RECTIFIER

 

A rectifier is an electrical device that converts alternating current to direct current, a process is known as rectification. Rectifiers are mostly used as components of power supplies and as detectors of radio signals. Rectifiers may be made of solid state diodes (Silicon or Germanium), vacuum tube diodes, mercury arc valves, and other technologies.

There difference between the term diode and the term rectifier i.e., the term rectifier describes a diode that is being used to convert AC to DC. Almost all rectifiers comprise a number of diodes in a specific arrangement for more efficiently converting AC to DC than using just a single diode. Before the development of solid state rectifiers, vacuum tube diodes and copper oxide or selenium rectifier were used.

Half-wave rectification

Half wave rectification can be achieved with a single diode in a one phase supply AC (by blocking the negative or positive portion of the waveform).

In half wave rectification, either the positive or negative half of the AC wave is passed easily while the other half is blocked, depending on the polarity of the rectifier. Because only one half of the input waveform reaches the output, it is very inefficient if used for power transfer

Full-wave rectification.

Full-wave rectification converts both polarities of the input waveform to DC, and is more efficient.

For single phase AC, if the transformer is center-tapped, then two diodes form a full wave rectifier as shown below. 

Full-wave Bridge rectification.

However, in a circuit with a non-center tapped transformer, four diodes are required instead of the one needed for half-wave rectification. This is due to each output polarity requiring two rectifiers each, for example, one for when AC wave is positive and one for when wave is negative. Four rectifiers arranged this way are called a bridge rectifier:

A full wave rectifier converts the whole of the input waveform to one of constant polarity (positive or negative) at its output by reversing the negative (or positive) portions of the alternating current waveform. The positive (negative) portions thus combine with the reversed negative (positive) portions to produce an entirely positive (negative) voltage/current waveform.

Rectifier output smoothing.

While half- and full-wave rectification suffices to deliver a form of DC output, neither produces constant voltage DC. In order to produce steady DC from a rectified AC supply, a smoothing circuit, sometimes called a filter is required. In its simplest form this can be what is known as a reservoir capacitor, Filter capacitor or smoothing capacitor, placed at the DC output of the rectifier. There will still remain an amount of AC ripple voltage where the voltage is not completely smoothed.

Types of filter Circuits. 

 

The most commonly used filters are Capacitor filter, choke input filter, and capacitor input filter.

       i.            Capacitor filter:

It consists of capacitor placed across the rectifier output in parallel with load R_L. The pulsating direct voltage of the filter is applied across the capacitor. As the rectifier voltage increases, it charges the capacitor and also supplies the current to the load. At the end of quarter cycle the capacitor is charged to the peak (V_m) of the rectifier voltage. Now the rectifier voltage starts to decrease. As this occurs, the capacitor discharge through the load and the voltage across it decreases. The voltage across the capacitor will decrease only slightly because immediately the next voltage peak comes and recharges the capacitor. This process is repeated again and again and the voltage output waveform becomes ABCDEFG. It may be seen that very little ripple is left in the output. Moreover the output voltage is higher as it remains substantially near the peak value of the rectifier output voltage. The capacitor filter is very popular because of its low cost, small size, little weight and good characteristics. For small load currents this, type of filter is preferred.

 

    ii.            Choke input filter:

It consists of a choke (inductor) connected in series with the rectifier output and a capacitor filter across the load. The pulsating output of the rectifier is applied across terminal 1 and 2 of the filter circuit. As discussed before the pulsating output of the rectifier contains a.c. components and d.c. components. The coke offers high opposition to the passage of a.c. component but negligible opposition to the d.c. component. The result is that, most of the a.c. component appears across the choke while whole of d.c. component passes through the choke on its way to the load. This results in reduced pulsations at terminal 3. At terminal 3, the rectifier output contains d.c. component and the remaining part of a.c. component which has managed to pass through the choke. Now the low reactance of filter capacitor bypasses the a.c. components but prevent the d.c. component to flow through it. Therefore, only the d.c. component reaches the load. In this way, the filter circuit has filtered out the a.c. component from the rectifier output, allowing d.c. component to reach the load.

                      

iii. Capacitor input filter:

It consists of a filter capacitor connected across the rectifier output, a choke in series and another filter connected across the load.

The pulsating output from the rectifier is applied across the input terminals of the filter. The filtering action of the three components of this filter is as follows:

a)   The filter capacitor C₁, offers low reactance to a.c. component of rectifier output while it offers infinite reactance to d.c. component. Therefore, capacitor C1 bypasses an considerable amount of a.c. component while the d.c. component continues its journey to the

choke L.

b)   The choke L, offers high reactance to the a.c. component but it offers almost zero reactance to the d.c. component. Therefore, it allows the d.c. component to flow through it, while that part of a.c. component which could not be bypassed by the capacitor is blocked.

c)    The filter capacitor C₂, bypasses the a.c. component which the choke has failed to block. Therefore only d.c. component appears across the load and that is what we desire.