Diodes is a simple P-N junction electronic component for DIY projects with leads from both P and N regions. P-N junction is the interface between a P type semiconductor and N type semiconductor. When impurities (either P-type or N-type) are added to an intrinsic semiconductor, free electrons or holes are generated in it. This will increase the conductivity of the substrate. A P-N junction is made by doping a semiconductor crystal with P type semiconductor on one side and the other side with N type semiconductor. Doping the substrate with two different materials (group 3 and group 5) on opposite sides provides the device with different property. Device allow current to pass through it only in one direction and blocks in the other. An ideal P-N junction diodes is a perfect conductor in one direction and a perfect insulator in the other. Structure and symbol of a diode is shown below.
Diodes- Electronic Component for DIY Projects
Diode is a two layer device formed by P-type and N-type semiconductor. In the N region, each impurity atom has an extra electron left behind after forming four covalent bonds with the silicon atom. Therefore electrons are majority carriers in this region. Similarly in the P region of diodes one electron is short to form covalent bonds with the four valence silicon atom. Absence of electron is called holes. Therefore at P-region of diodes, holes are the majority carriers. Due to the concentration difference of majority carriers in each region, electrons from N region will diffuse into the P region of diodes and holes from the P region will diffuse into the N region of diodes. The free electrons from the N-region will move into the P region where it fits itself in the holes. The movement of each electron will reduce charge carrier in both P region and N region of diodes. The process is continued and a region develops itself as a perfect insulator. The insulator region thus developed in both N region and P region of diodes together is called depletion region.
The growth of the depletion region will not continue for a longer time. When an electron is diffused into P region, a positive ion is created in the N region. The electrostatic force between positive ion and free electron will resist the movement of electron. So diffusion will reach equilibrium when this electrostatic force equals the force of diffusion. Due to the generation of ions, a voltage is developed across the depletion region. This voltage difference is called Barrier potential.
For different operating conditions predetermined voltage is applied across the P-N junction diodes. Diodes can be biased in two possible methods depending on the polarity of the applied voltage.
Under zero biasing condition, no voltage is applied across the diodes. No free carriers are present in the depletion regionand there is small voltage across the junction (barrier potential). When diodes is configured in forward bias mode, positive and negative terminal of the battery is connected to the P and N region respectively. Now negative voltage push electrons from the N region into the depletion region and similarly positive voltage push holes from the P region into the depletion region of diodes. This decreases the width of the depletion region. Presence of free carriers in the depletion region increases its conductivity therefore current flows through the diodes. In other words, electrons from the N region are pushed into the P region through the depletion region. Electrons in P region are attracted by the positive terminal of the battery, and flows in the opposite direction of current flow.
Forward Biasing in Diodes
For the diodes to operate in reverse biased condition, negative and positive terminal of the battery is connected to P and N regions respectively. Negative voltage connected to the P region will give electron to vacant spaces in it. Similarly positive voltage at N region of diodes attracts electrons. So it removes free electrons from the N region of diodes. As the charge carriers are removed, depletion region (region without free charge carriers) width will increase. Since depletion region behaves like a perfect insulator no current can flow through the diodes in reverse biased configuration.
When forward bias voltage is increased it is opposed by the barrier potential across the depletion region. When the applied voltage is greater than the barrier potential, carrier density in the depletion region equals to the carrier density of P or N region. Resulting device is close to a perfect conductor. Therefore from this point, a small increase in voltage results in a very large increase in current. This voltage is called Knee voltage Vf .
In the reverse characteristics, we can see that current is not increased even if we increase the voltage across the device. Diodes in reverse biased configuration behaves like a perfect insulator till the applied voltage is less than breakdown voltage.
The breakdown voltage of an insulator is the minimum voltage that causes a portion of an insulator to become electrically conductive. We explained the concepts of forward bias and reverse bias by considering the electrons in the outermost shell, because under normal conditions only free electrons in the outer most shell of an atom can move around. In reverse biased mode, no current flows through the device. All free electrons in the outer shell are removed when the applied voltage is increased. When the voltage is increased further electrons from the lower layer will gain energy and jump to the outer shell. This process is called tunneling and this type of break down is called Zener break down. After Zener breakdown, the conductivity of the diodes increases and large amount of current flows through the diodes.
There is a second type of breakdown called Avalanche breakdown. When reverse voltage is applied, free electrons from the N region move towards the positive terminal and holes from the P region move to the negative terminal. Moving electrons may collide with other electrons and transfer its energy. Energy will be sufficient to break its bond. So a new free electron is created. The newly created electrons collide with the neighboring electrons and energy is transferred. This process continues as a chain reaction and keeps on generating more free electrons. This process can turn out to be uncontrollable and the heat produced due to repeated collision can damage the device. In a particular device, occurrence of Zener and avalanche breakdown may either be independent or simultaneous.
It is the maximum voltage that can be given to the PN Junction diodes without any damage.
The current that a diodes can pass at normal temperature is the average surge current.
It is the large amount of current that a diodes can safely pass through it.
It is the maximum forward voltage that a diodes can have without any damage.
It is the power that the diodes can dissipate.
Time taken by the diodes to switch from ON to OFF.
Eventhough the theory part is known to everyone, most of us are unaware on how to identify diodes. Below are some tips and tricks to identify diodes. Now you guys may be thinking why this topic is important???? The answer is diodes are essentially "one-way", hence it's important to know to identify diodes easily.
Similar to transistor coding, the diodes also follow a special coding system. One is known as the Pro-electron coding scheme. The other coding system is the JEDEC system.
The Pro-electron coding system uses the format shown below and the table shows the meaning of each:
Examples
JEDEC coding system uses the format shown in the figure below and the table below shows the meaning of each character.
Thus a 1N914 is a diode. Here 1 represents the number of PN junctions present in a diode. Letter ‘N’ represents that the material used is a semiconductor and the sequential number represents the device number.
By going through these simple tips and tricks you can easily read your diode code. Quickly read below:
After knowing how to identify a diode in terms of its anode and cathode terminals, now it’s time to learn how to connect a simple diode circuit to the breadboard.
Circuit for Study: Bridge Rectifier Circuit
Here we are choosing a simple bridge rectifier circuit. This simple circuit helps you to connect the diodes efficiently to the breadboard and gives you a better knowledge to identify the diodes in terms of its anode and cathode terminals either by using a multimeter or by understanding the band.
Bridge rectifier circuit is a simple full wave rectifier circuit which uses a combination of four diodes to form the bridge. It helps in the conversion of both the half cycles of AC input into DC output. Being a major part of the power supplies, it is really important to know about it. Also, bridge circuits find applications in many motor controllers, welding purposes, modulation purposes, in home appliances etc.
Step Down Transformer
You can use a 230 volt to 12 volt or 110 volt to 12 volt step down transformer. Also it will be reliable if you can use a center tapped transformer. Now while using the center tapped transformer, connect either the terminal points across the bridge rectifier or connect one of the terminal point and the middle point to the bridge rectifier circuit.
Diodes
Here we are using four IN4007 diodes. It is a pn junction diode with better voltage range of 50 to 1000 volts and current 1 Ampere. Its features like low cost, less leakage, low voltage and high current makes them better for the bridge rectifier circuit.
Capacitor & Resistor
Capacitor used here is a 470microFarad capacitor. It is used for the conversion of full wave ripple output into smooth DC output. In a bridge rectifier circuit, working voltage and the capacitance value has to be determined while choosing the capacitor.
Note: The working voltage must be higher than the non-load output value of the rectifier. Also, capacitance value must be chosen in such a way that the ripple will be less.
The resistor used here is 1K and across it the output voltage is taken.
Case1: Positive Cycle
Case2: Negative Cycle
Output: Pulsating DC Output.
At first, measure all the circuit components prior to the construction of the circuit using the multimeter.
Note:
Clipper and Clamper circuits are considered to be one of the applications for diodes. Clipper also known as limiter is used to remove portions of the AC signal. Clamper also known as DC Restorer on the other hand is used to change the dc reference of the AC signal.
Clamping circuits place the negative and positive peaks of the waveform at a desired DC level. In other words, these circuits shift the input signal by an amount defined by the independent voltage source. Today, Clamping circuits are also known popularly as Clamped Capacitors.
Application of Diodes
The basic idea of the clamper circuits are shown in the figure above. Here the input signal is a sine wave having a peak to peak value of 10V. In case of a positive clamper, it pushes the signal to upwards so that the negative peak falls on the zero level. While in the case of the negative clamper, it pushes the signal downwards so that the positive peak of the signal falls on the zero level. One thing can be seen that the orginal shape of the signal is not changed in any of the clamper outputs.
Inorder to change the dc reference of the input signal, a clamper circuit usually uses:
General principle of the clamper circuit is that, the charging time of the capacitor in the clamper must be small compared to the discharging time of the capacitor in the clamper.
Figure below shows the circuit of a positive clamper. The input signal Is to the circuit is a square waveform. The output signal Os will be obtained across the load resistor. Here C represents the capacitor, D a diode, V voltage supply and RL represents the load resistor.
Application of Diodes: Positive Clamper
Proper operation of the clamper follows two points:
Positive Clamper Operation
During Negative Half Cycle
During Positive Half Cycle
Application of Diodes: Positive Clamper Waveform
Figure below shows the circuit of a negative clamper. The input signal Is to the circuit is a sine waveform. The output signal Os will be obtained across the load resistor. Here C represents the capacitor, D a diode, V voltage supply and RL represents the load resistor.
Application of Diodes: Negative Clamper
Negative Clamper Operation
During Positive Half Cycle
During Negative Half Cycle
Application of Diodes: Negative Clamper Output
Vin – Vc – Vout= 0
5- Vc- 2 = 0
Vc =3V.
Vin-Vc-Vout = 0
Vout= - 8V.
The output waveform tells that the circuit given is a negative clamper and the dc reference level here is 2V.
Clippers are classified into three types:
Positive Clipper
Positive Clipper is used to remove the positive half cycles of the input voltage. The circuit is shown in the figure below. In this, during the positive half cycle, diode is forward biased and it conducts. Diode acts as a short and the voltage across the load is zero. While during the negative half cycle, diode is reverse biased and behaves as an open.
Negative Clipper
Negative Clipper is used to remove the negative half cycles of the input voltage. In this, during the positive half cycle, diode is reverse biased and it becomes open. While during the negative half cycle, diode is forward biased and it acts as a short and the voltage developed across the load resistor is zero.
A P-N junction diode conducts current only in one direction, that is, when anode is positive with respect to its cathode. This property of diode is used in half wave rectifier to convert AC to DC. In the rectifier circuit, diode is connected in series with the load resistance. Sine wave given as the input is rectified by the diode. During the positive half cycle, diode is forward biased, so current passes through it and reaches the load. And during the negative half cycle, diode is reverse biased so it blocks current from reaching the load. Half wave rectifier is shown in the following figure, which is the simplest circuit used as rectifier and it is considered to be the simplest application of diodes.
In half-wave rectifier, negative half of the input signal is chopped off, therefore efficiency of the rectifier is only less than fifty percentage. In full wave rectifier, both positive and negative half of the input signal is used. Since both halves are used, efficiency of the full wave rectifier is double than that of half wave rectifier. A multiple winding transformer whose secondary winding is split equally into two halves is used in full wave rectifier. A diode which conducts only in one direction is connected in each half. During positive half cycle, diode D1 is forward biased and Diode D2 is reverse biased. Therefore current reaches load through the diode D1. Similarly, during negative half cycle, diode D2 is forward biased and diode D1 is reverse biased. In both half cycles, current passes through the load in the same direction and returns back to the secondary winding through the center tap. Thus we get a unidirectional output current. 
In full wave bridge rectifier, four diodes in bridge configuration replaces the combination of center tap transformer and two diodes. Therefore, size and cost of the device is reduced. Following figure shows the circuit of a bridge rectifier connected to its load.
Positive half cycle
Diodes D1 and D2 are forward biased and diodes D3 and D4 are reverse biased during positive half cycle. Current from input end pass through diode D1, reaches load resistance and then flows out through diode D2.
Negative half cycle
During negative half cycle, didoes D3 and D4 are forward biased and diodes D1 and D2 are reverse biased. Current reaches the load through diode D3 and flows out through diode D4.Notice that in both half cycles, the current flows through the load in the same direction. That means bi-directional input AC signal is converted into unidirectional DC output.
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