Precautions for Power Circuit Maintenance and Inspection
1. Add an 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 board, the primary circuit of the switching power supply transformer is still a hot bottom board. Therefore, if an isolation transformer is not added, an oscilloscope cannot be used to measure any circuit before the primary side of the switching transformer. Otherwise, not only will the oscilloscope shell be charged, posing a threat to personnel, but the power supply will also be burned out. When measuring voltage with a multimeter, an isolation transformer can be omitted.
2. Avoid electric shock
When repairing a switch power supply, using 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 a certain strength of current flows through the human body. Isolation transformers can eliminate the potential difference between the thermal ground and the power grid, which can to some extent prevent electric shock. But it cannot eliminate the inherent potential difference between various points in the circuit, which means that if maintenance personnel touch the parts with potential difference in the switch power supply circuit with both hands at the same time, it can also cause electric shock. Therefore, if the maintenance personnel must carry out live operation during repair, they should first make their bodies reliably insulated from the ground, such as sitting on a wooden seat, stepping on a dry board or packaging foam and other insulating materials; Secondly, it is necessary to develop the habit of one handed operation. When it is necessary to touch live parts, it is important to prevent the formation of circuits through the other hand or other parts of the body. These are effective measures to avoid electric shock.
3. Choose the appropriate reference potential
To measure the voltage of the power circuit, a reference potential should be selected. The ground before the primary side of the switching transformer is a hot ground, and the ground after the primary side of the switching transformer is a cold ground. The two are not equipotential. Therefore, when measuring the voltage of the primary circuit of a switching transformer, the thermal ground is used as the reference point, that is, the negative probe of the multimeter is connected to the thermal ground; When measuring the secondary circuit (load circuit) of a switch transformer, the cold ground should be taken as the reference point, that is, the negative probe of the multimeter should be connected to the cold ground.
4. When the power supply does not oscillate, the voltage discharge at both ends of the large filtering capacitor should be dealt with
Repair a power supply without output, turn on the power and then turn off the power. Due to the non oscillation of the power supply, the voltage discharge at both ends of the large filtering capacitor (300V filtering capacitor) will be extremely slow. At this time, if you want to measure the power supply using the resistance mode of a multimeter, you should first discharge the voltage at both ends of the large filtering capacitor (a high-power small resistor can be used for discharge) before measuring it. Otherwise, not only will the multimeter be damaged, but it will also endanger the safety of maintenance personnel.
5. During maintenance, the startup time should be controlled
When troubleshooting many faults where the output voltage of the switch power supply is higher than the normal value, the startup time should be as short as possible to avoid breaking down the switch tube and load components, causing unnecessary losses. The standard for startup time is the shortest time required to measure the voltage value at a certain point. During actual monitoring, you can hold the probe in one hand and press the switch with the other hand to turn on the power switch. After seeing the reading clearly, immediately turn off the power.
6. Monitor the output voltage value of the switch power supply when restarting after replacing the faulty component
When inspecting the switch power supply and discovering or suspecting that there is a problem with a certain component, after replacing the component, the output voltage of the switch power supply (105-150V) should be monitored during startup. If it is much higher than the normal value, it should be quickly shut down. Afterwards, check for faults with high voltage output.
