Maintenance Skills

The maintenance of switch power supplies can be divided into two steps: In a powered-off state, 'look, smell, ask, measure'. Look: Open the power supply casing and check if the fuse is blown, then check the internal condition of the power supply. If you find burnt areas or damaged components on the PCB, you should focus on checking these components and related circuit elements.

Smell: Check if there is a burnt smell inside the power supply and see if there are any burnt components.

Ask: Inquire about the process of the power supply failure and whether there were any improper operations performed on the power supply.

Measure: Before powering on, use a multimeter to measure the voltage across the high-voltage capacitor first. If the switch power supply is not oscillating or if the switch tube is open-circuited, in most cases, the voltage across the high-voltage filter capacitor has not discharged, which can be over 300 volts, so caution is needed. Use a multimeter to check the forward and reverse resistance of the AC power line and the charging condition of the capacitor; the resistance value should not be too low, otherwise, there may be a short circuit inside the power supply. The capacitor should be able to charge and discharge. Disconnect the load and separately test the ground resistance of each output terminal; under normal conditions, the meter needle should show the capacitor charging and discharging oscillation, and the final indication should be the discharge resistance value of that path. After powering on, check if there are any burnt fuses or smoke from individual components; if so, promptly cut off the power supply for maintenance. Check if there is a 300-volt output across the high-voltage filter capacitor; if not, focus on checking the rectifier diode, filter capacitor, etc. Check if there is output from the secondary coil of the high-frequency transformer; if not, focus on checking if the switch tube is damaged, whether it is oscillating, and whether the protection circuit is functioning. If there is output, focus on checking the rectifier diodes, filter capacitors, and three-terminal voltage regulators on each output side. If the power supply shuts down immediately after being powered on, it is in a protection state; you can directly check the voltage at the PWM chip's protection input pin. If the voltage exceeds the specified value, it indicates that the power supply is in a protection state, and you should focus on investigating the cause of the protection.