Common Defects

In general, the fuse blowing indicates that there is a problem with the internal circuit of the power supply. Since the power supply operates under high voltage and high current conditions, fluctuations and surges in the grid voltage can cause a sudden increase in current within the power supply, leading to the fuse blowing. It is crucial to check the rectifier diodes at the power supply input, high-voltage filtering electrolytic capacitors, inverter power switch tubes, etc., to see if these components have breakdowns, open circuits, or damage. If the fuse is indeed blown, the first step is to check all components on the circuit board to see if any components are charred or if there is any electrolyte leakage. If none of the above conditions are found, use a multimeter to check for breakdowns or short circuits in the switch tube. It is particularly important to note: do not replace a damaged component and power on directly, as this may damage the replaced component again due to other high-voltage components still having defects. A thorough inspection of all high-voltage components in the above circuit must be conducted before completely ruling out the defect of the blown fuse. No DC voltage output or unstable voltage output If the fuse is intact and there is no output of DC voltage at various levels under load, this situation is mainly caused by: open or short circuit phenomena in the power supply, defects in over-voltage or over-current protection circuits, defects in auxiliary power supplies, oscillation circuits not functioning, excessive power supply load, breakdown of rectifier diodes in high-frequency rectification and filtering circuits, leakage in filtering capacitors, etc.

When using a multimeter to check secondary components, after ruling out breakdown of high-frequency rectifier diodes and load short circuits, if the output is zero at this time, it can be confirmed that there is a defect in the control circuit of the power supply. If there is partial voltage output, it indicates that the front-end circuit is functioning normally, and the defect lies in the high-frequency rectification and filtering circuit. The high-frequency filtering circuit mainly consists of rectifier diodes and low-voltage filtering capacitors for DC voltage output. Breakdown of the rectifier diode will cause this circuit to have no voltage output, and leakage in the filtering capacitor will lead to unstable output voltage, among other defects. Static testing of the corresponding components with a multimeter can reveal the damaged components. For example, if a 24-volt DC motor power supply has no DC 24-volt output after being powered on, open the power supply casing, check that the fuse is not blown and that there are no obvious burn marks or broken components on the circuit board. Measure the resistance at the AC input and DC output terminals when not powered on, and find that the values are normal. Measure important components such as the switch tube, rectifier bridge, and rectifier tube, and conclude that there is no significant internal short circuit, estimating that the protection circuit has acted. Upon inspection, this switch power supply uses a control chip, and after searching for relevant information, it is found that when the voltage at pin 3 of the chip exceeds 1 volt, the internal current comparator outputs a high level, resetting the PWM latch and shutting off the output. The voltage at pin 3 during power-on testing is above 1 volt, and there is no output at pin 6. After checking the relevant circuit, it is found that the voltage regulator D2 has broken down, causing PC1 to conduct, resulting in pin 3 of U3842 being high, and thus no output at pin 6, with the switch tube not operating and no DC output on the DC side. Replacing the same type of voltage regulator D2 eliminates the defect. Power supply load capacity issues are a common defect, usually occurring in older or long-operating power supplies. The main reason is the aging of various components, unstable operation of the switch tube, and inadequate heat dissipation. Key checks should include whether the voltage regulator diode is overheating or leaking, whether the rectifier diode is damaged, and whether the high-voltage filtering capacitor is damaged.