1. Add isolation transformer
Most switching power supplies are parallel switching power supplies. For parallel switching power supplies, although the circuit board where the load is located is a cold bottom plate, the primary circuit of the switching power supply transformer is still a hot bottom plate. Therefore, if an isolation transformer is not added, the switch cannot be measured with an oscilloscope. Any circuit before the primary side of the transformer, otherwise, it will not only make the oscilloscope shell live, posing a threat to the human body, but also burn out the power supply. It is not necessary to add an isolation transformer when measuring voltage with a multimeter.
2. avoid electric shock
When repairing the switching power supply, the use of an isolation transformer cannot guarantee 100% safety. The necessary and sufficient condition for electric shock is that there is a potential difference exceeding the safe voltage between two or more conductors in contact with the body, and there is a certain strength. Electricity flows through the body. The isolation transformer can eliminate the potential difference between the hot ground and the power grid, which can prevent electric shock to a certain extent. However, it cannot eliminate the inherent potential difference between the points in the circuit, that is to say, if the two hands of the maintenance personnel touch the part with the potential difference in the switching power supply circuit at the same time, it will also cause an electric shock. Therefore, when repairing, if the maintenance personnel must operate with electricity, they should firstly insulate the body from the ground reliably, such as sitting on a wooden seat, stepping on a dry wooden board or insulating materials such as packaging foam; The habit of one-handed operation, when it is necessary to touch live parts, prevent the formation of a loop through the other hand or other parts of the body, etc., these are effective measures to avoid electric shock.
3. Select the reference potential
When measuring the voltage of the power supply circuit, it is necessary to select the reference potential. The ground before the primary side of the switching transformer is the hot ground, and the ground after the primary side of the switching transformer is the cold ground. The two are not equipotential. Therefore, when measuring the voltage of the primary circuit of the switching transformer, take the hot ground as the reference point, that is, connect the negative test lead of the multimeter to the hot ground; when measuring the secondary circuit (load circuit) of the switching transformer, take the cold ground as the reference point, that is, the multimeter The negative test lead is connected to the cold ground.
4. When the power supply is not oscillating, the voltage across the large filter capacitor should be discharged
Repair the power supply with no output, and then turn off the power after the power is turned on. Since the power supply does not oscillate, the voltage across the large filter capacitor (300V filter capacitor) will discharge extremely slowly. The voltage at both ends of the filter capacitor is discharged (it can be discharged by a small resistance with a large power) before measurement can be performed, otherwise not only will the multimeter be damaged, but also the safety of maintenance personnel will be endangered.
5. The start-up time should be controlled during maintenance
When overhauling the output voltage of the switching power supply is higher than the normal value and many faults, the startup time should be as short as possible, so as not to break down the switching tube and load components, causing unnecessary losses. The standard of startup time is the shortest time required to measure the voltage value at a certain point. In actual monitoring, you can hold the test pen with one hand, press the switch with the other hand, turn on the power switch, and turn off the power immediately after seeing the reading.
6. After replacing the faulty components, monitor the output voltage value of the switching power supply when it is turned on again.
Check the switching power supply and find or suspect that there is a problem with a component. After replacing this component, monitor the output voltage of the switching power supply (105-150V) when starting up. If it is much higher than the normal value, it should be quickly shut down. Then check according to the fault of high voltage output.
7. Repair of damaged fuse
Fuse of the fuse in the AC input circuit is rarely caused by the natural damage of the fuse, and most of it is caused by the short circuit fault of the rear circuit, such as the short circuit of the mains rectifier diode, the short circuit of the power switch tube, etc. Therefore, if you encounter the failure of the fuse damage, you should first check and replace the above vulnerable components, and then replace the new fuse test machine. When the fuse is damaged, it should be replaced with a new product of the same specification instead of copper wire, so as to avoid greater damage to the circuit. When the inside of the damaged fuse turns black or the picture tube bursts, it means that the circuit of the rear stage is seriously short-circuited, and it should be checked carefully.
8. Maintenance of Electrolytic Capacitors in Switching Transformers
The failure rate of electrolytic capacitors in the hot-end circuit of switching transformers is relatively high, but some capacitors are normal when measured with a multimeter, but there are actually problems. Therefore, it is recommended to use the substitution method to repair the 2.2~100μF electrolytic capacitor in the hot end circuit of the switching transformer to avoid misjudgment.
