The power supply

All the devices connected to the motherboard, need to be supplied with the electrical current for their operation. The power supply is the component of the computer that fulfils this function.

We take the electrical current from the electric supply network, to which we plug the PC Power Supply, which transforms alternating current into direct current, at suitable voltages for the computer devices. Therefore, although we normally refer to the Power Supply with that name, it does not generate electricity, it is not a source, but a transformer from alternating current to direct current, and from high voltage to low voltage.

Usually, a computer needs a 12-volt direct current to power the motors of devices, such as hard drives, and a 5-volt or 3.3-volt direct current for the different electronic components.

In the following figure, you can see the inside of an ATX power supply. The plates of the transformer can be appreciated in the figure (remember that it does not generate electricity but it transforms the alternating current into a direct current of lower voltage). You can also see the fan that helps dissipate the generated heat.

PC power supply
PC power supply

Types of power supply

It can be found different types of power supplies depending on the motherboard for which they have been designed. Here the form factor of the motherboard comes into play again, which we already saw in a previous post and which determines the type of components that can be connected to the motherboard.

The form factor determines the dimensions of the power supply, so it fits well in one box or another, which were sized taking motherboard type in mind. Thus, we will have larger sources that allow us to work with more power since it will be easier to dissipate the heat and place larger fans to facilitate dissipation. And others of smaller dimensions will have to work with less power, but will take up less space and will fit into smaller boxes.

AT Power supply

It is the oldest power supply type, designed to work with AT motherboards. Nowadays totally in disuse, the reason why I will not go into describing these power supplies. Although it is highly unlikely that you will come across one of them, you will be able to distinguish it quickly because it has a physical switch to turn it on or off.

ATX Power supply

This type of power supply are designed to work with ATX form factor motherboards (ATX, micro ATX, or FlexATX). They turn on and off electronically when we turn the computer on or off.

The differences between both types lie in the types of connectors used, and the power and voltages supplied.

ATX12V Power supply

New processors and motherboards designed for them need more power. For this type of motherboard, an improvement of the ATX Power Supply specification has been developed, baptized with the name of ATX12V Supply.

This power supply is equipped with an additional connector that allows it to supply more power to the motherboard, the P4 power connector. It is a 4-pin 12-volt power connector, which is the only noticeable difference from the traditional ATX Power Supply.

Today, on most modern power supplies, the P4 connector has been replaced by the EPS 12v connector, which is two P4 connectors put together. In any case, later in this post, we will go through the different connectors that we can find, and I will explain this connector in more detail.

ATX 12V power supply
ATX 12V power supply

SFX Power supply

They are power supplies with a format specially designed for compact equipment, so they will fit into small boxes. Having smaller dimensions than ATX, they require a more compact design to be able to reach a maximum power of up to 700 watts.

Formats comparison

ATX power supplies have mandatory measurements of 15 cm wide x 8.6 high and may have different lengths. SFX format is 12.5cm wide x 6.35cm tall. The depth dimensions may vary for the same model since what is limited for anchoring in the box are the width and height dimensions.

Due to the larger size of ATX power supply, they can reach a higher power, being able to reach up to 2,000 watts of power. On the contrary, the SFX power supply will not reach powers higher than 700 watts.

Power supply connectors

It is convenient to know which are the main connectors of the power supply, the voltages with which they work and the components of the computer that they power and connect to.

The motherboard power connector

The most important connector of the power supply is the one that powers the motherboard, providing direct current to the different integrated circuits of the motherboard. Among them is the CPU, the RAM, the chipset, etc.

Nowadays, you can still find the P1 connector, which is the one used in the old ATX motherboards, although we could consider it already obsolete. It is a 20-pin connector, which has the appearance and the voltages that I show you in the following figure:

P1 connector
P1 connector

Modern motherboards come with a 24-pin connector, so power supplies have evolved from the 20-pin P1 connector to the 24-pin ATX connector. This connector is normally supplied with a 20-pin main connector and an additional 4-pin connector. It is done on purpose to maintain compatibility with older motherboards, so if we came across an older motherboard with a 20-pin P1 connector, we could use the power supply.

The 24-pin ATX connector looks like this:

ATX-24 pins connector
ATX-24 pins connector

As can be seen in the image above, most of the connector pins are designed to supply different voltages: 3.3V | 5V | 12V | -12V , or the COM position that would be assimilable to the neutral or reference.

But there are two pins that I have marked in the figure and I want to highlight:

  • Pin 9 supplies 5V to the Standby rail, which supplies power to devices left on standby. This pin remains to supply this voltage even with the computer turned off, as long as the power supply is connected to power.
  • Pin 16, Power Supply ON, is used to turn the power supply on or off electronically, without the need for a mechanical switch.

The connector to power the CPU

Today’s power supplies incorporate an eight-pin connector known as an EPS 12v to provide power specifically to the CPU. In quite a few cases it comes as two 4-pin connectors. These two 4-pin connectors are the P4 connectors that I presented earlier in this post, and the reason for keeping them is that there are still quite a few motherboards that incorporate this connector instead of the EPS 12V.

EPS 12V connector
EPS 12V connector

The connector to power the internal devices

To power internal hard drives and some expansion cards, a 4-pin connector known as Molex was designed. This connector can still be found but is deprecated and many power supplies no longer include it.

The Molex connector had a smaller version, called a mini Molex, which was used to power floppy disk drives, which is why it was also called a Floppy connector. This version is obsolete and it would be very rare to find it in a power supply today.

Molex and mini Molex connectors
Molex and mini Molex connectors

The connector that power supplies use today to supply power to these internal components is the SATA connector. Its appearance is as follows

SATA connector
SATA connector

This connector provides voltages of 3.3V, 5V and 12V, thus covering all types of internal devices.

Connector to devices with extra power needs

Sometimes, we are going to find components that require higher power for their operation. Such can be the case with some very powerful graphics cards. For these situations, we have the 6-pin PCI-E connector. Some graphics cards may require an additional 2-pin connector, so the PCI-E connector is usually delivered in a 6+2-pin configuration

PCI-E connector
PCI-E connector

It is used for components that require high power, so it provides a voltage of 12V

Power of a power supply

The purpose of the power supply is to power the different components of the computer with the power they require to properly operate at all times.

Selecting our power supply, we will put ourselves in the worst case, so that it can supply the necessary power in all situations. And what would this situation be? Simply that all the components were working simultaneously and power had to be provided to all of them at the same time. I recommend adding some extra power, a small margin that would allow to expand the computer, install a new component, for example, a second hard drive.

Power management

However, the situation described above is not nearly the most common, for most of the time the computer is on, there is going to be some components without using. So, why continuously supply power to these idle components? After all, we are consuming energy that we do not use, we are wasting energy.

Power Management is the functionality of power supplies that solves the issue described in the previous paragraph. Simply, what this function does is turn off the hardware that is not needed at that moment, that is not being used.

I imagine that if you have read up to this point you are thinking about the hibernation state when our computer seems to shut down but it is not really. Hibernation mode is also controlled by this functionality: power management.

To finish, simply comment that this functionality is not found in the old AT power supplies, so I refer to ATX supplies at all times.

Electric protection

I have already commented previously that the power supply has nothing to do with a source, in reality, it does not generate electricity, but rather adapts the supply of the electrical network to the voltages and powers that the different components of the computer need. It is, therefore, a transformer.

In addition to transforming the electrical current to the appropriate values for the components of the computer, the power supply protects them from voltage variations that occur in the electrical supply and that would seriously damage the components. For this, they have systems that filter these unwanted voltage peaks.

Although the protection provided by the source in a stable environment such as the domestic one is more than sufficient, in much less stable environments, we will have to use additional protection components. I am going to introduce two components that fulfil this purpose, the two most common in this type of unstable environment.

  1. Voltage Regulator: As its name indicates, this component maintains the average voltage values at adequate levels. In any case, we are talking about average voltage values, when it comes to very short voltage peaks, the voltage regulator has no effect and we will have to use a peak suppressor to protect ourselves.

Logically, the regulator is placed between the electrical power supply and the equipment.

  1. Peak suppressor: This component complements the voltage regulator, protecting us from short voltage peaks, which the regulator could not do.

Uninterruptible Power Supply

Another piece of equipment related to the supply of energy that should be known is the Uninterruptible Power Supply (UPS).

So far I had talked about equipment that protected the computer from the instabilities of the supply of electrical current, but nothing like protecting our data against a power failure. Well, the UPS fulfils this function. It makes our equipment continue receiving power when we run out of electrical current, and in the process, it protects our computer and data.

I guess you’ll say, well this is a battery, like a laptop. In a way it is so, although it is a power supply with a battery that comes into operation when the electrical current is cut, providing us with a very valuable time to close all the applications and save all the data, avoiding losing the work done.

We will use it with equipment with a certain degree of criticality, usually servers. In class, this is all I explain, since I only pretend that the students understand the concept of UPS. But for those who want to dig a little deeper, you can investigate the types of UPS (offline, Inline and Online).


This post is part of the collection “Computer Architecture” that reproduces my class notes. You can see the index of this collection here.