I was reading this thread http://arduino.cc/forum/index.php/topic,22680.0.html and a thought occured to me.
Whenever calculating the current limiting resistors for simple LED drivers, I've always worked by R_{lim} = ( V_{cc} - \sum{ V_{f} ) / I_{f}.
How much of a margin do people leave between the voltage at the driving rail and the sum of the forward voltages? Would it be practical/prudent to remove the current limiting resistor all together?
Pol.
Would it be practical/prudent to remove the current limiting resistor all together?
No defiantly not you always need some form of current limiting circuit, normal a resistor or constant current driver.
http://www.thebox.myzen.co.uk/Tutorial/LEDs.html
Even if the sum of all the V(forward) is equal (or even slightly greater than) the supply rail?
Now we're on a roll maybe I could finally get someone to explain Ebers-Moll, transconductance and the Early effect to me in a way I can understand (jk). Aledgedly, I've got a masters in electronic and electrical engineering, but that was 11 years ago and I've been officially been a programmer since before I graduated...
Pol.
YES :- Even if the sum of all the V(forward) is equal (or even slightly greater than) the supply rail?
If indeed that were the case then the design is rubbish and you shouldn't do it. This is because the voltage dropped for a given current in an LED is not constant from device to device or with age of device or temperature. Look at the data sheet it says TYPICAL for the voltage drop.