Symptom 1: When the hand is placed in the hot air outlet, no hot air is blown out. Only cold air comes out. Diagnosis and Repair: The presence of cold air indicates that the hair dryer's motor is running normally, and the infrared detection and control circuit is also functioning properly. However, the absence of hot air suggests a problem with the heating element or loose wiring. Upon inspection, it was found that the heater’s wiring was loose. After reconnecting the wires, the hair dryer began to blow hot air, and the issue was resolved. Symptom 2: After turning on the device, hot air blows out even when the hand is not placed in the outlet. Diagnosis and Repair: Further investigation showed that the thyristor was not broken down, but there was a suspicion of internal leakage in the optocoupler’s photoelectric tube between pins 3 and 4. Replacing the optocoupler restored normal operation, and the fault was eliminated. Symptom 3: When the hand is placed in the hot air outlet, no hot air is emitted. Diagnosis and Repair: The fan and heater were checked and found to be in good condition. A voltage check at the thyristor gate showed no trigger signal. The control board was tested and produced a rectangular wave output, suggesting a problem with the optocoupler. Using a multimeter in R×10 mode, the resistance between pins 3 and 4 of the optocoupler was measured as infinite. Normally, the forward resistance should be several ohms, and the reverse resistance should be infinite. This indicated that the internal photoelectric tube was open, preventing the thyristor from receiving a trigger signal and thus failing to conduct. After replacing the optocoupler, the hair dryer functioned correctly again. Fiberglass Reinforced Plastic Casingm,GRP Pipe Channel,Glass Reinforced Plastic Duct,Glass Reinforced Polymer Tubing,GRP Pressurized Pipe Zhejiang Huafeng new material Co., Ltd. , https://www.cnhfpipe.com