How to Troubleshoot SMPS Problems

Troubleshooting SMPS

A power supply unit is the vital component of a system that supplies power to various components of the system. The power supply unit is an electronic component and may show some problems due to which the system may not start. Due to a faulty power supply, the components of a system may also get damaged. As a result, troubleshooting the power supply problems is also an essential task for maintaining the system. You can use various tools for troubleshooting SMPS. Various tools include a low power fine tip soldering iron and fine rosin core solder, Digital multimeter (DMM), twister, Brush, Screw driver set and Desoldering pump are used to diagnose the problems in SMPS.

System does not start:

When you switch on the system it does not start due to a faulty power supply unit. The power supply may not switch ON and the system may not respond. Sometimes the power supply unit may start but not be able to supply the appropriate voltage needed by various components of the system. In order to troubleshoot this problem:

  1. Make sure that the power connector of the system is properly plugged-in the main power supply junction.
  2. Check the internal connections of the system after removing the system case cover.
  3. Check the connection of the motherboard connector of the power supply unit with the motherboard.
  4. Check for any occurrence of the short circuit which might have happened inside the system case.
  5. Check if there is a problem with the main power supply junction.

Power supply fan does not work or emits a lot of sound:

The power supply fan is not working when the system is switched on or makes a lot of noise. The problem occurs if the power supply fan is not functional or if lots of dust is accumulated in the fan motor. To troubleshoot this problem:

  1. Shut down the system, remove the system case cover and clean the dirt or dust from the fan and power supply unit with a vacuum cleaner or with hand blower.
  2. Check the power supply fan and replace if it is defective.

UPS not handling power problems:

The UPS is not handling the power problems very well and does not function when there is a blackout. The system may also automatically shutdown at times. To troubleshoot this problem:

  1. Check the capacity of the UPS battery for supporting the electric current load for the system.
  2. Charge the battery of the UPS if needed by reducing some load from the UPS.
  3. Check the voltage selector switch, placed at the back of the system case and ensure that the voltage is according to the region.
  4. Check the connections of the motherboard and the disk drives with the power supply unit.
  5. Remove any extra disk drives if not needed during the current operation.

Before opening SMPS:

  1. Check the AC line.
  2. Check the power cord and Line voltage selector whether it has been set to the appropriate voltage according to the region.
  3. Remove all the daughter cards, FDD, HDD and supply connector and check the PG signal (+5 VDC Gray)
  4. Short pin no. 13 and 14 to see that fan is rotating and then check the O/P voltage by DMM.
  5. If the SMPS does not work when connected to the motherboard then check whether there is a problem with SMPS.

SMPS is dead, FAN moves for a moment and then stops:

This problem may occur due to faulty switching transistor circuitry in the feedback section. To troubleshoot this problem:

  1. Desolder both the switching transistor and check the fan.
  2. If the switching transistors are working properly, then check from oscillator and error amplifier IC.
  3. If there is no problem in IC, then check the driver transistors and diodes.
  4. Check the driver transformer and replace if the device is faulty.

SMPS voltage drops or increases:

This problem occurs due to faulty PWM section. To troubleshoot this problem:

  1. Check all the electrolytic capacitors in the PWM circuit and replace them if they are faulty.
  2. Check if there is a fault in IC TL 494.
  3. Check the diodes in the secondary rectifier circuit.

All the output voltages are OK except +5V:

This problem occurs if the secondary rectifier section is faulty. To troubleshoot this problem:

  1. Check Schottky diodes.
  2. Check the secondary rectifier diodes connected in +5V circuit.

SMPS makes hissing noise:

SMPS may make hissing noise because of loose transformer vibrating at a sub-harmonic frequency or dried up electrolytic capacitors. To troubleshoot this problem:

  1. Ensure that there is no short circuit or loose connection.
  2. Ensure that the power supply in not overloaded.

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