What is a Rectifier: Working, Types and Uses

What is a Rectifier?

Rectifier are among the most essential components being used in many electronic devices specially power supply systems. Almost all modern-day devices operate on DC power for being more efficient. Power grids only provide AC power, which may cause damage to certain devices, such as home appliances, if attached directly to grid power. Hence, rectifiers are used in these devices, which convert AC current to DC current to comply with the power requirements of these devices. The rectifier types and their uses are also discussed for a better understanding of these devices.

Rectifier Types and Working

On the basis of output waveform, rectifiers are categorized into two major types. Construction can also bring further subtypes into consideration.

Before moving on to the rectifier types and working, we must know the difference between AC current and DC current.

DC current: refers to the unidirectional flow of electrons within the circuit.

AC current: refers to the bi-directional flow of electrons within the circuit.

The AC current keeps changing polarity (direction) continuously whereas the DC current does not change its polarity (direction) or in other words, direction of flow. This change in polarity can cause significant damage to many devices that are designed to operate on DC current solely.

Essentially, a rectifier is just a PN junction diode whose power and current rating are higher than those diodes used for other applications. It is connected in forward bias configuration with respect to the direction of conventional current. When the AC current flows from the positive terminal of the battery to the negative terminal, which can be referred to as the “forward direction of current” or “positive half cycle of the input AC signal”, the rectifier allows the current to pass through it. When the AC current flows in “opposite or reverse direction of current” or “during the negative half cycle of the input AC signal”, the diode is reverse biased with respect to the direction of conventional current and consequently no current flows through it and current within the circuit is zero.

We have two basic rectifier types:

• Half wave rectifier
• Full wave rectifier

The working for both types is almost same. However, the difference arises between construction, power output and the efficiency which is discussed in later section. Both rectifier types are discussed below:

Half Wave Rectifier

The half wave rectifier consists of a diode connected in series with the load, such as a resistor or capacitor.

Figure 1: Circuit diagram for half wave rectifier

When the input AC voltage is positive, the diode allows the current to flow through the circuit and delivers the power to the load during positive half cycle.

Figure 2: Positive half cycle for half wave rectifier

However, when the input voltage becomes negative and the current flows in the reverse direction, the diode stops the flow of current within circuit and hence there is no power delivered to the load during the negative half cycle of the input voltage.

Figure 3: Negative half cycle for half wave rectifier

The output DC voltage is given by the following equation:

V_DC=0.318(V_m-V_k)

Where Vk=0.7V for the silicon diode and Vm corresponds to the peak value of the input wave. If V_m is much greater than V_K, the equation becomes:

V_DC=0.318V_m

The input and output waveforms are shown below. Here, a sinusoidal wave is selected to ease understanding. Otherwise, it could be any periodic AC signal like triangular wave.

Figure 4: AC input voltage

Figure 5: Rectified DC output of positive half-cycle

The output waveform of a half wave rectifier consists of a series of pulses corresponding to either the positive half cycle or negative half-cycle of the input waveform.

Full Wave Rectifier

Unlike the half wave rectifier, which utilizes only one half cycle of the input voltage, a full wave rectifier converts the input AC voltage to DC voltage by rectifying both the positive and negative half-cycles of the input voltage. A full wave rectifier consists of multiple diodes connected to the load. Some diodes correspond to the positive half cycle, and other diodes correspond to the negative half cycle of the input AC voltage. Consequently, the output signal has a series of pulses that correspond to both positive and negative half cycles.

There are two commonly used rectifier configurations for this rectifier type: bridge rectifier and center-tapped full wave rectifier.

Center-tapped Full Wave Rectifier

The center-tapped full wave rectifier consists of two diodes, one AC source, one load resistor, and a center-tapped transformer. The following figure shows the general configuration for a center-tapped full wave rectifier:

Figure 6: Center-tapped full wave rectifier

During the positive half cycle, the diode labeled as D1 conducts the flow of current, whereas the diode D2 blocks the current as it is in reverse bias configuration with respect to the flow of conventional current. It is shown in the figure below:

Figure 7: Flow of current during the positive half cycle

During the negative half cycle, the diode labelled as D2 conducts the flow of current whereas the diode D1 blocks the current as it is in reverse bias configuration with respect to the flow of current. It is shown in the figure below:

Figure 8: Flow of current during negative half cycle

The output DC voltage is given by the following equation:

V_DC=2V_m/π

Bridge Rectifier

Another configuration for a full wave rectifier is a bridge rectifier that uses four diodes and one load resistor. This configuration resembles the Wheatstone Bridge. This is shown in the figure below:

Figure 9: Bridge Rectifier

During the positive half cycle, the current flows from the positive terminal of the battery to the negative terminal via D2 and D3, whereas D1 and D4 are open as both are in reverse bias configuration with respect to the flow of conventional current. This is shown in the figure below:

Figure 10: Flow of current during the positive half cycle

During the negative half cycle, the current flows via D1 and D4, whereas D2 and D3 are open as both are in reverse bias configuration with respect to the flow of current. This is shown in the figure below:

Figure 11: Flow of current during negative half cycle

The output DC voltage is given by the following equation:

V_DC=0.636(V_m-2V_k)

 Where V_k=0.7V for silicon diode. If V_m is much greater than V_K, the equation becomes:

V_DC=0.636V_m

Consequently, the input AC voltage is rectified during both half cycles. The output of the full wave rectifier is shown in the figure below:

Figure 12: AC input voltage

Figure 13: Output waveform of a full wave rectifier

Efficiency Comparison of Half and Full Wave Rectifier

The efficiency of half and full wave rectifiers depends on some factors, such as:

• Output Power: Power delivered by the rectifier to the load.
• Transformer Utilization Factor: Ratio of DC power delivered to the load by rectifier to AC power rating of transformer secondary winding.

TUF = P_DC/P_AC

• Ripple Factor: Measure of remaining AC component in the DC output of the rectifier. It has two types given by the equations:

Voltage Ripple = V_RMS/V_DC

Current Ripple = I_RMS/I_DC

Based on these factors:

• The output power for a full wave rectifier is higher than a half wave.
• The transformer utilization factor for a full wave rectifier is higher than a half wave rectifier.
• The ripple factor for a half wave rectifier is higher than full wave rectifier.
• Hence, the efficiency of a full wave rectifier is higher than a half wave rectifier.

Rectifier Uses

Rectifiers are widely used in various electronic devices to convert the AC voltage into DC voltage. Some common rectifier uses are listed below:

• Rectifiers are important components of a power supply circuit, which are used to power various electronic devices like mobile phones and laptops. The rectifiers convert AC voltage from the mains power supply or the grid station into DC voltage that can be used to safely operate the electronic devices.
• The rectifiers are used in battery chargers to convert AC voltage into DC voltage and provide a steady DC input voltage to the charger, ensuring that the correct voltage level for the device is maintained.
• The rectifiers are used in voltage multiplier circuits to convert low AC voltages to high DC voltages.
• The rectifiers are widely used in signal demodulation circuits for the demodulation of AM (amplitude-modulated) signals.
• The half wave rectifiers are also used in firing and pulse generation circuits.
• The full wave rectifiers are used in audio amplification circuits to power the amplifiers.
• The rectifiers are used in electric welding to provide stable DC voltage to the welding equipment.

Conclusion

In summary, rectifiers are integral components in electronic devices, converting AC to DC power for efficient operation. The distinction between half-wave and full-wave rectifiers, along with their working principles, underscores their significance. The comparison highlights the superior efficiency of full-wave rectifiers. From preventing potential damage in devices to powering electronic equipment and enabling signal demodulation, rectifiers find diverse applications. Their crucial role in various electronic systems emphasizes the importance of understanding their types, working mechanisms, and efficiency factors.

Fatima Razzaq

Fatima Razzaq is a freelance technical writer who served as an electrical engineering lecturer at Air University—a federally chartered public sector research university in Pakistan. Razzaq holds a Bachelor’s degree with distinction in electronic engineering from Ghulam Ishaq Khan Institute of Engineering Sciences and Technology (GIKI) and a Master’s degree in Sustainable Transportation and Electrical Power Systems from the University of Nottingham, Universidad de Oviedo, and La Sapienza University of Rome. Razzaq’s diverse work experiences in academia and industry continue to inform her prolific technical writing journey in the areas of electrical engineering, storage mechanisms, power electronics, electric vehicles, energy, and related topics.

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