Sunday, 2 June 2019

Diode circuits - Half Wave Rectifier | Full Wave Rectifier | Bridge Rectifier

Diode circuits - Half Wave Rectifier | Full Wave Rectifier | Bridge Rectifier

1) The half wave rectifier:


A rectifier is nothing but a simple diode or group of diodes which converts AC input to DC output. 

Half wave rectifier consist only one diode,transformer and load resistor as shown in figure.


The Half wave rectifier

A half wave rectifier is a type of rectifier which allows only half cycle i.e.either positive half cycle or  negative half cycle of the input AC signal while the another half cycle is blocked.


However a half wave rectifier will not allow both positive and negative half cycle at the same time.

Half wave rectifier operation:


Positive half cycle:

When high AC voltage is applied ,the step down transformer reduces this high voltage into low   voltage.

Thus , a low voltage is produced at the secondary winding of the transformer.

The low AC voltage produced by the step down transformer is directly applied to the diode. 
  

Positive half wave rectifier

                        
When low AC voltage is applied to the diode (D), during the positive half cycle of the signal, the diode is forward biased and allows electric current.

Whereas, during the negative half cycle, the diode is reverse biased  and blocks electric current.

In simple words, the diode allows the positive half-cycle of the input AC signal and blocks the negative half-cycle of the input AC signal.


The positive half-cycle of the input AC signal  applied to the diode is analogous to the forward DC voltage applied to the  diode .

Similarly the negative half-cycle of the input AC signal applied to the diode is analogous to the reverse DC voltage applied to the  diode.


Negative half cycle:

The construction and working of negative half wave rectifier is almost similar to the positive half wave rectifier. 

The only thing we change here is the direction of a diode.

When AC voltage is applied, the step-down transformer reduces the high voltage to low voltage. This low voltage is applied to the diode.

The positive half wave rectifier, the negative half wave rectifier allows electric current during the negative half-cycle of input AC signal and blocks electric current during the positive half-cycle of the input AC signal.


Negative half wave rectifier


During the negative half cycle, the diode is forward biased and during the positive half cycle the diode is reverse biased,

So the negative half wave rectifier allows electric current only during the negative half cycle.

Thus, the negative half wave rectifier allows negative half cycles and blocks positive half cycles.


2) The full wave rectifier:


A full wave rectifier is a type of rectifier which converts both half cycles of the AC signal into pulsating DC signal.

It consist of two rectifier diodes  i.e. D1 and D2.


The Full wave rectifier

As shown in the above figure, the full wave rectifier converts both positive and negative half cycles of the input AC signal into output pulsating DC signal.


The full wave rectifier is further classified into two types: center tapped full wave rectifier and full wave bridge rectifier.

In this tutorial, center tapped full wave rectifier is explained.
  

Circuit Operation:


Positive cycle:

During the positive half cycle of the input AC signal, terminal A become positive, terminal B become negative and center tap is grounded (zero volts). 

The positive terminal A is connected to the p-side of the diode D1 and the negative terminal B is connected to the n-side of the diode D1.


So the diode D1 is forward biased during the positive half cycle and allows electric current through it.
                                       
Positive half cycle


 Negative cycle:

During the negative half cycle of the input AC signal, terminal A become negative, terminal B become positive and center tap is grounded (zero volts).


The negative terminal A is connected to the p-side of the diode D1 and the positive terminal B is connected to the n-side of the diode D1.


So the diode D1 is reverse biased during the negative half cycle and does not allow electric current through it. 


Negative half cycle

The output waveforms of the full wave rectifier is shown in the below figure.




3) The bridge rectifier:


In bridge rectifier, center tap is not required. 
    
If stepping down or stepping up of voltage is not required, then even the transformer can eliminated in the bridge rectifier. 

                                                 
The bridge rectifier


The rectifier efficiency of a bridge rectifier is almost equal to the center tapped full wave rectifier. 

The only advantage of bridge rectifier over center tapped full wave rectifier is the reduction in cost.

In bridge rectifier, instead of using the center-tapped transformer, four diodes are used.

The half wave rectifier and the center tapped full wave rectifier (full wave rectifier) are already discussed in the previous tutorials.

This tutorial is mainly focused on the bridge rectifier.

A bridge rectifier is a type of full wave rectifier which uses four or more diodes in a bridge circuit configuration to efficiently convert the Alternating Current (AC) into Direct Current (DC).


The input AC signal is applied across two terminals A and B and the output DC signal is obtained across the load resistor RL which is connected between the terminals C and D.

The four diodes D1, D2, D3, D4 are arranged in series with only two diodes allowing electric current during each half cycle.
 

For example, diodes D1 and D3 are considered as one pair which allows electric current during the positive half cycle.

whereas diodes D2 and D4 are considered as another pair which allows electric current during the negative half cycle of the input AC signal.


Operation of bridge rectifier:

When input AC signal is applied across the bridge rectifier, during the positive half cycle diodes D1 and D3 are forward biased and allows electric current,while the diodes D2 and D4are reverse biased and blocks electric current. 

On the other hand, during the negative half cycle diodes D2 and D4 are forward biased and allows electric current while diodes D1 and D3are reverse biased and blocks electric current.

During the positive half cycle, the terminal A becomes positive while the terminal B becomes negative.


This causes the diodes D1 and D3 forward biased and at the same time, it causes the diodes D2 and D4 reverse biased.


The current flow direction during the positive half cycle is shown in the figure A (I.e. A to D to C to B).

   
Positive half cycle

                                                       
During the negative half cycle, the terminal B becomes positive while the terminal A becomes negative. This causes the diodes D2 and D4 forward biased and at the same time, it causes the diodes D1 and D3 reverse biased.

The current flow direction during negative half cycle is shown in the figure B (I.e. B to D to C to A).

 
Negative half cycle

From the above two figures (A and B), we can observe that the direction of current flow across load resistor RL is same during the positive half cycle and negative half cycle. 

Therefore, the polarity of the output DC signal is same for both positive and negative half cycles.

The output DC signal polarity may be either completely positive or negative.

In our case, it is completely positive. If the direction of diodes is reversed then we get a complete negative DC voltage. 

Thus, a bridge rectifier allows electric current during both positive and negative half cycles of the input AC signal.




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