Full wave rectifier
The process of converting the AC current into DC current is called rectification. Rectification can be achieved by using a single diode or group of diodes. These diodes which convert the AC current into DC current are called rectifiers.
Rectifiers are generally classified into two types: half wave rectifier and full wave rectifier.
A half wave rectifier uses only a single diode to convert AC to DC. So it is very easy to construct the half wave rectifier. However, a single diode in half wave rectifier only allows either a positive half cycle or a negative half cycle of the input AC signal and the remaining half cycle of the input AC signal is blocked. As a result, a large amount of power is wasted. Furthermore, the half wave rectifiers are not suitable in the applications which need a steady and smooth DC voltage. So the half wave rectifiers are not efficient AC to DC converters.
We can easily overcome this drawback by using another type of rectifier known as a full wave rectifier. The full wave rectifier has some basic advantages over the half wave rectifier. The average DC output voltage produced by the full wave rectifier is higher than the half wave rectifier. Furthermore, the DC output signal of the full wave rectifier has fewer ripples than the half wave rectifier. As a result, we get a smoother output DC voltage.
Let’s take a look at full wave rectifier………..
Full wave rectifier definition
A full wave rectifier is a type of rectifier which converts both half cycles of the AC signal into pulsating DC signal.
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.
Before going to the working of a center tapped full wave rectifier, let’s first take a look at the center tapped transformer. Because the center tapped transformer plays a key role in the center tapped full wave rectifier.
Center tapped transformer
When an additional wire is connected across the exact middle of the secondary winding of a transformer, it is known as a center tapped transformer.
The wire is adjusted in such a way that it falls in the exact middle point of the secondary winding. So the wire is exactly at zero volts of the AC signal. This wire is known as the center tap.
The center tapped transformer works almost similar to a normal transformer. Like a normal transformer, the center tapped transformer also increases or reduces the AC voltage. However, a center tapped transformer has another important feature. That is the secondary winding of the center tapped transformer divides the input AC current or AC signal (VP) into two parts.
The upper part of the secondary winding produces a positive voltage V1 and the lower part of the secondary winding produces a negative voltage V2. When we combine these two voltages at output load, we get a complete AC signal.
I.e. VTotal = V1 + V2
The voltages V1 and V2 are equal in magnitude but opposite in direction. That is the voltages (V1 and V2 ) produced by the upper part and lower part of the secondary winding are 180 degrees out of phase with each other. However, by using a full wave rectifier with center tapped transformer, we can produce the voltages that are in phase with each other. In simple words, by using a full wave rectifier with center tapped transformer, we can produce a current that flows only in single direction.
What is center tapped full wave rectifier
A center tapped full wave rectifier is a type of rectifier which uses a center tapped transformer and two diodes to convert the complete AC signal into DC signal.
The center tapped full wave rectifier is made up of an AC source, a center tapped transformer, two diodes, and a load resistor.
The AC source is connected to the primary winding of the center tapped transformer. A center tap (additional wire) connected at the exact middle of the the secondary winding divides the input voltage into two parts.
The upper part of the secondary winding is connected to the diode D1 and the lower part of the secondary winding is connected to the diode D2. Both diode D1 and diode D2 are connected to a common load RL with the help of a center tap transformer. The center tap is generally considered as the ground point or the zero voltage reference point.
How center tapped full wave rectifier works
The center tapped full wave rectifier uses a center tapped transformer to convert the input AC voltage into output DC voltage.
When input AC voltage is applied, the secondary winding of the center tapped transformer divides this input AC voltage into two parts: positive and negative.
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.
On the other hand, the negative terminal B is connected to the p-side of the diode D2 and the positive terminal A is connected to the n-side of the diode D2. So the diode D2 is reverse biased during the positive half cycle and does not allow electric current through it.
The diode D1 supplies DC current to the load RL. The DC current produced at the load RL will return to the secondary winding through a center tap.
During the positive half cycle, current flows only in the upper part of the circuit while the lower part of the circuit carry no current to the load because the diode D2 is reverse biased. Thus, during the positive half cycle of the input AC signal, only diode D1 allows electric current while diode D2 does not allow electric current.
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.
On the other hand, the positive terminal B is connected to the p-side of the diode D2 and the negative terminal A is connected to the n-side of the diode D2. So the diode D2 is forward biased during the negative half cycle and allows electric current through it.
The diode D2 supplies DC current to the load RL. The DC current produced at the load RL will return to the secondary winding through a center tap.
During the negative half cycle, current flows only in the lower part of the circuit while the upper part of the circuit carry no current to the load because the diode D1 is reverse biased. Thus, during the negative half cycle of the input AC signal, only diode D2 allows electric current while diode D1 does not allow electric current.
Thus, the diode D1 allows electric current during the positive half cycle and diode D2 allows electric current during the negative half cycle of the input AC signal. As a result, both half cycles (positive and negative) of the input AC signal are allowed. So the output DC voltage is almost equal to the input AC voltage.
A small voltage is wasted at the diode D1 and diode D2 to make them conduct. However, this voltage is very small as compared to the voltage appeared at the output. So this voltage is neglected.
The diodes D1 and D2 are commonly connected to the load RL. So the load current is the sum of individual diode currents.
We know that a diode allows electric current in only one direction. From the above diagram, we can see that both the diodes D1 and D2 are allowing current in the same direction.
We know that a current that flows in only single direction is called a direct current. So the resultant current at the output (load) is a direct current (DC). However, the direct current appeared at the output is not a pure direct current but a pulsating direct current.
The value of the pulsating direct current changes with respect to time. This is due to the ripples in the output signal. These ripples can be reduced by using filters such as capacitor and inductor.
The average output DC voltage across the load resistor is double that of the single half wave rectifier circuit.
Output waveforms of full wave rectifier
The output waveforms of the full wave rectifier is shown in the below figure.
The first waveform represents an input AC signal. The second waveform and third waveform represents the DC signals or DC current produced by diode D1 and diode D2. The last waveform represents the total output DC current produced by diodes D1and D2. From the above waveforms, we can conclude that the output current produced at the load resistor is not a pure DC but a pulsating DC.
Characteristics of full wave rectifier
Ripple factor
The ripple factor is used to measure the amount of ripples present in the output DC signal. A high ripple factor indicates a high pulsating DC signal while a low ripple factor indicates a low pulsating DC signal.
Ripple factor is defined as the ratio of ripple voltage to the pure DC voltage
The ripple factor is given by
Finally, we get
γ = 0.48
Rectifier efficiency
Rectifier efficiency indicates how efficiently the rectifier converts AC into DC. A high percentage of rectifier efficiency indicates a good rectifier while a low percentage of rectifier efficiency indicates an inefficient rectifier.
Rectifier efficiency is defined as the ratio of DC output power to the AC input power.
It can be mathematically written as
η = output PDC / input PAC
The rectifier efficiency of a full wave rectifier is 81.2%.
The rectifier efficiency of a full wave rectifier is twice that of the half wave rectifier. So the full wave rectifier is more efficient than a half wave rectifier
Peak inverse voltage (PIV)
Peak inverse voltage or peak reverse voltage is the maximum voltage a diode can withstand in the reverse bias condition. If the applied voltage is greater than the peak inverse voltage, the diode will be permanently destroyed.
The peak inverse voltage (PIV) = 2Vsmax
DC output current
At the output load resistor RL, both the diode D1 and diode D2 currents flow in the same direction. So the output current is the sum of D1 and D2 currents.
The current produced by D1 is Imax / π and the current produced by D2 is Imax / π.
So the output current IDC = 2Imax / πWhere,
Imax = maximum DC load current
DC output voltage
The DC output voltage appeared at the load resistor RL is given as
VDC = 2Vmax /πWhere,
Vmax = maximum secondary voltage
Root mean square (RMS) value of load current IRMS
The root mean square (RMS) value of load current in a full wave rectifier is
Root mean square (RMS) value of the output load voltage VRMS
The root mean square (RMS) value of output load voltage in a full wave rectifier is
Form factor
Form factor is the ratio of RMS value of current to the DC output current
It can be mathematically written as
F.F = RMS value of current / DC output current
The form factor of a full wave rectifier is
F.F = 1.11
Advantages of full wave rectifier with center tapped transformer
High rectifier efficiency
Full wave rectifier has high rectifier efficiency than the half wave rectifier. That means the full wave rectifier converts AC to DC more efficiently than the half wave rectifier.
Low power loss
In a half wave rectifier, only half cycle (positive or negative half cycle) is allowed and the remaining half cycle is blocked. As a result, more than half of the voltage is wasted. But in full wave rectifier, both half cycles (positive and negative half cycles) are allowed at the same time. So no signal is wasted in a full wave rectifier.
Low ripples
The output DC signal in full wave rectifier has fewer ripples than the half wave rectifier.
Disadvantages of full wave rectifier with center tapped transformer
High cost
The center tapped transformers are expensive and occupy a large space.
