In daily life, it is often necessary to use a 5V power supply. Generally typical are 5V charger, 5V power adapter, battery series, and 5V power module. The following shallowly discusses the design principle of a 5V power supply circuit.
5V power supply circuit
The components to be prepared are 220V to 6V single-phase AC transformer, rectifier bridge (can be replaced by four diodes), LM7805, and capacitors. The principle of 220V AC through the transformer step-down to 6V AC, then through the full-bridge rectifier, and then filtered, the DC voltage is about 6V AC about 1.2 times, and finally through the LM7805 three-terminal linear regulator voltage regulator output 5V. The circuit schematic diagram shows below.
The choice of transformer must be combined with 5V power module output power and choose the appropriate transformer output power, such as the choice of 5W output power, the maximum output current is only 0.7A. Linear three-terminal regulator input and output current remains unchanged, when the input voltage of 6V 1.2 times, 5W transformer can only withstand 0.7A of current.
The rectification part is used for full-bridge rectification. After full-bridge rectification of the RMS voltage of 0.9U, you can use four rectifier diodes instead. The filter capacitor is selected around 2200uF, and the strict selection requires a complex calculation formula. When choosing the capacitor to withstand voltage, one should consider a derating factor of about 0.7-0.8. In the rectified output of about 7.2V DC, the required capacitor withstand voltage value is 7.2 x 1.3 = 9.36, and the minimum need to choose a resist voltage value of 10V or more.
pay attention to three places that easy to ignore
In selecting the LM78XX series of three-terminal regulators, pay attention to three places that easy to ignore.
1, the maximum output current of 1.5 A
2, LM78XX series three-terminal regulator input voltage must be at least 2V higher than the output voltage
3, the need to consider the power consumption of the three-terminal regulator itself, calculated as power consumption = (input voltage – output voltage) × current, the three-terminal regulator’s power consumption should not be greater than 5W greater than must add 2W to the heat sink. Suppose the input voltage of LM7805 is 8V, the voltage difference is 3V. The output current is 1A, then LM7805 own power consumption P = 3Vx1A = 3W.
The above is a scheme of +5V power supply module. Following is a scheme of -5V power supply. This scheme uses a step-down, rectification, filtering, linear regulator 7905. The specific circuit diagram shows below.
This is a direct conversion from the mains. If it is a negative voltage conversion from the absolute value of the voltage, it can be achieved directly using a voltage regulator. And positive voltage needs to be converted to negative 5V output power, which can use to invert the switching regulator power supply.