What is a Diode and Types of Diodes

This post covers what is a diode, forward voltage drop and revers voltage of a diode. Also, it states the functions of diode as well as types of diodes. Testing diode also is not an exception.

First of all, lets know what a diode is before the types of diodes.

What is a Diode

A Diode in electronics is a two-terminal electronic component with asymmetric conductance. That is; it has low (ideally zero) resistance to current in one direction and high (ideally infinite) resistance in the other. Construction of diodes are mostly from semiconducting materials to form two junctions. These are P-Junction and N-Junction or PN-Junction.

Diodes allow electricity to flow in only one direction (forward bias) while resists electricity in the other direction (reverse bias). The arrow of the circuit symbol shows the direction in which the current can flow.


Through a diode, electricity makes use of little energy pushing its way through it as if a person pushing through a door with a spring. This means that; a small voltage across a conducting diode is present and this is called “Forward Voltage Drop”. This voltage is about 0.7V for all normal diodes of Silicon element. The forward voltage drop of a diode is almost constant whatever the current passing through the diode. Therefore, diodes have very steep characteristics.


In a normal and perfect diode, there is no conduction of current when a reverse voltage is applied to it. Though, all real diodes leak a very tiny current of a few micro-amps, µA or even less. This can be neglected in most of the electrical circuits because the current is very smaller than the one in forward direction. However, all diodes have a maximum reverse voltage (usually 50V or more). Therefore, diodes damage if it exceeds this voltage and it passes a large current in the reverse direction. This is call “voltage breakdown”.


The most common function of diodes is to allow an electric current to pass in one direction (diode’s forward direction) while blocking current in the opposite direction (the reverse direction). Thus, one can view diode as an electronic version of a check valve. This unidirectional behaviour is call “Rectification” and is use to convert alternating current (A.C) to direct current (D.C). Also, it includes extraction of modulation from radio signal in radio receivers. These diodes are forms of rectifiers.

A Zener Diode will conduct in reverse direction when it reaches its breakdown voltage unlike the signal and rectifier diodes. This characteristic makes them to be use as voltage regulator or high voltage dampers. That is, when the supply voltage reaches their breakdown voltage rating; they conduct to regulate the voltage.

Let us now look into types of diodes since we have talked about what a diode is.


Ordinary diodes may be categorized into four types that is:

1. Signal Diodes: These types of diodes pass small currents of 100µA or less

2. Rectifier Diodes: They can pass large currents


3. Light Emitting Diodes: These types of diodes emit light whenever current is passing through them.

4. Zener Diodes: Unlike other types of diodes, Zener Diodes can conduct in reverse direction when they reach their breakdown voltage. For example, a Zener diode that rates 5.6V will conduct in reverse direction when the reverse supply voltage reaches up to 5.6V that its breakdown voltage. These diodes maintain a fixed voltage and can be use where a fixed voltage is needful to serve as regulator.


Light Emitting Diode emits light when current is being passing in them. When connecting or soldering a LED, care should be taken so is been connected the right way round otherwise, it not work. In the circuit symbol of LED, “a” or “+” implies the anode and “k” or “−” is the cathode. Another way to identify these is to look directly on the LED, the short lead or terminal is the cathode and if inside the LED is visible, the large electrode indicates the cathode. Light Emitting Diodes can be damaged by excessive heat when soldering and excessive current.


LED must not be connected directly to the supply voltage otherwise due to large current that would pass through it may damage it. To avoid this, a resistor is connected in series with it to limit the current to a safe value. If your supply voltage is 12V to below, a 1KΩ resistor can be used to avoid the LED’s damage and always make sure that the LED is connected correctly.


LEDs can exist in these colours; White, Blue, Red, Orange, Green, Yellow, and Amber. Determining the colour of the LED is not by the colour of the plastic body but the type of semiconductor used in manufacturing it.


These LEDs are mostly combination of red and green LEDs in one package. They have three terminals. The centre lead is always the common cathode for both colour leads and the other two leads are anodes to make them lit separately or at the same time to give third colour. This is because; mixing red and green colour gives out yellow colour.


These are LEDs (two LEDs) wired so that one is forward and the other backward in one package. Hence, only one of them can lit at one time.



Working out LED resistor

From Ohm’s law,


  • Where V is the voltage across the resistor, but there is supply voltage and LED’s voltage in of Light Emitting Diode (usually 2V, but 4V for blue and white LEDs)

Therefore; V=VS – VL

Where VS is the supply voltage and VL is the LED’s voltage

  • I = the current through the resistor

Taking VS – VL as V substitute,

Hence; R= (VS – VL)/I

Example 1

A red LED which will use 9V and 0.1A current, what will be resistance value to avoid it from damage?


Red LED’s voltage = 2V (VL)

Supply Voltage = 9V (VS)

Current = 0.1A (I)

Resistor = ?

R= (VS – VL)/I

= 9 – 2/0.1

= 7/0.1

R= 70Ω

However, if 70Ω is not available, use any nearest greater value.

Example 2

Three LEDs: Red, White, and green are connected in series. If 12V with a current of 0.2A supply is to be use to power them, find the resistance value to be used in order to prevent them from damage.


Red LED’s voltage = 2V

White LED’S voltage = 4V

Green LED’s voltage = 2V

Therefore,  the total LED’s voltage (VL) =2+4+2=8V

Supply Voltage = 12V (VS)

Current = 0.2A


Current = 0.1A (I)

Resistor = ?

R= (VS – VL)/I

R= 12 – 8/0.2

= 4/0.2

= 40Ω

When this value is not available, use any nearest greater value.


Bridge Rectifier is use to maintain the proper DC polarity at the input to the circuit. It comprises of a four Rectifier Diodes connections to form a bridge. They are rate by their maximum current and reverse voltage. They have four leads or terminals. The two DC outputs are mark + and –, the AC inputs are mark ~.

The diagram below shows the operation of a bridge rectifier as it converts AC to DC.


Diodes can malfunction when they lose their characteristics i.e. functions. Diodes of this state in a circuit can cause malfunctioning or short circuits in the circuit. Use this simple diode testing circuit which consists of battery, resistor, and Light Emitting Diode (LED) to check that a diode is conducting in only one direction. Also, you can use a Multi-meter is also a device for testing diodes.

Apart from this post (What is a Diode and types of Diodes), you might likewise want to read these posts: Resistors and types of Resistors and also Calculating Power Inverter or UPS runtime

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