Full wave rectifier is a type of rectifier which converts alternating current voltage into pulsating direct current voltage during both half cycles of applied input voltage. This rectifier acts a heart of circuitry which allows the sensors to attach to the RCX in either polarity. In our previous articles, we explain half wave rectifier circuit, Rectifier efficiency equation, diode, semiconductors, diode types in detail. In this article, we are going to explain full wave rectifier circuit, working, operation in detail.
In this rectifier, full wave rectification can be achieved by using two crystal diodes which conduct current alternatively. During positive as well as the negative half cycle of input AC, the two circuits are employed to obtain the same direction of current flow in the load resistor.
Full wave Rectifier Circuit:
There are two main forms of Full wave rectifier circuit that can be used, each has its own working operation and features etc. The following two circuits are:
(i) Centre Tap Full-wave Rectifier
(ii) Full wave Bridge Rectifier
Centre Tap Full-wave Rectifier :
This rectifier is widely used in the vacuum tubes and thermionic valves. It employs a tapped transformer with secondary winding AB tapped at the centre point C, two diodes D1 and D2 are connected in the upper half and the lower half portion of the circuit. For rectification, the Diode D1 utilises the AC voltage appearing across the upper half of the secondary winding while D2 uses the lower half of secondary winding.
The circuit diagram of full wave rectifier circuit with waveform is shown in the figure below. The alternating voltage Vin appears across the terminals AB of the secondary winding of the transformer when AC supply is switched on.
During positive half cycle at a given voltage, end A of the transformer become positive while end B become negative which makes the D1 diode forward biased and D2 diode reverse biased. Due to this reason, the diode D1 conducts current while D2 does not conduct current. As shown in the figure above, the current starts flowing through diode D1, load resistor RL in the upper portion of the secondary by bold arrows.
During Negative Half Cycle, the end B of the transformer become positive while the end A become negative which makes the D1 diode reverse biased and D2 diode forward biased. Due to this reason, the diode D1 does not conduct current while D2 conduct current. As shown in the figure above, the current starts flowing through diode D2 , load resistor RL in the lower portion of the secondary by dotted arrows.
Note: During Negative as well as positive half-cycle of the input AC voltage, the current flows through the load resistor RL in the same direction.
Therefore across the load resistor RL, DC output voltage is obtained. The waveform diagram of the current flowing through the load, applied input voltage and output voltage developed across the load is shown in above figure.
Full wave bridge Rectifier :
To produce desired output, this type of full wave rectifier uses four individual rectifying diodes connected in a closed loop bridge configuration.It does not require a special centre tapped transformer which makes it smaller in size & cheaper at cost.
Full wave Rectifier Characteristics:
For the analysis of Full wave rectifier following parameters value or properties are considered. Following of its characteristics are:
(i) Ripple factor:
Ripple factor is defined as the ratio of the RMS value of AC component to the DC component. Therefore,
Ripple factor = RMS value of AC component / value of DC component
This factor mainly decides the effectiveness of a rectifier i.e. smaller the value of this factor, lesser is the AC component in comparison to the DC component. Hence, more effective is the rectifier.
Calculation of ripple factor:
The ripple factor of Full wave rectifier can be calculated as follows:
As we know,
Irms = √ Idc2 + Iac2
Irms = RMS value of total load current
Idc = value of dc component of the load current
Iac = RMS value of AC component of load current
Iac = √ Irms2 – Idc2
As we know,
Ripple factor = Iac / Idc
= √ Irms2 – Idc2 / Idc
= √ (Irms / Idc)2 -1
For full wave rectifier, ripple factor is calculated as:
As we know,
Irms / Idc = ( Im / √ 2) / (2 Im /π) = 1.11
Ripple factor = √ (1.11)2 -1 = 0.482
An efficiency of full wave rectifier is defined as the ratio of DC power output to the input AC power. Therefore,
Efficiency = DC power output / AC power input
= 0.812 RL / (rf + RL)
If rf is neglected as compared to RL ,the efficiency of the rectifier is maximum given as;
ηmax = 0.812 = 81.2%
(iii) Peak Inverse Voltage (PIV) :
Peak inverse voltage is defined as the maximum value of the voltage coming out of the diode when it is reverse biased during the negative half cycle. For centre tap full wave rectifier its value is 2 Vm and for the bridge rectifier, its value is Vm.
(iv) Transformer Utilization Factor:
It is defined as the ratio of power delivered to load and VA rating of the transformer. For centre tap full wave rectifier its value is 0.573 and for the bridge rectifier, its value is 0.812.
Advantages and Disadvantages of full Wave Rectifier:
Following of advantages of full wave rectifier are:
- The output and efficiency of centre tap full wave rectifier are high because AC supply delivers power during both the halves.
- For the same secondary voltage bridge rectifier has double output.
Following of disadvantages of full wave rectifier are:
- Full wave rectifier requires more diodes i.e two for centre tap rectifier and four for bridge rectifier.
- When a small voltage is required to be rectified this full wave rectifier circuit is not suitable.
- In centre tap full wave rectifier, centre on the secondary winding for tapping is difficult.
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