# Structure and how to test the ATX power supply unit

This post is lesson 6 of 16 in the subject Computer Hardware Practice

## 1. Lesson objectives

• Review disassembling and rebuilding Desktop
• Understand the structure of the power supply unit (PSU) and the types of power connectors on the Desktop
• Correctly disassemble the power connectors on the Desktop
• How to test the ATX power supply unit

## 2. Preparation of instruments

Each student prepares the following tools:

Main tools:

• 01 Desktop case
• 01 screwdriver
• 01 screw box

Support tools:

• 01 computer cleaning brush
• 01 cleaning cloth

## 3. How to test a power cord

– Suppose, we have a 10m long power cord. How do we test if the cord is broken or not?

Using a multimeter: With analog multimeter, you must see if the multimeter is set on “ohms X1”. With digital multimeter, you must see if the multimeter is set on diode test mode. Then, put 2 probes of the multimeter into the 2 leads of power cord. If the power cord is not broken, the buzzer on the multimeter will sound and led on the multimeter will light.

## 4. Power supply unit (PSU) of Desktop

Power Supply Unit (PSU) is a device that provides power for motherboards, hard drives, optical drives and other devices,…. It provides power for all components of computer to operate. PSU has the function of converting Alternating Current (AC) into Direct Current (DC). PSU provides voltages: +3.3V, +5V, +12V, -12V, -5V,…

Structure of power supply unit

– Power supply unit parameters:

• Volt (V)
• Ampe (A)
• Watt (P)

– Power connectors: The pins of the PSU is color-coded in detail, the red power wire has a voltage of +5V, the yellow power wire is +12V, the black wire is the ground, the purple wire is +5V, the orange wire is +3.3V, etc. There are types of power connectors:

• Main power connector: 20 or 24 pins
• CPU power connector 12V: 4 pins
• Molex connector, SATA connector , PCI Express connector, etc.

Color of PSU connectors

– Power supply unit of Desktop are classified by design standards:

• AT standard is often found in older machines (supports Pentium MMX, Pentium II, Celeron,etc.)
• ATX allows automatic power on/off by software without having to use a switch. There are types of ATX:
• ATX: 20-pin main connector.
• ATX12V: 20-pin main connector, 4-pin auxiliary connector.
• ATX12V 2.X: 24-pin main connector, 4-pin auxiliary connector.
• BTX is a new standard designed to supply power for the BTX mainboard. The BTX PSU is optimally designed for today’s latest technologies: SATA, USB 2.0, PCI Express, etc.

## 5. How to test the ATX power supply unit

– When the Desktop computer does not start, the first thing is to check whether the power supply unit is working or not? The simplest way to test a ATX power supply unit is to use a metal wire to connect pin 14 (green) to pin 16 (or any black pin) of the power supply unit.

How to do: Supply power to the PSU. If the PSU fan is spinning, the PSU is still working. On the contrary, the PSU was damaged. However, the case of the rotating PSU fan does not guarantee 100% of the PSU to work properly. We need to measure the DC voltage generated by the PSU to supply the Desktop mainboard.

## 6. Requirement of practice

• Each student receives practical tools.
• Disassemble and rebuild the Desktop step by step and follow the regulations.
• Use the multimeter to test for continuity of any network cable, power cord, or any metal wire.
• Check whether the power supply unit is working or not?
• Disassemble PSU and clean PSU.
• Complete the practice sheet for lesson 06.
• Take pictures, record videos for documentation.

## 7. Practice sheet

Each student downloads the practice sheet here, photo it and bring it with you when you practice.

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Posted by Vinh Le

Composer at ITLearningCorner.