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

End