

Andy

oh, yeah ok i thought that you meant the IR and the resistor should have the same Vdrop, in my case they are identical so all 3 readings are the same

Do you mean that i could drive the ir led with  ..say..50mA , which would mean that the voltage drop is going to be about 3.6V, given the 180mW of the LED,wont that hurt the LED?

Depending on your needs, but you don't always have to power your LEDs with the max amount of current specified in the sheet - just don't go over.

For example I have several projects that I use 2K 1/8 watt resistors on my power indicator LEDs simply because using the calculated resistor of 680 ohm 1/2 watt makes the LED way to bright. Plus I like to consume as little power as possible, allowing me to use smaller wattage resistors.

Like a mentioned earlier, try a 100 ohm or even a 500 ohm resistor.

yes i am powering it off of the USB port, but isnt it drawing as much current as it needs (i thought that its ok as long as i dont draw more current that it can provide)

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

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

then .25mW resistors are perfect?

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.