How can you tell how much voltage to use by what mm an LED is?

Sorry if these things don't correspond with each other (i'm kinda new to this kind of thing)

AndyGadget8 years ago
You can't tell by the size what voltage to use for a LED - but you can tell by the colour! The colour of light a LED emits depends on the materials it's made of, and the materials it's made of determine the 'voltage' of the LED.
As a rough guide :-
Red 1.8V
Orange 2.2V
Yellow 2.2V
White 3.6V
Green 2.1V
Blue 3.6V
This is called the 'forward voltage drop (Vf)' and is the voltage you would get if you measured across the pins while it was working. It is NOT the voltage it should be connected to.
What is more important is the CURRENT (If) through the LED, which is around 20mA for most normal small LEDs. To power a LED correctly, you would use a supply voltage higher than the Vf of the LED and limit the current with a resistor. The resistor value is calculated by the formula (Supply voltage - Vf) / LED current. No need to work this out yourself as there is a calculator HERE.
You will see lots of circuits, especially on Instructables which power LEDs with no resistor. This is a 'quick and dirty' method which usually works if the battery voltage is a little bit higher than the LED Vf. The internal resistance of the battery and the small voltage difference limits the current.