Design of DC electronic load based on STC12C5A60S

1. Scheme design and demonstration

1.1 Overall solution design

DC electronic load based on manual adjustment of microcontroller control.

This scheme uses two self-locking switches to control the working state of the circuit, switching between constant voltage, cross current and constant resistance. The value of constant voltage, constant current, etc. is controlled by the stc12c5a60s single-chip computer through the D/A chip. The stc12c5a60s is a new generation of 8051 single-chip computer with high speed, low power consumption and super anti-interference. The instruction code is fully compatible with the traditional 8051, but the speed is 8-12 times faster, and 8-way high-speed 10-bit A/D conversion. The high-power NMOS tube IRF540 is used. The on-resistance of the tube is small enough and the source-drain resistance is strong enough. The software and hardware junction method is convenient and concise to realize the conversion between different modules, and the basic functions such as constant voltage and constant current are well completed, and additional functions such as constant resistance are completed.

The single chip microcomputer collects the voltage and current values, detects the circuit overload, controls the relay to work, realizes the circuit overload protection and alarms.

1.2 Module Solution

1.2.1 Constant pressure design scheme

Solution 1: Use transistors to achieve voltage amplification and comparison. The base and emitter are equivalent to the negative and positive input terminals of the comparator respectively. The base itself will receive a portion of the current, and there will also be a current Ibe that affects the voltage of the emitter. Such a circuit can achieve a constant voltage function, but the error is relatively large, and there is also a large power loss.

Solution 2: Directly use the operational amplifier OP07 chip to realize voltage amplification and comparison. The circuit looks simple and easy to understand. The circuit can realize the constant voltage function module with relatively small error. Comprehensive consideration to choose solution 2 1.2.2 Constant current design solution

Solution 1: Use the same type of transistors and use the relatively stable Ube of the transistors as a reference. This constant current mode is simple and easy, and the current value can be freely controlled. The product cost is low. Different types of tubes have different Ube values, and even the same type has certain individual differences. At the same time, under different working currents, this voltage will also fluctuate, which is not a precise constant current requirement.

Solution 2: Use an operational amplifier as feedback, that is, select the OP07 chip to realize the amplification and comparison of the constant current function module. Its circuit has sufficient accuracy and adjustability, and the original components are generally easy to build and debug.

After comprehensive consideration, choose option 2.

1.2.3 Display Module

Solution 1: Use digital tube display. Digital tubes can be used for display, which have the characteristics of simple wiring, low cost, simple and flexible configuration, easy programming, low requirements for the external environment, and easy maintenance. Digital tubes can be used to display voltage and current, but digital tubes can only display simple numbers, occupy more resources, have less actual information, and are not easy to display a large amount of information.

Solution 2: Use a 2.4-inch TFT screen with a font library as the display module. The hardware connection method is simple, and the display content is rich and vivid. A friendly human-computer interaction interface can be designed, which is easy for human-computer communication.

Considering the system, display content and system practicability, we adopt solution 2.

2. Circuit Design

2.1 Constant voltage circuit

TEXT and GND are test points. The circuit as a whole is a negative feedback: when TEXT is higher than the set value, the op amp outputs a high voltage, Q1 conductivity increases, the load impedance decreases, and the voltage is divided by the internal resistance of the power supply, TEXT decreases until V+=V-; when TEXT is lower than the set value, the op amp outputs a low voltage, Q1 conductivity decreases, the voltage divided by the load and the internal resistance of the power supply increases, and TEXT increases until V+=V-.

2.2 Constant current circuit diagram

TEXT and GND are test points. In OP07, V+=V-. When V+>V-, the op amp outputs a high voltage, Q1 conductivity increases, current increases, V- increases, and reaches V+=V-. When V+ 

2.3 Constant resistance circuit diagram

When the sliding rheostat is set to 50%, the resistance voltage V+=1/2Vin=V-, the current I=Vin/4, R=Vin/I=4 ohms, and the power supply voltage changes in direct proportion to the current. It can be implemented with a single-chip microcomputer, R=VText/I, and realized by the constant current principle. (If long-term testing is required, it is best to connect the MOSS tube to a large heat sink)

3. Software Design

In the software design, the voltage and current acquisition data are sent to the C8051F360 microcontroller after A/D conversion, compared with the set value, and then controlled as required, and the voltage, current and resistance parameters are displayed at the same time. The main program flow is shown in Figure 5.

Figure 5 System program flow chart

4. Test data and result analysis

4.1 Constant voltage test data

4.2 Constant current test data

Result analysis: The data shows that the measured current values ​​are stable around the set values. After calculation, the relative error is less than 3%. This shows that the system works normally in constant current mode. The measured voltage values ​​are stable around the set values. After calculation, the relative error is less than 3%. This shows that the system works normally in constant current mode.

The measured resistance values ​​are all stable around the set values. After calculation, the relative error is less than 3%, which shows that the system works normally in constant current mode.

4. Summary

This question proposes a design scheme of DC electronic load based on STC12C5A60S. The DC electronic load designed in this scheme is mainly controlled by the high-speed, low-power, and super anti-interference STC12C5A60S single-chip microcomputer. The working state of the circuit is controlled by the self-locking switch, and the working state between the constant voltage, constant current, and constant resistance circuit is switched by the manual adjustment switch. The system's voltage regulation range is 1V-30V, the current regulation range is 100mA-3.5A, and the error of 0-5% is within the range required by the question, achieving the control of the constant voltage value or constant current value within a certain range. Overload protection is set, and the overload is displayed by lighting the light. It has been verified that this scheme has practical application value.