Full Wave Rectifier

Full wave Rectifier

In full wave rectifier , current flows through the load in the same direction ( i.e. d.c.) for both the half cycles of input a.c. supply voltage. There are two types of circuits commonly used for full-wave rectifications:

  • Centre-tap full wave rectifier
  • Full wave bridge rectifier

Centre-tap  Full wave Rectifier :

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Fig(3)

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Fig(4)

Fig(3) shows the circuit diagram of a Centre-tap full wave rectifier and  Fig(4) shows the input and output waveform of a centre-tap full wave rectifier .

The circuits uses two diodes D1 and D2. A centre-tap secondary winding AB is connected with the two diodes such that each diode uses one half-cycle of input a.c. voltage. That means diode Dutilises the the upper half of secondary winding for rectification and diode D2  uses the lower half.

Operation:

During the positive half cycle of secondary voltage, the end A of the secondary winding  becomes positive and end B negative.

So diode  D1 is forward biased and diode D2 is reverse biased. Hence, diode D1 conducts and diode D2 does not.

The current flows through diode D1, load resistance RL and the upper half of the secondary winding OA. This is shown by the red colored arrows.

During the negative half cycle of secondary voltage, the end A of the secondary winding  becomes negative and end B positive.

So diode D2 is forward biased and  D1 is reverse biased. Hence D2 conducts while D1 does not.

The conventional current flows through diode D2 , load resistance RL and the lower half of thr secondary winding OB as shown by the green colored arrows.

As we can see that current in the load RL  flows in the same direction for both the half cycles of input supply voltage. So d.c. is  obtained across RL.

Peak Inverse Voltage: 

Let Vm is the maximum voltage across the half secondary winding. Fig() shows the circuit at the instant secondary voltage reaches its maximum value Vm in the positive direction.At this instant diode D1 is conducting and D2is not conducting.So whole of the secondary voltage appears across the non-conducting diode.Hence the peak inverse voltage is twice the maximum voltage across the half secondary winding.

PIV = 2 Vm

Disadvantages:

It is difficult to locate the centre tap on secondary winding.

The d.c. output is low as each diode utilizes only half of the secondary voltage.

The diodes must have high peak inverse voltage.

Full Wave Bridge Rectifier :

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Fig(5)

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Fig(6)

Fig(5) shows the circuit connection of a full wave bridge rectifier and Fig(6) shows the input and output waveform of  full-wave bridge rectifier.

The full wave  bridge rectifier circuit  contains four diodes D1 , D2,D3 and D4, connected to form a bridge as shown in Fig(5).

The a.c. supply is applied to the diagonally opposite ends A and B of the bridge through the transformer.

The load resistance RL is connected between other two ends of the bridge i.e. P and Q.

Operation:

During the positive half cycle of secondary voltage, the end A of the secondary winding becomes positive and end B negative.

This makes diodes D1 and Dforward biased while Dand D4 are reverse biased. Hence  only diodes  D1 and D2 conducts.

The conventional current flows through load RL is shown by the red colored arrows .

During the negative half cycle of secondary voltage, end A becomes negative and end B positive.

This makes diodes D3 and D4and forward biased  and  diodes D1 and Dare reverse biased. Hence only diodes D3  and D4  conducts.

The conventional current flow through load RLis shown by the green colored arrows.

It can be seen that the current flows through the load RL in the same direction for both the half-cycles. Hence d.c. output is obtained across load RL.

Peak Inverse Voltage:

The peak inverse voltage(PIV) of each diode is equal to the maximum secondary voltage of transformer  i.e. Vm.

PIV  =  Vm

Advantages:

There is no need of centre taped transformer in full-wave bridge rectifier.

The output is twice that of centre-tap circuit for the same secondary voltage.

The PIV is half that of the centre-tap circuit.

Disadvantages:

It needs four diodes.

Since during each half cycle two diodes that conducts are in series so voltage drop in the internal resistance of the rectifying unit is twice as in the centre-tap circuit. This is objectionable when secondary voltage is small.

Output Frequency:

The output frequency of a full wave rectifier is double the input frequency.

fout= 2fin

Efficiency:

Let  v = Vmsinθ

rf = diode resistance

RL= Load resistance

i= instantaneous current

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Efficiency η:

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d.c. output power:

Let  Idc=  Iav =average current

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So d.c.power output,

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a.c. input power:

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For  a full-wave rectified wave,

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The efficiency will be maximum if rf is negligible as compared to RL

So Maximum Efficiency  = 81.2%

This is double the efficiency of a half wave rectifier .Therefore,a full wave rectifier is twice as effective as a half-wave rectifier.

Ripple Factor:

In full-wave rectification,

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It is clear that d.c. component exceeds the a.c. component in the output of a full wave rectifier .This results in lesser pulsation in the output of a full wave rectifier as compared to a half wave rectifier . Therefore, full-wave rectification is invariably used for conversion rectification.