Tag: mitsubishi fx1

This article presents an in-depth analysis of the Mitsubishi FX1S PLC switching power supply board, focusing on its circuit schematic diagram and the STRG6551 chip. The power supply board features two isolated 24V outputs and provides a 5V supply for the motherboard through voltage reduction. These outputs are isolated via L2 bidirectional filters, which also serve to block high-frequency interference pulses, enhancing system stability.

Circuit Working Principle:
The 220V power supply enters the power board through the L and N terminals. C1, C2, C4, and L1 form a bidirectional low-pass filtering network, with L1 and L2 performing similar functions. Dual fuses F1 and TH1 protect the circuit, with F1 serving as an overload protection quick fuse and TH1 as a temperature fuse. After rectification, the mains power is converted to a DC voltage of about 280V, which is then applied to the primary switch transformer TB1’s oscillation and voltage stabilization circuit, centered around the STRG6551 power oscillation module.

STRG6551 Chip Analysis:
Despite limited available information on the STRG6551 chip, its pin functions can be inferred from the circuit structure. Pins 4 and 3 are used for power supply, while pins 1 and 2 are internally connected to the source and drain of a power switch (MOS) transistor. Pin 5 introduces feedback voltage. The STRG6551 functions similarly to the common U3844 switch mode power supply oscillator.

Circuit Analysis:
The circuit can be divided into three main branches: (1) the oscillation circuit, (2) the voltage stabilizing circuit, and (3) the protection circuit.

1. Oscillation Circuit: The rectified 280V DC voltage is reduced to 30V through R1, R11, and D5, and then supplied to pin 4 of STRG6551. The secondary winding of TB1 provides the working power supply for STRG6551.
2. Voltage Stabilizing Circuit: TB1’s winding 2 induces voltage, which is rectified and filtered by D4 and C10. This voltage serves as the overall working power supply for the PLC. The stability of this voltage is crucial for the PLC’s performance. R9, IC2, and the PC1 output voltage sampling circuit are used to maintain voltage stability.
3. Protection Circuit: R2 serves as the sampling resistance for the internal MOCS switching tube’s working current. In case of abnormal load or input, the protection circuit will disconnect the driving circuit of the switch tube, stopping oscillation and protecting the circuit. Additionally, D6 provides a discharge path for the magnetic field energy stored in the transformer, protecting the switch tube from overvoltage breakdown.

Repairing the Switching Power Supply:
A common fault in switching power supplies is the lack of output voltage. This can be caused by various reasons, including blown fuses, abnormal load, or circuit malfunctions. To troubleshoot, it is important to first distinguish whether the fault lies in the oscillation circuit or the voltage stabilization circuit. By disconnecting the PLC motherboard and using a voltage regulator to set the input voltage below AC100V, one can test the circuit’s response and identify the faulty component.

In conclusion, understanding the working principle and circuit structure of the Mitsubishi FX1S PLC switching power supply board, as well as the function of the STRG6551 chip, is crucial for effective troubleshooting and repair. By adopting reasonable maintenance steps and scientific methods, one can improve maintenance efficiency and ensure the stable operation of the PLC system.