RECTIFIERS

                An electronic device used for converting A.C voltage into D.C voltage is defined as rectifier.

Types of rectifiers:

Based on the period of conduction rectifiers are classified into the following types,

1) Half wave rectifier

2) Full wave rectifier

3) Bridge rectifier

HALF WAVE RECTIFIER:

                 A half wave rectifier is an electronic circuit which converts A.C voltage into a pulsating D.C voltage using one half cycle of the applied A.C voltage.

Construction:

                 Figure shows the basic circuit of a half wave rectifier. It consists of a power transformer, a semiconductor diode D and load resistor RL. The A.C voltage to be rectified is applied to the input of the power transformer and the voltage V1 across the secondary is applied to the diode connected in series with a load resistor RL.

Working:

                 When an A.C voltage is applied to the input circuit, during positive half cycle, terminal A is positive with respect terminal B. This makes the anode of the diode positive with respect to cathode. The diode is forward biased and therefore it conducts. A current IL flows through the circuit and there is a voltage drop across the load resistance RL.

               During negative half cycle of the A.C input voltage terminal A is negative with respect to B. The diode is reverse biased and therefore it does not conduct. Now no current flows in the circuit and there is no voltage drop across the load resistance, for the period of negative half cycle.

                Thus when an A.C voltage is applied to the input of the rectifier current always flows through the load resistance only in one direction. Therefore the output across load resistance will be a D.C voltage. The input and output waveforms are shown in figure. It is seen that the output wave is not steady D.C but only pulsating D.C. Since one half cycle of the input wave is utilized, it is called a half wave rectifier. Though the output is pulsating it can be used for certain small power applications.

Input and output waveforms:

Advantages:

Ø  Circuit is very simple and occupies less space.

Ø  Less cost.

Disadvantages:

Ø  Low output voltage

Ø  Rectification efficiency is low

Ø  Ripple factor is high

Ø  D.C saturation of transformer core results in hysteresis loss and production of harmonics

Ø  Suitable only for very low power applications 

FULL WAVE RECTIFIER:

                         A full wave rectifier is an electronic circuit which converts A.C voltage into pulsating D.C voltage using both half cycles of the applied A.C input voltage.

Construction:

                 Figure shows the basic circuit of a full wave rectifier. It consists of a transformer, two semiconductor diodes and a load resistor. The secondary winding of the transformer is center tapped. Hence two voltages V1 and V2 fed to the two diodes are equal magnitude but opposite in 180 phase difference. Terminal A of the transformer secondary is connected to the anode of the diode D1 and the other end terminal B is connected to the anode of the diode D2. The load resistor RL is connected between the center tap C of the secondary winding and inter junction of two cathodes of diode D1 and D2. 

Working:

                   When an A.C voltage is applied to the input circuit, the terminals A and B of the transformer secondary becomes positive and negative alternately. During positive half cycle of the A.C input voltage terminal A is at positive potential, C is at zero potential and terminal B is at negative potential. This makes the anode of the diode D1 positive with respect to cathode. Now the diode D1 is forward biased and D2 is reversed biased. Therefore D1 conducts and at the same time diode D2 remains non conducting. A current I1 flows through the load resistance RL in the direction as shown in figure.

                         During the negative half cycle of the A.C input voltage, terminal A is at negative potential, C is at zero potential and terminal B is at positive potential. This makes the anode of the diode D2 positive with respect to cathode.  Now the diode D2 is forward biased and D1 is reverse biased. Therefore the diode D2 conducts and diode D1 remains non conducting. A current I2 flows through the load resistor RL in the directions as shown in figure.

                 Thus when an A.C voltage is applied to the full wave rectifier, during positive half cycle diode D1 conducts and during the negative half cycle diode D2 conducts. Further, current flows through the load resistor RL in the same direction in both half cycles of the A.C input. A pulsating D.C voltage is developed across the load resistor RL. The input and output waveforms are shown in figure.

Input and output waveforms:

Advantages:

Ø  High output voltage than half wave rectifier

Ø  Rectification efficiency is high

Ø  Ripple factor is low

Ø  D.C saturation of the core is avoided.

Disadvantages :

Ø  Cost is high when compared to half wave rectifiers

Ø  Requires tapped transformer which is more costly

Ø  Higher PIV rated diodes are necessary for the operation which increases the cost. 

BRIDGE RECTIFIER:

                      A bridge rectifier is an electronic circuit which converts A.C voltage in to pulsating D.C voltage using both half cycles of the applied A.C voltage as in the case of full wave rectifiers. It is also a full wave rectifier. Bridge rectifier avoids the usage of center tapped transformer.

Construction:

                     A bridge rectifier is shown in figure. It consists of a transformer, four semiconductor diodes and a load resistor. Because of the bridge like arrangement of diodes it is called bridge rectifier. The A.C voltage to be rectified is applied across one diagonal of the bridge through a power transformer. The load resistor is connected across the other diagonal of the bridge.

Working:

                      When an A.C voltage is applied to the input circuit the end terminals A and B of the transformer secondary becomes positive and negative alternately. During positive half cycle of the A.C input voltage terminal A is positive with respect to terminal B. In this situation diodes D1 and D3 are forward biased whereas diodes D2 and D4 are reverse biased. Therefore diodes D1 and D3 conduct and current flows through the load resistor RL as shown in figure. This current produces a voltage drop across RL.

                         During the negative half cycle of the A.C input voltage terminal B is positive with respect to terminal A. In this situation diodes D2 and D4 are forward biased whereas diodes D1 and D3 are reverse biased. Therefore diodes D2 and D4 conduct and a current flows through the load resistor RL as shown in figure. This current produces voltage drop across RL.  

                               Thus when an A.C voltage is applied to the bridge rectifier, during positive half cycle diodes D1 and D3 conduct and during negative half cycle diodes D2 and D4 conduct. However current flows through the load resistor RL in the same directions in both half cycles of the A.C input. Thus a pulsating D.C voltage is developed across the load resistor RL and we have full wave rectification. The input and output waveforms are shown in figure. 

Input and output waveforms:

Advantages:

Ø  Rectification efficiency is high.

Ø  Ripple factor is low.

Ø  No center tap is needed in the transformer secondary.

Ø  D.C saturation of the transformer core is avoided.

Ø  It is highly suited for high voltage applications.

Disadvantages:

Ø  It requires four diodes instead of two.

Ø  The load resistor and the source have no common point.

Ø  Two diodes in series conduct during each half cycle of the A.C input. It increases the total voltage drop and loses.