I'm new to electronics. When I look at the back of the box of LED's, it shows the need of a resistor. But not what kind of a resistor.
How do I know what resistor to use with an LED?
Thank you
Depends on the forward voltage/current of the LED and what voltage you're supplying it with.
if you have an LED with a forward voltage of 3V at 20mA forward current with a supply voltage of 5V:
"Hmm... What value of resistor will drop 2V (5V-3V) across it at a current of 20mA?"
R = V / I = 2V / 20mA = 100ohms
This is the minimum resistor you should use as you don't normally want to exceed 20mA. LEDs also have a varying forward voltage of lets say 1.8V to 3.5V typical.
If you were instead being conservative and had an LED with a drop of 2V and you wanted 10mA:
R = V / I = (5V - 2V) / 10mA = 300ohms
Typically people use 330ohm resistors when using standard LEDs for indicators at 5V.
If brightness isn't important and you're running on batteries, it's probably best to use up to 1kohms.
Forward voltage drop usually depends on the colour of the LED, so a red LED will have typically a lower forward voltage (around 2V) than a blue LED (up to around 4V).
You need to check the LED type, and if in doubt, use a higher value resistor.
Is the forward voltage is the what's left after the LED takes what it needs to generate light?
I assume the resistor is there to soak up the extra power (that it dissipates as heat) so you do not blow out the LED?
The forward voltage is the voltage the LED itself drops, whats left over is what the resistor should drop.