The operation of any electronic device relies heavily on a DC power supply. Transistors and integrated circuits, which are fundamental components in modern electronics, require a stable DC voltage to function properly. There are two main methods for providing DC power: dry batteries and regulated power supplies. Dry batteries offer the advantage of stable output voltage and portability, but they have limited capacity and a short lifespan. In contrast, a DC stabilized power supply can convert 220V AC into a steady and reliable DC output. It typically consists of four key components: a transformer, a rectifier, a filter, and a voltage regulator. A reference circuit is shown in Figure 1.
Transforming AC to DC involves several steps. The output voltage of the regulated power supply is usually determined based on the specific needs of the equipment. Some devices may require multiple voltages simultaneously, so a separate regulated power supply is often used to adjust the output within a certain range. When the adjustment range is large, it may be divided into several discrete levels. To achieve this, an AC-to-DC conversion process is necessary. A power transformer is used to step down the high-voltage AC (typically 220V ± 10%) to a lower voltage suitable for the circuit. This ensures that the power supply meets both the voltage and current requirements of the system.
To ensure reliable and stable performance at 220V ± 15%, the DC voltage after transformation, rectification, and filtering must be carefully selected. First, the input and output voltage difference for an integrated voltage regulator should be minimized. Second, the forward voltage drop across two diodes in a bridge rectifier must be considered. Lastly, a margin should be left to prevent excessive heat dissipation or instability at low voltages. Additionally, the transformer must provide more than 20% current margin to handle variations in load.
Rectification is the process of converting alternating current (AC) into pulsating direct current (DC). This is typically done using diodes, which allow current to flow in only one direction. Common rectifier configurations include half-wave, full-wave, and bridge rectifiers. The bridge rectifier, consisting of four diodes, is widely used due to its efficiency. Each diode must be rated to handle at least half the load current and withstand a reverse voltage greater than the peak voltage of the transformer's secondary winding.
Filtering is essential to smooth out the pulsating DC from the rectifier. Capacitors or inductors are commonly used for this purpose. In practice, large electrolytic capacitors are often employed for low-frequency filtering. For example, C2 and C4 are used to reduce low-frequency ripple, while C1 and C3 help eliminate high-frequency noise. The capacitance value depends on factors such as the load resistance and the AC input frequency. The nominal voltage of the capacitor must also exceed the maximum voltage in the circuit with some safety margin.
Voltage regulation ensures that the output voltage remains stable despite fluctuations in the input AC voltage or load. Voltage stabilizing circuits can be made using discrete components or integrated regulators. Integrated voltage regulators, such as the popular three-terminal 7800 and 7900 series, are widely used due to their reliability, ease of use, and cost-effectiveness. These regulators come in fixed and adjustable versions, including the LM117, LM317, and LM337 series.
Finally, a DC stabilized power supply should always include a fuse and a heat sink. The fuse protects the circuit from overcurrent conditions, automatically disconnecting the power in case of a short or overload. The heat sink helps dissipate excess heat generated by the voltage regulator, ensuring it operates within safe temperature limits. Together, these components enhance the safety and longevity of the power supply.
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