Design of program-controlled power supply based on D/A converter

    introduction

In various electronic circuit experiments, power supply is an indispensable instrument. At present, most of the power supplies used in experiments have only fixed voltage output (for example, commonly used ones are: ±5V, ±12V or ±15V). The disadvantage is that the output voltage cannot be changed artificially, and the output accuracy and stability are not high: in measurement, traditional power supplies generally use pointer or digital to display voltage or current, and use potentiometers to adjust the desired voltage and current output values. If you want to adjust the precise voltage output, you must use a precise display instrument for monitoring: because the resistance characteristics of the potentiometer are nonlinear, it takes a certain amount of time to adjust, and it will cause drift, so you have to make do with what you have.

With the rapid development of science and technology, the requirements for power supply reliability, output accuracy and stability are getting higher and higher. The design of programmable power supply using the high resolution of D/A converter and the automatic detection technology of single-chip microcomputer shows its superiority. The programmable power supply can not only conveniently input and select the preset voltage value, but also has high accuracy and stability. It can also realize programmable monitoring of the power supply, such as simulating voltage drop, interruption or fluctuation. The programmable power supply can also be regarded as a power-type low-frequency signal generator. The programmable power supply can set the output voltage or current arbitrarily. All functions are realized by the keyboard on the panel or the upper microcomputer connected through the RS-232C serial port, which brings great convenience to circuit experiments and improves work efficiency.

How it works

This power supply is based on a conventional voltage-stabilized power supply.

The power supply uses a high-performance single-chip microcomputer and a digital-to-analog converter as the control circuit. Under the support of control and detection software, the preset value is sent to the corresponding D/A converter through the output port of the single-chip microcomputer to convert it into a corresponding given analog voltage to replace the comparison reference voltage in the conventional regulated power supply, so that the output voltage or current changes with the change of the reference voltage. The principle is shown in Figure 1. Users can set the output value of the regulated power supply through the keyboard as needed.

Figure 1 Block diagram of programmable power supply

System hardware configuration

The whole block diagram of the program-controlled power supply is shown in Figure 2. It mainly consists of seven parts: keyboard, LCD display, single-chip microcomputer system, D/A conversion circuit, alarm circuit, detection circuit and DC voltage stabilization circuit. RS232 interface can be expanded as needed.

Single chip microcomputer system

The single-chip microcomputer system is the core of the program-controlled power supply. It controls the operation of the entire instrument through the operation of the software, thereby completing the set functions. The single-chip microcomputer system uses AT89C52 as the CPU. It is a low-voltage, high-performance 8-bit CMOS single-chip microcomputer produced by ATMEL of the United States. The chip contains 8K bytes of FLASH or PEROM and 256 bytes of RAM. The device is produced using ATMEL's high-density, non-volatile storage technology and is compatible with the standard MCS-51 instruction system and 8052 product pins. AT89C52 receives the external interrupt signal of the detection circuit, and also receives information from the keyboard, and processes the input information to determine the original working state of the instrument and the size of the output voltage. In addition, AT89C52 can automatically adjust the fan speed according to the working temperature, and if overheating occurs, it can also drive the alarm circuit to work.

Figure 2 System Block Diagram

D/A conversion circuit

The D/A converter uses the MAX508, which is a 12-bit voltage output type with built-in double-buffered input latch and Zener reference power supply. The double-buffered latch is used to interface with the microprocessor. The data input allows the lower 8 bits and the upper 4 bits to be segmented right-shifted. The on-chip range resistor provided by the MAX508 can be connected to generate bipolar output voltage ranges of ±5V or unipolar output voltage ranges of 0-5V and 0-10V. This power supply uses a 0-10V unipolar output mode, as shown in Figure 3. At this time, ROFS is connected to AGND, RFB is connected to VOUT, and direct binary coding is used.

Figure 3D/A power supply connection schematic