Design of Bidirectional SCR Zero-Crossing Trigger Circuit

Bidirectional thyristor is a power semiconductor device, also known as bidirectional thyristor. In the single-chip control system, it can be used as a power driver. Since bidirectional thyristor has no reverse withstand voltage problem and the control circuit is simple, it is particularly suitable for use as an AC contactless switch. Bidirectional thyristor is generally connected to some high-power electrical appliances and connected to a strong power network. The anti-interference problem of its trigger circuit is very important. Usually, the trigger signal in the single-chip control system is loaded to the control electrode of the thyristor through an optocoupler. In order to reduce the driving power and the interference generated when the thyristor is triggered, the triggering of the bidirectional thyristor in the AC circuit often uses a zero-crossing trigger circuit. Zero-crossing triggering refers to the instantaneous connection when the voltage is zero or near zero. Since zero-crossing triggering is used, the above circuit also requires a sinusoidal AC zero-crossing detection circuit.

1 Zero-crossing detection circuit

The circuit design is shown in Figure 1. In order to improve efficiency and synchronize the trigger pulse with the AC voltage, a trigger pulse is required to be output every half cycle of the AC power, and the trigger pulse voltage should be greater than 4V, and the pulse width should be greater than 20us. In the figure, BT is a transformer, and TPL521-2 is a photocoupler, which plays an isolation role. When the sinusoidal AC voltage is close to zero, the two light-emitting diodes of the photocoupler are cut off, and the bias resistor potential of the base of the transistor T1 turns it on, generating a negative pulse signal. The output end of T1 is connected to the input pin of the external interrupt 0 of the single-chip computer 80C51 to cause an interrupt. In the interrupt service subroutine, the timer is used to accumulate the phase shift time, and then the synchronous trigger signal of the bidirectional thyristor is issued. The voltage output waveforms of points A and B of the zero-crossing detection circuit are shown in Figure 2.

2 Zero-crossing trigger circuit

The circuit is shown in Figure 3. In the figure, MOC3061 is a photoelectric coupling bidirectional thyristor driver, which is also a type of photoelectric coupler. It is used to drive the bidirectional thyristor BCR and play an isolation role. R6 is a trigger current limiting resistor, and R7 is a BCR gate resistor to prevent false triggering and improve anti-interference ability. When the P1.0 pin of the single-chip computer 80C51 outputs a negative pulse signal, T2 is turned on, MOC3061 is turned on, triggering BCR to turn on, and connecting the AC load. In addition, if the bidirectional thyristor is connected to an inductive AC load, since the power supply voltage leads the load current by a phase angle, when the load current is zero, the power supply voltage is a reverse voltage, and the inductive load self-inductance electromotive force el is added, so the voltage value borne by the bidirectional thyristor far exceeds the power supply voltage. Although the bidirectional thyristor is reversely conducted, it is easy to break down, so the bidirectional thyristor must be able to withstand this reverse voltage. Generally, an RC resistor-capacitor absorption circuit is connected in parallel between the two poles of the bidirectional thyristor to realize overvoltage protection of the bidirectional thyristor. C2 and R8 in Figure 3 are the RC resistor-capacitor absorption circuit.

3 Conclusion

The bidirectional thyristor zero-crossing trigger circuit is mainly used in the AC load control circuit of the single-chip control system. It can control high-power AC equipment such as electric furnaces and AC motors. It has been proven to be safe and reliable in practice. This article focuses on the zero-crossing detection and trigger circuit. As for the software design, it is relatively simple. When the zero-crossing detection circuit detects zero crossing, an interrupt request is generated. As long as a trigger pulse is sent through the P1. 0 pin of the single-chip 80C51 in the interrupt service program, the bidirectional thyristor can be triggered to conduct. Due to space limitations, it will not be repeated here.