Diode: Rectification

Rectification

The process of converting alternating current into direct current is called rectification. In this section, we will discuss two types of rectifiers namely, half wave rectifier and full wave rectifier.

1. Half wave rectifier circuit

The half wave rectifier circuit is shown in Figure 9.17(a). The circuit consists of a transformer, a p-n junction diode and a resistor. In a half wave rectifier circuit, either a positive half or the negative half of the AC input is passed through while the other half is blocked. Only one half of the input wave reaches the output. Therefore, it is called half wave rectifier. Here, a p-n junction diode acts as a rectifier diode.

During the positive half cycle

When the positive half cycle of the ac input signal passes through the circuit, terminal A becomes positive with respect to terminal B. The diode is forward biased and hence it conducts. The current flows through the load resistor R_L and the AC voltage developed across R_L constitutes the output voltage V₀ and the waveform of the diode current is shown in Figure 9.17(b).

During the negative half cycle

When the negative half cycle of the ac input signal passes through the circuit, terminal A is negative with respect to terminal B. Now the diode is reverse biased and does not conduct and hence no current passes through R_L. The reverse saturation current in a diode is negligible. Since there is no voltage drop across R_L, the negative half cycle of ac supply is suppressed at the output. The output waveform is shown in Figure 9.17b.

The output of the half wave rectifier is not a steady dc voltage but a pulsating wave. This pulsating voltage can not be used for electronic equipments. A constant or a steady voltage is required which can be obtained with the help of filter circuits and voltage regulator circuits.

Efficiency (η) is the ratio of the output dc power to the ac input power supplied to the circuit. Its value for half wave rectifier is 40.6 %

If the direction of the diode is reversed, the negative half of the ac signal is passed through and the positive half is blocked.

2. Full wave rectifier

The positive and negative half cycles of the AC input signal pass through the full wave rectifier circuit and hence it is called the full wave rectifier. The circuit is shown in Figure 9.18(a). It consists of two p-n junction diodes, a center tapped transformer, and a load resistor (RL ) .

The centre is usually taken as the ground or zero voltage reference point. Due to the centre tap transformer, the output voltage rectified by each diode is only one half of the total secondary voltage.

During positive half cycle

When the positive half cycle of the ac input signal passes through the circuit, terminal M is positive, G is at zero potential and N is at negative potential. This forward biases diode D₁ and reverse biases diode D₂. Hence, being forward biased, diode D₁ conducts and current flows along the path MD₁AGC. As a result, positive half cycle of the voltage appears across R_L in the direction G to C

During negative half cycle

When the negative half cycle of the ac input signal passes through the circuit, terminal N is positive, G is at zero potential and M is at negative potential. This forward biases diode D₂ and reverse biases diode D₁. Hence, being forward biased, diode D₂ conducts and current flows along the path ND₂ BGC. As a result, negative half cycle of the voltage appears across R_L in the same direction from G to C.

Hence in a full wave rectifier both postive and negative half cycles of the input signal pass through the load in the same direction as shown in Figure 9.18(b). Though both positive and negative half cycles of ac input are rectified, the output is still pulsating in nature.

The efficiency (η) of full wave rectifier is twice that of a half wave rectifier and is found to be 81.2 %. It is because both the positive and negative half cycles of the ac input source are rectified.

Centre tap transformer: There is a facility to tap at halfway point in the secondary windings. This helps to measure the induced voltage from one end of the secondary to the centre point. If the centre tap point is grounded then the voltage applied across the secondary will be divided by half. For example, if the voltage applied across the secondary is 240 V, then the voltage across one end and the centre tap point is +120 V and at the other end it is –120 V.