A diode is an electronic component that allows current to pass in one direction only. Among other things, this capability makes diodes useful in rectification --- the process of converting alternating current (AC) to direct current (DC). Early diodes consisted of a vacuum chamber connected to two electrodes, known as the cathode and the anode, but diodes used in modern circuits tend to be semiconductor diodes. A vast variety of semiconductor diodes are available, leading to an identification system to differentiate the different types.
Identify the cathode and the anode. One side of the diode may have several bands. The side to which the bands sits closest is the cathode. If the diode has no coloured bands, look for a spot, marking, the word "cath," the letter "k" or an unusual shape or tapering on the cathode end. Note that the direction of current flows from the anode to the cathode.
Hold the diode with the cathode (and the coloured bands) on your left.
Inspect the coloured bands, which you can translate into the identification number of the diode. The right-most band indicates the suffix letter, and all bands left of this band represent numbers. The only exception to this rule occurs when the right-most band is black, which indicates the lack of a suffix letter and that the diode is designated by numbers only.
Translate the coloured bands into the relevant numbers using the following code: black = 0, brown = 1, red = 2, orange = 3, yellow = 4, green = 5, blue = 6, violet = 7, grey = 8 and white = 9.
Translate the suffix letter (if used) using the following code: brown = A, red = B, orange = C, yellow = D, green = E, blue = F, violet = G, grey = H and white = J (no "I" appears).
Add "1N" to the beginning of the identification number. The "1" indicates a diode (as opposed to "2" for a transistor and "3" for a tetrode), and the "N" identifies the component as a semiconductor. For example, a diode with yellow, green, red and violet bands would have the code 1N452G.
If the diode lacks coloured identification bands, the number may be stamped or engraved onto the diode. Try to locate it.