Content Strategy Manager
Diode rectifier is a device that is used for converting alternating current or voltage to direct current or voltage. And the main application of p-n junction diodes, which is in rectification circuits, are basically used to explain the conversion of a.c signals to d.c in power supplies. The resistance of a p-n junction diode becomes low when forward biased and high when reverse biased and it can be used as a half-wave and a full-wave rectifier.
| Table of Content |
Key Takeaways: Semiconductors, diode, p-n junction, semiconductor diode, half-wave and a full-wave rectifier, AC current, DC current
Definition
[Click Here for Sample Questions]
The concept of both the Alternating current and Direct current is not unknown to all. The devices out there have instructions about how to use the AC or DC current. Although, there are times when one needs to connect the device that requires a DC power point despite having an AC power point. This is at this time when Rectifier appears in the scene. A rectifier is a device that is used to transfer alternating current or voltage into direct current or voltage. Simply, a rectifier is used to restrict the current or voltage to a single direction.
Junction diode as a rectifier
Also Read:
| Related Articles | ||
|---|---|---|
| Junction Transistor | Digital Electronics and Logic Gates | Junction Diode as Rectifier |
| Semiconductor Electronics | Bridge Rectifier | Transistor |
P-N Junction Diode
[Click Here for Sample Questions]
A P-N junction diode is formed when a p-type semiconductor is fused or combined to an n-type semiconductor which creates a potential barrier which allows the voltage or current across the diode junction.
P-n junction diode
Here a p-n junction diode is a two-electrode or two-terminal semiconductor device, which allows the electric current in one direction only and it blocks the electric current in a reverse or opposite direction. The electric current or voltage is allowed to flow only when the diode is forward-biased. On the other hand, it blocks the electric current or voltage flow when the diode is reverse biased. P-N junction semiconductor diodes are also known as p-n junction semiconductor devices.
Junction Diode as a Rectifier
[Click Here for Sample Questions]
Rectification is the process of conversion of alternating current or alternating voltage into a direct current or direct voltage. The device which is used for this conversion process is known as a rectifier. The p-n junction diode has the property which will allow a current to flow through it by offering a low resistance in the forward biased condition. The same property is also used in the rectification process. A junction diode is used as a rectifier in two ways.
Half-wave Rectifier
[Click Here for Sample Questions]
Half wave rectifier
The above circuit diagram shows that an alternating power supply is applied across a junction diode which is connected to a load in a series connection. In this case, a voltage will appear across the load only during the positive half cycles of the Alternating current input that is when the diode is forward biased. And the circuit which rectifies only one-half of the input current is known as the Half wave Rectifier.
Here the alternating current is supplied at both points A and B. During the alternating cycle, the diode will be forward biased only when the voltage at point A has positive cycles. This is when the diode conducts. On the other hand, when the voltage at point A is negative the diode is reverse biased and hence the diode does not conduct. For all practical purposes or experiments, the reverse saturation current will be considered zero since it is negligible.
Hence, an output voltage or current is available only through one-half of the input. Also, there will be no current available in the other half cycle. Hence, the output will be varied between positive to zero and zero to positive but the negative cycle is cut off and by this, the output voltage is said to be rectified.
Advantages of Half Wave Rectifier
- Affordable
- Simple connections
- Easy to use as the connections are simple
- Number of components used are less
Full-wave Rectifier
[Click Here for Sample Questions]
Full-wave rectifier
The above circuit diagram shows that two junction diodes are connected to a load. Hence this circuit gives the output in both positive and negative halves of the AC cycle. Hence it is known as Full wave Rectifier. In full wave rectifiers, the p-sides of both the diodes are connected to the input. And, the n-sides are connected to each other and then connected to the load.
In this circuit the mid point of the transformer is also connected to the load to complete the circuit. This mid point of connection is also known as Center tap and hence, the transformer is also known as Center tap transformer.
The reason for connecting two diodes in a full wave rectifier is very simple. When one diode D1 rectifies the voltage for one half of the cycle and the other diode D2 rectifies the voltage for the other half of the cycle. This can also be said as when one diode is forward biased the other diode will be reverse biased. And this process takes place simultaneously. Therefore, the output between the centre tap of the transformer and the common terminals (Diodes) becomes a full wave output. Let’s see how this works.
If the voltage at point A is positive then the voltage at point B will be negative. In such a scenario (case), the diode D1 will be forward biased and diode D2 will be reverse biased. Hence, D1 conducts while D2 blocks the current. And hence we get the output current during the positive half of the input AC cycle.
Subsequently, when the voltage at point A becomes negative then the voltage at point B becomes positive. In such a scenario (case), the diode D2 conducts and diode D1 blocks the current. By this we get an output current in the negative half of the input AC cycle.
Since the circuit rectifies both the halves of the input voltage (complete cycle of the input voltage), it is known as Full wave Rectifier. But, the output is pulsating and not steady. A capacitor is connected across the output terminals (parallel to the load) to derive a steady flow to DC output.
Advantages of Full Wave Rectifier
- Full-wave rectifiers have high efficiency than half-wave rectifiers
- The power loss is very low
- Number of ripples generated are less
Application of a Junction Diode as a Rectifier
[Click Here for Sample Questions]
The Volt-Ampere characteristics (that is VI characteristics) of a junction diode explains that the current passes through the diode only when it is forward biased. Hence, if an alternating voltage is supplied to a junction diode, then the current will flow only when it is in forward biased condition. This property of a junction diode is used to rectify alternating voltage/ alternating current. A rectifier is used for this purpose.
Things to Remember
- Diode rectifier is a device that is used for converting alternating current or voltage to direct current or voltage.
- The main application of p-n junction diodes, which is in rectification circuits, are basically used to explain the conversion of a.c signals to d.c in power supplies.
- The resistance of a p-n junction diode becomes low when forward biased and high when reverse biased and it can be used as a half-wave and a full-wave rectifier.
- Full-wave rectifiers have high efficiency than half-wave rectifiers. The power loss is very low.
- The Volt-Ampere characteristics (that is VI characteristics) of a junction diode explains that the current passes through the diode only when it is forward biased.
Also Read:
Sample Questions
Ques. What happens to the width of the depletion layer of a p-n junction when it is (i) forward biased and (ii) reverse biased? (All India 2011)
Ans: (i) In forward bias, the width of the depletion layer of a p-n junction decreases.
(ii) In reverse bias, the width of the depletion layer increases.
Ques. Why are half-wave rectifiers not used in dc power supply? (1 mark)
Ans: In a DC power supply half-wave rectifiers are not used because half-wave rectifiers do not provide a satisfactory power supply since they convert only positive cycles of the alternating power supply into DC power supply.
Ques. What are the advantages of a full-wave rectifier? (1 mark)
Ans:
- Full-wave rectifiers have high efficiency than half-wave rectifiers
- The power loss is very low
- Number of ripples generated is less.
Ques. Why are semiconductor diodes used as a rectifier? (1 mark)
Ans: Diode is a device that allows the current only when it is forward biased, That is why if we use an alternating voltage or alternating current source instead of a battery during the negative cycles the diode will be reverse biased and it will not allow the current or voltage to flow through it. Hence semiconductor diodes are used as rectifiers.
Ques. Define half-wave rectifier? (1 mark)
Ans: A half-wave rectifier forms a circuit that transforms one-half of the alternating input current into a direct current. In this case, a voltage will appear across the load only during the positive half cycles of the Alternating current input that is when the diode is forward biased. And the circuit which rectifies only one-half of the input current is known as the Half wave Rectifier.
Ques. Draw a circuit diagram of a half-wave rectifier and explain its working. (All India 2016)
Ans: Half wave rectifier refers to the type of rectifier that rectifies only half cycle of the waveform. It consists of a step-down transformer, a diode connected to the transformer, and a load resistance connected to a cathode end of the diode. The circuit diagram of a half-wave rectifier is given below,
Working of a half-wave rectifier-
The input provided to the rectifier will have both positive as well as negative cycles. The half rectifier will allow only the positive half-cycles and leave out the negative half cycles. In positive half-cycles, when the input Ac power is given to the primary winding of the step-down transformer, we will obtain the decreased voltage at the secondary winding that is given to the diode. Secondly, the diode will allow the current to flow in clock wise direction from anode to cathode in the forward bias( diode conduction will occur in forwarding bias) which will generate only the positive half cycle of the AC. Thirdly, the diode will eliminate the variations in the supply and give the pulsating DC voltage to the load resistance RL. Thus, we can get the pulsating DC at the load resistance.
In the negative half cycle, the current will flow in the anti clock wise direction and the diode will go into the reverse bias. In the reverse bias, the diode will not work and therefore no current will flow from anode to cathode. We can not get any power at the load resistance. Only a small amount of reverse current is flown from the diode but this current is almost negligible. And the voltage across the load resistance is also zero.
Ques. What do you mean by a full-wave rectifier? (3 marks)
Ans: A full-wave rectifier is a circuit that transforms the entire alternating input current into a direct current. Hence this circuit gives the output in both positive and negative halves of the AC cycle. In full-wave rectifiers, the p-sides of both the diodes are connected to the input. And, the n-sides are connected to each other and then connected to the load.
In this circuit, the mid point of the transformer is also connected to the load to complete the circuit. This mid point of connection is also known as Center tap and hence, the transformer is also known as the Center tap transformer.
Ques. How does a diode act as a rectifier? (3 marks)
Ans: A rectifier helps to transfer the AC into a DC with the help of one or more contact diodes. When the voltage is applied in such a way to the P-N junction diode or semiconductor diode, that the positive terminal of the battery is connected to the p-type end of the P-N junction, whereas, the negative terminal of the battery is connected to the n-type end which is said to be forward-biased. Again, when the positive terminal of the battery is connected to the n-type and the negative terminal of the battery is to the p-type, it is reverse biased. Hence, the diode allows the current to pass in the forward bias condition and blocks in the reverse bias one. This very property of the diode helps it to act as a rectifier by transforming an alternating current into a DC source.
Ques. (i)Explain Briefly with the help of a circuit diagram, the working of a full-wave rectifier. Draw its input and output waveforms.
(ii) Identify the logic gate which is equivalent to the circuit shown in the figure. Draw the truth table for all possible values of inputs A and B. (Delhi 2015C)
Ans:
During the first half cycle of input ac, one diode will behave as a forward biased and conduct, whereas the other diode that acts as a reverse-biased will not conduct. In another half cycle of the input signal, the reverse-biased diode will get forward biased and therefore will conduct while the other diode gets reverse biased and hence will not conduct. In such cases, we will achieve undirectional output voltage for the positive and negative half-cycles.
(ii) Identification: AND gate
| Input | Output | |
|---|---|---|
| A | B | Y |
| 0 | 0 | 0 |
| 0 | 1 | 0 |
| 1 | 0 | 0 |
| 1 | 1 | 1 |
Ques. Draw a circuit diagram of a p-n diode used as a half-wave rectifier. Explain it's working. (All India 2013C)
Ans: The circuit diagram for a p-n diode that is used as a half-wave rectifier is shown below,
Positive half cycle: During the positive half cycle, the p-n junction is forward biased, meaning the forward current flows from p to n, the diode offers a low resistance path to the current. Therefore, we get output across load i.e., ac input will be obtained as dc output.
Negative half cycle: During the negative half cycle, the p-n junction is reverse biased, that is the reverse current flows from n to p and the diode offers a high resistance path to the current. This is why the output across the load is not obtained.
For Latest Updates on Upcoming Board Exams, Click Here: https://t.me/class_10_12_board_updates
Check-Out:
Comments