In modern scientific research and industrial production, a highly accurate and stable digital DC current source has been widely used. Most of the current sources used in the past have been adjusted using potentiometers, and the output current value cannot be accurately stepped. Although some current sources can realize numerical control, but often output current values ​​are too small, and the set output current value is not accurate and cannot be determined without testing, which is not intuitive. For this reason, a new type of high-precision digital DC current source based on single-chip microcomputer control was designed and manufactured using the closed-loop feedback adjustment control principle combined with the single-chip technology and V/I conversion circuit. The system uses the AT89S52 microcontroller as the controller to set the output current through the man-machine interface (buttons and LCD display), set the step level of 1 mA, and can display the preset current value and the actual output current value at the same time. This system sets the output current value by the key set, after the calculation of the one-chip computer exports the analog signal through the D/A converter (TLV5618), then transforms into the electric current through the V/I conversion circuit. The MCU samples the voltage across the sampling resistor through the A/D converter (MAX187), which is processed by the MCU and converted into a current value and displayed on the LCD screen for reference by the user. The actual test results show that the output current of this system can be arbitrarily set within the range of 20 mA to 1 000 mA, with an accuracy of ±10 mA, and the output current is stable. It can be applied to fields that require high stability and low power constant current sources. 1.1 System Principle This design uses analog closed-loop control. Uses AT89S52 one-chip computer as the main controller, uses the D/A converter to export the analog voltage signal, then obtains the electric current through the V/I conversion circuit. The V/I conversion circuit is based on the principle of series negative feedback of the current and consists of an operational amplifier and a high-power transistor to form an analog closed loop, which stabilizes the output current. The system can be divided into three parts: the power supply part, the control part, and the V/I conversion part. The power supply should not only provide ±5 V and ±12 V for the use of analog devices in the control section and V/I conversion section, but also provide a large voltage for use by the V/I conversion section and must have high power output capability; the control section The function is to output the corresponding voltage signal according to the user setting value; the function of the V/I conversion part is to convert the voltage signal into the corresponding current. The relationship between the various parts is shown in Figure 1. 1.2 Working power Use two transformers. The output of the high-power transformer is directly supplied to the V/I conversion section after being rectified and filtered. The output of the low-power transformer is rectified and filtered to obtain voltages of ±12 V and ±5 V through the 78 and 79 series chips. The output of the high-power transformer is regulated after rectification and filtering, and is then provided to the V/I conversion circuit. The output power of this scheme can meet the requirements, and the stability of the power supply of the V/I conversion part can be guaranteed. The current source output voltage is set within 40 V, so the maximum output power is 80 W, leaving a margin, large transformers choose dual 18 V, 100 W. Using a three-terminal regulator chip LM338K to obtain a voltage of 40 V, so that the LM338K input and output voltage differential is about 6V, the output current is 2A, the dissipation power is about 12W, the LM338K maximum output current is 5 A, and the dissipated power is 50 W. The power supply circuit is shown in Figure 2.
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