23.7 Rectification of Alternating Currents
23.7.1 Rectification
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| Figure 23.29 The figure shows the circuit symbol of a diode, indicating the direction of current flow from anode to cathode. |
- Rectification is the process of converting an alternating current or voltage to a direct current/voltage.
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Rectification may be:
- (a) half wave
- (b) full wave
- The electrical/electronic component which rectifies ac is known as a rectifier. One such component is the diode.
- A diode is represented by the symbol shown in Figure 23.29.
- Main characteristic of a diode:
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| Figure 23.30 (a) The diode is forward biased (anode positive), allowing current to flow through the circuit and producing a potential difference across the resistor. (b) The diode is reverse biased (anode negative), preventing current flow, so no potential difference is produced across the resistor. |
Refer to the diode circuit shown in Figure 23.30(a). When the anode is positive with respect to the cathode, we have current flowing through the diode and the resistor. In this circuit arrangement, the diode conducts electricity. A p.d. is produced across the resistor.
Refer to the diode circuit shown in Figure 23.30(b). When the anode is negative with respect to the cathode, no current flows through the diode and the resistor. In this circuit arrangement, the diode does not conduct electricity. No p.d. exists across the resistor.
23.7.2 Half Wave Rectification
1 Circuit for half wave rectification
Half wave rectification can be achieved by connecting a diode in series to a resistor. A sinusoidal voltage source is then connected to this series circuit, as shown in Figure 23.31(a).
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| Figure 23.31 (a) The circuit shows a diode connected in series with a resistor and an AC voltage source for half-wave rectification. (b) The current-time graph shows that current flows only during one half of each cycle, producing a pulsating direct current. (c) The voltage across the resistor follows the same waveform as the current, as it is directly proportional to the current. |
2 Principle of half wave rectification
(a) Since the applied voltage is an alternating one, the polarity of the anode periodically changes with time, being positive one moment, then changing to negative the next moment, back to positive again, and so on.
(b) When the anode of the diode is positive with respect to the cathode for half of the applied voltage cycle, the diode conducts electricity. There is current flowing through the resistor during this half of the cycle.
(c) For the next half voltage cycle, the anode is negative with respect to the cathode. Now the diode does not conduct electricity. No current flows through the resistor during this half voltage cycle.
(d) This means that current flows through the diode in one direction for half a cycle, stops flowing for the next half cycle, and flows again in the same direction for another half cycle, and so on. The current flowing now is always in one direction. Hence, it is a direct current.
3 Current and voltage waveforms for half wave rectification
(a) Figure 23.31(b) shows the current waveform in the current-time curve. Notice the following:
(i) The current only has a positive value. This means that the current flows only in one direction. In other words, the original alternating current has been converted to direct current.
(ii) Current flows only for half of the applied voltage cycle. Because current flows only for half a cycle, the rectification is known as half wave rectification.
(b) The p.d. VR across the resistor is given by:
VR = IR
where R is the resistance of the resistor. The waveform of VR is similar to that of the current, as shown in Figure 23.31(c), since VR is proportional to I.
23.7.3 Full Wave Bridge Rectifier
1 Circuit for full wave rectification
Figure 23.32(a) shows the circuit which can achieve full wave rectification. It is known as a rectifier bridge circuit. It consists of:
- (a) four diodes
- (b) a resistor R acting as an electrical load.
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| Figure 23.32 (a) The first half cycle where junction A is positive, causing diodes D1 and D3 to conduct while D2 and D4 do not, allowing current to flow through the resistor from X to Y. (b) The second half cycle where junction A is negative, causing diodes D2 and D4 to conduct while D1 and D3 do not, with current still flowing through the resistor from X to Y. |
2 Principle
(a) For half a cycle of the applied voltage, junction A is positive with respect to junction B, as shown in Figure 23.32(a).
(b) This condition causes the anodes of diodes D1 and D3 to be positive with respect to the cathodes. The two diodes conduct electricity and so current passes through them.
(c) At the same time the anodes of diodes D2 and D4 are negative with respect to the cathodes. Hence, they do not conduct electricity.
(d) Consequently, current flows along the path as shown. Current flows through the resistor R from X to Y. A p.d. is produced across R.
(e) For the next half cycle of the applied voltage, junction A becomes negative with respect to junction B, as shown in Figure 23.32(b).
(f) This condition causes the anodes of diodes D2 and D4 to be positive with respect to the cathodes. The two diodes conduct electricity and so current passes through them.
(g) At the same time the anodes of diodes D1 and D3 are negative with respect to the cathodes. Hence, they do not conduct electricity.
(h) Consequently, current flows along the path as shown. Current flows through the resistor R, also from X to Y. A p.d. is produced across R.
(i) In other words, for both halves of the applied voltage cycle, there is current flowing through R, always from X to Y. Hence, it is a direct current that flows through R.
3 Current and voltage waveforms for full wave rectification
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| Figure 23.33 The figure shows a current-time graph where the current remains positive throughout, indicating that the alternating current has been converted into direct current. |
(a) Figure 23.33 shows the current waveform in the current-time curve. Notice the following:
(i) The current only has a positive value. This means that the current flows only in one direction. In other words, the original alternating current has been converted to direct current.
(ii) Current flows for both halves of the applied voltage cycle. Because current flows in one complete voltage cycle, the rectification is known as full wave rectification.
(b) The waveform of VR is similar to that of the current.
23.8 SMOOTHING BY CAPACITORS
23.8.1 Smoothing Output Voltage
The output voltage:
(a) The voltage developed across the resistor R in the diode circuit is the output voltage.
(b) The polarity at each end of the resistor always remains the same (one end always positive, the other end always negative) because the current flowing through it is a direct current.
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| Figure 23.34 (a) The figure shows a capacitor connected in parallel with a resistor to smooth the output voltage. (b) The graph shows the output voltage waveform where the capacitor charges and discharges, reducing fluctuations and producing a smoother voltage. |
(c) But the magnitude of the voltage fluctuates with time between zero and a maximum value. It is not suitable to make use of a voltage whose magnitude changes over a wide range of values.
Smoothing the fluctuating voltage:
We can drastically reduce the range of fluctuation so that the output voltage becomes nearly constant. The process of reducing the fluctuation of the voltage is referred to as smoothing the voltage.
Method:
To achieve smoothing of the output voltage, we connect a capacitor directly across the resistor R, as shown in Figure 23.34(a).
Principle:
First quarter of a cycle:
(a) The output voltage across the resistor increases from zero to the maximum value.
(b) This voltage is applied across the capacitor since the capacitor is connected parallel to the resistor. Hence, the capacitor undergoes charging as the voltage increase with time. The charging continues up to the moment when the output voltage reaches the maximum value.
Second quarter of the cycle:
(a) The output voltage starts to decrease and falls below the maximum value. Now we have the situation where the voltage across the capacitor would be greater than the output voltage. To prevent that from happening, the capacitor has to discharge so that the voltage across the capacitor is equal to the output voltage across the resistor.
(b) When the capacitor discharges, it produces a current which flows in the same direction as the current that is flowing through the resistor. Thus there is an increase in the net current.
(c) Consequently, the output voltage decreases at a very much slower rate. Because of this, it does not decrease to zero in this quarter of the cycle, as shown in Figure 23.34(b).
Third quarter of the cycle:
(a) The capacitor continues to discharge. The output voltage continues to decrease slowly and extends into this quarter of the cycle.
(b) Before long, the voltage starts to increase again. When this happens, the capacitor begins charging again. The charging continues right up to the moment when the voltage reaches the maximum value. After that the capacitor discharges, and the whole process described above is repeated.
(c) The charging and discharging of the capacitor occurring alternately has caused the output voltage to fluctuate only within a narrow range around a value close to the maximum value, thus smoothing the output voltage.
Application of Rectification
Converting Alternating Current (AC) into Direct Current (DC)
Rectification is a fundamental process in electronics that converts alternating current (AC) into direct current (DC). This process is achieved using diodes, which allow current to flow in only one direction.
Real-World Applications
Mobile & Laptop Chargers
Chargers convert AC from wall outlets into DC required for charging batteries safely and efficiently.
Computer Power Supplies
Power supply units use full-wave rectifiers to provide stable DC voltage for internal components.
Televisions & Home Electronics
Devices such as TVs and radios rely on rectification circuits to operate electronic systems.
Audio Amplifiers
Amplifiers require smooth DC voltage to produce clear and stable sound output.
Types of Rectification
- Half-Wave Rectification: Uses one half of the AC cycle
- Full-Wave Rectification: Uses both halves of the AC cycle
- Bridge Rectifier: Most efficient and widely used configuration
Concept Simulation
AC Input Behavior:
- Positive half-cycle → diode conducts → current flows
- Negative half-cycle → diode blocks → no current
Output Result:
- Pulsating DC is produced
- Adding a capacitor smooths the output voltage
Rectification plays a crucial role in modern electronics by converting alternating current (AC) into direct current (DC). Through the use of diodes, electrical energy can be controlled and directed efficiently, enabling devices to operate safely and reliably.
- Diodes allow current to flow in only one direction
- Half-wave and full-wave rectifiers serve different efficiency levels
- Bridge rectifiers are widely used in practical applications
- Capacitors help smooth the output into stable DC voltage
Without rectification, most electronic devices such as chargers, computers, and communication systems would not function properly. It remains one of the most essential concepts in electrical and electronic engineering.
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