Michael Angstadt's Blog: The Power is Yours!

Every computer has a box called a power supply, which is responsible for supplying electricity to the internal components of the computer. Its main task is to convert the AC (alternating current) power from the electrical outlet to DC (direct current) power, and then dole out the DC power to the computer's internal components. Different parts of the world use different voltage standards for their electrical outlets, so a power supply has to be compatible with the voltage standards in your part of the world. For example, power outlets in North America run at around 115V, and those in Europe generally run at around 230V. Some power supplies have a physical switch on the outside that tell it what voltage to expect (called fixed-input). Others will adjust automatically (called auto-switching).

Due to the nature of AC power, power supplies can take damage over time from something called harmonics. Harmonics is caused by the way in which electrical devices draw power from an AC connection, and is what causes electrical devices to make faint humming sounds. Most power supplies come with circuitry that protect against this, called active power factor correction (active PFC). You should never buy a power supplies that does not have this.

I need more power, Captain!

Every power supply has a maximum amount of wattage it can draw. If the internal components of the computer try to draw more than that, the computer won't work right. For example, if you want to install a brand new, high performance graphics card, you should make sure your power supply has enough available voltage. Note that power supplies are replaceable, so if your current power supply isn't good enough, you can always replace it.

Power supplies do not use all of the AC power it consumes. Some power is lost due to inefficiencies and released in the form of heat. Most power supplies are at least 80% efficient, and they will advertise what their efficiency is on the packaging. A more efficient power supply will consume less power.

It's important to note that power supplies only draw the amount of energy that is actually being used by the computer—they do NOT draw the maximum amount they are capable of. For example, if you have a power supply can that provide a max of 500 W and your computer is only using 200 W, then the power supply will only draw enough power for 200 W. You won't be wasting electricity if you buy a power supply that can supply more power than your computer needs. In fact, it is good to have a such a power supply for two reasons: (1) To allow room for future upgrades and (2) to account for the fact that power supplies produce less wattage over time due to wear and tear.

Rails

The DC power that the power supply generates is doled out through three voltage rails. Each rail supplies a different voltage: 12V, 5V, and 3.3V. The 12V rail is typically used to power devices that have motors of some sort, such as hard disk drives and optical drives, but there is no restriction regarding what each voltage rail can be used for (for example, a high-end graphics card might want to use the 12V rail).

Each rail has a maximum amount of amperage it supports, and this is monitored by circuitry called over-current protection (OCP). Single-rail systems have a single OCP that monitors all the rails. Multi-rail systems have one OCP per rail to monitor each rail. If the amperage in any rail is exceeded, the power supply will shut itself off to prevent damage to itself. When multi-rail systems were first introduced, they were very unstable due to poorly written specifications, but they have gotten much better since then. For computers that use a lot of power, like servers and gaming PCs, multi-rail systems give your system extra protection against short-circuits. For an ordinary, low-wattage desktop PCs, it doesn't really make a difference whether you have a single-rail or multi-rail system.

Power supply standards

Various power supply standards have been released over the years. ATX (also called ATX12V) introduced the idea of providing a constant supply of power (5V) to the motherboard, even when the computer is off. This is called soft power, and it allows the computer to implement various power saving features. This is the reason why you always should always unplug a computer before servicing it! This standard was later improved upon by subsequent standards (below).

ATX12V 1.3 added the P4 connector, which supplies extra power to the motherboard. It also added the AUX connector. The downside to this standard was that it was not specific enough, which resulted in power supply manufacturers producing wildly different power supplies.

EPS12V was created for servers that need more power than the average desktop machine. It added a 24-pin motherboard power connector. It also introduced the idea of “voltage rails” (explained above).

ATX12V 2.0 adopted many of the advancements that EPS12V brought to the table. Notably, it added a 24-pin P1 connector and voltage rails.

Connectors

Many of the different connectors you will see coming out of a power supply are listed in the table below. Yeah! Tables!

Connector	Voltages	Pins	Description
P1 power connector	3.3V, 5V, 12V	20/24	The older variant of this connector has 20 pins. The newer variant (which is backward compatible) has 24 pins and provides more current.
Molex	5V, 12V	4	Typically used to power storage devices, like hard drives.
Mini	5V, 12V	4	This connector used to be used for 3.5” floppy disk drives and isn't used much anymore. You have to be careful when plugging in this connector because it is easy to plug in upside down, which will ruin the device.
SATA power connector	3.3V, 5V, 12V	15	Only used for SATA hard drives. In practice, only the 5 V and 12V voltages are used.
SATA slimline connector	5V	6	A smaller version of the SATA power connector.
SATA micro connector	3.3, 5V	9	Even smaller! Can't reliably find a photo of this one.
P4 connector	12V	4	Used in conjunction with a 20-pin P1 connector to supply the motherboard with extra power.
AUX connector	3.3V, 5V	6	Also used for supply the motherboard with extra power.
EPS12V EATX12V ATX12V 2x4	12V	8	This connector goes by many different names. One half is compatible with the P4 connector.
PCIe Connector	12V	6/8	In some 8-pin connectors, two of the pins are detachable so make them compatible with the 6-pin version. It looks similar to the EPS12V connector, but is not compatible with it.