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.

well my resistors are 0.25 W .. I will get 0.5 W tomorrow. thanks for the help!

well my resistors are 0.25 W .. I will get 0.5 W tomorrow. thanks for the help!

did upping the resistor wattage help you dirtdriver?

i just tried it with .5W and they are still verry hot.I just don't see the reason!

Curious, have you measured the voltage drop across the resistors and LEDs individually?

Then you will need to accurately calculate the V drop (measure it with a meter) across the IR, and the Resistor. Each group should be close to the same. Then calculate the current draw of each to get the watts being dissipated by the resistors.

quick calculations say that the v drop on the Ir should be around 1.7v so you should be using a 33 ohm resistor, but I bet a 100 ohm would work fine if you have some handy to try.

Each one should be the same? why?

I measured it and its like this:

I gues the USB port isnt exactly 5 volts

so 3.9 /30 = 130mA, 3.9*130=507mW

oh man 7 mW above the max and the resistors are capable of cooking my dinner

its back to the store then..and again thanks for the help!

dirtdiver wrote:

 

Each one should be the same? why?

Each group, so each resitor should meassure 3.9 v drop, assumeing each resistor is identical. And each Ir should messure 1.7, again assuming they are the same values. IN your first drawing you have 3 in parallel.

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

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?

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!