How to Check if a Computer SMPS/PSU is Working or Not

The personal computers at your office or  home  are  powered by electronic power supply units called Switched Mode Power supply (SMPS). Commonly known as Power Supply Units (PSU) this hardware converts  the input AC power to output DC  varying  with respect to voltage. Though there are PSUs available in different form factors, ATX is the most commonly available and used one right now.

Being the most vital part of  workstation its important that a PSU work continuously without any hiccups. Have you ever noticed a PC that restarts quite often or a PC that shuts down after a short duration without  you initiating it? Well chances are high that the SMPS/ PSU  is  faulty. The test or confirmation about the status of SMPS is a must before speculating on what might be the possible reason for PC failure. Here is how exactly you can confirm that.


Firstly,  make sure that  the power cord is detached from  PSU and then take out the unit from cabinet. You will notice a 24 pin main connector along with many other 4 pin peripheral cable connectors coming out from the PSU. Here is what you have to do next:

1) Take one among the 4 pin peripheral cable connector and you will observe a yellow, red and two black wires from it. The yellow wire being the 12 volt supply and black the ground, you must connect a 10 ohm /10 watts ceramic load resister across them.

This is how a ceramic resistor looks like:

ceramic resistor

2) Connect the power cord to the PSU now and take the 24 pin main power cable connector to the motherboard. You will notice a green wire and few adjacent black wires. Power the SMPS/PSU now and short the green (pin 16) and black (pin 17) point using a jumper (wire) .

24 pin SMPS power cable connector

If the PSU fan rotates you can 8o % confirm that the unit is functioning well. There are still chances that the PSU can be faulty which can be tested by checking the output volts at the pins. If the fan does not rotate then you must replace ( or repair ) the unit before trying to power your PC again.

Pin description of the 24 pin power cable connector is given below:

Pin Name Color Description
1 3.3V Orange +3.3 VDC
2 3.3V Orange +3.3 VDC
3 COM Black Ground
4 5V Red +5 VDC
5 COM Black Ground
6 5V Red +5 VDC
7 COM Black Ground
8 PWR_OK Gray Power Ok is a status signal generated by the power supply to notify the computer that the DC operating voltages are within the ranges required for proper computer operation (+5 VDC when power is Ok)
9 5VSB Purple +5 VDC Standby Voltage (max 10mA)
10 12V Yellow +12 VDC
11 12V Yellow +12 VDC
12 3.3V Orange +3.3 VDC
13 3.3V Orange +3.3 VDC
14 -12V Blue -12 VDC
15 COM Black Ground
16 /PS_ON Green Power Supply On (active low). Short this pin to GND to switch power supply ON, disconnect from GND to switch OFF.
17 COM Black Ground
18 COM Black Ground
19 COM Black Ground
20 -5V White -5 VDC  (this is optional on newer ATX-2 supplies, it is for use with older AT class expansion cards and can be omitted on newer units)
21 +5V Red +5 VDC
22 +5V Red +5 VDC
23 +5V Red +5 VDC
24 COM Black Ground

Data credit  :

Hope this post helps you to test  computer SMPS next time you come across such a situation. Share your thoughts and do share if you liked.

8 Responses

  1. ramesh says:

    Very much helpful to me to test my repaired SMPS, thanks a lot.

  2. ST says:

    Thanks a Lot buddy ,Great work
    I have a question and hoping to get a answer to this .

    -I have a Single pc and have 8 internal hardrives total which i have aquired down the line most of them are sata,Since i have only 4 sata ports on my pc , i can connect only 3 hdd at a time as one port is taken by the DVD drive
    What i would like to do is have access to all my hdd at the same time , so that i dont have to keep unpluging re-pulging them when need . requesting to kindly advise , Any help is appreciated.

  3. kalyan says:

    thanks ………………..

  4. khemraj says:

    i want to know about cpu

  5. Shafeeque says:

    Very helpful one@@

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