Preface EDA technology has developed rapidly and is playing a vital role in various fields such as scientific research, product design and manufacturing, and education. EDA represents the latest development direction in electronic product design. Using EDA tools, electronic engineers can not only design electronic products on computers, but also complete the entire process of electronic product design, from circuit design, simulation experiments, performance analysis, to PCB printed circuit board design, all on a computer. In terms of education, almost all science and engineering universities offer EDA courses. Through learning and practicing EDA, students master the use of EDA technology for electronic circuit design and simulation experiments in the "Fundamentals of Electronic Technology" course, thus laying a foundation for future work in electronic technology design. Multisim 2001 is an EDA software for electronic circuit design and simulation. Because Multisim 2001's most powerful function is for circuit design and simulation, this software is called a virtual electronic laboratory or electronic work platform. On any computer, a virtual laboratory for "Fundamentals of Electronic Technology" can be created using Multisim 2001, thereby changing the traditional teaching model. Students can verify the knowledge they have learned in "Fundamentals of Electronic Technology" using Multisim 2001 circuit simulation software. For example, in the design of a series-type DC regulated power supply, the system consists of three parts: rectification, filtering, and voltage regulation. The bridge rectifier circuit, with added capacitor filtering, makes the output waveform smoother. The voltage regulation section generally has four stages: adjustment stage, reference voltage, comparator amplifier, and sampling circuit. When the mains voltage or load changes cause a change in the output voltage Uo, the sampling circuit feeds a portion of the output voltage Uo to the comparator amplifier for comparison with the reference voltage. The resulting error voltage, after amplification, controls the base current of the regulating transistor, automatically changing the collector-emitter voltage of the regulating transistor to compensate for the change in Uo, thereby maintaining the output voltage essentially constant. 1. DC Regulated Power Supply Design Design and fabricate a series-type DC regulated power supply with an output voltage UO = 10V, an output adjustment range of 8–12V, a rated output current IL = 100 mA, a mains power fluctuation of ±10%, a voltage regulation coefficient Sr < 0.05, and an output resistance RO = 0.05. The operating temperature is 25–40℃. 1.1 Initial Circuit Selection Based on the design requirements, the output current is a relatively large 100mA. Therefore, a composite transistor consisting of two transistors is selected. Considering the voltage regulation range, a sampling circuit with a variable resistor is chosen. A preliminary circuit schematic is shown in Figure 1. Through parameter calculations and simulation tests, the selected circuit is reconsidered to ensure it meets the requirements. Finally, the circuit and component parameters are further determined during the debugging process. 1.2 Component Parameter Selection 1.2.1 The rectifier and filter circuit uses a bridge rectifier and a capacitor filter circuit. To ensure the regulating transistor always operates in the amplification region, a certain voltage drop is required. Based on calculations, U1 = 15V. Considering IL = 100mA, plus the current through R6, the Zener diode VZ (10mA), and the sampling circuit current (20mA), the current through the rectifier diode ID = 130mA. In the actual circuit, the rectifier diode is selected based on the calculated U1 and ID. In this example, a 3N259 is selected, and the filter capacitor is selected as 470μF/30V. 1.2.2 The selection principle for the regulating transistor V1 in the adjustment section is reliable operation. Based on BUCEO≥UOMAX and ICM≥1.5IOM, V1 is selected as 2N6703. 1.2.3 The selection principle for the reference voltage is to make the sampling voltage as high as possible to better reflect the changes in Uo. Generally, a voltage division ratio of (0.5～0.8) and a regulated voltage of around 6V are preferred. Therefore, a Zener diode with a regulated voltage of 6.2V and model IN4735A is selected. 1.2.4 The principle for selecting the amplifier circuit parameters is to ensure that the amplifier circuit operates in the amplification region and maximizes the amplification factor as much as possible when the mains voltage or load current changes to meet the requirements of voltage regulation accuracy. Here, 2N2222 is selected. For the sampling circuit, to improve stability, the current through the sampling resistors R7, RP, and R8 should be much larger than the base current of V4 to ensure the required voltage division ratio. However, if the current is too large, the loss in the sampling resistors will also be large. Here, a current of 20mA is selected. Based on calculations, select R7=100Ω, R8=200Ω, and RP=220Ω. 2. Editing the Circuit Schematic 2.1 Placing Components On the Windows desktop, double-click the Multisim2001 icon to enter the main program window. The largest area in the main window is the circuit workspace, where you can edit and test the circuit schematic. First, retrieve all the components from the component library in the initially selected circuit schematic. This is done by clicking the icon containing the component in the component library toolbar to open the component library, and then dragging the component from the library to the circuit workspace. For example, to place V1, click the transistor component library icon to open the Transistors component library. The background of the transistor icon is gray and green; gray represents components that exist in reality, and green represents virtual components that do not exist in reality. Click the gray-background NPN transistor to open the ComponentBrowser dialog box, select 2N6703, click OK, move the mouse to a suitable position, and click to place the transistor. If a component's orientation is incorrect, right-click on it to bring up a shortcut menu. Select options like "Mirror," "Rotate," etc., as needed. Component V1 requires the "90CounterCW" command to rotate it 90° counter-clockwise. Component V1 is now placed. Use this method to place all components sequentially. After placing all components, carefully arrange their positions to ensure a reasonable and aesthetically pleasing distribution. 2.2 Connecting Wires After placing the components, connect them. According to the schematic, point the mouse at a component's pin until a solid crosshair appears, press the left mouse button, drag out a wire, and connect it to the relevant component's pin. Repeat this process to correctly connect all wires. The schematic is now complete, as shown in Figure 1. 2.3 Importing Instruments Selecting instruments allows you to drag and drop corresponding instrument icons from the instrument library to the circuit workspace. Instrument icons have connection terminals for connecting the instrument to the circuit, as shown in Figure 2. A multimeter and oscilloscope are used in this example. The multimeter has two input terminals, and the oscilloscope has four terminals (Channel A, Channel B, T trigger, and G ground). To observe the test waveform, double-click the instrument icon to open the instrument panel, as shown in Figure 2(a). The oscilloscope displays the unfiltered waveform after bridge rectification, and Figure 2(b) shows the output UO value of the multimeter. The usage of the instruments is basically the same as that of the actual instruments. To use the multimeter, first select whether to measure DC or AC based on the actual situation of the two points being measured, and then select to measure voltage V, current I, or resistance R. To use the oscilloscope, first select the working mode Y/T, and then select the input coupling switch for channels A and B, whether it is DC, AC, or ground. 3. Circuit Simulation Analysis 3.1 Simulation Steps Before starting the simulation analysis, double-click the instrument icon to open the instrument panel. Prepare to observe the test waveform. Press the start/stop switch in the upper right corner of the program window to set the status to 1, and the simulation analysis will begin. If you press it again, the start/stop switch will set the status to 0, and the simulation analysis will stop. After the circuit starts, the time base and channel control of the oscilloscope need to be adjusted to ensure the waveform display is normal. The working state of the instrument after simulation is shown in Figure 3. 3.2 Simulation Output Results 3.2.1 Rectification and Filtering Apply an AC voltage with amplitude U1=15V and frequency of 50Hz, RL=100Ω, to the input terminal. The output results of the filtering circuit can be observed using the multimeter and oscilloscope provided on the Multisim2001 electronic workbench. At this time, adjust RP so that the output UO is around 10V. As shown in Figure 3, the voltages at key points measured by the multimeter are U1=14.998V, UI=18.381V, and UO=10.156V. Use the A and B channels of the oscilloscope to display the waveforms of the rectified and filtered voltage UI and the regulated output voltage Uo, respectively. From the oscilloscope display window, you can see that the upper sawtooth wave curve is the UI waveform, and the lower line is the Uo waveform. 3.2.2 The voltage regulator circuit simulates AC grid fluctuations of ±10% at 13.5V and 16.5V, with a frequency of 50Hz. First, the input voltage signal is changed to simulate grid fluctuations. Using the Multi-sim2001 platform is relatively simple; just double-click the voltage source with the mouse and change it from 15V to 13.5V and 16.5V according to the screen display. The corresponding measured UI values ​​are 16.280V and 20.406V, and the output voltage UO is 10.133V and 10.181V, respectively. 3.2.3 Overcurrent protection circuit: When U1 = 15V and the frequency is 50Hz, RL is changed... When RL=∞, Uo=10.160V; when RL=100Ω, IL=101.816mA, Uo=10.156V; when RL=10Ω, IL=160.075mA, Uo=1.601V; when RL=5.1Ω, IL=158.433mA, Uo=808.005mV; when the load is short-circuited, IL=156.741mA, Uo=156.74lpV. Based on the measured data, this circuit is a current-limiting protection circuit. 4. Comparison and Verification with Design Specifications 4.1 Theoretical Calculation of Output Voltage Uo=（R7′+R8′）（Vz+Ube4）／R8=10.196V. In the formula, R7′=R7+RP′=265Ω, R8′=R8+RP″=255Ω, VZ=4.3V, Ube4=0.7V. RP′ is the upper half of the resistance of RP, and RP″ is the lower half. Simulation was performed using Multisim 2001. The actual output voltage measured with a multimeter was Uo=10.160V. The adjustment range of the regulated power supply output voltage Uo was as follows: When U1=15V and the frequency is 50Hz, adjusting RP (i.e., when the keyboard letters are in lowercase mode, continuously pressing the A key moves the potentiometer slider downwards to the bottom; at this point, Uo=13.29V is measured); conversely, when the keyboard letters are in uppercase mode, continuously pressing the A key moves the potentiometer slider upwards to the top; at this point, Uo=6.064V is measured. 4.2 Calculation formula for the theoretical value of the voltage regulation coefficient Sr: β4 is taken as 30, [rbe4 + (1 + β4)rz] is taken as 1K, which is the voltage division ratio of the sampling circuit. Using Multisim 2001 to simulate the electronic circuit, the voltage was changed from 15 V to 13.5 V according to the screen display. The corresponding measured UI values ​​were 18.381V and 16.280V, and the output voltage UO was 10.156V and 10.133V, respectively. Therefore: 4.3 Output resistance: Data measured using Multisim 2001 simulation: When RL = ∞, UO = 10.160V; When RL = 100Ω, IL = 101.816mA, Uo = 10.156V; Calculation results: Through the above analysis, the measured values ​​of the series-type DC regulated power supply are consistent with the theoretical calculations. The actual circuit meets the design requirements. If the circuit design described above does not meet the design requirements through simulation analysis, the design can be gradually modified by changing the component parameters or component models to make it meet the requirements, ultimately determining the component parameters. The modified circuit can then be immediately simulated to observe whether the virtual results meet the design requirements, which is difficult to achieve in a real circuit board. 5. Conclusion As seen from the above examples, Multisim 2001 is an open virtual electronic experimental platform. It has both advantages and limitations. Designers can perform various types of electronic circuit experiments and design actual electronic products, but it cannot completely replace the final circuit and physical testing. This is because interference is a difficult problem to solve in actual electronic circuits, especially high-frequency circuits. The reason for using Multisim 2001 for simulation is to ensure that the circuit has roughly correct parameter attributes before building the actual circuit, thereby reducing unnecessary detours in the design process. In the teaching of "Fundamentals of Electronic Technology," using Multisim 2001 circuit simulation software can, on the one hand, verify theoretical knowledge, and on the other hand, set some faults, such as the open-circuit capacitor (ce) of the regulating transistor V1 in a series-type DC regulated power supply. First, students are asked to theoretically analyze what problems might arise. Then, they are asked to use simulation software to perform simulations and verify the results. This expands students' thinking and further promotes the teaching of "Fundamentals of Electronic Technology." Therefore, we can see that for engineering technicians, the proper use of Multisim2001 circuit simulation software can save a significant amount of manpower and resources and shorten the design cycle; for teachers, it can connect theory with practice, strengthen students' practical abilities, and cultivate practical talents.