Once you get to the Vf (forward voltage) of a LED, a very small change in voltage makes a very large change in current.
Knowing that alone, you might think that by carefully adjusting the voltage, you can still get the current you want. Which would be true if it weren't for another characteristic of LEDs. As the temperature increases, Vf drops. Which means more current. More heat. Resulting in more current still. And so on. This is known as thermal runaway.
You haven't run into this problem with the heaters in your e-cigs because they act exactly the opposite as LEDs - as they heat up, resistance increases. So no thermal runaway.
Now since you'll be using an Arduino and a digital pot, if you also measure the actual current through the LED, and continuously adjust voltage based on both the current and the desired brightness, you have indeed created a current controlled power supply. Although it's a bit risky doing it this way. If your code is buggy or Arduino crashes, it could burn up your LEDs.Statistics: Posted by Darkcobra — Tue Jan 15, 2013 2:03 am
]]>