Design of Thyristor Trigger Based on AT89C52 Single Chip Microcomputer

0 Introduction

The thyristor trigger based on the single-chip microcomputer is undoubtedly a popular trigger device now. It has many advantages, such as small temperature drift, high reliability, and convenient intelligent control. The general trigger device often only collects one-phase synchronization signal, and then sends out six-way pulse control signals with a certain conduction angle α after processing by the single-chip microcomputer, which undoubtedly has a certain error for the three-phase AC power. This design collects the synchronous pulse signals of the three phases at the same time, avoiding the delay caused by detecting only one phase. At the same time, the three-phase fully controlled bridge rectifier circuit in the system adopts a resistor-capacitor absorption device to avoid overvoltage, making the system more stable and reliable.

1 System Hardware Circuit

The hardware circuit of the whole system is mainly composed of the main circuit and the microprocessor control circuit. The main circuit includes the synchronization signal generation circuit, the trigger pulse signal drive circuit and the three-phase fully controlled bridge rectifier circuit with the resistor-capacitor absorption device.

The timer/counter of the AT89C52 single-chip microcomputer used in this device works in the mode of 12M crystal oscillator timer. The synchronous signal generating circuit is used to convert the 220V AC voltage obtained from the power grid into 6 synchronous pulses with a phase difference of 60°. The AT89C52 is used to receive the synchronous signal and the angle α, and convert the angle α into a pulse delay, thereby controlling the gate level of the three-phase fully controlled bridge rectifier circuit and controlling the output current. The driving circuit is used to power amplify the pulse signal from the single-chip microcomputer. The three-phase fully controlled bridge rectifier circuit with a resistor-capacitor absorption device realizes the control of the output current and receives overcurrent and overvoltage.

1.1 AT89C52 main control circuit

The main control circuit (Figure 1) makes full use of the internal resources of AT89C52 and realizes the clock circuit by connecting an external 12M crystal oscillator and capacitor. As shown in Figure 1, the synchronization signal is input through the P0.0~P0.2 ports, and the microcontroller realizes timing and outputs six pulse control signals to the P1.2~P1.7 ports through internal software. If the program is in an infinite loop, it can be automatically reset or manually reset when powered on. The circuit structure is very simple and easy to implement.

1.2 Three-phase fully controlled bridge rectifier circuit

The DC voltage from the transformer turns on the six thyristors. At the same time, the six pulse signals with trigger angles α from the pulse isolation drive circuit control the gate levels UT1 to UT6.

In order to avoid the adverse effects caused by overvoltage and improve the stability of the system, this experiment uses a three-phase fully controlled bridge rectifier circuit with a resistor-capacitor absorption device, as shown in Figure 2. The resistor-capacitor absorption device uses capacitors to absorb overvoltage, converting the magnetic field energy that causes overvoltage into electric field energy and storing it in the transformer, and then the capacitor discharges through the resistor and releases the energy on the resistor.

1.3 Synchronous circuit design

Traditional trigger circuits generally require a three-phase synchronous transformer to provide a synchronization signal. In a three-phase fully controlled bridge rectifier circuit, using a thyristor triggered by a single-chip microcomputer, the natural phase change point of the trigger pulse must first be synchronized with the zero-crossing point of the line voltage of the three-phase power supply.

In order to overcome the shortcomings of the traditional synchronous transformer, which is complex in connection and difficult to debug, three synchronous circuits are used as shown in Figure 3. Each circuit collects a phase synchronous signal, which makes the error smaller and the accuracy higher. These three identical circuits are connected to P0.0~P0.2 of the single-chip microcomputer respectively. The synchronous circuit is mainly composed of zero-crossing detector SF339 and optocoupler isolation. The line voltage obtained from the power grid by SF339, which has a simple structure and is easy to use, is converted into a square wave signal, and then is isolated by optocoupler to form the synchronous signal required by the trigger circuit. Two synchronous pulses are output at the zero-crossing point of each power supply cycle, as shown in Figure 4. In such a cycle, the three-phase power supply outputs 6 synchronous pulses, and the phase difference of these 6 synchronous pulse signals is 60°. The synchronous signal is then shaped and output and sent to the three input ports P0.0~PO.2 of AT89C52 respectively.

1.4 Trigger pulse drive circuit

The six-channel pulse control signal must be amplified before being sent to the thyristor control stage, because the pulse signal strength output from AT89C52 is not enough to drive the thyristor. At this time, the photoelectric coupling integrated amplifier drive circuit shown in Figure 5 is used. The control signal from the single-chip microcomputer is amplified by the integrated amplifier through photoelectric coupling to reach the trigger pulse required by the thyristor. This method abandons the large pulse transformer and simplifies the circuit structure.

2 Software Timing and Implementation

2.1 Software Timing

Since the timer uses the automatic counting function of AT89C52, the external timing chip of the microcontroller is omitted, simplifying the design circuit. The crystal oscillator used is 12MHz, and the frequency division is 12, so the corresponding clock cycle is:

That is, the maximum value of the timing counter is 20000, which corresponds to the 360° electrical angle of the synchronization pulse.

In order to make the thyristor conduct again after the current is interrupted, the pair of thyristors that should be turned on in the two groups must have trigger pulses at the same time. There are two methods: wide pulse triggering method and double pulse triggering method. The wide pulse triggering method is to make the width of each pulse greater than 60°, but must be less than 120°, generally 80°~100°. The double pulse triggering method is to send a pulse to the previous thyristor at the same time when triggering a certain thyristor, so that the two thyristors that should be turned on in the common cathode group and the common anode group have trigger pulses, which is equivalent to two narrow pulses equivalent to replace the wide pulse greater than 60°. The software implementation of the wide pulse triggering method is simpler than the double pulse triggering method. Now take the pulse width as 90°, so the initial count value N0 is:

That is, the initial count value of the delay time of the first pulse is 5000, and 6 pulses with a phase difference of 60 are obtained from the synchronization signal generating circuit. After the input angle α is delayed, the output current of the three-phase fully controlled bridge thyristor rectifier circuit is controlled. The delay angle α can be achieved through software delay, and its initial delay value Nα is:

The output waveform principle is shown in Figure 6 (taking α=30° as an example):

2.2 Software Implementation

The main program includes the system initialization subroutine, the input and calculation of the control angle, the detection of the synchronous input signal, the output of the pulse signal, and the control of system startup, reset or shutdown.

The main program flow chart is shown in Figure 7:

3 Conclusion

This experiment makes full use of the internal resources of the AT89C52 single-chip microcomputer, uses the single-chip microcomputer to realize the control of the output current of the circuit by the conduction angle α, uses the internal counter/timer of the single-chip microcomputer and saves some peripheral devices, thus making the structure simple. The control of the thyristor is intelligentized through software. The control scheme is simple, uses few components, is easy to implement, and has a wide range of applications. It has high practical and promotion value.