How to Test SMPS Power Supply: Testing Pins, Voltage Checks and Troubleshooting

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Power Supply is one of the most important parts of your computer system because a computer cannot operate properly without a reliable power supply. Here I am describing the functions of an SMPS (Switch Mode Power Supply), the voltage levels of its ATX connector pins, and some common techniques used to troubleshoot power supply problems.

Functions Performed

The following functions are provided by an SMPS (Switch Mode Power Supply):

1. Its main function is to convert AC input power into the DC voltages required to operate computer components.
2. Different voltage levels are supplied to the motherboard, processor, storage devices and other components.
3. The output voltage is regulated within the power supply's specified operating range when input voltage and load conditions are within its limits.
4. Depending on the model, it provides protection against overvoltage, overcurrent, overload and short-circuit conditions.
5. Its cooling system helps control the heat generated during operation.

Safety note: Never open the SMPS casing or touch its internal components. Internal parts can retain dangerous electrical energy even when the unit is unplugged. Do not use an improvised metal pin to bridge connector terminals. Use a compatible commercial PSU tester or ask a qualified technician to perform electrical testing.

Voltage Levels

If you want to understand how an SMPS is tested, you should know the typical voltage levels and functions of the pins on its connector. The following table preserves the original parallel Side A and Side B layout and the colour coding of the wires.

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# signal
5	Black	Ground	17	Black	Ground
6	Red	+5 Volts	18	Black	Ground
7	Black	Ground	19	Black	Ground
8	Grey	Power Good (PWR_OK)	20	White	NC on most modern ATX supplies
9	Purple	+5VSB (Standby)	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

Note: This is a common ATX 24-pin layout. Wire colours, connector orientation and pin assignments should be verified against the documentation for your particular power supply. Pin 20 is generally unused in modern ATX 24-pin connectors, while older 20-pin designs may use it differently.

Fig. given below illustrates the pins on the ATX connector.



Now you are familiar with the typical voltage levels and functions of the connector pins. Actual measurements should be checked against the specifications for your power supply.

Case 1: SMPS Not Powering On

If the computer does not power on, check the AC power connection, power cable, wall socket and power supply switch, if fitted. A compatible commercial PSU tester can help determine whether the supply starts and whether its outputs are within the expected range. The green wire is commonly the PS_ON# control signal, and black wires are ground. Do not use a loose metal pin or improvised jumper to bridge them. A power supply fan spinning is not enough to prove that the unit is functioning correctly under load.

Case 2: No Display (Motherboard and RAM Assumed OK)

Another problem that may be associated with the SMPS is a computer that powers on but shows no display. However, a no-display problem can also be caused by the monitor, display cable, graphics hardware, motherboard, RAM, processor or a loose power connection. Check that the main ATX connector and the additional CPU power connector are securely fitted. On many systems, the CPU power connector carries +12V on yellow wires and ground on black wires. Verify the connector type and pinout for your particular system. The +12V rail should remain within its specified tolerance under the applicable test conditions. A single voltage reading cannot guarantee that the supply will operate correctly under load.

Case 3: System Automatically Shuts Down After Some Time

If a computer shuts down after running for some time, possible causes include overheating, a faulty CPU fan, loose connections, an overloaded or failing SMPS, motherboard faults or other defective components. Check the cooling system and external power connections first. If the problem continues, arrange for the power supply and other hardware to be tested by a qualified technician.

How to Identify Which Section May Be Faulty?

Case 1: Input or Standby Power Problem

If the computer does not respond to the power button, check the wall socket, power cable, rear switch and motherboard power connections. The purple wire on a typical ATX connector carries +5V standby (5VSB). A technician can check whether standby voltage is present using suitable equipment. The green wire is the PS_ON# control signal; it is not a direct measurement of the AC input section. Its reading depends on the operating state of the system, so it should not be used by itself to diagnose an input-section fault.

Case 2: Output Section Problem

If standby power is present but the computer does not start correctly, the problem could involve the main output rails, motherboard, CPU power connector or another component. The yellow, red and orange wires commonly carry +12V, +5V and +3.3V respectively. The grey wire is commonly associated with the Power Good (PWR_OK) signal. These signals and voltage readings must be interpreted according to the applicable ATX specification and the power supply manufacturer's documentation. A single reading cannot confirm that the complete output section is healthy. A compatible PSU tester or professional load test provides a more reliable assessment.

Conclusion: Understanding the ATX pinout and wire colours can help with computer power supply troubleshooting. Always verify the pinout for the specific unit and use suitable testing equipment. Never open the SMPS casing or attempt improvised live-pin testing.

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