Research on the Use of Diodes in AC to DC Conversion

AC and DC are the two basic methods of power usage in daily life. However, AC and DC each have their own advantages and are irreplaceable in certain scenarios. Therefore, we are often interested in how to convert AC to DC or DC to AC. Through research, it is understood that the conversion from AC to DC is primarily achieved through a rectifier circuit, and the key component in a rectifier circuit is the diode. This article focuses on how AC is converted to DC, with diodes as the researched electronic components, aiming to investigate the role of diodes in rectifier circuits to gain a deeper understanding of the principles of rectifier circuits, the ideal circuit model, and the relationship and differences between actual circuit components and modules. This strengthens the deep learning of circuit theory and the Multisim simulation tool.

1. Introduction

It is well known that AC and DC are the two basic methods of electricity usage in daily life. However, AC and DC have their respective advantages and are applied in different scenarios. For example, most lighting and power applications use AC, while computers and mobile phones predominantly use DC. Therefore, at times, we need to convert AC and DC to meet user demands for power characteristics.

This article analyzes the conversion of AC to DC, highlighting the crucial role of a module called the rectifier circuit in this process. It further investigates how the rectifier circuit is constructed using the characteristics of the electronic component, the diode. By combining the Multisim simulation tool, this process deepens the understanding of the relationship and differences between the ideal circuit model and actual circuit components and modules.

The research approach of this article is to start from the basic characteristics of diodes—how rectifier circuits utilize the characteristics of diodes—how rectifier circuits are used in converting AC to DC—followed by an in-depth exploration of voltage regulation diodes and DC voltage regulators.

2. Basic Characteristics of Diodes

A. Basic Structure and Voltage-Current Characteristics of DiodesDiodes are electronic components made from semiconductor materials (silicon, selenium, germanium, etc.). They exhibit unidirectional conductivity, meaning that when a forward voltage is applied to the anode and cathode of the diode, it conducts. When a reverse voltage is applied to the anode and cathode, the diode is cut off. Therefore, the conduction and cutoff of the diode are equivalent to the on and off of a switch.

B. Equivalent Circuit of Diodes As seen in the figure, the voltage-current characteristics of diodes are nonlinear, which complicates the analysis of circuits using diodes. To facilitate analysis, it is common to approximate the characteristics of diodes with a circuit made of linear components under certain conditions, replacing the diode in the circuit. Such circuits that can simulate diode characteristics are called the equivalent circuit of the diode, also known as the diode’s equivalent model. Various equivalent circuits can be constructed based on the diode’s voltage-current characteristics.

3. How Rectifier Circuits Utilize Diode Characteristics

When analyzing rectifier circuits, to highlight key points and simplify the analysis process, it is generally assumed that the load is purely resistive; the rectifier diode has the ideal voltage-current characteristics as shown in the solid line of Figure 1.2.4 (a), meaning that the forward voltage drop is 0 when conducting, and the reverse current is 0 when cut off. Moreover, for the convenience of circuit simulation and analysis, a reasonable AC power source is directly used as the input power source to study the waveform without going through transformer processing.

A. AC Alternating current (AC) refers to the current whose direction varies periodically with time, and its waveform is shown in the figure as a sine wave.

B. Single-Phase Half-Wave Rectifier Circuit As noted in section (2. Basic Characteristics of Diodes), when a forward voltage is applied to the anode and cathode of the diode, it conducts. When a reverse voltage is applied, the diode is cut off. Therefore, by simply connecting an ideal diode D1 in series in the simulation circuit, the voltage and current across the load resistor R1 will exhibit a unidirectional pulsating characteristic. This constitutes the simplest rectifier circuit—the single-phase half-wave rectifier circuit.

C. Single-Phase Bridge Rectifier Circuit To overcome the drawbacks of the single-phase half-wave rectifier circuit, the single-phase full-wave rectifier circuit is more commonly used in practical applications, with the single-phase bridge rectifier circuit being the most common.

The single-phase bridge rectifier circuit consists of four diodes, and its construction principle is to ensure that the voltage V1 maintains the same direction for both voltage and current across the load throughout the entire cycle. To achieve this, the current flowing to the load must be correctly guided during both the positive and negative halves of V1. The model of the single-phase bridge rectifier circuit is shown in the figure below:

4. How Rectifier Circuits are Used in AC to DC Conversion

A. DC

The DC obtained above is called pulsating DC, whose current magnitude changes over time, but its direction remains constant. This differs from the constant DC studied in high school—where both the magnitude and direction of the current do not change, such as in constant voltage sources and horizontal flow voltage sources. However, in practical applications, pulsating DC often exists, such as in common dry batteries, where the terminal voltage gradually decreases during use, but the direction remains unchanged.

B. Rectification + Filtering + Voltage Regulation Although the output voltage of the rectifier circuit is unidirectional, it contains a significant AC component and cannot meet the needs of most electronic circuits and devices. Therefore, after rectification, a filtering circuit is generally added to convert the pulsating DC voltage into a smooth DC voltage.

After this, although the rectification and filtering circuit can convert the sine AC voltage into a smoother DC voltage, on one hand, the average value of the output voltage depends on the effective value of the input voltage, so when the input voltage fluctuates, the average output voltage will also fluctuate correspondingly; on the other hand, due to the internal resistance of the rectification and filtering circuit, when the load varies, the voltage across the internal resistance will change, thus the average output voltage will also change inversely. To obtain a stable DC voltage, voltage regulation measures must be taken.

By connecting a voltage regulator diode (1N4884) in parallel in the circuit, the connection of the rectifier circuit + filtering circuit + voltage regulation circuit is completed, successfully obtaining the desired stable and smooth DC from AC.

5. In-Depth Exploration of Voltage Regulator Diodes and DC Voltage Regulators

A. Voltage Regulator Diodes Voltage regulator diodes are silicon-based, surface contact type crystal diodes, commonly referred to as voltage regulator tubes. These tubes exhibit voltage regulation characteristics within a certain current range (or power dissipation range) during reverse breakdown, demonstrating stable voltage characteristics. They are widely used in voltage regulation power supplies and clamping circuits.

B. DC Voltage Regulators The input of a DC power supply is a 220V grid voltage (i.e., mains electricity). Generally, the required DC voltage differs significantly from the effective value of the grid voltage, necessitating a step-down through a power transformer before processing the AC voltage. The effective value of the secondary voltage of the transformer is determined by the needs of the subsequent circuit. Currently, some circuits do not use transformers, utilizing other methods for stepping up or stepping down.

The secondary voltage of the transformer is converted from AC voltage to DC voltage through the rectifier circuit, which means converting sine wave voltage into unidirectional pulsating voltage. The output waveforms of half-wave and full-wave rectifier circuits are illustrated in the figure:

It can be seen that they all contain significant AC components, which will affect the normal operation of the load circuit; for instance, the AC component will mix into the input signal being amplified, and even the power supply AC component mixed into the output of the amplifier circuit can exceed the useful signal; hence, they cannot be directly used as a power supply for electronic circuits. It should be noted that the waveform drawn at the output of the rectifier circuit in the figure is the waveform when the filtering circuit is not connected; the waveform will change after connecting the filtering circuit.

To reduce the voltage ripple, a low-pass filter circuit is needed to smooth the output voltage. Ideally, all AC components should be filtered out, allowing the output voltage of the filter circuit to be purely DC voltage. However, since the filter circuit is a passive circuit, connecting a load will inevitably affect its filtering effect. For electronic circuits that do not require high stability, the DC voltage after rectification and filtering can serve as a power supply.

Although the AC voltage is transformed into a DC voltage with a smaller AC component through rectification and filtering, when the grid voltage fluctuates or the load changes, its average value will also change accordingly. The function of the voltage regulation circuit is to make the output DC voltage largely unaffected by fluctuations in grid voltage and changes in load resistance, thus achieving sufficient stability. This is the basic principle of DC voltage regulators.

From the block diagram of the DC voltage regulator above, it is not difficult to see that its principle is precisely the rectifier circuit + filtering circuit + voltage regulation circuit discussed earlier. The core component is the electronic component—the diode. The difference is that the mains electricity in daily life is 220V, which needs to be stepped down before use.

6. Conclusion

The complex core component of a DC voltage regulator is merely a small diode. The electronic products and household appliances we commonly encounter are large in size and complex in function; however, their functionality is fundamentally composed of small electronic components like diodes. Small components can have great uses, and the process of building large circuit models from these tiny electronic components to create miraculous electronic products resembles the birth of a nation. Perhaps every circuit model can be seen as an independent kingdom, with each electronic component as a citizen of that kingdom. How to use these miraculous electronic components effectively requires a deep understanding of each component, starting from their manufacturing and basing on their respective characteristics. Considering and planning from multiple aspects will lead to circuits that are full of miracles and vitality.

I believe this is why we study courses like Circuit Theory, Semiconductor Physics, and Semiconductor Device Physics—to start from principles, allowing us to reach a level of understanding with the components, so that when constructing the desired circuit with components, it feels effortless and enhances our capabilities.