Matters needing attention in power supply circuit maintenance
1. Add isolation transformer
Switching power supplies are mostly parallel switching power supplies. For parallel switching power supplies, although the circuit board where the load is located is a cold backplane, the primary circuit of the switching power supply transformer is still a hot backplane. Therefore, if an isolation transformer is not added, any circuit before the primary side of the switching transformer cannot be measured with an oscilloscope. Otherwise, the oscilloscope shell will not only be charged, which will pose a threat to people and will also burn out the power supply. When measuring voltage with multimeter, it is unnecessary to add isolation transformer.
Step 2 avoid electric shock
When maintaining switching power supply, the use of isolation transformer can not guarantee 100% safety. The necessary and sufficient condition for electric shock is that there is a potential difference exceeding safe voltage between two or more conductors in contact with the body, and a certain intensity of current flows through the human body. Isolation transformer can eliminate the potential difference between thermal ground and power grid and prevent electric shock to some extent. However, it can't eliminate the inherent potential difference between points in the circuit, that is, the maintenance personnel touch the parts with potential difference in the switching power supply circuit with both hands at the same time, which will also lead to electric shock. Therefore, when repairing, if maintenance personnel must operate with electricity, they should first make their bodies reliably insulated from the earth, such as sitting on a wooden seat and stepping on a dry wooden board or insulation such as foam for packaging; Secondly, it is necessary to develop the habit of operating with one hand, and when it is necessary to contact with live parts, to prevent the loop from forming through the other hand or other parts of the body, etc. These are effective measures to avoid electric shock.
3. Choose a good reference potential
To measure 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 hot ground, and the ground after the primary side of the switching transformer is cold ground. They are not equipotential. Therefore, when measuring the voltage of the primary circuit of the switching transformer, take the thermal ground as the reference point, that is, connect the negative probe of the multimeter to the thermal ground; When measuring the secondary circuit (load circuit) of the switching transformer, it is necessary to take the cold ground as the reference point, that is, connect the negative probe of the multimeter to the cold ground.
4. When the power supply is not oscillating, the voltage at both ends of the large filter capacitor should be discharged.
Repair the power supply without output, and then cut off the power supply after it is energized. Because the power supply does not oscillate, the voltage discharge at both ends of the large filter capacitor (300V filter capacitor) will be extremely slow. At this time, if you want to measure the power supply with the electric barrier of the multimeter, you should first discharge the voltage at both ends of the large filter capacitor (you can use a high-power small resistor to discharge it), and then you can measure it. Otherwise, it will not only damage the multimeter, but also endanger the safety of maintenance personnel.
5. The startup time should be controlled during maintenance.
When overhauling many faults that the output voltage of the switching power supply is higher than the normal value, the startup time should be as short as possible, so as not to break through the switching tube and load components and cause 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 stylus in one hand and press the switch in the other, turn on the power switch, and turn off the power immediately after reading clearly.
6. Monitor the output voltage of the switching power supply when it is turned on again after replacing the faulty components.
Check the switching power supply and find or suspect that there is something wrong with a component. After replacing this component, monitor the output terminal voltage of the switching power supply (105 ~ 150 V) when starting. If it is much higher than the normal value, shut it down quickly. Then check according to the fault of high voltage output.
7. Repair of damaged fuse tube
Fuse fuse in AC input circuit is rarely caused by natural damage of fuse, but mostly caused by short-circuit fault of subsequent circuit, such as short-circuit of rectifier diode of commercial power and short-circuit of power switch tube. Therefore, if the fuse tube is damaged, the above vulnerable components should be checked and replaced first, and then a new fuse tube test machine should be replaced. When the fuse tube is damaged, it should be replaced with new products of the same specification, and copper wire should not be used for replacement, so as not to cause greater damage to the circuit. When the damaged fuse tube turns black or the picture tube bursts, it means that the back-stage circuit is seriously short-circuited, so it should be carefully checked.
8. Maintenance of electrolytic capacitor in switching transformer
The failure rate of electrolytic capacitor in the hot end circuit of switching transformer is high, but some capacitors are measured normally with multimeter, but there are problems in fact. Therefore, it is suggested that the 2.2 ~ looμ f electrolytic capacitor in the hot-end circuit of switching transformer should be overhauled by substitution method to avoid wrong judgment.
