What is a full wave Rectifier? Comparison of HW-rectifiers and FW-rectifiers,Simple FW-rectifier with center-tapped transformer its working,Efficiency of FW-rectifier

Full-Wave Rectifiers

               There are two types of full-wave rectifier circuits.one is uses two diodes with a center tapped transformer and the transformer have equal turns Ns1 and Ns2 of the secondary windings which is shown in the fig. below The second type of FW-rectifier is having bridge-rectifier circuit and uses four diodes and it does not require a center-tapped transformer it only have simple transformer.

fig.

Comparison of HW Rectifiers and FW 

Rectifiers

                 The FW-rectifier circuits are better if we compare with HW rectifier circuits in all most all respects.FW-rectifiers have double the value of dc voltage delivered to load their efficiency is also double and their ripple factor is low.Due to these reasons FW-rectifiers are preferred over HW-rectifiers.

Simple FW-Rectifier with Center-Tapped Transformer

fig

                   Primary side of a step down transformer is connected to the ac main voltage of 220V 50hz.As shown in the above fig. the transformer used has two secondary windings with a common center point c which is also the earth of the system.Secondary windings have the same number of turns Ns1=Ns2 .As we know for a step down transformer Np (primary turns) is greater than Ns (secondary turns).The secondary voltage across Ns1 and Ns2 are equal in magnitude but opposite in phase and denoted as Vin1 and Vin2 .It means when point a is positive with respect to the center point then lower point b is negative.

Working Principle of HW Rectifier

 Positive H.C of Vin1

• For the positive half cycles of Vin1 from 0 to π and 2π to 3π the upper diode D1 will be forward-biased it will conduct a forward current .This current will flow from top of Rl  towards ground.So, point D is positive with respect to ground.
• Between the same period 0 to π and 2π to 3π the 2nd input voltage vin2 will be negative.During this period the diode D2 is reversed bias and will turn off and show the open circuit so no current will flow through the diode D2.
• The waveform of Vin1 and Vin2 are the waveform of the output voltage Vo developed in the load resistor Rl shown in the above fig.the complete path for the current through the circuit is shown with solid arrows.
• If we consider D1 and D2 are ideal diodes Vd=0V and rd= 0 ohm ,the max. voltage across the load resistor Rl will be equal to the max. amplitude Vmax of the input voltage Vin1 . for a practical silicon diode Vd=0.7V and rd= 0ohm the peak or max. amplitude of Vo will be   Vmax -0.7V.
• At π/2 the point d will be at +Vmax and point b will be at -Vmax.This shows that the P-side of the diode D2 will be 2Vmax with respect to its N-side.We can say that D2 will be reverse-biased by twice the value of the max. amplitude of Vin.This is the disadvantage of such a kind of circuit ,the reverse voltage of the diode when it is off is Vmax , So the PIV-rating of the diodes used in full-wave rectifier of this kind is greater than 2Vmax.

Negative Half-cycle of Vin1

• During the negative half cycle of Vin1 or between π to 2π and 3π to 4π  ,the other input voltage Vin2 will be positive going.
• Now D2 will be ON and D1 will be OFF ,The positive half cycles of Vin2 will forward-bias D2 and the negative half cycles of Vin1 will reverse bias D1.therefore diode D2 will be ON and will conduct forward current through Rl as shown in the above fig.
• This will result in a voltage Vo across Rl in the positive direction,the current through Rl will again flow from top to bottom.Therefor the waveform of Vo between π to 2π and 3π to 4π will be the same as it was before between 0 to π and 2π to 3π when the upper D1 conducted current through the load resistor Rl.The output voltage across Rl is uni-directional but it is pulsating.
• If we compare the FW-rectifier with the HW-rectifier since there is no vacant area between π to 2π .So, dc voltage across Rl will be twice as much compares to the HW-rectifier. It is the advantage of the FW-rectifier over the HW-rectifier.

Efficiency Of FW-Rectifier

                  Efficiency of FW-rectifier is defined as the ratio of the output dc power delivered to the load resistor Rl and the ac power drawn from the source that is the secondary winding of the transformer in mathematical form

                               η of FW= Pdc/ Pac

        Pdc=dc power delivered to load by the FW-rectifier   and Pac=rms value of the power drawn from the secondary winding of the transformer.

                             Pdc=(Vdc)2/Rl  =  (2Vmax/ π)2 (1/Rl)           bcs      Vdc=2Vmax/ π

                               

                              Pac= (Vorms)2/Rl

                            

                              Pac=(Vmax)2/(2Rl)   

                               
                                η of FW = 4(Vmax)22Rl / Rl(π )2(Vmax)2  =  8/(π)2 =  81.06%

        It showes that efficiency of the FW-rectifier is twice the value of the HW-rectifier.

                               

                                

                                  η of FW= 81.06% = 2 η of HW
       
                    Above eq. shows that out of 100% power drawn from the secondary winding of the transformer 81.06% power is usefully delivered to the load Rl.The remaining power of 18.94% is wasted in the rest of the circuit.If we consider the resistance of the secondary of the transformer then  η of FW- rectifier will be slightly less than the theoretical max. of 81.06%.