Design and implementation of a digitally controlled DC current source

This design uses a single-chip microcomputer as the main control component, presets the output current value through the keyboard and uses a liquid crystal module for real-time display. The entire system hardware consists of a microcontroller module, a voltage-current conversion module, a keyboard module, a display module, a DC regulated power supply module, and a voice prompt module. The system structure diagram is shown in Figure 1.

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Figure 1 Digitally controlled DC current source system

The microcontroller is the core of the whole system and is responsible for the operation of the whole system. In order to make the hardware circuit simple, the system performance stable and reliable, and to facilitate the coordination of functions such as voice broadcast, keyboard setting and real-time information display, after multiple schemes were demonstrated, the Lingyang 16-bit single-chip microcomputer SPCE061A was selected. The single-chip microcomputer is designed using modern electronic technology - system on a chip SOC (system on a chip) technology, and has integrated ADC, DAC, PLL, AGC, DTMF, LCD-DRIVER and other circuits (related to the IC model). It uses a reduced instruction set (RISC), and the instruction cycle is in units of CPU clocks. In addition, it also has DSP chip functions, built-in 16-bit hardware multipliers and adders, and is equipped with special instructions owned by DSP, which greatly accelerates the running speed of various algorithms. At the same time, the Lingyang 16-bit single-chip microcomputer application development tool can be used in the Windows environment. The tool supports standard C language and Lingyang single-chip microcomputer assembly language, integrating assembly, programming, simulation and other functions, greatly accelerating the software development process. Lingyang's single-chip microcomputer has the characteristics of high speed, low price, reliability, practicality, small size and low power consumption. It is more suitable to use this single-chip microcomputer as a controller. It is easy to realize A/D, D/A conversion, voice prompts, PID calculation and other functions under the premise of simple hardware circuit.

The main function of the display module is to display the set current output value and other human-computer interaction information. This part can use a seven-segment digital LED display to display information such as numbers, simple letters and decimal points, but because its display information is single and the human-computer interaction is not friendly, the character LCD display LCDSMC1602A module is used in the system. This module has the advantages of being light, thin, short, low-voltage, low-power, small size, no radiation hazard, flat right-angle display and stable image without flickering. It is convenient to use to display information such as letters, numbers, symbols, etc., and does not require the expansion of too many peripheral circuits. It can be directly controlled by the single-chip microcomputer for output display.

The voltage-current conversion module is composed of a precision op amp and a Darlington circuit composed of three transistors. The conversion circuit uses the flat output characteristics of the transistor and the deep negative feedback circuit to stabilize the output current. As shown in Figure 2, this V/I conversion circuit has a strong load capacity and a current output range of 0 to 3A. The output current Io obtains a feedback voltage Vf through the feedback resistor RF, Vf= V11-V12, which is added to the two input terminals of the operational amplifier through R5 and R6. Assume that the voltages at both ends of the op amp are V1 and V2, and Vi is output by the microcontroller DAC. Because the input current of the ideal op amp is approximately zero, and V1=V2, then V12[1-R6/(R2+R6)]+ViR6/(R2+R6)=V11R1(R1+R6). Since V12 = V11-Vf, then V11R2/(R2+R6)+(ViR6-VfR2)/(R2+R6)=V11R1/(R1+R5). Let R1=R2=10kΩ, R5=R6=1kΩ, then Vf=ViR6/R2=Vi/10. If feedback is not considered for the time being, Io=Vi/(10Rf).

Figure 2 V/I conversion circuit diagram

It can be seen that the calibration of the output current is determined by the output voltage Vi and Rf of the D/A conversion, which is a linear conversion. Rf is made of large-diameter constantan wire, and its temperature coefficient is very small (5×10-6/℃). The large wire diameter can minimize its temperature influence. The three triodes should use high-power tubes TIP122, and use heat sinks to ensure that the tubes work in the linear region.

There is another scheme for the composition of the voltage-current conversion module, which uses three operational amplifiers to form a current source with variable output current, as shown in Figure 3. The output current I=Vi/R1. In order to keep the voltage across R1 constant, the differential amplifier IC1b monitors the potential across R1 through the emitter follower IC1c. This potential is added to the inverting input of the comparator ICa through the 7th pin of IC1b and compared with Vref. The comparison result changes the output of the comparator until it is balanced, that is, Vr1=Vi. The capacitor in the circuit is used to compensate the frequency of ICa and reduce the delay of the control loop. As long as R1=R2=R3=R4=R5, the performance of this circuit is good. However, the load capacity of this circuit is not strong, and the loop delay compensation has a great influence on the stability of the circuit.

Figure 3 Three op amp V/I conversion circuit

The system keyboard module can be connected in an independent or row-column (matrix) manner. The function of this module is mainly to complete the setting of output current and other information. The DC regulated power supply module supplies power to the entire system; the voice module implements voice prompts, making the system design more humane and the system has a friendly working interface. The Lingyang microcontroller integrates ADC, DAC, PLL, AGC, DTMF and other modules. The voice function can be realized by software programming without any external circuit, which effectively utilizes system resources.

System operation and software process

During operation, the SPCE061A MCU converts the preset current value through D/A conversion, drives the V/I conversion circuit with the output voltage to realize the circuit output, and inputs the voltage value corresponding to the current value into the MCU system through the closed loop after A/D conversion, and then adjusts the current output through the PID algorithm. The whole system workflow is shown in Figure 4.

Figure 4 System workflow diagram