A diode is a two-terminal electronic component that conducts primarily in one direction (asymmetric conductance); it has low (ideally zero) resistance to the current in one direction, and high (ideally infinite) resistance in the other.

A diode can be used as a rectifier (convert AC current to DC), demodulator (AM radio, microphone volume detector), as protection for reverse or too high input signals or reverse power connections, or for back EMF protection (flyback diode for coils, relays and solenoids). In specific (high frequency) applications, a special high speed diode is needed.

Given its asymmetric behaviour a diode has two distinct sides. The cathode is the negative side, the anode the positive side. The cathode side is indicated in the symbol by a bar and on the physical diode through a ring on the housing.

Most important characteristic of a diode is the current-voltage or I-V graph. A regular  silicon semi-conductor diode has a forward voltage drop of 0.7 volts. This implies that if you add a diode in a circuit, the cathode has a 0.7 volts lower potential than the anode (0.3 V for germanium and 0.2 V for Schottky diode). When the voltage difference in forward direction across a diode is larger then this forward voltage drop  the resistance of the diode is very low and the current becomes (too) high very fast. When the voltage difference is in reverse direction, only a very small current will flow until the voltage difference is too big (breakdown voltage) and the diode breaks. Zener diodes are specially constructed to use this effect.

Most important specifications

• V_r: Maximum reverse voltage before breakdown (V)
• V_f: Maximum forward voltage, voltage across diode when conducting maximum forward current (V)
• I_f: Maximum current in forward direction (A)