Rectifier Type

FULL WAVE RECTIFIER

     Full wave rectifier rectifies the full cycle in the waveform i.e. it rectifies both the positive and negative cycles in the waveform. 

     A full wave rectifier converts both halves of each cycle of an alternating wave (AC signal) into pulsating DC signal. 

    The full wave rectifier can be further divided mainly into following types.

1.      Center Tapped Full Wave Rectifier

2.      Full Wave Bridge Rectifier

Center Tapped Full Wave Rectifier

CIRCUIT DIAGRAM

                            

                           

CONSTRUCTION

A center-tapped full wave rectifier system consists of:

1. Center-tapped Transformer
2. Two Diodes
3. Resistive Load

Ø    In the center tapped full wave rectifier two diodes are used. These are connected to the center  tapped secondary winding of the transformer.

Ø    The positive terminal of two diodes is connected to the two ends of the transformer.

Ø    Center tap divides the total secondary voltage into equal parts. 

Ø    Center tap is the contact made at the middle of the winding of the transformer.

WORKING

Case 1- D1 Conduct only

ü  During positive half cycle Diode D1 is connected to the positive terminal and D2 is connected to the negative terminal.

ü  Thus diode D1 is in forward bias and the diode D2 is reverse biased.

ü  Only diode D1 starts conducting and thus current flows from diode and it appears across the load RL.

ü  So positive cycle of the input is appeared at the load.

Case 2- D2 conduct only

ü  During the negative half cycle Diode D2 is applied with the positive cycle.

ü  D2 starts conducting as it is in forward bias.

ü  The diode D1 is in reverse bias and this does not conduct.

ü  Thus current flows from diode D2 and hence negative cycle is also rectified, it appears at the load resistor RL.

Conclusion

       By comparing the current flow through load resistance in the positive and negative half cycles, direction of the current flow is same. Thus the frequency of rectified output voltage is two times the input frequency. The output that is rectified is not pure, it consists of a dc component and a lot of ac components of very low amplitudes. 

PEAK INVERSE VOLTAGE

     PIV is defined as the maximum possible voltage across a diode during its reverse bias.

    During the first half that is positive half of the input, the diode D1 is forward bias and thus conducts providing no resistance at all. Thus, the total voltage Vs appears in the upper-half of the ac supply, provided to the load resistance R.

   Similarly, in the case of diode D2 for the lower half of the transformer total secondary voltage developed appears at the load. The amount of voltage that drops across the two diodes in reverse bias is given as

D2 = Vm + Vm = 2Vm

                                                                      D1 = 2Vm

        Vm = voltage developed across upper and lower halves. 

Full Wave Bridge Rectifier

CIRCUIT DIAGRAM

     A full wave bridge rectifier is a type of rectifier which will use four diodes or more than that in a bridge formation. A full wave bridge rectifier system consists of

1. Four Diodes
2. Resistive Load

Ø  The working of a bridge rectifier is simple.

Ø  The secondary winding of the transformer is connected to the two diametrically opposite points of the bridge at points 1 and 3. Assume that a load is connected at the output.

Ø  The load R_Load is connected to bridge through points 2 and 4.

WORKING

Case 1- D1, D4 Conducts only

ü  During first half cycle, the upper portion of the transformer secondary winding is positive with respect to the lower portion.

ü  Thus only diodes D1 and D₄ are forward biased. Current flows through path 1-2, enter into the load R_L. It returns back flowing through path 4-3.

ü  During this half input cycle, the diodes D₂ and D₃ are reverse biased.

ü  Hence there is no current flow through the path 2-3 and 1-4.

Case 2- D2, D3 Conducts only

ü  During the next cycle, lower portion of the transformer is positive with respect to the upper portion.

ü  Thus only diodes diodes D2 and D3 are forward biased. Current flows through the path 3-2 and flows back through the path 4-1.

ü  During this half input cycle, the diodes D1 and D4 are reverse biased.

ü  So there is no current flow through the path 1-2 and 3-4.Thus negative cycle is rectified and it appears across the load.

PEAK INVERSE VOLTAGE

     At any instant when the transformer secondary voltage attains positive peak value Vmax, diodes D1 and D3 will be forward biased (conducting) and the diodes D2 and D4 will be reverse biased (non conducting).

If we consider ideal diodes in bridge, the forward biased diodes D1 and D3 will have zero resistance.

This means voltage drop across the conducting diodes will be zero.Thus,

PIV of a bridge rectifier = Vmax (max of secondary voltage)

Advantages

·                     The ripple frequency is two times the input frequency.

·                     Efficiency is higher.

·                     Ripple factor is less.

·                     Higher output voltage.

·                     Higher transformer utilization factor.

·                     Utilizes both halves of the AC waveform.

Disadvantages

·                     More expensive than half-wave rectifier.

·                     It requires more diodes, two for center tap rectifier and four for bridge rectifier.

·                     PIV rating of the diode is higher.

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