Nature of Rectifier Output | Ripple Factor | Comparison of Rectifiers

Nature of Rectifier Output

Nature of Rectifier:- It has already been discussed that the output of a rectifier is pulsating d.c. as shown in Fig. 6.37. In fact, if such a waveform is carefully analyzed, it will be found that it contains a d.c. component and an a.c. component. The a.c. the component is responsible for the *pulsations in the wave. The reader may wonder how a pulsating d.c. voltage can have an a.c. component when the voltage never becomes negative. The answer is that any wave which varies in a regular manner has an a.c. component.

The fact that a pulsating d.c. contains both d.c. and a.c. components can be beautifully illustrated by referring to Fig. 6.38. Fig. 6.38(i) shows a pure d.c. component, whereas Fig. 6.38 (ii) shows the *a.c. component. If these two waves are added together, the resulting wave will be as shown in Fig. 6.38 (iii). It is clear that the wave is shown in Fig. 6.38 (iii) never becomes negative, although it contains both a.c. and d.c. components. The striking resemblance between the rectifier output wave shown in Fig. 6.37 and the wave is shown in Fig. 6.38 (iii) may be noted. It follows, therefore, that a pulsating output of a rectifier contains a d.c. component and an a.c. component.

What is Ripple Factor

The output of a rectifier consists of a d.c. component and an a.c. component (also known as ripple). The a.c. component is undesirable and accounts for the pulsations in the rectifier output. The effectiveness of a rectifier depends upon the magnitude of a.c. component in the output; the smaller this component, the more effective is the rectifier.


The ratio of r.m.s. value of a.c. component to the d.c. component in the rectifier output is known
as ripple factor i.e.

Therefore, the ripple factor is very important in deciding the effectiveness of a rectifier. The smaller the ripple factor, the lesser the effective a.c. component and hence more effective is the rectifier. Mathematical analysis. The output current of a rectifier contains d.c. as well as a.c. component. The undesired a.c. the component has a frequency of 100 Hz (i.e. double the supply frequency 50 Hz) and is called the ripple (See Fig. 6.39). It is a fluctuation superimposed on the d.c. component.
By definition, the effective (i.e. r.m.s.) value of total load current is given by :

It is clear that a.c. component exceeds the d.c. component in the output of a half-wave rectifier.
This results in greater pulsations in the output. Therefore, half-wave rectifier is ineffective for conversion of a.c. into d.c.


For full-wave rectification

In full-wave rectification,

This shows that in the output of a full-wave rectifier, the d.c. component is more than the a.c.
component. Consequently, the pulsations in the output will be less than in half-wave rectifier. For
this reason, full-wave rectification is invariably used for conversion of a.c. into d.c.
Example 6.22. A power supply A delivers 10 V dc with a ripple of 0.5 V r.m.s. while the power
supply B delivers 25 V dc with a ripple of 1 mV r.m.s. Which is a better power supply?

Solution. The lower the ripple factor of a power supply, the better it is.

Comparison of Rectifiers

A comparison among the three rectifier circuits must be made very judiciously. Although bridge
circuit has some disadvantages
, it is the best circuit from the viewpoint of overall performance. When the cost of the transformer is the main consideration in a rectifier assembly, we invariably use the bridge circuit. This is particularly true for large rectifiers which have a low-voltage and a high-current rating.

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