Do not worry about the voltage as much, Current is more important. They normally have a maximum forward current of 20ma. You can use a 12 Volt DC supply to drive the LED however you will need to use a current limiting resistor around 680 Ohm to drop the forward current to around 15 - 18 milliamps. Lets say that you have a 10 000 mcd LED and you want to run it from a 3 volt supply. You will need to work out the resistor value by doing this. 5 Volts - 1.5 Vled forward equals 3.5 volts. Now R=V/I (Resistance = Voltage Divided by Current) SO: R= 3.5/20ma = 175 Ohms. You will not be able to get a 175 Ohm res so use a 200 Ohm instead, which may drop the current to around 18ma. Also, the mcd of the LED should not affect the battery life, it depends on the current drain of the LED.(So if it was run at 30ma (Milliamps) it will drain the battery quicker..) I always get a superbright LED and run it at a lower current, like 15ma so it lasts longer and battery life is extended, it is not so important if it is being used in a detecting circuit with mains supply, however if it is in a torch it is. You can also use a 555 timer to pulse the current at a high frequency so you can run them brighter without blowing it. (Also), an LED will generate a small voltage when exposed to sunlight (Any semiconductor will actually), asit is photovoltaic (Solar Panel), this voltage should be around 0.7 Volts, which will be the barrier voltage of the pn junction of the LED. (Thats why transistors are black cased) Hope this helps mate.
· 1 decade ago