Precautions for power supply circuit maintenance
1. Plus 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, not only will the oscilloscope case be electrified, pose a threat to the person, but also burn out the power supply. When measuring voltage with a multimeter, an isolation transformer may not be added.
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 intensity. Electricity flows through the body. The isolation transformer can eliminate the potential difference between the hot ground and the power grid, and to a certain extent can prevent electric shock. But it cannot eliminate the inherent potential difference between the points in the circuit, that is to say, if the maintenance personnel touch the parts with potential difference in the switching power supply circuit with both hands at the same time, it will also cause electric shock. Therefore, when repairing, if the maintenance personnel must operate with electricity, they should firstly insulate their body from the ground reliably, such as sitting on a wooden seat, stepping on a dry board or insulating materials such as packaging foam under their feet; Acquiring the habit of operating with one hand, preventing the formation of a circuit through the other hand or other parts of the body when it is necessary to touch live parts, etc., are all effective measures to avoid electric shock.
3. Choose the reference potential
To measure the voltage of the power circuit, it is necessary to select a reference potential. 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 the switching transformer, use the hot ground as the reference point, that is, connect the negative meter pen of the multimeter to the hot ground; when measuring the secondary circuit (load circuit) of the switching transformer, use the cold ground as the reference point, that is, connect the multimeter The negative meter lead is connected to the cold ground.
4. When the power supply does not oscillate, the voltage across the large filter capacitor should be discharged
To repair the power supply without output, turn it on and then turn off the power. Since the power supply does not oscillate, the voltage discharge at both ends of the large filter capacitor (300V filter capacitor) will be extremely slow. The voltage at both ends of the filter capacitor should be discharged (a small resistor with high power can be used for discharge), and then it can be measured, otherwise it will not only damage the multimeter, but also endanger the safety of maintenance personnel.
5. The start-up time should be controlled during maintenance
When repairing many failures of switching power supply whose output voltage is higher than the normal value, the start-up time should be as short as possible, so as not to break down the switch tube and load components, causing unnecessary losses. The standard of start-up time is the shortest time required to measure the voltage value of a certain point. During actual monitoring, you can hold the test lead in one hand, press the switch with the other hand, turn on the power switch, and turn off the power immediately after seeing the reading clearly.
6. After replacing the faulty components, monitor the output voltage value of the switching power supply when starting up again
Check the switching power supply and find or suspect that there is a problem with a certain 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, shut down quickly. Then check according to the fault of high voltage output.
7. Inspection and repair of damaged fuse tube
The fusing of the fuse in the AC input circuit is rarely caused by the natural damage of the fuse, but most of it is caused by the short circuit fault of the subsequent circuit, such as the short circuit of the mains rectifier diode and the short circuit of the power switch tube. Therefore, if you encounter a fault that the fuse is damaged, you should first check and replace the above vulnerable components, and then replace the new fuse with a 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 not to cause greater damage to the circuit. When the inside of the damaged insurance tube turns black or the picture tube bursts, it means that the short circuit of the rear stage circuit is serious, and it should be checked carefully.
8. Overhaul of Electrolytic Capacitor in Switching Transformer
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 in fact there are problems. Therefore, it is recommended to use the substitution method to overhaul the 2.2-100μF electrolytic capacitor in the hot end circuit of the switching transformer to avoid misjudgment.
