How to Test Power Supply (SMPS)?

Power Supply is the most important part of your computer System because you can not work until your computer is not powering on.  Here I am describing some techniques that you can use to troubleshoot and overcome the SMPS Problems.

Functions Performed

Following Functions are provided by the SMPS (Switch Mode Power Supply):
Its main function is to provide required DC voltage to operate all components in the system.
Different Voltage levels obtained from same power supply.
Output is not affected by the input means if input voltage is between 120V AC to 240V AC the output obtained will be constant. It also provides electrical protection to all components from under voltage or over voltage.
It Provides Necessary Cooling to its components and protects computer from overheating. 

Voltage Levels
If you want to test SMPS than you should know voltage level on every pin of the connector so that testing can be performed:

24 PIN ATX Connector
SIDE A			SIDE B
Pin No.	Color	Voltage Level	Pin No.	Color	Voltage Level
1	Orange	+3.3 Volts	13	Orange	+3.3 Volts
2	Orange	+3.3 Volts	14	Blue	-12 Volts
3	Black	Ground	15	Black	Ground
4	Red	+5 Volts	16	Green	PS_ON
5	Black	Ground	17	Black	Ground
6	Red	+5 Volts	18	Black	Ground
7	Black	Ground	19	Black	Ground
8	Grey	Power good	20	White	NC
9	Purple	VSB  +5V	21	Red	+5 Volts
10	Yellow	+12 Volts	22	Red	+5 Volts
11	Yellow	+12 Volts	23	Red	+5 Volts
12	Orange	+3.3 Volts	24	Black	Ground

 Fig Given Below illustrates these pins on Connector

Now you are aware about the voltage levels at each pin and are ready to test SMPS

Case1: SMPS Not Powering ON

First Short Green and Any Black Pin of the SMPS with the help of a Pin and if SMPS starts then it is OK. This test was just to confirm that SMPS is Powering on or not rest testing is done by checking voltage on each pin.

Case2: No Display (Condition: Motherboard is OK, RAM OK)

Another Problem that SMPS can cause is no display. In this case SMPS will Power on but there will be no display (Condition: Motherboard is OK, RAM OK). To resolve this problem check voltage on additional 4 Pin Connector that carries 2 Yellow wires and two black wires. If Voltage at any of these yellow pins is below +12V DC (+- 5% Tolerance) display will not be generated.

Case 3: System Automatically Shut Down after Some Time (Condition: CPU FAN Working)

This Problem is due to Faulty Components in the SMPS and most of them will be: Faulty Diodes, Leaked Capacitors, SMPS FAN Faulty, Loose Connections or Loose Fuse

How to Identify Which Section is Faulty?

Case1: Input Section Faulty

Check Voltage between Green and Any Black wire if this voltage is 0 or less than +5V DC then Input Section is Faulty.

Case2: Output Section Faulty

If Green and Black wire voltage is ok then your input section is OK and there is some problem with Output Section. To find this check voltage on Purple and any black pin. If this Voltage is +5V DC then Output Section is OK otherwise it is faulty.