voltage
Newbie question:
I have an Arduino board supplying 3.3v. I'm looking at LEDs. I've found one which I like, but it has a forward voltage of 4.4v.
Aside from any consideration towards resistors, would this be an optimal setup / would it break either the LED or the board?
I'm unclear as to whether the board would be able to supply enough voltage to power the LED?
The LED forward voltage drop will remain (roughly) the same, but the current can change, so the calculation becomes (same equation solving for I):
$$I_{LED} = {(V_s - V_f)\over{R}}$$
So for a 3V \${V_f}\$ and a 5V supply, the \$100\Omega\$ resistor would give \${(5V - 3V)\over{100 \Omega }} = 20 mA\$.
So if you know what current you want, just plug the values in, e.g. for 10mA:
$$R = {(5V - 3V)\over{0.01 A}} = {200 \Omega}$$
Basically, the fact that the supply and the LED forward voltage can be relied upon to be pretty static, means that whatever value resistor you put in will also have a static voltage across it (e.g ~2V in this case), so it just leaves you to find out that voltage and select a resistance value according to the current you want.
Below is the V-I curve of a diode (from the wiki LED page), notice the current sharply rises (exponentially) but voltage stays roughly the same when the "on" voltage is reached.
For more accurate control of the current you would use a constant current, which is what most LED driver ICs provide.
Yes, use a 2Ω resistor in series.
Best Answer
Nothing will break, nor will anything light up.
If you must use the LED, you need a secondary voltage source. One can be made using a step-up circuit or boost-converter, and there are likely many questions about that here.
Another option is taking a secondary voltage in, for example 5 volt from any USB charger and just use the Arduino to control it, using a transistor. This might work, though it's cutting it a bit close.