Poor load capacity of power supply is a common fault, which usually occurs in old-fashioned or long working power supplies. The main reasons are aging of various components, unstable operation of switch tubes, and failure to timely heat dissipation. Emphasis should be placed on checking for heating and leakage of the voltage regulator diode, damage to the rectifier diode, and damage to the high-voltage filtering capacitor.

After inspection, there is only about 2.3 volts left in the DC 5V power supply for the spectrometer motherboard. Disconnect the load from the 5V DC power supply, power on, and measure the 5V DC power supply again. At this point, there is 5V. It is preliminarily judged that the load carrying capacity of this 5V DC power supply is poor. Remove the power supply shell for maintenance. Since there is a DC 5V output when powered on without a load, focus on checking the output rectifier circuit on the secondary coil side, Connecting a fake load to a 5-volt power supply for measurement revealed that the voltage between pins 1 and 2 of the three-way stabilized 7805 was 5.2 volts, while there was still 2.3 volts left between pins 2 and 3. As shown in the following figure, it is determined that the performance of the three-way regulator 7805 has deteriorated. The fault has been resolved by replacing the three-way regulator 7805.
Tip:
1. When the LED display screen power supply fails to start, we usually need to check whether the switching frequency is correct, whether the protection circuit is blocked, whether the voltage feedback circuit and current feedback circuit are normal, and whether the switch tube is broken down.
2. The power transformer of the LED display screen generates heat or emits a “sizzling” sound, usually due to incorrect switching frequency.
3. The output voltage of the LED display screen power supply is usually indicated by a short circuit on the secondary side when the power indicator light flashes.