2 Answers

Read here>> http://en.wikipedia.org/wiki/Diode

Are you asking how to make an LED work in a flashlight or smiler ap? Diodes are current sensitive and the manufacture stipulates the current rating. Most flash-light LEDScan handle .2 amps at 5volts. Some are rated as high as .39amps at 12VDC . So E=IR is the formula for working with nearly all things electrical. E is voltage in volts, I is current in amps, R is resistance in ohms. So to get a LED to operate at  .2 amps from a 6 volt source, transposing the formula to calculate resistance called for R=E/I  thus 6/.2= 30 ohms of resistance is called for in series with the LED. Some diodes have some resistance built in that can only be checked by metering the current in the circuit after it is in operation and if you find the current draw is less than you want reduce the resistance to bring the current up to the desired current level. There are many ways to do this and understanding electronics design will give you a fascinating experience in acquiring a skill in the subject.