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Diode as a Rectifier | Physics Class 12 - NEET PDF Download

Application of junction diode as a Rectifier

  • Rectifier is a device which is used for converting alternating current or voltage into direct current or voltage
  • A p-n junction diode can be used as a half-wave and full-wave rectifier
  • The resistance of a p-n junction diode becomes low when forward biased and becomes high when reverse biased. This is the principle of the working of rectifier

Half-wave Rectifier

Circuit Diagram
Diode as a Rectifier | Physics Class 12 - NEET

  • Transformer with primary and secondary coils
  • Diode
  • Load resistance RL

The AC voltage to be rectified is connected between the primary of the transformer
To one coil of the secondary, the p junction of the diode is connected
The output is measured across the load resistance RL
Input and output

Diode as a Rectifier | Physics Class 12 - NEET

Working

Case 1

  • During the positive half cycle of the input AC voltage, suppose P1 is negative and P2 is positive
  • On account of inductance, S1 becomes positive and S2 becomes negative
  • The p-n junction is forward biased and hence the resistance of the p-n junction diode becomes low
  • Hence, current flows in the circuit and we get output across the load resistance RL
  • This is indicated in the graph above

Case 2

  • During the negative half cycle of the input AC voltage, suppose P1 is positive and P2 is negative
  • On account of inductance, S1 becomes negative and S2 becomes positive
  • The p-n junction is reverse biased and hence the resistance of the p-n junction diode becomes high
  • Hence, no current flows in the circuit and we do not get any output across the load resistance RL
  • This is indicated in the graph above

The above process is repeated. Thus , we have current only in the positive half of the cycle. Hence, if is called as half-wave rectifier
The output signal is not continuous and available as bursts. Hence, this is not of much use.


Full Wave Rectifier


Circuit Diagram
Diode as a Rectifier | Physics Class 12 - NEET


  • The main difference between half and full wave rectifier in circuit, is the usage of two diodes – D1 and D2

Input and Output
Diode as a Rectifier | Physics Class 12 - NEET

Working

Case 1

  • During the positive half of the input cycle of AC voltage, the junction diode D1 is forward biased as shown in the diagram above
  • Hence, current flows in the above circuit as indicated
  • The diode D2 is reverse biased and hence no current due to D2
  • We get output when the same is measured across the load resistance RL due to the diode D1 alone

Case 2

  • The circuit diagram for the negative half of the input cycle of AC voltage:
    Diode as a Rectifier | Physics Class 12 - NEET
  • During the positive half of the input cycle of AC voltage, the junction diode D2 is forward biased as shown in the diagram above.
  • Hence, current flows in the above circuit as indicated.
  • The diode D1 is reverse biased and hence no current due to D1.
  • We get output when the same is measured across the load resistance RL due to the diode D2 alone

We observe that one of the diode conducts and the flow of current across the load resistance is in the same direction Also, current flows during both cycles of the input AC voltage. However, the output though unidirectional has ripple contents. Ripple contents indicate both AC and DC components
We can get only the DC component by passing it through a filter circuit
The filter circuit consists of Resistance and Capacitance

Circuit Diagram

  • C has a high capacitance value and serves as a filter circuit
  • RL is a load resistance

Working

  • The capacitance offers low impedance to AC component and offers infinite impedance to DC component
  • Due to this the AC component is bypassed or filtered out
  • The DC component produces a voltage drop across the load resistance which is almost DC voltage

Problem: In half-wave rectification, what is the output frequency if the input frequency is 50 Hz. What is the output frequency of full wave rectifier for the same input frequency?
Solution: Half-wave rectifier –The output voltage is obtained is once in one cycle of input voltage, hence output ripple frequency after half-wave rectification = 50 Hz
Full-wave rectifier – For one cycle of input voltage, we get output twice in the same direction. Hence, the output after full wave rectification = 100 Hz

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FAQs on Diode as a Rectifier - Physics Class 12 - NEET

1. What is a rectifier?
A rectifier is an electrical device that converts alternating current (AC) into direct current (DC). It is commonly used in power supplies and electronic circuits to convert the AC voltage into a form that can be used by electronic devices.
2. How does a junction diode function as a rectifier?
A junction diode can function as a rectifier by exploiting its inherent electrical properties. When a diode is forward-biased (positive voltage applied to the anode and negative voltage to the cathode), it allows current to flow freely. On the other hand, when it is reverse-biased (negative voltage applied to the anode and positive voltage to the cathode), it acts as an open circuit, preventing current flow. By using this property, a junction diode can rectify AC voltage by allowing current flow only during the positive half-cycle, effectively converting it into a pulsating DC voltage.
3. What is the difference between a half-wave rectifier and a full-wave rectifier?
A half-wave rectifier only allows current flow during one half of the AC cycle, while a full-wave rectifier allows current flow during both halves of the AC cycle. In a half-wave rectifier, the negative half-cycle is blocked, resulting in a pulsating DC output. In contrast, a full-wave rectifier uses additional diodes or a center-tapped transformer to rectify both positive and negative half-cycles, resulting in a more smooth and continuous DC output.
4. What are the advantages of using a full-wave rectifier over a half-wave rectifier?
The advantages of using a full-wave rectifier over a half-wave rectifier include: 1. Higher efficiency: Since a full-wave rectifier utilizes both halves of the AC cycle, it can convert a larger portion of the input power into DC output, resulting in higher efficiency. 2. Smoother output: A full-wave rectifier produces a more continuous DC output compared to the pulsating output of a half-wave rectifier, which is beneficial for many electronic devices. 3. Lower ripple voltage: The ripple voltage, which represents the AC component present in the DC output, is lower in a full-wave rectifier compared to a half-wave rectifier. This can reduce the potential interference with other electronic components.
5. Can a junction diode be used as a rectifier in high-power applications?
No, a junction diode is not suitable for high-power rectification due to its limitations in current and voltage handling capacity. Junction diodes are typically used for low-power applications and are limited by their small size and current-carrying capabilities. In high-power applications, other rectifier devices such as diode bridges, thyristors, or power transistors are commonly used, which can handle higher current and voltage levels.
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