Diodes are simple semiconductors that allow current to flow only in one direction. This is accomplished by joining one material for the negative side to another different material for the positive side. The negative side of a diode is called the cathode and the positive side of the diode is called the anode.
A diode can be thought of as a one-way check valve. This is an important characteristic. It’s why diodes are used in automotive alternators to convert AC voltage to DC voltage. When the AC current reverses, the diode blocks the negative pulses only allowing positive pulses through. This converts the AC voltage created by the generator to DC voltage used throughout the automotive electrical system. A rectifier bridge in an alternator consists of six diodes, two for each stator winding. The conversion of AC current to DC current is known as rectification.
If a diode is placed in a circuit with a reverse bias, it will act as an insulator, blocking the flow of electricity. This allows the diode to act as a protective device, protecting sensitive components from voltage surge. If the diode is placed in a forward bias direction, with the positive or P-side facing the positive voltage, it would act as a conductor allowing electricity to flow through the circuit. This is known as forward bias.