Drive a LED from Atmega168 without RES

[Scarr]

Thanks everyone

OK looks like we are there, even though LED resistor calculators say 68R gives 52ma, in the real world 39R acutally gives 54ma, I don't want to know why

So running it at 54ma for short periods (50ms) with a 39R should be OK unless anyone is going to say different?

Thanks again for all your help and I love electronics even though when I think I understand something, I find I don't!

 

[audioguru]

You should not drive the LED in the opto-coupler at its absolute max continuous current of 50mA. Then it will also be at its absolute max allowed temperature if its ambient temperature is 25 degrees C.

How did you calculate a current of 43mA with a 82 ohms current-limiting resistor? The darlington driver (which one?) IC has an output voltage loss of about 0.75V and you don't know the exact forward voltage of the IR LED. The datasheet says the forward voltage is a max of 1.5V but it is probably less. What is its typical voltage?

Assuming that the forward voltage is 1.3V at 43mA then the voltage across the current-limiting resistor is 5.0V - 1.3V - 0.75V= 2.95V. Then for 43mA the resistor value is 2.95V/43mA= 68.6 ohms.

 

[Pax Writer]

Just a question here: Couldn't he sink the LED directly and simply pulse it?
Edit: Beaten to it by a gazillion posts... Never mind my post.

 

[audioguru]

Just a question here: Couldn't he sink the LED directly and simply pulse it?

Only if he limits the current to 3A and programs the microcontroller for max 1us pulse width for 300pps.

He doesn't say which darlington driver he has and doesn't say if it sinks or if it sources.