Digital display adjustable regulated power supply design based on LM317 and AT89C51

　　1 Introduction

　　DC regulated power supply is a commonly used electronic device that can ensure the output of stable voltage when the grid voltage fluctuates or the load changes. It is widely used in instrumentation, industrial control and measurement fields. Therefore, designing and manufacturing a low-ripple, high-precision DC regulated power supply occupies a very important position in power supply technology.

　　2Design requirements

　　The regulated power supply designed in this article has the characteristics of large output current, small output resistance, and low voltage stabilization coefficient. It converts the analog voltage into a digital signal and displays it intuitively through LED, making the entire power supply more user-friendly. The specific parameter requirements of the design are: the output voltage is continuously adjustable from 1.5 volts to 30 volts, the maximum output current can be up to 1.5 amps, the voltage stabilization coefficient is less than 0.01, the output resistance is less than 0.1 ohms, and the display accuracy reaches the 0.2 level standard.

　　3 system hardware design

　　3.1 Overall design

　　This design includes the design of the main power supply and auxiliary power modules, the design of the voltage value conversion and analog-to-digital conversion modules, and the design of the signal processing and digital display modules. The main power module is the core of the entire system and is used to power external loads, while the auxiliary power module supplies power to other modules inside the device. The voltage value conversion and analog-to-digital conversion circuit modules first convert the output voltage to a voltage range that the microcontroller can process, and then convert the analog voltage into a digital signal. The signal processing and digital display module processes the sent digital signal and then outputs it for display. The overall design block diagram of the circuit is shown in Figure 1.

　　3.2 Main power supply and auxiliary power circuit design

　　The main power supply is used to power external loads, and the auxiliary power supply supplies power to other modules inside the device, so that the voltage conversion and processing display module signals will not be affected by load changes. The circuit contains four links: voltage transformation, rectification, filtering and voltage stabilization. The difference is that the voltage stabilizing module of the main circuit is adjustable voltage stabilizing, and the auxiliary power supply is fixed voltage stabilizing. At the heart of the main power circuit is the LM317 adjustable three-terminal regulator, capable of delivering over 1.5A at an output voltage range from 1.2V to 37V. This regulator is very easy to use, requiring only two external resistors. Sets the output voltage and uses internal current limiting, thermal shutdown, and safe operating area compensation mechanisms to prevent chip burnout. The regulated output voltage is given by Equation 1:

　　Because the adjustment current represents the error term in the formula, LM317 is controlled within 100uA and kept constant during design. The typical value of R2 is 240 ohms, so the size of the adjusting resistor RV1 can be calculated based on the required output voltage. This design requires a maximum output voltage of 30 volts, so the value of RV1 should be 5.6 kiloohms.

　　The voltage value conversion and analog-to-digital conversion circuit modules and the host and digital display circuit modules require independent and stable positive and negative 5-volt power supplies. Here, an auxiliary power supply circuit with symmetrical output is designed with 7805 and 7905 as the core.

　　3.3 Voltage value conversion and analog-to-digital conversion circuit design

　　Generally, the DC voltage output by a regulated power supply requires an external voltmeter or multimeter to display the voltage. In order to make the use of the regulated power supply more convenient and intuitive, this design has a specially designed display circuit to display the output voltage in real time. The signal processed by the microcontroller is a digital signal within 5 volts, so the output voltage of the power supply must first be converted numerically and analog-to-digital.

　　The operational amplifier AD822S is used in the circuit to form two inverting proportional operation circuits. The relationship between the output voltage and the input voltage is as shown in Equation 2:

　　In order to convert the maximum voltage of 30 volts output by the power supply into a voltage of 5 volts, R31 is about 1.66 kiloohms. The specific resistance value can be improved by fine-tuning according to the needs of the subsequent analog-to-digital conversion circuit Huang Luji: Lecturer Master Graduate Road Display accuracy.

　　The voltage output from the voltage value conversion circuit is still an analog voltage. The analog voltage signal is converted into a digital voltage signal through the analog-to-digital conversion circuit and transmitted to the microcontroller.

　　The analog-to-digital conversion circuit uses ADC0804 as the core, connects the output voltage of the voltage-to-value conversion circuit to the VIN+ pin, and sends the conversion result to Port 2 of the microcontroller AT89C51 through the data port. The RD, WR and INTR of ADC0804 are connected to the RXD, TXD and INTR of the microcontroller respectively, and the query mode is used to detect whether the ADC0804 has completed the conversion, and the interrupt mode can also be used for processing.

　　3.4 Signal processing and digital display circuit design

　　The core of the signal processing and digital display module is the AT89C51. The signal transmitted from the voltage value conversion and analog-to-digital conversion circuit to Port2 of the AT89C51 is a digital signal corresponding to the analog voltage. As shown in Figure 6, the signal is converted into a specific display driver code after being processed by AT89C51, and is transmitted to the data bus of the four-bit common anode seven-segment LED digital tube through Port0. The scanning signal passes through P1.0~P1.3 Connect them to the bases of Q1~Q4 respectively, and drive the LED digital tube through the triode for display.

　　3.5 System circuit

　　The overall hardware circuit of the system is shown in Figure 2:

　　4Software design

　　The processing of digital voltage signals is mainly implemented by the host through software. The software design includes the design of query reading module, evaluation operation and output display module. First, make some global settings according to the needs of subsequent programs, then use query mode to collect digital signals from ADC0804, perform operations on the collected digital signals, and finally output and display the operation structure. The conversion query reading and numerical calculation are implemented in the main program, and the output display is implemented using the display function.

　　5 system test

　　5.1 Performance indicators

　　System testing includes three aspects. The first is to test the output voltage and maximum output current of the voltage stabilizing circuit; the second is to test the voltage stabilization quality of the voltage stabilizing circuit, and the third is to test the display accuracy. In the test of the voltage stabilization quality of the voltage stabilization circuit, its voltage stabilization characteristics are examined from three aspects. First, when the grid voltage fluctuates, check whether the output voltage is stable. Generally, the voltage stabilization coefficient Sr determined by Equation 3 is used for measurement; On the other hand, when the load changes, whether the output voltage is stable is generally measured by the output resistance Ro determined by Equation 4. The third aspect is to test the size of the AC component in the DC output voltage, which is generally measured by the ripple coefficient determined by Equation 5. δ is measured. The error between the value displayed by the display module and the actual value of the output voltage of the regulated power supply indicates the quality of the instrument, and is measured by the accuracy K determined by general formula 5.

　　5.2 Test data

　　The measured data are shown in Table 1 and Table 2

　　Table 1 Output parameter test data

　　The test results prove that all parameter indicators meet the design requirements.

　　6 Conclusion

　　This system has the characteristics of simple structure, low cost, reliable performance, adjustable output, easy operation, and intuitive display. It can be used in various precision equipment with relatively high DC voltage requirements.