Application of phase-shift trigger chip TCA785 in circumferential current control of magnetic particle flaw detector

Abstract: Aiming at the problem of unstable demagnetization of existing fluorescent magnetic particle flaw detectors, the working principle of the phase-shift trigger TCA785 and its application method in the circumferential current circuit control system of the wheel-set fluorescent magnetic particle flaw detector are given. The results show that the circumferential current control of the magnetic particle flaw detector based on the phase-shift trigger TCA785 has a simple circuit structure, stable demagnetization and high stability. It has high economic and social benefits in improving the defect detection rate of fluorescent magnetic particle flaw detection and reducing train accidents.
Keywords: magnetic particle flaw detector; phase-shift trigger; TCA785; thyristor phase shift

0 Introduction
Magnetic particle flaw detectors have been widely used in aviation, machinery, automobiles, internal combustion engines, railways, ships and other departments due to their relatively simple structure, fast detection speed, low cost and low environmental pollution. Since the demagnetization of some wheel pairs of fluorescent magnetic particle flaw detectors is unstable, it is necessary to improve the thyristor voltage regulation scheme in the circumferential current circuit control system. At present, in the magnetic particle flaw detection equipment used in production, the circumferential current mostly uses two thyristors in reverse parallel to form a voltage regulation circuit. This article gives a method of using TCA785 phase-shift trigger to achieve voltage regulation of thyristors.

1 Principle and triggering method of thyristor voltage regulation
Thyristor has the advantages of small size, light weight, high voltage resistance, low price, sensitive control and long service life. It enables the application of semiconductor devices from the weak current field to the strong current field, and is widely used in high-power electronic circuits such as rectification, inversion and voltage regulation. Thyristor is an active switching device. It usually remains in a non-conducting state until a smaller control signal triggers it (or "ignites") to turn it on. Once it is turned on, it remains on even if the trigger signal is removed. To turn it off, a reverse voltage can be added between its anode and cathode or the current flowing through the thyristor diode can be reduced to below a certain threshold. The magnetization circuit of the magnetic particle flaw detector mostly uses thyristor voltage regulation to control the circumferential magnetization current.
1.1 Principle of thyristor voltage regulation
The conditions for thyristor to turn on and off are: when the anode potential is higher than the cathode potential and the control electrode has sufficient forward voltage and current, it can be turned from off to on; when the anode potential is higher than the cathode potential and the anode current is greater than the holding current, the thyristor can be maintained on: when the anode potential is lower than the cathode potential or the anode current is less than the holding current, the thyristor changes from on to off.
Generating a trigger pulse is one of the necessary conditions for thyristor conduction, and its quality will directly affect the working condition and performance of the thyristor. Therefore, the reliability of the trigger circuit that generates the trigger signal is directly related to the quality of the thyristor voltage regulator.
1.2 Triggering mode of thyristor
There are usually two triggering modes for using thyristor to achieve AC voltage regulation, namely zero-crossing triggering mode and phase-shifting triggering mode.
Zero-crossing triggering is to trigger the thyristor to turn on near the zero point of the power supply voltage, and to achieve AC voltage regulation by changing the frequency of the thyristor conduction in the set cycle. The working waveform of thyristor fixed-cycle zero-crossing triggering is shown in Figure 1. In Figure 1, Tc is the period of the control signal, t1 and t2 are the on and off time of the thyristor, and Tc=t1+t2. This circuit achieves voltage regulation by changing the on and off time of the thyristor, that is, changing the on and off cycles. Usually, the control circuit first connects the load and the input voltage U for t1 seconds (on for n cycles) in the period Tc, and then disconnects it for t2 seconds (off for m cycles), that is, by changing the on and off time to adjust the output voltage of the load.

Phase-shift triggering achieves voltage regulation by changing the conduction angle. Figure 2 shows the conduction conditions when the phase-shift trigger angles of the trigger pulse are 45°, 90°, and 135°. As shown in Figure 2, the voltage across the load changes with the change of the phase-shift trigger angle.

Phase-shift triggering has a part that keeps on and off in each positive or negative cycle of the thyristor, that is, the output is continuously adjustable, so it can adapt to various loads, but in the control process, it will generate electromagnetic interference to the power grid. According to the load properties, usage conditions and surrounding environment, this design chooses phase-shift triggering as the triggering control method of the thyristor.

2 Design of thyristor phase-shift trigger circuit
With the improvement of integrated circuit manufacturing technology, integrated triggers have overcome the shortcomings of discrete component triggers and have been widely used. This article uses the integrated trigger circuit TCA785 to transform existing equipment to improve equipment performance.
2.1 Introduction to TCA785 phase-shift trigger
The TCA785 phase-shift trigger is a single-chip phase-shift trigger, which is a dual-in-line 16-pin large-scale integrated circuit. Compared with other chips, TCA785 has the
advantages of good output pulse uniformity, wide phase shift range, wide output pulse and manual adjustment, so it has a wider range of applications.
2.2 Working principle of TCA785
The principle of TCA785 is shown in Figure 3, where pin 11 is connected to the phase shift control level V11, pin 6 is connected to the modulation signal, pin 5 is connected to the synchronization signal, pin 12 is grounded through a capacitor, pins 9 and 10 are connected to the sawtooth slope resistor and capacitor respectively, and pins 15 and 14 are pulse output terminals Q1 and Q2.

The synchronous voltage VSYNC passes through the resistor R5 to the zero phase detector ZD. When ZD detects its zero crossing, it can be sent to the synchronous register SR for storage, and the sawtooth generator RG is controlled by SR. The capacitor C10 of RG
is charged by the constant current source SC determined by the resistor R9. When the sawtooth voltage V10 of the capacitor C10 is greater than the phase shift control voltage V11, a pulse signal is generated to the output logic unit, and a trigger pulse is generated at pin 14 (Q1) and pin 15 (Q2). It can be seen that the phase shift of the trigger pulse is controlled by the size of the phase shift control voltage V11. The trigger pulses Q1 and Q2 can be phase-shifted within the range of 0° to 180°, and the phase difference of the output pulses of pins 14 and 15 is 180°.
The main pin waveforms of TCA785 are shown in Figure 4. Among them, pin 5 is the external synchronization signal terminal, which is used to detect the zero crossing of the AC voltage. Pin 10 is the synchronous sawtooth wave generated inside the chip. Its maximum
and minimum slope values ​​are determined by the external resistors and capacitors of pins 9 and 10. By comparing with the control voltage of pin 11, synchronous pulse signals can be output at pins 15 and 14. Therefore, by changing the control voltage of pin 11
, phase shift control can be achieved. The pulse width is determined by the external capacitor value of pin 12. When the double narrow pulse driving mode is selected, pin 12 should be connected to a 150 pF capacitor. In fact, a pulse width of several tens of microseconds can make the thyristor conduct normally.

2.3 Design of thyristor phase-shift trigger circuit
The control object of this control system is CJW-4000 wheel set magnetic particle flaw detector, which consists of power control system, wheel in/out system, spray magnetization system, fluorescent lamp system, magnetic separation liquid spray recovery system and darkroom. The equipment control circuit adopts programmable controller PLC centralized control.
The thyristor phase-shift trigger circuit of wheel set fluorescent magnetic particle flaw detector is shown in Figure 5. This circuit uses the Q1 and Q2 (i.e. the output of pins 14 and 15) pulses output by TCA785 to directly trigger the thyristor in the positive and negative half-cycle of the AC power supply respectively. The phase-shift control voltage U11 comes from the external DC power supply V1 (here +15 V). When it changes continuously within its effective range (0.2 V ~ V1-2 V), the phase of the pulse output Q1 and Q2 can be shifted within the range of 0° ~ 180°.

Since the signals coming from the synchronous transformer are all sinusoidal signals, and TCA785 uses the principle of zero-crossing detection to achieve synchronization, if the amplitude of the sine wave is too small, it will
not provide a clear zero-crossing point, and electromagnetic interference may also cause zero-crossing point detection errors. However, if the amplitude of the sine wave is too large, it will exceed the synchronous voltage input range of the chip, so the synchronous signal should be shaped into a square wave. As shown in Figure 5, between pin 1 (grounded) and pin 5, an anti-parallel limiting diode (tube voltage drop is about 1 V) is used to convert the sine wave into a square wave. After a 220 kΩ resistor is connected to pin 5, a 220 V AC synchronous voltage signal can be directly connected to detect the zero-crossing point of the AC voltage. At the same time, two positive and reverse parallel limiting diodes are connected to the ground terminal for protection.
Pin 12 is the pulse width control terminal of the output pulses Q1 and Q2. Its application range is 150~4700 pF.
Pin 6 is the pulse signal prohibition terminal. It can be connected to +15 V power supply through the resistor R2 with a resistance value of 10 kΩ. When the voltage at this terminal is less than 2.5 V, the output pulse is blocked; and when the voltage at this terminal is greater than 4 V, the blocking function does not work. Therefore, this pin can be used as the thyristor overcurrent and overheat protection of the main circuit.
The maximum and minimum slopes of the synchronous sawtooth wave generated by the chip at pin 10 are determined by the external resistors and capacitors at pins 9 and 10, and then by comparing with the control voltage at pin 11, synchronous pulses can be output at pins 15 and 14 to change the control voltage at pin 11, thereby realizing phase shift control. Pin 11 can be connected to +15 V power supply through a capacitor.

3. Issues to be noted in TCA785 application
The TCA785 phase-shift trigger adopts negative logic working mode, that is, the control voltage increases, the control angle of the output pulse increases, and the conduction angle of the thyristor decreases. This should be noted in the application. In addition, the reverse-parallel limiting diode is generally used between pin 1 and pin 5 of TCA785 to convert the 220 V AC power connected to pin 5 into a square wave, thereby providing a clear zero-crossing signal to TCA785.
If the trigger circuit is used for circumferential current charging and demagnetization, a rectifier bridge should be added after the AC current output by the thyristor to convert the current into DC, and then connect it to the circumferential coil of the magnetic particle flaw detector.

4 Conclusion
The thyristor phase-shift trigger circuit with TCA785 as the core in this paper can effectively control the magnetization and demagnetization of the circumferential magnetic field of the wheel set fluorescent magnetic particle flaw detector. In this way, not only can the magnetization effectively detect whether the wheel has cracks by attracting fluorescent magnetic particles, but also the magnetic field of the wheelset after inspection can be reduced to within the standard, thereby improving product quality. The circuit design is relatively simple, reliable, and can effectively guarantee the inspection quality of the flaw detector.