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LED resistor help

25ma is a lot of current through that LED. That's the maximum rated current, the more you put through it, the shorter the lifespan of the LED. Also, light output from a LED is not linear with the current - there's really not much gain in brightness going from 15ma to 25ma. If you're going for maximum brightness, then 25ma is acceptable, but it will probably have about 95% the same brightness running at a much lower current. Two 100 ohm resistors in series (200 ohms) will give you a current draw of about 15ma - plenty of brightness for most people.

You could also put two LEDs in parallel with each other and a 100 ohm resistor in series with them and each would draw 15ma. Two LEDs in series would probably work with a resistor between 35 (bright) and 80 (dim) ohms, but this has a higher chance of thermal runaway so personally I would put the LEDs in parallel.
 
ValeX said:
So, you say i could put 2 100ohm resistors in paralell? is this what you had in mind?

[img=http://img71.imageshack.us/img71/7601/singleledsetup7sq.th.jpg]

im going to draw up one for putting the 2 LED's in parlell with a 100ohm resistor next, check back.

Thanks again

ValeX
Click to expand...

Those would be two resistors in series, which will work just fine.

The 120 ohm would work, but you would be putting alot of current through the LED, shortening its life with really no appreciable increase in brightness. The 1/8 or 1/4 watt is merely a maximum rating for the resistor. You probably won't dissipate more than 1/16 of a watt, so whatever power rating you want to get is fine.
 
V=IR

(Vsource-Vforward) = I * R


so 5 - 2 (to be safe) = .025 * R

3 = .025 (R)
3/ .025 = 120

So you need a 120 Ohm... Myself I would probably use a 180 ohm and if it lights up bright enough, call it a day.

I like to work with 1/2 or 1/4 watt myself...

Don't use 2 100 ohm resistors in parallel, that will create a 50 ohm resistor that will overdrive the LED. you could use them in series to create a 200 ohm resistor though.
 
I would suggest against putting two LEDs in parallel. Because of variance in manufacturing, one may have a slightly higher voltage than the other, and you'll end up with unequal brightness.

I would suggest going somewhere between 100 and 200 Ohms for your resistor. 200 might reduce the brightness too much, and 100 will likely overdrive your LED.

If you're using both LEDs, you'll want to either (1) give them each their own resistor, or (2) put them in series with each other and with a resistor. At 20mA and 2.1V apiece, you'll drop 4.2V and sink 20mA.....0.9V taken by resistor....you'll need a ~50Ohm resistor (the setup will then be 5V---Resistor----LED---LED---GND)

 
ValeX said:
(i'll post pics somewhere when im done)
Click to expand...
You had better post pictures--and before you're done, too!

I worry a bit about the fan. If the mouse is PS/2, you may run up against the current limit for PS/2 devices (which is 100mA @ 5V). If it's USB, a fan that small shouldn't be a problem. Great idea!


 
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