Power Supply Units

Power Supply Units provide almost all the power to the computer. They convert the electrical current from a wall socket/plug (AC) to amounts that are usable (DC) inside your computer.

There are 2 types of power, AC (alternating current) and DC (direct current).

Alternating Current (AC) Alternating current describes the flow of charge that changes direction periodically. As a result, the voltage level also reverses along with the current. AC is used to deliver power to houses, office buildings, etc.

example of alternating current as water flow

Direct current is a bit easier to understand than alternating current. Rather than oscillating back and forth, DC provides a constant voltage or current.

Generating DC

DC can be generated in a number of ways:

An AC generator equipped with a device called a "commutator" can produce direct current
Use of a device called a "rectifier" that converts AC to DC
Batteries provide DC, which is generated from a chemical reaction inside of the battery

Uninterruptible Power Supply (UPS)

These are used for protection against:

Blackouts - a complete loss of power in an area. Can last for a few minutes to a few weeks.
Brownouts - a drop in voltage that can cause poor equipment performance or device failure.

A UPS contains a battery. If the voltage drops or is completely cut, then the battery kicks in and will power the computer. If this happens you shouldn't just keep running your computer, but save anything you are working on and shut down the computer.

Surge Protectors

Most of these work using a component (or components) called an MOV (Metal Oxide Varistor) which essentially soaks up (or shorts) the surges.

Each time the MOV in the board soaks up a surge it reduces it’s lifespan. In some extreme cases, such as a lighting strike, the MOV can die in one surge. Once the MOV dies, the board needs to be replaced. In some power strips when the MOV is used up / dies the board stops working entirely. In others, it continues to work without any surge protection leaving your equipment vulnerable.

Most good surge protection power strips will include an indicator light to allow you to monitor whether the surge protection is still working or if the board needs to be replaced. If the light is blinking or burned out your surge protector is no longer protecting your PC.

Standard ATX power supply. There is also an AT power supply. These look the same, but are a little smaller.

Modular ATX power supply. Same function as the standard version, but you can connect only the cables you need. These are excellent for cable management and reducing the number of wires inside your computer.

ATX Power Connector

Provides power to the motherboard. There are 3 versions - a 20 pin, a 24 pin, and 20+4 pin version.

Molex Connector

These are used to connect peripherals like hard drives and CD drives. It is an older, but can still be used for fans, video cards, or case lighting.

SATA Power Connector

Provides power to devices such as hard drives and CD drives.

ATX 4-pin Connector

Usually used for the CPU. If this isn't plugged in the computer will not start.

8 Pin PCI-E connector

Used to power video cards.

4-pin Floppy Drive Connector

Used to power old floppy drives, and sometimes AGP graphics cards.

buying a power supply

What should you consider when you buy a PSU?

Size - most computers will use the ATX form factor
Wattage - This is the amount of power a computer needs when under full load. Full load is when a computer uses more energy while watching a 4K movie or running a video game.
Continuous Wattage vs. Peak Wattage - If you have a 500w continuous PSU then it will run at 500w for however long you need it to. If it says 500w peak then it will only run for a few mins at 500w before lowering the wattage.
Energy Efficiency - Look for energy efficient PSU's. These will save you money.
Never go cheap! Spend the money for a good PSU and it will last for years without any problems.

You can determine the power required for you computer by entering what components and devices your computer is using on a website like this.

It's always important to have MORE power than what you would need. This prevents periodic system failures, and ensures you have enough power for future upgrades.

Troubleshooting

A bad power supply can cause intermittent lockups and reboots, as well as intermittent startup difficulties.
Bad power supplies erase CMOS information and sometimes even erase data on mass storage devices.
Not enough power can cause your computer to lockup or shutdown. This will usually only happen when it's under full load. For example while playing a game or watching a movie.
Lights on your motherboard indicate that electricity is flowing from the PSU. If these are off there's a good chance the PSU isn't working.
If you suspect a problem with the power supply, always try swapping the cord first.

Power Supply Unit Questions

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