What is a rectifier ? And Its Types, Explain the working of a junction diode as (i) half wave rectifier and (ii) full wave rectifier.

types of rectifier-half wave rectifier-full wave rectifier-what is half wave rectifier

In this what is rectifier, half wave rectifier, full wave rectifier, bridge rectifier, rectifier circuit, types of rectifier, rectifier diode, rectifier symbol.

What is a rectifier?

Rectifier is a device which converts alternating current (a.c.) into direct current (d.c.)

rectifier

rectifier

What is rectification ?

Rectification is the process of converting a.c. into d.c..

Principle : The fact which makes a junction diode act as a rectifier is that it conducts only when forward biased which

means that it does not conduct when reverse biased. A semiconductor diode acts as a valve.

Using a labelled diagram, Explain the working of a junction diode as (i) half wave rectifier

The rectifier which converts only one half of a.c. into d.c. is called half wave rectifier.

The circuit diagram of half wave rectifier is shown in figure 15. The ac. input signal to be rectified is fed to the primary (P) coil of the transformer. The secondary (S) coil is connected to the junction diode through a load resistance RL. The d.c. output signal is obtained

across the load resistance R,.

Working : When the positive half of a.c. input signal flows through the primary coil, an induced e.m.f. is set up in the secondary coil due to mutual induction. Let the direction of induced e.m.f. be such that the upper end of the secondary coil becomes positive and lower end becomes negative. Since upper end of secondary coil is connected to p-region and lower end is connected to the n-region of the junction diode, so the junction diode is forward biased during the positive half of input a.c. Thus the junction diode conducts and flow of current starts in the direction shown by arrows. The output voltage which varies in accordance with the input half cycle is obtained across the load resistance R

When negative half cycle of a.c. input flows through the primary coil, again induced e.m.f is set up across the secondary coil due to mutual induction. Now the direction of induced e.m.f. is such that upper end of the secondary coil becomes negative and lower end becomes positive. So the Junction diode is reverse biased. Hence the junction diode does not conduct and therefore, we get no output across the load resistance during negative half of input a.c. The input and corresponding output voltages are shown Figure 16.

Since only half of the portion of the wave (input signal) is obtained as output in the form of d.c., so the junction diode is called a half wave rectifier.

Disadvantages :

1. Since the output signal is discontinuous, so the efficiency of half wave rectifier is less.

2. The output is not pure d.c. but it is a fluctuating (or pulsating) which contains a.c. components or ripples also.

Expression for output d.c. voltage

Output d.c. voltage = Mean load current x load resistance

i.e. Vdc = ld.c. X RL

where I, is the maximum value of the secondary half wave current.

Using a labelled diagram, Explain the working of a junction diode (ii)as a full wave rectifier

A rectifier which converts both the halves of a.c. into d.c is called full wave rectifier

The circuit diagram of full wave rectifier is shown in Figure 17. The a.c. input signal is fed to the primary (P) coil of the transformer.

The p-regions of both the diodes D1, and D2, are connected to the two ends of the secondary coil (S). The load resistance Ru is connected between common pint of n sides of diodes and central tapping of the secondary coil Output voltage is obtained across load resistance R.

Working : When positive half cycle of input a.c. signal flows through the primary coil, induced em.t is set up in the secondary coil due to mutual induction. Let the direction of induced e.m.f. be such that the upper end of the secondary coil becomes positive while the lower end becomes negative. This makes diode D, forward biased and diode D, reverse biased. The current due to diode D flows through the circuit in a direction shown by arrows. The output voltage which varies in accordance with the input half cycle is obtained across the load resistance (RD).

During negative half cycle of ac, signal input, diode D is reverse biased and diode D, is forward biased. The current due to diode D, flows through the circuit in a direction shown by arrows. The output voltage is obtained across the load resistance (RD). The input and corresponding output voltage are shown in Figure 18.

Since both the halves of input a.c. (wave) are available in the output in rectified form, so the junction diode is called a full wave rectifier.

Advantage : In full wave rectifier, output is continuous, so its efficiency is more than that of the half wave rectifier.

Disadvantage : The output is again fluctuating (or pulsating d.c.). Additional filter* circuit is required to get smooth d.c. output.

Expression for output d.c. voltage

Output d.c. voltage = Mean load current  load resistance

i.e. Vdc = Idc X RL

But Idc=2I0/π,where lo is the maximum value of the secondary full wave current

Vdc=2I0/π X RL

Thus , output d.c. voltage in case of half wave rectifier.

Explain the working of a junction diode as a(3.) Bridge Rectifier.

In bridge type rectifier, four diodes are required instead of two diodes as discussed in centre-tap rectifier. The circuit diagram of bridge rectifier is shown in figure 20.

Bridge Rectifier

Bridge Rectifier

In this case, during the positive half cycle of input signal, diodes D, and D, are forward biased, whereas diodes D and D, are reverse biased. Hence current flows through the diode D, load resistance R. diode D, and the secondary of the transformer. During the negative half cycle of input voltage, diodes D, and D, are forward biased and diodes D, and D. are reverse biased. Therefore, the current flows through the diode D, load resistance diode D and the secondary of the transformer. The input and output wave forms are shown in figure 21.

2. Ripple Factor. It is defined as the ratio of the root-mean-square value of a.c. component or ripple to the d.c. value of the output voltage

It is given by  γ =r.m.s. value of a.c. component/ d.c. value of the output voltage

For a half wave rectifier, y = 1.21:

For a full wave rectifier, y = 0.48

This shows that the ripple factor of a full wave rectifier is much less than that of half wave rectifier.

 

3. Efficiency of rectifier is given by η=Output d.c. power x 100/Input a.c. power

Maximum efficiency of half wave rectifier is 40.6%

Maximum efficiency of full wave rectifier is 81.2%.

What is a filter circuit ? Briefly mention some filter circuits.

A Filter circuit is used to make the pulsating or fluctuating output of a rectifier smooth.

The output voltage of a rectifier is unidirectional, but it does not have a steady value. In fact, the output of a rectifier has both a.c. and d.c. components. In other words, the output d.c. voltage of a rectifier has some amount of a.c. voltage superimposed on it. The a.c. voltage superimposed on the d.c. voltage is called ripple component. To have an output voltage of steady value the a.c. component is to be filtered out. The device used to filter the a.c. component from the output of the rectifier is called filter circuit or simply filter.

Commonly used filters are :

i) C filter : It consists of a capacitor (C) shunting the load resistance RL as shown in figure 19(a).

(ii) L filter : It consists of an inductor (L) connected in series with the load resistance R as shown in figure 19(6),

(iii) n-type LC-filter : It consists of an inductor and two capacitors connected in the shape of a and the combination is connected across the load resistance R as shown in figure 19(c).

Action of inductor as a filter

We know, inductive reactance of an inductor L is given by,

XL = Lω = L X 2vπ

For          d.c., v = 0. therefore, XL=0

For           a.c., v = finite, therefore, XL = finite

Therefore, the d.c. component of output of the rectifier is passed through the inductor easily without any loss and the a.c. component is blocked by the inductor. Hence, the inductor filter reduces the fluctuation of the voltage across the load resistance RL.

Action of capacitor as a filter

We know, capacitive reactance of a capacitor C is given by.

Xc =1/Cω=1/Cx2 2πv1

For    d.c., v = 0, therefore, Xc = 1/ 0=∞

For, a.c. frequency v = finite, therefore Xc is small but finite.

Thus, the d.c. component of output of the rectifier is completely blocked by the capacitor and it prefers to pass through the load resistance. Hence, d.c. component of the output of the rectifier appears across the load resistance RL.

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