Rectifier Rectification is the process of converting AC to DC A diode is used as the rectifier A diode will conduct only when it is given forward bias i.e. when the anode is more +ve than the cathode Hence when AC is applied to a diode it will conduct only in one direction, giving DC
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Rectifier Rectification is the process of converting AC to
DC A diode is used as the rectifier A diode will conduct only when it is given
forward bias i.e. when the anode is more +ve than the cathode Hence when AC is applied to a diode it will
conduct only in one direction, giving DC
Half-wave rectifier
+
_
D1
Half wave rectifier
AC is applied to the Power Transformer and it steps down the voltage to the required AC
This AC is applied to the diode D1 During the +ve half cycle, the diode gets forward
bias Now current flows from Anode to Cathode
through the Load resistor RL back to the secondary winding
This current produces a voltage across RL
Thus we get one half of the AC input, at the output During the –ve half, the diode gets reverse bias Now the diode stops conducting Thus, the diode conducts only for one half and
gives the output for that alf
As the diode conducts only for one half of the applied AC, it is called as Half wave rectifier circuit
This way the diode rectifies AC to DC But the obtained DC is a fluctuating DC & not a
constant DC
Full-wave rectifier
+
_
D1
D2
RL
A
B
The full wave rectifier uses 2 diodes & a centre-tapped transformer
In any transformer the ends of the secondary will give out of phase voltage
i.e. the voltage will be alternating with one end at a higher (positive) potential than the other, for one half cycle
And at lower (negative) potential during the next half and so on
Full-wave rectifier
+
_
D1
RL
A
B
During the first half, the top end A gets the +ve half while the bottom end B gets the –ve half
Now D1 gets forward bias and D2 gets reverse bias Hence only D1 will conduct during this half The current will now flow from A through D1,
through RL to the centre tapping of the transformer This produces a voltage with positive polarity as
shown
Full-wave rectifier
+
_D2
RL
A
B
During the next half, the end A gets –ve half, while B gets +ve half
Now D2 gets forward bias and starts conducting, while D1 gets reverse bias & stops conducting
The current now flows from B through D2, through RL in the same direction back to the centre tapping
Hence this half also gives a positive voltage As the diode conducts during the full cycle (both
halves), it is referred to as Full wave rectifier
Bridge rectifier
+
_RL
D3
D4 D1
D2
A
B
The bridge rectifier uses 4 diodes The circuit has 4 balanced arms (diodes) Hence it is called as Bridge Rectifier circuit As before, when the secondary end A is +ve going,
the end B will be –ve going The +ve voltage at A forward biases D1 & reverse
biases D4 The –ve voltage at B provides forward bias for D2
and reverse biases D3
Thus for one half D1 & D2 conduct and for the other half D3 & D4 conduct
The circuit diagram can be redrawn omitting the non-conducting diodes for easier understanding
During the first half the current flows from A through D1, through RL, through D2 back to the secondary
Bridge rectifier
+
_RL
D1
D2
A
B
During the next half, current flows from B through D3, through RL, through D4 back to the secondary
In both the cases, the current flows through RL in the same direction, producing a +ve signal as shown
Thus the bridge rectifier also gives a full wave output
Bridge rectifier
+
_RL
D3
D4
A
B
Comparison
Out of the 3 rectifiers, half wave rectifier is the simplest and cheapest
But the output contains only one half This makes it more difficult to convert the
fluctuating DC to a pure DC A centre-tapped transformer is a must for Full
wave rectifier
Comparison
The half wave and bridge rectifier circuits can also be drawn without the power transformer
In that case, we may have to use a high wattage resistor to reduce the voltage to the required value
Comparison
In full wave rectifier, voltage is taken across one half of the secondary
Hence the number of turns in the secondary will be double for the centre-tapped (full wave) transformer compared to the one used for the bridge circuit
Or, the voltage output per turn is more for the bridge rectifier transformer