In general, the fuse tube is blown to indicate that there is a short circuit or overcurrent fault in the internal circuit of the charger. This is because the charger operates for a long time under high voltage and high current, and the internal device has a high failure rate. In addition, fluctuations in the grid voltage, surges will cause the current in the charger to increase instantaneously and the fuse will blow.
Maintenance method: First carefully look at the various components on the board to see if the appearance of these components is burnt or there is electrolyte overflow, smelling and smelling. Then measure the resistance value of the power input terminal. If it is less than 20OkΩ, it means that there is a partial short circuit at the back end. Then measure the positive and negative resistance values ​​of the four rectifier diodes and the resistance of the two current limiting resistors to see if there is any short circuit or burnout. Finally, measure whether the power supply filter capacitor can perform normal charge and discharge, whether the switch power tube breaks down, whether the UC3842 and surrounding components are broken down, burned out, etc. It should be noted that because the measurement is in the road, the measurement result may be wrong or misjudged, so the components can be welded and measured if necessary. If the above situation still exists, measure whether the input power line and the output power line are internally short-circuited. In general, in the fuse fuse failure, rectifier diode, power supply filter capacitor, switching power tube, UC3842 are wearing parts, the probability of damage can reach more than 95%, it is easy to eliminate the fault by checking these components.
2. No DC voltage output or unstable voltage output
If the fuse is intact, under load conditions. The causes of such faults are: overvoltage and overcurrent protection circuits appear open circuit, short circuit phenomenon; vibrating circuit does not work; power supply load is too heavy, rectifier diode in high frequency rectification and filtering circuit is broken down: filter capacitor leakage.
Maintenance method: First, use a multimeter to measure whether the components of the high-frequency pulse transformer are damaged: after eliminating the breakdown of the high-frequency rectifier diode and the short-circuit of the load, measure the DC voltage at each output, if the output is still zero. , it can be sure that the control circuit of the power supply has failed. Finally, the multimeter is used to statically measure whether the rectifier diode and the low-voltage filter capacitor in the high-frequency filter circuit are damaged. If the above components are damaged, replace the new components, and the general fault can be eliminated. . However, it should be noted that the output line is broken or open welding, and the virtual welding can also cause such a failure. This should be noted during maintenance.
3. No DC voltage output, but the fuse is intact
This phenomenon indicates that the charger is not working or has entered the protection state after work.
Maintenance method: First of all, it should be judged whether the charger's variable control chip UC3842 is in the state of Wang Zuo or has been damaged. The specific judgment method is: power-on measurement of the 7-pin ground voltage of UC3842. If the voltage of the 7-pin is normal and the voltage of the 8-pin has +5∨, the voltages of 1, 2, 4, and 6 will also have different voltages, indicating that the circuit has been activated. Vibration, UC3842 is basically normal. If the voltage of pin 7 is low and there is no voltage on the other pins, the UC3842 is damaged. The most common damage is the breakdown of the 7 feet to the ground, the breakdown of the 6 and 7 feet to the ground and the breakdown of the 1 and 7 feet to the ground. If these feet are not broken, and the charger still does not start normally, it also indicates that the UC3842 is damaged and should be replaced directly. If it is judged that the chip is not broken, it is checked whether the current limiting resistor of the gate of the switch is open soldered, soldered or changed, and the performance of the switching power tube itself is poor. In addition to this, the power output line is broken or poorly connected, which can cause such a malfunction, so care should be taken during maintenance.
4. DC voltage output is too high
This kind of fault is often caused by the abnormality of the voltage-stamped sampling and voltage-stabilizing control circuit. In the charger, the DC output, the sampling resistor, the error sampling amplifier, the optocoupler, and the power control chip together form a closed control loop. Any problem will cause the voltage to rise.
Maintenance method: Since the charger has an overvoltage protection circuit, the output voltage is too high to cause the overvoltage protection circuit to operate first. Therefore, in the event of such a fault, we can disconnect the overvoltage protection circuit so that the voltage protection circuit does not work, and then measure the mains voltage of the power supply at the moment of startup. If the measured value is higher than the normal value by more than 1V, the reason why the output voltage is too high is indeed in the control loop. At this time, it is important to check whether the sampling resistor is changed or damaged, and whether the precision reference voltage source (TL431) or the optocoupler (PC817) is poor in performance, deterioration or damage. The precision reference voltage source (TL431) is extremely fragile. We can use the following method to distinguish the precision voltage regulator amplifier: connect the reference terminal (Ref) of TL431 to its cathode (Cathode), string 1OkΩ resistor, and connect to 5 ∨ voltage. If the anode (Anode) is 2.5V between the cathode and the cathode is still 2.5 片, it is a good tube, otherwise it is a bad tube.
5. DC voltage output is too low
According to maintenance experience, in addition to the voltage regulator control circuit will cause the output voltage to be too low, there are the following reasons:
(1) The output voltage terminal rectifies the three-pole 莒 and the filter capacitor fails, which can be judged by the substitution method.
(2) The performance of the switching power tube is degraded, resulting in the switch tube not being able to conduct normally, so that the internal resistance of the power supply is increased, and the load carrying capacity is decreased.
(3) The source of the switching power tube is usually connected to a resistor with a small resistance but a large power as an overcurrent protection resistor. The resistance of the resistor is generally 0.2 to 0.8 Ω. If the resistance is changed or the welding is not performed, the contact voltage will cause the output voltage to be too low.
(4) The high-frequency pulse transformer is not good, which not only causes the output voltage to drop, but also causes the switching power tube to be insufficiently excited to damage the switch tube.
(5) The high-voltage DC filter capacitor is poor, resulting in poor power supply load capacity.
(6) The power supply output line is in poor contact, and there is a certain contact resistance, which causes the output voltage to be too low.
Maintenance method: First check with a multimeter - whether the high-voltage DC filter capacitor is degraded, whether the capacity is reduced, whether it can be normally charged and discharged. If there is no such problem, measure the current limiting resistance of the electrode of the switching power tube and the overcurrent protection of the source to determine whether the resistance is variable, deteriorated or open, and the contact is poor. If there is no problem, check again - whether the core of the high-frequency transformer is intact. In addition to this z, there may be a decrease in the output filter capacitor capacity, or open welding, virtual connection; power output current limiting resistor variable value or virtual connection; power output line virtual connection.
Don't let go of the troubles, you should check carefully to ensure that nothing is lost.
6, the hot fan does not turn
The fault is mainly caused by damage to the triode (usually 8550 or 8050) that controls the fan, or the fan itself is damaged or the blades are stuck by debris. However, some chargers use smart heat dissipation. For chargers that use this type of heat dissipation, the probability of damage to the thermistor is very large.
Method: Firstly, use a multimeter to measure whether the triode of the control fan is damaged. If the tube is not damaged, it may be that the fan itself is damaged. The fan can be removed from the circuit board and connected with a 12V DC ( Pay attention to the positive and negative poles. See if it is turning, and if there is any foreign matter stuck. If you swing the wires of the lower fan and the fan rotates, it means that there is a broken wire inside the wire or the connector is not in good contact. If it still does not turn, the fan will be broken. For chargers that use intelligent heat dissipation, in addition to the above checks, check the thermistor for poor contact or damage, open soldering, etc. However, it should be noted that this thermistor has a negative temperature coefficient and should be taken care of when replacing.
(7) The grid voltage is too low. Although the charger can still output the rated charging voltage under low jade, when the grid voltage is lower than the minimum voltage limit of the charger, the output voltage will be too low.
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