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Diodes are two terminal devices that exhibit low resistance to current flow in one direction and high resistance to current flow in the other.  The direction in which the current flows is often referred to as the forward direction whilst in the negligible current flows is known as the reverse direction.  When the diode is conducting a small voltage is dropped across it and this is known as the forward voltage drop.  The diode is one of the simplest forms of semiconductor and it is used to control the flow of electrons.

A variety of applications use diodes and they are classed in different subtypes.

Zener diodes:  Zener diodes  limit voltage to a pre-determined amount.  A zener diode can be used to build a voltage regulator into a circuit.

Zener diode

Light emitting diodes, LED‘s: All semiconductors emit infrared light when they conduct current.  LEDs make this light visible. 

Silicon-controlled rectifier (SCR):  The SCR is a type of switch used to control AC or DC currents and are commonly used in light dimmers.

Rectifier:  This basic diode transforms AC current to provide DC current only.  The DC current does not alternate and is only positive or negative.  Diodes can be referred to as rectifiers because they perform this rectifying function, a thyristor is a type of rectifier.

rectifier diode

Bridge rectifier:  This component is made up of four diodes and rectifies AC to DC with great efficiency.

bridge rectifier diode

 Ac inputs and dc inputs:

 A diode does not have a value (apart from the zeners), they are simply used to control the flow of electrons.  Diodes are distinguished by two main criteria their PIV (peak inverse voltage) and current.  These criteria indicate which type of diode should be used in a specific circuit.

 The PIV rating indicates the maximum working voltage for the diode.  For instance if it is rated at 100 volts, you would not use it in a circuit that would apply more than 100 volts to the diode.

 The current rating is the maximum amount of current the diode can handle.  If it was to attempt to conduct more amps than its rating it would overheat and become damaged.

 Diodes are identified by a numerical system which is industry standard.   A typical example would be a 1N4001 rectifier diode, which is rated at 1.0 PIV and 50 volts.  A 1N4002 is rated at 100 volts and a 1N4003 is rated at 200 volts and this pattern continues.

 A rectifier diode rated to about 3 to 5 amps will be encased in grey or black epoxy and can be directly mounted onto a PCB.  The higher current diodes are usually contained inside a metal housing that includes a heat sink or a mounting stud so that the diode can be fixed securely on a heat sink.

 All diodes will have a positive (anode) and a negative terminal (cathode) and the cathode  can commonly be   identified  by a red or black stripe  near one of the leads.  The stripe will correspond with the line in the schematic symbol for the diode.  When following a schematic diagram to build a circuit the diode must be orientated to the line facing the specified way.  If a diode is placed in the circuit in the wrong direction it will not work and it can damage components.

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