In a large number of electronic circuits, we require DC voltage for operation. We can easily convert the AC voltage or AC current into DC voltage or DC current by using a device called P-N junction diode.
One of the most important applications of a P-N junction diode is the rectification of Alternating Current (AC) into Direct Current (DC). A P-N junction diode allows electric current in only forward bias condition and blocks electric current in reverse bias condition. In simple words, a diode allows electric current in one direction. This unique property of the diode allows it to acts like a rectifier.
Rectifier Definition
A rectifier is an electrical device that converts an Alternating Current (AC) into a Direct Current (DC) by using one or more P-N junction diodes.
What is Rectifier?
When the voltage is applied to the P-N junction diode in such a way that the positive terminal of the battery is connected to the p-type semiconductor and the negative terminal of the battery is connected to the n-type semiconductor, the diode is said to be forward biased.
When this forward bias voltage is applied to the P-N junction diode, a large number of free electrons (majority carriers) in the n-type semiconductor experience a repulsive force from the negative terminal of the battery similarly a large number of holes (majority carriers) in the p-type semiconductor experience a repulsive force from the positive terminal of the battery.
When the voltage is applied to the P-N junction diode in such a way that the positive terminal of the battery is connected to the n-type semiconductor and the negative terminal of the battery is connected to the p-type semiconductor, the diode is said to be reverse biased.
When this reverse bias voltage is applied to the P-N junction diode, a large number of free electrons (majority carriers) in the n-type semiconductor experience an attractive force from the positive terminal of the battery similarly a large number of holes (majority carriers) in the p-type semiconductor experience an attractive force from the negative terminal of the battery.
The forward bias and reverse bias voltage applied to the diode is nothing but a DC voltage. A DC voltage produces a current which always flows in one direction (either forward direction or backward direction).
Half Wave Rectifier
The simplest rectifier is a diode connected to AC power supply. This is also known as a half wave rectifier. A simple half wave rectifier is a single p-n junction diode connected in series to the load resistor. The operation of a half wave rectifier is easy to understand a p-n junction diode conducts current only when it is forward biased.
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| Half wave Rectifier Circuit |
This principle is used in a half wave rectifier to convert AC to DC. The input provided here is an alternating current. This input voltage is stepped down using a transformer. A p-n junction diode conducts current only when it is forward biased. The same principle is made use of in a half wave rectifier to convert AC to DC. The input here is an alternating current. This input voltage is stepped down using a transformer. Since the diode is forward biased for half cycle of the AC, output is available only during that half cycle.
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| Half Wave Rectifier Output |
Advantages and Disadvantages of Half Wave Rectifier
Advantages:
Simple circuit and low cost.
Disadvantages:
1. The output current in the load contains, in addition to dc component, ac components of basic frequency equal to that of the input voltage frequency.
2. Ripple factor is high and an elaborate filtering is, therefore, required to give steady dc output.
3. The power output and, therefore, rectification efficiency is quite low. This is due to the fact that power is delivered only half the time.
4. Transformer Utilization Factor (TUF) is low.
5. DC saturation of transformer core resulting in magnetizing current and hysteresis losses and generation of harmonics.
Full Wave Rectifier
Like the half wave circuit, a full wave rectifier circuit produces an output voltage or current which is purely DC or has some specified DC component. Full wave rectifiers have some fundamental advantages over their half wave rectifier counterparts. The average output voltage is higher than for half wave, the output of the full wave rectifier has lesser ripple than that of the half wave rectifier producing a relatively smoother output waveform. There are two major types of full wave rectifier designs used frequently. The smaller design uses two diodes instead of the single diode used in half wave diode, i.e. one for each half of the cycle. A multiple winding transformer is used where secondary winding is split equally into two halves with a centre tapped connection.
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| Center top full wave rectifier circuit |
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| Bridge full wave rectifier circuit |
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| Full wave rectifier waveform |
Full Wave Rectifier Characteristics
1. Ripple Factor: Ripple factor is defined as the ratio of the RMS value of AC component to the DC component.
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.
2. Efficiency: 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
3. Peak Inverse Voltage: 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.
4. Transformer Utilisation 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
Advantages:
Following of advantages of full wave rectifier are:
1. The output and efficiency of centre tap full wave rectifier are high because AC supply delivers power during both the halves.
2. For the same secondary voltage bridge rectifier has double output.
Disadvantages:
Following of disadvantages of full wave rectifier are:
1. Full wave rectifier requires more diodes i.e two for centre tap rectifier and four for bridge rectifier.
2. When a small voltage is required to be rectified this full wave rectifier circuit is not suitable.
3. In centre tap full wave rectifier, centre on the secondary winding for tapping is difficult.
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