Design and simulation analysis of thyristor dimming circuit based on Multisim 10

Dimming circuits are widely used in daily life. In teaching, it is not only an entry-level circuit for learning the application of thyristors, but also a classic project for intermediate maintenance electrician and electronic skills training. The content of dimming circuits covers a wide range, including the working principles of thyristors, single-phase half-wave controlled rectifier circuits, single-junction transistor trigger circuits, and the concepts of control angle and synchronous triggering, and the influence of control angle on the controlled voltage. For students, it is undoubtedly very challenging to understand and master these knowledge points and use traditional instruments to obtain waveforms for analysis. Using Multisim 10 software for experimental simulation, you can dynamically and intuitively observe the influence of different parameters on the performance of dimming circuits, which is very helpful for understanding the principles and being familiar with the debugging process.

1 Introduction to Multisim 10
Multisim 10 is the latest version launched by National Instruments. Multisim 10 uses software to virtualize electronic and electrical components, virtual electronic and electrical instruments and meters, and realizes "software is components" and "software is instruments". It is a virtual simulation software for principle circuit design and circuit function testing.
The component library of Multisim 10 provides thousands of circuit components for experiment selection, and it can also create or expand the existing component library, so it is also very convenient to use in engineering design. Multisim 10 has a full range of virtual test instruments, including general instruments for general experiments, such as multimeters, function signal generators, dual-trace oscilloscopes, and DC power supplies; and there are also instruments that are rare or not available in general laboratories, such as Bode plotters and word signal generators.
Multisim 10 can not only design, test and demonstrate various electronic circuits, but also has a more detailed circuit analysis function. Circuit analysis methods such as transient analysis and steady-state analysis, time domain and frequency domain analysis can be completed to help designers analyze the performance of the circuit.

2 Dimming circuit design
2.1 Circuit composition
The dimming circuit is shown in Figure 1, which consists of three parts: the rectifier circuit, the trigger circuit and the main circuit. The bridge rectifier circuit composed of VD1~VD4 and the voltage regulator circuit composed of the voltage regulator tube VD2 generate a trapezoidal wave voltage, which is used as the power supply voltage of the single junction transistor and is also used to ensure that the trigger circuit is synchronized with the main circuit. The charging circuit (R2+R3) C1 and the programmable unijunction transistor PUT form a trigger circuit to generate a synchronous trigger pulse for the thyristor. The main circuit consists of a thyristor VT1 and a lighting lamp X1, and the power supply is directly provided by the 220 V mains.
2.2 Dimming principle
Before the power is turned on, the voltage on the capacitor C1 is zero. After the power is turned on, the capacitor C1 is charged through R2 and R3, and the voltage uC of the capacitor gradually increases. When the peak voltage UP is reached, the e~b1 of the PUT is turned on, and the voltage uC on the capacitor is discharged to the resistor R5 through e~b1. When the voltage uC on the capacitor drops to the valley voltage UV, the PUT resumes the blocking state. After that, the capacitor C1 is recharged and the above process is repeated, resulting in a sawtooth voltage on the capacitor C1 and a pulse voltage on R5. This pulse voltage serves as the trigger signal for the thyristor VT1. In each half-wave time of the bridge rectifier output of VD1~VD4, the first pulse generated by the oscillator is a valid trigger signal. By adjusting the resistance of R2, the phase of the trigger pulse can be changed, the conduction angle of the thyristor VT1 can be controlled, and the size of the load voltage UX1 can be adjusted, thereby controlling the brightness of the bulb.

3 Build a simulation circuit
Build a simulation circuit as shown in Figure 1.

Transformer T1 selection and parameter setting. The selection path is: double-click the basic component library shortcut icon "", open the "Select a Component" dialog box, select "" (transformer), select "TS_IDEAL" (ideal transformer) in the component list, and click "OK" to confirm. The purpose of parameter setting is to convert the primary side AC 220 V into the secondary side AC 36 V. Therefore, the transformation ratio n=36/220=0.163 636 36≈0.163 4, so in the "Vahle" tab of the ideal transformer properties dialog box, set the "coefficient of coupling" to 0.163 4, and keep other parameters unchanged.
The voltage regulator VDZ is selected as 1N4745A (corresponding to the domestic models 2CW112-16 V, 2DW6E), with a stable current of 15 mA, a power of 1 W, and a stable voltage of 16 V.
Selection of programmable single junction transistor PUT. The only two PUTs available in Mutilsim 10 are 2N6027 and 2N6028. The path is: Transistor Component Library "" → Single Junction Transistor "". The programmable single junction transistor PUT, also known as the programmable single junction transistor, is essentially the function of an N-pole gated thyristor, but because it has a similar purpose to the single junction transistor BUJ, it is included in the list of single junction transistors. The programmable single junction transistor can replace the internal base resistors Rb1 and Rb2 with external resistors. By simply changing the resistance ratio of the two, its parameter value can be adjusted externally. The voltage divider ratio of the PUT in Figure 1 is:

Selection and parameter setting of lighting lamps. The selection path is: Indicator Library "" → Virtual Light "". The parameter setting is: 220V, 5W.
The incremental step "Increment" of the potentiometer R2 is set to 1%. The model and parameter selection and setting of the remaining components refer to Figure 1.

4 Simulation Analysis
Click the simulation button "" to start the simulation. Open the oscilloscope panel, set appropriate parameters, and observe the waveform after voltage stabilization and the charging and discharging waveform of capacitor C1. Press the "A" key on the keyboard to change the value of R2 and observe the voltage waveform changes across capacitor C1. Figure 2 is the waveform of the oscilloscope when R2 is a certain value. It can be seen that the waveform after voltage stabilization is a trapezoidal wave and the capacitor voltage uC is a sawtooth wave.

Connect the input channel A of the oscilloscope to line number 8, so that channel A displays the voltage waveform across R5, that is, the waveform of the trigger voltage, as shown in Figure 3. Obviously, the trigger pulse is a series of sharp pulses. The first sharp pulse in each half cycle is the trigger pulse. Pull the two cursor coordinates so that cursor T1 coincides with the zero point of the trapezoidal wave, and cursor T2 coincides with the first sharp pulse after the zero point. At this time, the reading of T2-T1 is the time t (unit: ms) of the trigger control angle.

According to the formula α=2πt／T (T is the period of the AC power supply, here T=1／50 Hz=20 ms), the size of the control angle α can be obtained. Record the trigger time t corresponding to different positions of R2. Open the DC voltmeter XMM1 and record the load voltage value UX1 corresponding to different positions of R2. According to the formula, calculate the theoretical calculated value of the load voltage corresponding to different trigger angles. The results are shown in Table 1.

According to the data analysis in Table 1, when the charging time constant (R2+R3)C1 increases, the trigger time t is also extended, the trigger angle α increases, and the load voltage value UX1 decreases. The measured voltage on the load is approximately equal to the theoretical calculated value within the allowable error range.

5 Conclusion
Using the Multisim 10 tool to simulate the circuit environment and circuit process is low-cost, high-efficiency, and the results are fast, accurate, and vivid. In the teaching of thyristor dimming circuits, Multisim 10 simulation software is used to study the influence of circuit parameters on charging time and waveform, and the influence of control angle on output voltage. The simulation results are consistent with theoretical analysis and calculation, thereby deepening
students' understanding of theoretical knowledge, improving teaching efficiency, and achieving good teaching results. Computer simulation-assisted teaching can make classroom teaching more vivid and intuitive, and simplify complex and profound knowledge.