Common Faults and Repair Methods of UPS Power Supply

UPS power failure phenomenon: A Maipu 1KVA online UPS power supply has normal bypass output after startup. Press the ON button to switch from bypass to inverter operation, but immediately jumps to bypass again, and the fault light is on, and the buzzer sounds a long alarm. Press the OFF button, the buzzer stops alarming, and the bypass output is normal.

UPS power supply fault analysis and repair: According to the fault phenomenon, it is initially believed that the control circuit is working normally, because after pressing the ON button, it can automatically jump to the inverter working state after a delay of 1 to 2 seconds, but the fault occurs immediately. From this, it can be roughly determined that the fault circuit is:

(1) There is a short circuit fault in the soft start control circuit;

(2) The output circuit of the power amplifier board has a short circuit fault; the aluminum plate spot welding machine

(3) Both parts have short circuit faults. Because the bypass output is normal, the possibility of short circuit in external devices such as microcomputers and sockets can be basically ruled out. After opening the case, it was found that the soft start sealant had been burned and deformed. After cutting the lead wires, each component on the soft start block was measured one by one with a multimeter. All of them were burned out. A new soft start block was replaced, the power was connected, and the ON switch was pressed. The fault still existed, proving that the short circuit fault still existed.

Turn off the power supply, use a multimeter to measure the diodes Q13, Q14, Q19, Q20 in the output circuit of the power amplifier board, all of which are normal, and the MOS high-power tubes (YTFP250) Q7, Q22, Q23 are also normal. Measure the MOS high-power tubes Q5, Q17, Q18 in the other arm, and find that the resistance between the D pole and the S pole of Q17 and Q18 is 0Ω, and Q5 is not abnormal. Because the D pole and S pole of the two power tubes Q17 and Q18 are connected in parallel, Q17 and Q18 are welded off and measured separately. Q18 is normal, and the D pole and S pole of Q17 have indeed been short-circuited.

Because it is difficult to buy YTFP250 on the market, I checked the manual and found that the parameters of IRFP250 are almost the same as YTFP250. After replacing it with an IRFP250, I used a multimeter to measure the online resistance values ​​of the two arms and found that they were equal. After connecting the power supply and turning on the machine, I pressed the ON switch and the inverter could work, but the output was about 230V. I adjusted the output fine-tuning potentiometer VR3 to make the output 220V, sealed VR3 with wax or sealant, connected the load, and turned on the machine. Everything was normal and the fault was eliminated.

UPS power failure phenomenon: A Maipu 1KVA online UPS power supply works normally when it is turned on and the bypass is working properly. However, when the ON switch is pressed, there is no response, the relay is not closed, and the inverter cannot work.

UPS power supply fault analysis and repair: According to the fault phenomenon, it can be roughly judged that the fault is in the panel circuit or relay circuit. Open the case, remove the panel, and use a multimeter to measure the relay first, which is normal. Since pressing the ON switch does not work, it is suspected that the ON switch is damaged. Use the red and black pens of the multimeter to connect the two ends of the ON switch respectively, press the ON switch, and the resistance is 0Ω, which proves that the ON switch is good.

Connect the power supply, use the multimeter DC voltage range to measure the DC voltage of the two ends of the ON switch to the ground, and find that there is voltage at one end and no voltage at the other end. The no-voltage end is connected to the resistor R100 through the resistor R99. Then use the multimeter to measure the voltage of the two ends of R99 to the ground. There is voltage at one end, but no voltage at the end connected to ON. Turn off the power supply, measure the online resistance value of R99 to be infinite, while the resistance value of R99 should be 100KΩ. Replace it with a 100KΩ resistor, turn on the power supply, press the ON switch, the inverter can work, and the output has 220V AC voltage. Connect the load and it can work normally. The fault is eliminated.

UPS power failure phenomenon: A SANTAK1000VA square wave backup power supply, the AC power supply is running normally, when inverting, the buzzer sounds for a long time, the alarm indicator light is on for a long time, and there is no output.

UPS power failure analysis and repair: The user reported that two days before the UPS was sent for inspection, when the mains was switched to the inverter, an abnormal "humming" sound could be heard from inside the machine, and the sound was very loud, but there was output. After a period of time, the above-mentioned fault phenomenon occurred. The case was opened and the machine was turned on without mains power and no load. It was found that at the moment the switch was turned on, the UPS had output and the fan also started to rotate (the fan used the UPS output voltage of 220V). About 2 seconds later, the inverter had no output and the above-mentioned fault phenomenon occurred. The multimeter was used to measure the final drive circuit and it was found that Q1~Q3 had been short-circuited (Q1~Q3 were connected in parallel).

It can be seen that the "humming" sound emitted by the UPS when converting from AC power to inverter before the fault occurred is due to the extremely unbalanced output of the two arms of the UPS's final drive circuit, which causes the abnormal sound of the transformer, that is, Q1~Q3 (or Q4~Q6) is partially damaged. Due to lack of timely maintenance, one arm of the final drive circuit, Q1~Q3, is completely damaged, causing a short circuit, thereby causing the overcurrent protection circuit to operate, blocking the inverter working pulse output, and causing the inverter to have no output. After replacing Q1~Q3 and measuring that other components are not damaged, turn on the power switch, the UPS inverter output returns to normal, and the fault is eliminated.

UPS power failure phenomenon: A SANTAK600VA sine wave backup power supply has no output when the AC power is converted to inverter, the buzzer sounds long, and the LED is always on.

UPS power supply failure analysis and repair: As a rule, this kind of failure should first check whether the battery is normal. The power supply uses two YUASANP7-12 (12V, 7.0AH) batteries connected in series. During static measurement, the voltage of one battery is 12V and the voltage of the other battery is 10V. It seems that there is no problem with the battery. Check the static resistance value of each foot of the 30A fuse, the inverter output Darlington compound power tube MJ11033, the front stage push tube TIP41C and the pulse width modulator (SG3524) in the inverter circuit, and no abnormal phenomenon is found.

After repeated power-on tests, the fault still exists, except that the inverter output can be maintained for about 10 seconds at no load. When the multimeter is used to detect the voltage value of the relevant parts, it is found that the voltage value of one battery is 12V when the fault occurs, while the voltage value of the other battery is only about 5V (the voltage of this battery in static measurement is 10V). The battery is replaced and the fault is eliminated.

UPS power failure phenomenon: A SANTAK500VAUPS power supply, the mains power supply is normal, there is output when inverting, but the output voltage is low, and the transformer is accompanied by noise. Fault analysis and maintenance: It does not work properly when inverting, and the inverter circuit of the power supply should be checked. The voltage output indicates that the transistor final stage push amplifier circuit is working normally, and the transformer noise indicates that the two arms of the final stage push amplifier circuit are not working symmetrically (the transformer itself is less likely to be damaged). It is estimated that the two amplifier tubes MJ11033 may be damaged. Use a multimeter to measure the forward resistance of the emitter junction of the two transistors. One of them is about 50Ω, and the other resistance value is very large, indicating that it has been burned out. After replacement, the fault is eliminated.

UPS power failure phenomenon: A SANTAK500VA UPS power supply, when the AC power is normal, the voltage-stabilized power supply only works in the inverter state.

UPS power supply fault analysis and repair: The mains power is normal, but the power supply only works in the inverter state and cannot return to the mains power supply state. In this case, the AC input fuse should be checked first. When the fuse is intact, check the mains power supply-inverter power supply conversion control circuit. Use a multimeter to measure the mains sampling transformer T1 and the voltage comparator component LM339, and find that the primary winding of the mains sampling transformer T1 is open. After replacement, the fault is eliminated.

UPS power failure phenomenon: It works normally in both AC power supply and inverter state, but in inverter state, there is still output after shutdown.

UPS power failure analysis and repair: As we all know, the UPS power switch controls the mains input and the positive pole of the battery. Under normal circumstances, no matter it is powered by the mains or in the inverter state, there should be no voltage output after shutting down. Using a multimeter to check the power switch, it was found that a set of switches connected to the positive pole of the battery had been deformed and not connected properly. After replacement (if the same type of power switch cannot be purchased, the deformed reed can be carefully flattened and the contact can be polished with fine sandpaper), the fault is eliminated.

UPS power failure phenomenon: Computer configuration: Pentium 133, 16MB memory, 3GB hard disk, S3VIRGE graphics card. Recently upgraded to MMXP166, motherboard replaced with VXPRO. After the upgrade, when starting WIN95, it often crashes inexplicably. Restart, report "Illegal instruction execution", "Abnormal error", etc., and often crashes under DOS and WINDOWS3.2.

UPS power supply failure analysis and repair: First, repeatedly installing WIN95, WINDOWS3.2, and DOS failed to solve the problem. Scanning found and cleared the GRAVE virus, and made multiple adjustments to the options in BIOSSETUP, but the problem still existed. Second, consider hardware failure. First consider new parts, because only the CPU and motherboard were new, so I replaced two motherboards of the same model, but the problem still existed.

I replaced the memory and CPU and found that they were all normal. I replaced the Asus TXP4 motherboard, and not only did it not work, but I could not boot from the hard disk. After replacing the hard disk, it showed that the IDE interface of the motherboard was normal. The hard disk worked normally on other machines. So far, it seems that every part is normal, but it does not work properly when assembled together. After careful observation, I found that the host power supply is 200 watts. After replacing it with a 230 watt power supply, the Asus motherboard started normally.

To confirm, I replaced the VXPRO motherboard, and found that the problem still occurred. I replaced another 200-watt power supply, and it also failed. This shows that the cause is indeed a problem with the power supply and the motherboard. Summary: The cause of the failure in this case is that the power of the old 200-watt power supply is too low, and the MMXCPU needs a larger current. In addition, the VXPRO motherboard cannot support the multi-power Pentium well. This makes me think that when upgrading a computer, we must comprehensively consider the relationship between various components and design a comprehensive upgrade plan. In addition to giving the computer a Pentium "core", we must also prevent the phenomenon of a small horse pulling a big cart. ■