Diode as a Rectifier: Half Wave Rectifier & Full Wave Rectifier

The main application of p-n junction diode is in rectification circuits. These circuits are used to describe the conversion of a.c signals to d.c in power supplies. Diode rectifier gives an alternating voltage which pulsates in accordance with time. The filter smoothes the pulsation in the voltage and to produce d.c voltage, a regulator is used which removes the ripples.

There are two primary methods of diode rectification:

  • Half Wave Rectifier
  • Full Wave Rectifier

Table of Contents

Half Wave Rectifier
Working of Half Wave Rectifier
Characteristics of Half Wave Rectifier
Advantages of Half Wave Rectifier
Disadvantages of Half Wave Rectifier
Applications of Half Wave Rectifier
Full Wave Rectifier
Working of Full Wave Rectifier
Characteristics of Full Wave Rectifier
Types of Full Wave Rectifier
Advantages of Full Wave Rectifier
Disadvantages of Full Wave Rectifier
Applications of Full Wave Rectifier
Difference

What is Half Wave Rectifier?

In a half-wave rectifier, one half of each a.c input cycle is rectified. When the p-n junction diode is forward biased, it gives little resistance and when it is reversing biased it provides high resistance. During one-half cycles, the diode is forward biased when the input voltage is applied and in the opposite half cycle, it is reverse biased. During alternate half cycles, the optimum result can be obtained.

Working of Half Wave Rectifier

The half wave rectifier has both positive and negative cycles. During the positive half of the input, the current will flow from positive to negative which will generate only positive half cycle of the a.c supply. When a.c supply is applied to the transformer, the voltage will be decreasing at the secondary winding of the diode. All the variations in the a.c supply will reduce and we will get the pulsating d.c voltage to the load resistor.

Half Wave Rectifier

In the second half cycle, current will flow from negative to positive and the diode will be reverse biased. Thus, at the output side, there will be no current generated and we cannot get power at the load resistance. A small amount of reverse current will flow during reverse bias due to minority carriers.

Characteristics of Half Wave Rectifier

Following are the characteristics of half wave rectifier:

Ripple Factor

Ripples are the oscillations that are obtained in the DC which is corrected by using filters such as inductors and capacitors. These ripples are measured with the help of ripple factor and is denoted by γ. Ripple factor tells us the amount of ripples present in the output DC. Higher the ripple factor, more is the oscillation at the output DC and lower is the ripple factor, less is the oscillation at the output DC.

Ripple factor is the ratio of rms value of AC component of output voltage to the DC component of the output voltage.

\(\gamma=\sqrt{(\frac{V_{rms}}{V_{_{DC}}})^{2}-1}\)

DC Current

DC current is given as:

\(I_{DC}=\frac{I_{max}}{\pi }\)

Where,

  • Imax is the maximum DC load current

DC Output Voltage

The output DC voltage appears at the load resistor RL which is obtained by multiplying output DC voltage with the load resistor RL. The output DC voltage is given as:

\(V_{DC}=\frac{V_{Smax}}{\pi }\)

Where,

  • VSmax is the maximum secondary voltage

Form Factor

Form factor is the ratio of rms value to the DC value. For a half wave rectifier form factor is 1.57.

Rectifier Efficiency

Rectifier efficiency is the ratio of output DC power to the input AC power. For a half wave rectifier, rectifier efficiency is 40.6%.

Advantages of Half Wave Rectifier

  • Affordable
  • Simple connections
  • Easy to use as the connections are simple
  • Number of components used are less

Disadvantages of Half Wave Rectifier

  • Ripple production is more
  • Harmonics are generated
  • Utilization of transformer is very low
  • Efficiency of rectification is low

Applications of Half Wave Rectifier

Following are the uses of half wave rectification:

  • Power rectification: Half wave rectifier is used along with transformer for power rectification as a powering equipment.
  • Signal demodulation: Half wave rectifiers are used for demodulating the AM signals.
  • Signal peak detector: Half wave rectifier is used for detecting the peak of the incoming waveform.

What is Full Wave Rectifier?

Full wave rectifier circuits are used for producing an output voltage or output current which is purely DC. The main advantage of full wave rectifier over half wave rectifier is that such as the average output voltage is higher in full wave rectifier, there is less ripple produced in full wave rectifier when compared to half wave rectifier.

Working of Full Wave Rectifier

Full wave rectifier utilizes both halves of each a.c input. When the p-n junction is forward biased, the diode offers low resistance and when it is reversing biased it gives high resistance. The circuit is designed in such a manner that in the first half cycle if the diode is forward biased then in the second half cycle it is reverse biased and so on.

Full Wave Rectifier

Characteristics of Full Wave Rectifier

Following are the characteristics of full wave rectifier:

Ripple Factor

Ripple factor for a full wave rectifier is given as:

\(\gamma =\sqrt{(\frac{V_{rms}}{V_{DC}})^{2}-1}\)

DC Current

Currents from both the diodes D1 and D2 are in the same direction when they flow towards load resistor RL. The current produced by both the diodes is the ratio of Imax to π, therefore the DC current is given as:

\(I_{DC}=\frac{2I_{max}}{\pi }\)

Where,

  • Imax is the maximum DC load current

DC Output Voltage

DC output voltage is obtained at the load resistor RL and is given as:

\(V_{DC}=\frac{2V_{max}}{\pi }\)

Where,

  • Vmax is the maximum secondary voltage

Form Factor

Form factor is the ratio of rms value of current to the output DC voltage. Form factor of a full wave rectifier is given as 1.11

Rectifier Efficiency

Rectifier efficiency is used as a parameter to determine the efficiency of the rectifier to convert AC into DC. It is ratio of DC output power to the AC input power. Rectifier efficiency of a full wave rectifier is 81.2%.

Types of Full Wave Rectifier

There are two main types of full wave rectifiers and they are:

  • Two diode full wave rectifier circuit (requires center-tapped transformer and is used in vacuum tubes)
  • Bridge rectifier circuit (doesn’t require centre-tapped transformer and is used along with transformers for efficient usage)

Advantages of Full Wave Rectifier

  • The rectifier efficiency of a full wave rectifier is high
  • The power loss is very low
  • Number of ripples generated are less

Disadvantages of Full Wave Rectifier

  • Very expensive

Applications of Full Wave Rectifier

Following are the uses of full wave rectifier:

  • Full wave rectifiers are used for supplying polarized voltage in welding and for this bridge rectifiers are used.
  • Full wave rectifiers are used for detecting amplitude of modulated radio signals.

Difference between Half Wave Rectifier and Full Wave Rectifier

Parameter Half Wave Rectifier Full Wave Rectifier
Definition Half wave rectifier is a rectifier which is used for converting one half cycle of AC input to DC output Full wave rectifier is a rectifier which is used for converting both the half cycles of AC input into DC output
No. of diodes used 1 2 or 4 depending on the type of circuit
Form factor 1.57 1.11
Rectifier efficiency 40.6% 81.2%
Ripple factor Ripple factor of half wave rectifier is more Ripple factor of full wave rectifier is less


Frequently Asked Questions – FAQs

Why half-wave rectifiers are not used in dc power supply?

Half-wave rectifiers are not used in dc power supply because the supply provided by the half-wave rectifier is not satisfactory.

What is PIV of a diode in a rectifier circuit?

PIV stands for peak inverse voltage and it is the maximum voltage that is possible to occur across the diode when it is operated in reverse biased.

What is a ripple in a rectifier circuit?

Ripple is defined as the ac component that has a pulsating output in a rectifier.

What are the advantages of bridge rectifier over a centre tapped full wave rectifier?

Following are the three advantages of bridge rectifier over a centre tapped full wave rectifier:
  • The TUF of a bridge rectifier is 81.2% while the TUF of a centre tapped is 67.2%.
  • The output of the bridge rectifier is twice that of the centre tapped full wave rectifier.
  • A bridge rectifier as a PIV half of the centre tapped full wave rectifier.

What is the cycle followed by the current in a full wave rectifier?

In a full wave rectifier, the current flow in the half cycle of the input signal.

What happens to the peak current if the value of the shunt capacitor filter is increased?

When the value of the shunt capacitor filter is increased, the peak current will also increase in a rectifying diode.

Why bridge rectifier is preferred over an ordinary two diodes full wave rectifier?

A bridge rectifier is preferred over an ordinary two diodes full wave rectifier because:
  • The PIV is less per diode
  • There is no need for centre tap
  • The transformer required is smaller with the same output

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