Rectifier Basic

Rectifier basic & its types are explained in this post – The rectifier is a device which converts the input AC signal into DC. The rectifier converts the sinusoidal signal to unidirectional flow and not pure D.C.

The electronic circuits require a D.C. source of power. The input signal can be A.C. and so the output signal will be amplified A.C. signal. To get this, D.C. batteries can be used.

But they will get dried quickly and replacing them every time is a costly affair. Hence it is economical to convert A.C. power into D.C. for such circuits.

The rectifier is further divided into the following types:

• Half Wave Rectifier
• Full Wave Rectifier

Half-Wave Rectifier

In half-wave rectification, the circuit conducts current only during the positive half-cycles of the input a.c. supply. The negative half-cycles of a.c. supply is suppressed i.e. during negative half-cycles, no current is conducted and hence no voltage appears across the load. Therefore, current always flows in one direction (i.e. d.c.) through the load though after every half-cycle.

Circuit details of a half-wave rectifier are shown in the above figure. This circuit shows the circuit where a single crystal diode acts as a half-wave rectifier. The a.c. supply to be rectified is applied in series with the diode and load resistance RL. Generally, a.c. supply is given through a transformer. The use of a transformer permits two advantages.
Firstly, it allows us to step up or step down the a.c. input voltage as the situation demands. Secondly, the transformer isolates the rectifier circuit from the power line and thus reduces the risk of electric shock.

Disadvantages

1. The pulsating current in the load contains an alternating component whose basic frequency is equal to the supply frequency. Therefore, elaborate filtering is required to produce steady direct current.
2. The a.c. supply delivers power only half the time. Therefore, the output is low.

Full-Wave Rectifier

In full-wave rectification, current flows through the load in the same direction for both half-cycles of input a.c. voltage. This can be achieved with two diodes working alternately. For the positive half-cycle of input voltage, one diode supplies current to the load and for the negative half-cycle, the other diode does so; current being always in the same direction through the load. Therefore, a full-wave rectifier utilizes both half-cycles of input a.c. voltage to produce the d.c. output. The following two circuits are commonly used for full-wave rectification :

• Center-tapped transformer full-wave
• Bridge type full-wave

Centre-Tap Full-Wave Rectifier

The circuit employs two diodes D1 and D2 as shown in the Figure below. A center-tapped secondary winding AB is used with two diodes connected so that each uses one half-cycle of input a.c. voltage. In other words, diode D1 utilizes a.c. voltage appearing across the upper half (OA) of secondary winding for rectification while diode D2 uses the lower half winding OB.

Operation: During the positive half-cycle of secondary voltage, the end A of the secondary winding becomes positive and end B negative. This makes the diode D1 forward biased and diode D2 reverse biased. Therefore, diode D1 conducts while diode D2 does not. The conventional current flow is through diode D1, load resistor RL and the upper half of secondary winding as shown by the dotted arrows.

During the negative half-cycle, end A of the secondary winding becomes negative and end B positive. Therefore, diode D2 conducts while diode D1 does not. The conventional current flow is through diode D2, load RL and lower half winding as shown by solid arrows. It is clearly seen that current in the load RL is in the same direction for both half-cycles of input a.c. voltage. Therefore, d.c. is obtained across the load RL. Also, the polarities of the d.c. output across the load should be noted.

Disadvantages

1. It is difficult to locate the center tap on the secondary winding.
2. The d.c. output is small as each diode utilizes only one-half of the transformer secondary voltage.
3. The diodes used must have high peak inverse voltage.

Full-Wave Bridge Rectifier

The need for a center-tapped power transformer is eliminated in the bridge rectifier. It contains four diodes D1, D2, D3 and D4 connected to form bridge as shown in Figure below. The a.c. supply to be rectified is applied to the diagonally opposite ends of the bridge through the transformer. Between the other two ends of the bridge, the load resistance RL is connected.

Advantages of a bridge rectifier

1. In the bridge circuit, a transformer without a center tap is used.
2. The bridge circuit requires a smaller transformer as compared to a full-wave rectifier giving the identical rectified dc output voltage.
3. For the same dc output voltage, the PIV rating of a diode in a bridge rectifier is half of that for a full-wave circuit.
4. The bridge circuit is more appropriate for high-voltage applications, thus, making the circuit compact.

Disadvantages of a bridge rectifier

1. Two or more diodes are required in case of a bridge rectifier, as a full-wave rectifier uses two diodes whereas a bridge rectifier uses four diodes.
2. The amount of power dissipated in a bridge circuit is higher as compared to a full-wave rectifier. Hence, the bridge rectifier is not efficient as far as low voltages are concerned.

Comparison between half-wave and full-wave rectifier

1. In a half-wave, a single diode exists and the load current flows through it for only the positive half-cycle. On the other hand, in a full-wave, the current flows throughout the cycles of the input signals.
2. Full-wave rectifiers require a center-tapped transformer. For a half-wave, only a simple transformer is required.
3. The PIV in a half-wave is the maximum voltage across the transformer secondary. Whereas, in the case of a full-wave the PIV for each diode is two times the maximum voltage between the center tap and at either end of the transformer secondary.
4. In a half-wave, the frequency of the load current is the same as that of the input signal and it is twice the frequency of the input supply for the full-wave.
5. The dc load current and conversion efficiency for a full-wave are twice that of a half-wave. Also, the ripple factor of the full-wave is less than that of the half-wave circuit. This indicates that the performance of the full-wave rectifier is better than the half-wave rectifier.
6. In a full-wave, two diode currents flow through the two halves of the center-tapped transformer secondary in opposite directions, so that there is no magnetization of the core. The transformer losses being smaller, a smaller transformer can be used for a full-wave rectifier.

Difference between Rectifiers in Table Form

S. No.	Particulars	Half-wave	Centre-tap	Bridge type
1.	No. of diodes	1	2	4
2.	Transformer necessary	No	Yes	No
3.	Max. efficiency	40.6%	81.2%	81.2%
4.	Ripple factor	1.21	0.48	0.48
5.	Output frequency	fin	2fin	2fin
6.	Peak inverse voltage	Vm	2Vm	Vm