Content:
As the automation level of textile equipment increases, components such as inverters and PLC circuit boards have become essential parts of textile machinery. However, as the equipment operates for many years, circuit board failures occur frequently, leading to rising maintenance costs for enterprises. Based on years of practical experience, I have summarized some tips and techniques for circuit board repair and component testing, which I now share with my peers for discussion and exchange.
1Repair of Switching Power Supply Circuit Boards
When the fuse blows after powering on, it indicates damage to the rectifier bridge, switching transistor power integration block (UC3842), or power filter capacitor; if the voltage across the power filter capacitor is normal when powered on, but the voltage does not disappear quickly after power off, it indicates that the startup resistor of the switching transistor is faulty; if the switching power supply board has no output, replacing all the electrolytic capacitors on the circuit board can yield significant results; if the output voltage of the switching power supply board is unstable, it may be due to poor contact of the adjustable resistor or malfunctioning or damaged components like the voltage regulator block (T431) or optocoupler; frequent damage to the switching transistor on the power supply board may be caused by faulty capacitors or resistors in the high-frequency pulse circuit; when the inverter servo controller is powered on in a cold state, if the display digits flicker and it cannot operate, and it takes longer to return to normal operation, the most common cause is leakage of the electrolytic capacitors on both the primary and secondary sides of the switching power supply, leading to reduced capacitance; when measuring resistance, it should be less than or equal to the nominal resistance value for normal operation; otherwise, it is abnormal.
2Faults in the Electrical Control PLC of Cotton Combing Dust Filter
If the power indicator light is off and there is no output, online testing shows that the 2A fuse is blown, and the 1N4007 and 0.47VF/400 electrolytic capacitor are damaged. After replacing the fuse and 1N4007, if there is no 0.47VF/400 electrolytic capacitor, two 1VF, 400V electrolytic capacitors can be used in series as a substitute, allowing the PLC to operate normally with good results.
3CPU Testing
The main conditions for CPU operation are three elements: normal power supply circuit; reset, tested with a multimeter in DC voltage mode at the reset point, checking for voltage presence during power on and off states, with significant voltage fluctuations indicating normal operation; oscillation circuit, for most transistor oscillators, one leg has a high voltage (above 2V), and the other leg has a low voltage (a few hundred millivolts); if the high voltage leg shows significant changes when probing the two legs of the transistor oscillator, it indicates that the internal clock oscillation circuit of the CPU is functioning normally.
4A New Method for Measuring Transistors
Based on the structure of the transistor, the resistance of the BC junction is smaller than that of the BE junction. Using an analog multimeter, set it to R×100 or R×1K range, first identify the base (B) of the transistor (using a PNP transistor as an example), connect the red probe to the base and the black probe to the two legs of the transistor; the leg with the higher resistance is the emitter (E), and the other leg is the collector (C); using a digital multimeter, the measurement method is the same as with the analog multimeter, but set the digital multimeter to the diode mode, similarly identify the base (B) of the transistor, connect the black probe to the base and the red probe to the collector (C) and emitter (E); the leg with the higher resistance is the emitter (E), and the other leg is the collector (C).
5Measuring Electrolytic Capacitors
For online measurement of electrolytic capacitors, set the multimeter to DC20V, connect the black probe to ground (negative), and the red probe to the positive terminal; a power filter capacitor below 3V and a secondary filter capacitor below 1V are considered normal; measurement results above 10V or outside the range indicate severe leakage or slight leakage of the electrolytic capacitor; if not measured online and the capacitor being tested exceeds the measurement range of the digital multimeter, and there is no capacitance meter available, to check if the capacitance is normal, a small standard capacitor can be connected in series with the capacitor being tested; if the measurement result is significantly lower than the nominal capacitance of the capacitor being tested and the calculated value of the series connection with the small standard capacitor is outside the error range, it indicates that the capacitor needs to be replaced due to leakage.
6Using Electrolytic Capacitors for Data and Program Retention
When replacing batteries, electrical data can only be retained for a few seconds, and if the process is slow, important data may be lost. A 4700VF, 10V electrolytic capacitor can be used, connecting the positive terminal of the capacitor to the positive terminal of the battery holder and the negative terminal to the negative terminal of the battery holder, allowing data and programs to be retained for 3 minutes during battery replacement.
7Using Landline Phone Lines to Measure Voltage Regulation Values Below 36V
The method is to connect the unknown voltage regulation diode in parallel with the landline phone line, set the digital multimeter to DC200V, and connect the red and black probes of the multimeter in parallel with the positive and negative terminals of the tested voltage regulation diode; the reading on the digital multimeter indicates the voltage regulation value of the diode, making this method simple, convenient, and practical.
(Author: Zhang Yongzheng, Shanxi Oasis Textile Co., Ltd.)
Source: Jinkunying Textile