The second is to start the protection circuit. During maintenance, we can judge whether it is protected by measuring the voltage of key points, and observe the fault phenomenon for maintenance.

(1), judge whether the protection is started, and measure the B-pole voltage of the thyristor in the protection circuit at the moment of starting, which is normally as low as 0 ⅴ. If the B-pole voltage of SCR becomes higher than 0.7V during startup or failure, the overvoltage protection circuit with analog SCR as the core is started.

For overvoltage, protection: first, the voltage stabilizing chip of the main power supply voltage stabilizing control circuit that causes overvoltage, and second, check whether the voltage stabilizing diode of the overvoltage protection sampling circuit leaks electricity.

(2) Release protection and maintenance: After protection is confirmed, the protection judgment can be released. Energize and measure the output voltage of the switching power supply to determine the fault location. In order to prevent the output voltage of switching power supply from being too high, which will damage the load circuit, it is suggested to connect a dummy load to measure the output voltage of switching power supply first, and then connect the load circuit after the output voltage is normal. All protections will be released to simulate the short circuit between the B pole of SCR and the ground for confirmation. Protection release one by one: 12ⅴ overvoltage protection detection circuit turns off D22,

24V overvoltage protection detection circuit closes D2 1, and then starts the test. If D22 is turned off, the protection is caused by 12ⅴ overvoltage protection circuit. If D2 1 is turned off and no protection is provided when it is turned on, it is the protection caused by the 24V overvoltage protection circuit.