Troubleshooting the PC Power Supply

Troubleshooting the your PC's power supply is actually quite easy if you are fortunate enough to have a system board equipped with hardware monitoring.  If this is the case for you you simply need to find the voltage readouts that come from the hardware monitoring chip.  While a number of system boards provide specific software for using the hardware monitoring functions within Windows, they more often provide access to these readings within the "Chipset Features" screen of the BIOS' CMOS setup.

One thing you'll probably note is that the voltage readings are rarely exactly what the rated voltages should be.  This is little cause for concern unless the readings are a full volt or more either above or below the rated setting.  If they are there is a good chance that your power supply is not performing properly and should be replaced.

A Properly Connected Power Supply
is essential for the PC to run.  ATX and AT- style power supplies each connect to your system board in a different fashion.  For an ATX power supply the connector only fits one way properly so you're fairly safe.  Most ATX powered system boards also monitor the voltage and will at the post display an 0xx or 0xxx code if something is wrong

ATX Power Supply

Pin Signal Pin Signal
1 3.3V 11 3.3V
2 3.3V 12 -12V
3 GND 13 GND
4 5V 14 Pwr_On
5 GND 15 GND
6 5V 16 GND
7 GND 17 GND
8 Power_Good 18 -5V
9 5V_Standby 19 5V
10 12V 20 5V

Improperly connecting an AT power supply is an all to common problem that usually results in irreparable damage to the mainboard so be extra careful.  AT power supplies use two 6-wire connectors generally identified as P8 and P9 that attach to the motherboard's power connector side by side. The problem is that these connectors look almost identical. The correct orientation of these connectors is with the four black wires together in the middle

AT Power Supply    


Connector I

Connector II


Power Good

















Power Supply Diagnosis & Repair
The following guidelines should be taken into consideration when diagnosing and repairing power supply units.

The first step is to make sure that the computer is properly plugged in and that the outlet into which it is plugged is working properly. This can easily be done by testing the outlet with an electrical device such as a lamp. (Note that most power supplies cannot be run unless they are under load, i.e. attached to a device such as the motherboard and/or a hard drive.)  If the other device works properly when plugged into the same outlet as the computer the problem may be with the computer's power supply unit.  Make sure the power supply connections to the motherboard are correct and secure, if there is still no response, the problem might be in the power supply. If you have a multi-meter, you can test the power supply to see if it is putting out the correct voltage to the motherboard.

Circuit Testing an AT Power Supply

Peripheral Power Connector

Pin Wire Color Signal
1 Yellow +12V
2 Black Gnd
3 Black Gnd
4 Red +5V
  1. Turn off the PC, but leave it connected to the wall outlet. 
  2. Open the case. 
  3. Set your multi-meter to read DC voltage in the next range higher than 12 volts. 
  4. Locate an unused drive connector, if available, or disconnect the power supply connection from a hard disk or floppy disk to use for the test. 
  5. Turn on the PC. 
  6. Insert the multi-meter's black probe into the power connector on one of the black wires. 
  7. Touch the multi-meter's red probe to the red wire on the power connector. You should get a reading of +12 volts. 
  8. Touch the red probe to the yellow wire on the connector. You should get a reading of +12 volts. 
    If you didn't get any readings don't even bother to continue. Replace the power supply. 
  9. Insert the multi-meter's black probe into P8 at one of the black wires. 
  10. Insert the red probe into the P8 connector to the motherboard at a red wire. You should get a reading of +5 volts. 
  11. Insert the red probe into the P8 at the yellow wire. You should get a reading of +12 volts. 
  12. Check the blue wire. You should get a reading of -12 volts. 
  13. Move the black probe to one of the black wires on P9. Use the red probe to test the white wire. You should get a reading of -5 volts. 
  14. Check each of the red wires on P9. You should get a reading of +5 volts on each one. 

You probably won't get exactly 5 or 12 volts on these connections, but you should be close ( 5.09v or 11.89v. for example). If the power supply reading is off by more than 1.0v., or there is no reading, you have a bad power supply.

Before discarding the power supply, it is necessary to ensure that it is not some other component causing the problem.  If the power supply's "power good" signal is good, continue measuring the voltage ranges of the pins on the motherboard and drive connectors and verify them against the manufacturers specifications. If any of these are outside the range, then the power supply connectors are likely bad, and the power supply must be replaced. If the values obtained for these readings are all within expected ranges, then the power supply itself may not be defective; there may he a short circuit in one of the system's devices.

To test for short circuits in devices, simply disconnect all the components from the power supply one at a time - starting with the floppy drives, then the hard drives, and finally the  mainboard - each time powering up the computer to see if it starts running. If disconnecting a component results in the computer operating, reconnect it to see if it is the cause. If the fan on the power supply no longer works, then the cause of the problem has been located. If after detaching all components, the fan still doesn't work then the problem is almost certainly a defective power supply.

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