Is it possible to convert AC into DC without using batteries or transformers? Can rectifiers be used for this purpose？

It is possible to convert alternating current to direct current without using a battery or transformer. A rectifier can be used for this purpose.

I. Working principle of rectifiers

A rectifier is an electronic device that can convert alternating current to direct current. It mainly realizes the rectification function through the unidirectional conductivity characteristics of semiconductor devices such as diodes.

Half-wave rectification

• In a half-wave rectifier circuit, when the positive half-cycle of the input alternating current is present, the diode conducts, and current flows through the load, forming a direct current output. During the negative half-cycle of the input alternating current, the diode is cut off, and there is no current flowing through the load. In this way, a pulsating direct current with only the positive half-cycle is obtained at the output. For example, a simple half-wave rectifier circuit can consist of a diode and a load resistor.

• The advantage of half-wave rectification is that the circuit is simple and low-cost. But the disadvantage is that the output direct current voltage fluctuates greatly, and the efficiency is low, only using half of the alternating current waveform.

Full-wave rectification

• The full-wave rectifier circuit can overcome the shortcomings of half-wave rectification. It uses two diodes or a center-tapped transformer to enable both positive and negative half-cycles of alternating current to pass through the load, thereby obtaining a relatively smooth direct current output. For example, in a full-wave bridge rectifier circuit, four diodes form a bridge. No matter whether the input alternating current is in the positive half-cycle or the negative half-cycle, there are always two diodes conducting, and current always flows through the load.

• Full-wave rectification has higher efficiency and less fluctuation of the output direct current voltage, but the circuit is relatively more complex.

II. Other possible methods

In addition to rectifiers, other methods can also be used to convert alternating current to direct current, but these methods usually also require some specific electronic components.

Capacitor filtering

• Connecting a capacitor in parallel at the output end of the rectifier circuit can play a filtering role and make the output direct current more smooth. When the peak voltage of the input alternating current is present, the capacitor charges; when the input voltage drops, the capacitor discharges to maintain the voltage across the load. For example, in a simple half-wave rectifier circuit with capacitor filtering, the capacitor can greatly reduce the fluctuation of the output voltage.

• The filtering effect of the capacitor depends on the capacitance of the capacitor and the size of the load. Generally speaking, the larger the capacitance, the better the filtering effect, but the cost will also increase.

Voltage stabilizing circuit

• To further stabilize the output direct current voltage, a voltage stabilizing circuit can be added on the basis of the rectifier circuit and filtering circuit. The voltage stabilizing circuit can automatically adjust the output voltage according to the change of the load to keep it within a relatively stable range. For example, commonly used voltage stabilizing diodes, three-terminal voltage regulators, etc. can all be used to construct a voltage stabilizing circuit.

• The voltage stabilizing circuit can improve the quality of direct current and is suitable for occasions with high requirements for voltage stability.

In conclusion, when a battery or transformer is not used, alternating current can be converted to direct current by methods such as rectifiers, combined with capacitor filtering and voltage stabilizing circuits.