Switching power supply circuit maintenance precautions

When repairing the switching power supply, pay attention to the following issues:

(1) When repairing a power supply without output, the power should be turned on and then off. Since the switching power supply does not oscillate, the voltage across the 300V filter capacitor will discharge very slowly, and the high voltage across the capacitor will remain for a long time. At this time, if the resistance block of a multimeter is used to measure the power supply, the 300V filter capacitor should be discharged first (a small resistor with high power can be used for discharge) before measurement. Otherwise, not only will the multimeter be damaged, but the safety of the maintenance personnel will also be endangered.

(2) When measuring voltage, be sure to pay attention to the correct selection of the ground wire, otherwise the test value will be wrong, and it may even cause damage to the instrument. When measuring the primary circuit of the switching power supply, the "hot ground" should be used as the reference point. The ground wire ("hot ground") can be selected from the negative pole of the +300V filter capacitor of the AC rectifier filter circuit, because the +300V filter capacitor is the "marker" of the primary circuit of the switching power supply and is the easiest to find. When measuring the secondary voltage of the switching power supply, the "cold ground" should be used as the reference point. In addition, when performing waveform testing, the corresponding ground wire should also be selected, and it is best to select the ground wire near the circuit being tested. If it is too far away from the waveform test point, interference is likely to occur on the test waveform.

(3) When repairing a switching power supply, the use of an isolation transformer cannot guarantee 100% safety. The necessary and sufficient conditions for electric shock are: there is a potential difference exceeding the safe voltage between two or more conductors in contact with the body, and a current of a certain strength flows through the human 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, but it cannot eliminate the inherent potential difference between points in the circuit. In other words, 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 power on, they should first insulate their bodies from the ground reliably, for example, sitting on a wooden seat, stepping on a dry wooden board or insulating materials such as packaging foam plastics; secondly, they should develop the habit of operating with one hand. When they must touch the live parts, they should prevent the formation of a loop through the other hand or other parts of the body. These are all effective measures to avoid electric shock.

(4) Many LCD monitors do not work when they are offline or connected but the host is not turned on. They can only work normally when the monitor is connected to the host and the host is turned on. Therefore, for such LCD monitors, they should be connected online and the host power should be turned on during maintenance to avoid misjudgment.