Diodes are simple semiconductors that allow current to flow in one direction only. This is accomplished by joining one material for the negative side and another 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.
This means that if a diode is placed in one direction with the negative side or N-side facing positive voltage it will act as a insulator blocking the flow of electricity. This is known as reverse bias and allows the diode to act as a protective device protecting components from voltage surge. If the diode were to be placed in the other direction with the positive or P-side facing the positive voltage it would act as a conductor allowing electricity to flow through. This is known as forward bias.
A diode can be thought of as a one-way check valve. This is an important characteristic of the diode. This is why diodes are used in automotive alternators to convert AC voltage to DC voltage. When the AC current reverses the diode blocks the negative pulse only allowing the positive pulses through. This converts the AC voltage generated by the generator to DC voltage used throughout the automotive electrical system. A rectifier bridge in a generator consists of six diodes, two for each stator winding. The conversion of AC current to DC current is also known as rectification.
A Zener diode is a diode that allows only a certain amount voltage through. If the zener diode is rated at 16 volts it would act as a conductor for 16 volts only. If the voltage were to increase past the 16 volts the diode would limit/regulate the output to the represented voltage. Zener diodes are used in instrument clusters for their voltage regulating characteristics.
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