How a computer power supply works

This report is to help you understand how your computer's power supply works, how it converts the AC current to DC current and how it makes the power available for all computer components.

For a computer to operate, it requires different amounts of power delivered to its different components. To fully understand this report, I will go over some key terms that will be used throughout.

AC current: Stands for Alternate Current. The current varies in cycles through time. Click here for a graph and more explanations.
DC current:
Stands for Direct Current. It is a steady stream of current over time. Click here for a graph and more explanations.
Ampere: Stands for the charge per second in the electricity. Click here for more explanations.
Watt: The rate that energy is produced. Click here for more explanations.

The power supply is used to transform the 110 volt AC current down to three different voltages of DC current: they are 3.3, 5 and 12 volts. The computer's components work on one of the three different voltages. Most digital components use either 3.3 or 5 volts, whereas fans or motors use 12 volts to function. There is always a continuous flow of 5 volts that is going to the computer's power button to be able to start your machine when it is off. This 5 volts is called the standby voltage or VBS.

This picture shows the use or the different outputs of this power supply along with mentioning some of the wire colors

Converting AC to DC
To be able to change the AC current to DC, you must augment it to a much higher frequency, many more cycles per second. This is achieved by passing the current through two fairly large transistors. Increasing the AC frequency also helps later on to filter the current and rectify any problems with it. Once the required frequency is reached, the current goes on to one of the three tranformers that reduce the voltage from 110 to the required voltage. This reduction must be precise because some components are very sensitive. The final step to getting DC current is to pass the current through diodes that convert the AC current into DC current.

Industry standards
To make it easier for customers, there have been some standardization on size, connectors and color codes. The form for the majority of personal computers is the ATX form. The ATX standard assures clients that the power supply will fit in any ATX case and connect to all ATX motherboards without problems. The different connectors are designed to only fit one way and only in the proper plug. The wires are also color coded to help facilitate plugging them in correctly.

Different connectors
To reduce the chance of making bad connections, each plug is designed to only fit in one specific slot and also to only fit one way. I will briefly talk about the most common connectors that supply power to different parts of the motherboard and also to drives and fans. Click on the link below to view a table of the different types of connectors, the amount of power they have and what they are used for. View the table

Wattage demands
Here is a table of how many watts some of the computer components need to operate normally. These amounts can change depending on the different components and their power requirements. When choosing a power supply, you should always get a power supply that produces more wattage then you need, this creates less stress on the electronic parts if they don't have to function at 100%.

PC Item Watts
Accelerated Graphics Port (AGP) card 20 to 30W
Peripheral Component Interconnect (PCI) card 5W
small computer system interface (SCSI) PCI card 20 to 25W
floppy disk drive 5W
network interface card 4W
50X CD-ROM drive 10 to 25W
RAM 10W per 128M
5200 RPM Integrated Drive Electronics (IDE) hard disk drive 5 to 11W
7200 RPM IDE hard disk drive 5 to 15W
Motherboard (without CPU or RAM) 20 to 30W
550 MHz Pentium III 30W
733 MHz Pentium III 23.5W
300 MHz Celeron 18W
600 MHz Athlon 45W











Troubleshooting power supply problems
Since the power supply is the link for providing power between your electrical outlet and the different computer components, problems with your power supply can render your computer unusable. Here are a few tips that you can use to try and solve the problem. Please note that before attempting to do anything with your power supply, unplug the power cord that goes to that wall and wait for a short period. Even when the system is off there is electricity flowing in some of the wires, and especially in the capacitors.

-If your power supply fan no longer works, you should consider changing either the fan on it, or the entire power supply. Or else it could lead to the following problem.

-If you smell a burning odor and your computer automatically shutsdown not long after, this could mean your power supply has overheated, frying the components inside it. If this is the case, you will need to change your power supply.

-If your computer does not start after an electrical storm, the fuse in your power supply may have blown. If this is the case, you'll need to open up your power supply and simply change the fuse inside. Note that not all power supplies have fuses.

-If your system does not turn on at all, check to make sure the power cord is plugged into the wall and into the power supply.


ATX Power Supplies - ATX Power Supplies
Electronic Engineering - Power Supply Connectors
How Stuff Works - How PC Power Supplies Work
The Tech Board - PC Terms

Copyright© 2005 - Matthew Stevens,