Power supply. What is psu in computer

In simple words for buyers, look for continuous power indicators, ignore peak power indicators and ignore a product that does not advertise its continuous power rating. If you can’t tell which one it is, don’t take chances. Just get moving.

Anatomy of a Power Supply Unit (PSU)

Every desktop, console or laptop has one of them. It does not increase the frame rate or generate cryptocurrency; it has no billions of transistors and is not made using the latest semiconductor processing node. Sounds boring, right? Totally not! This thing is very important because without it our computers would do absolutely nothing.

Power supplies don’t make headlines like the latest processors do, but they’re amazing technology solutions. So let’s put on our gowns, masks, and gloves and open up a modest power supply – taking its various parts apart and seeing what each piece does.

Anatomy of the TechSpot hardware series

You can have a desktop computer at work, school, or home. You can use it to compile tax returns or play the latest games; you can even build and upgrade computers. But how well do you know the components that make up a computer?

What’s the name of the game?

Many computer parts have names that require a bit of tech knowledge to understand exactly what it does (e.g. SSD), but in the case of a power supply, it’s pretty self-explanatory. It’s a unit. Provides power!

Since we can’t just vacuum our hands and proudly say “article ready” with that statement, we’d better start looking at one. We use the Cooler Master G650M – it’s a fairly general design, the specs of which can be found in many similar ones, but has one particular feature that not every power supply has.

This power supply is standard size, which means it is compatible with ATX 12V v2.31 format, so it fits in many computer cases.

However, there are other shape factors: those for smaller boxes or unique to specific vendors. Not every unit is exactly the size specified by standard aspect ratios, they can be the same width and height, but they can be longer or shorter.

Cisco Power Supply – Specially designed for rack-mounted servers

They are also usually labeled as the maximum power they can deliver; in the case of Cooler Master, it can deliver up to 650 watts of electrical power. We’ll see what that actually means in this article, but you can get power supplies that only deliver a small number of watts as not everything computer requires hundreds of watts to run. However, most desktops will perform well in the 400-600W range.

Power supplies like this are housed in a metal box, usually black or bare metal, so they can be heavy. Laptops almost always have a power supply that is located outside the computer and is almost always plastic, but the inside is very similar to what we will see in this.

Most desktop power supplies come with an AC isolating switch and a fan to keep things cool and nice, but not all of them work (or have to). Not all of them will have a metal body full of holes – those found on servers rarely have them.

But as you can see in the picture above, we already took a screwdriver for our example, so let’s tear off the cover and jump in.

You can have a desktop computer at work, school, or home. You can use it to compile tax returns or play the latest games; you can even build and upgrade computers. But how well do you know the components that make up a computer?

Where is the power supply located in a computer?

The power supply is located on the back of the computer, usually on the top. However, many newer tower PC cases include a power supply in the bottom of the case. In an all-in-one desktop computer case, the power supply is located on the rear left or rear right.

Below is a list of the parts you can find on the back of the power supply.

  • Connecting the power cord to the computer.
  • Fan opening for heating from the power supply.
  • Red switch to change the supply voltage.
  • Rocker switch for turning the power on and off.

There are several cables on the front of the power supply, which is not visible unless the computer is open. These cables connect to the computer’s motherboard and other internal components. The power supply connects to the motherboard via an ATX type connector and may be equipped with one or more of the following cables to connect the power to other devices.

Parts found inside a power supply

Below is a list of the parts inside the power supply.

  • A rectifier that converts AC (alternating current) to DC.
  • A filter that smoothes the direct current (direct current) coming from the rectifier.
  • A transformer that controls the input voltage by increasing or decreasing it.
  • A voltage regulator that controls the DC output, enabling the correct amount of power, volts or watts to be delivered to the computer equipment.

The sequence of operation of these internal components of the power supply is as follows.

  1. Transformer
  2. Rectifier
  3. Filter
  4. Voltage regulator

Try the Outervision extreme power calculator or the Cooler Master power calculator. If you know what you are doing, the Cooler Master version is better, but if you are unsure what parts the calculator is asking for, stick with the Outervision Basic Calculator.

Input Voltage

Some power supplies, mostly older ones, may have a switch on the back. The switch has two positions 115V or 230V. This is because the power output varies from country to country. For example, in the USA the socket supplies 115V, and in the EU countries 230V. If you don’t know what is the output power in your country, you can check it on this page or ask a neighbor.

This means that if you live in the US (or wherever the output is near 115V) you need to switch the power supply to 115V, and if you live in the EU (or wherever the output is near 230V) you need to switch to 230V.

Now the important part, if the power supply doesn’t have a switch on the back, it doesn’t mean it can use any voltage automatically. You need to make sure your particular power supply is able to use the output you have. You can check it in the manual or other documentation related to the power supply. It can be around 115V or 230V and can only use close voltage.

However, most new power supplies can use any voltage from ~ 90V to ~ 260V. Before purchasing a new power supply, check what advertisers are saying about the voltages it may be using. Otherwise, you can burn your computer or even your home.


The power supplies come with several different cables. The various cables can seem very complicated at first, especially if you haven’t built a PC before. Don’t worry as this can be confusing at first, they are actually very simple and easy to understand.

In this post, we won’t delve into powers or anything specific, I’ll just cover what cable goes into which socket and why. Once you know this, you can dig deeper and learn more about cables.

Fully modular power supply

Fully modular PSU connector sockets

PC main connector or ATX connector

The main cable is a 24 pin power cable to the motherboard. This cable just powers the motherboard, it is the largest cable and is connected to the largest connector on the motherboard. It is easy to locate, it is located on the side of the motherboard. Most motherboards have a 24-pin connector, but some motherboards can have a 20-pin connector. Therefore, the motherboard power cable that comes with the PSU is typically a 20 + 4-pin connector cable, so it will fit both 24-pin and 20-pin connectors.

P4 or EPS connector

The next cable gives the CPU the power it needs, as the motherboard can no longer provide enough power to the CPU, this is done with an external cable straight from the power supply. However, this cable is attached to the motherboard near the CPU socket. Typically, you will see a “CPU” sign on the motherboard to indicate that it is a socket for the CPU power cable.

On high-end motherboards that allow overclocking, the cable is typically 8-pin, which allows it to deliver enough power. Regular, cheaper motherboards usually only have a 4-pin connector. Power supplies can have both 8 pin and 4 pin cables, or more commonly just an 8 pin cable that can be split as needed.

PCI-e connectors

PCI-e cables provide power to devices connected to PCI-e expansion slots on motherboards. This is because the PCI-e interface cannot provide enough power to some devices, mainly graphics cards. These cables are connected directly to the device and not to the motherboard as the cables said before. The number of pins needed on the PCI-e cable varies depending on the device used. High-end graphics cards require more power, more power can be delivered with more pins, just like that.

The PCI-e cables used are 6-pin, 6 + 2-pin and 8-pin. Low-end graphics cards only require a 6-pin connector, and some high-end enthusiast graphics cards may need two 8-pin connectors to have enough power. Most often, mid-range cards and graphics cards need an 8-pin power cable to have sufficient power. This is usually done with a 6 + 2 pin cable. Make sure what your graphics card needs before connecting any cables to be sure.

SATA power connector

SATA power connectors provide power to drives, SSDs and HDDs. The new hard drives now use SATA cables, which greatly simplifies power cabling as it is likely that all you need to do is use SATA power cables. The SATA power cable is L-shaped and can only be connected one way, so you don’t have to worry about connecting it the wrong way round. It is similar to the SATA data cable, the L-shaped data cable is only shorter. The SATA power cable connects directly to the storage device.


When looking for a power supply, look for three key characteristics: output power, rails and efficiency. Other specifications and features are also important, but these three have a direct impact on the performance of the power supply.

Save Energy With True Rated PSUs

As our society needs constant energy, buying environmentally friendly electronics helps our planet. Even if you don’t care about nature or use a 100 percent green energy supplier, a true rated power supply will still save you a lot of money on your energy bills.

So what is a truly rated power supply? During operation, the power supply draws AC from the wall outlet and converts it to DC, which is then sent to all parts.

Typically, the power supply wastes some energy in the conversion process. Therefore, the performance of a power supply depends on how much it can process and how little is wasted.

Therefore, an 80 percent efficiency indicates that it can convert 80 percent alternating current to direct current. Likewise, a 50 percent efficiency indicates it converts 50 percent AC to DC. Put simply: a higher percentage of efficiency is better and requires less power from the outlet.

The most efficient power supplies are those rated 80 Plus by an independent certifier. Even in the 80 Plus power supplies, there are different levels: 80 Plus, 80 Plus Bronze, 80 Plus Silver, 80 Plus Gold, 80 Plus Platinum, 80 Plus Titanium. (They are ordered from worst to best.)

An additional benefit of these efficient power supplies is that they generate significantly less heat than other power supplies, and tend to operate at lower volumes as well. Manufacturers are proud to advertise 80 Plus Certified PSUs, but if you’re having trouble finding the information, check out the full list of 80 Plus PSUs.

Figuring Out the Minor Details

Until now, everything was only about the bases of the power supplies. As with any technology, you can fool around and get more detailed information on what you want or need, but if you are a beginner, the above-mentioned three aspects will be of most importance to you when making your purchasing decision.

That being said, there is another lingo that you may come across when purchasing a PSU. Some of them are of little importance to new PSU users, and some of them are very important; therefore, it is a good idea to learn what to ignore and what to look for.

  • AT vs. ATX vs mATX: Sometimes you will see power supplies designed for “AT”, “ATX” or “mATX” computers. These terms are used to describe motherboards of different sizes and designs, and therefore have different energy requirements. Check carefully which motherboard you have (it should be listed under “form factor” in the specification) and purchase the correct power supply.
  • Rails: multi-rail or monorail power supplies can be obtained. Both have their pros and cons, and you don’t have to worry about the technical details right now. If you live in an area where power fluctuations or interruptions are normal, you should consider a multi-path connection. For any other scenario or if you are using a good uninterruptible power supply (UPS), get a single rail power supply.
  • Voltage Stability: If you check all of the above boxes, voltage stability will not be an issue. This basically refers to the ability of the power supply to hold 12 volts without loss.
  • Cabling or Connectors: Unless you are purchasing a high-end specialized power supply, you should be fine with the cabling that comes in the box. High-end power supplies offer what is called “modular cabling,” which allows you to mate custom cables and plug connectors to connect parts to them. Not important for the average user.
  • Repair of units and accessories: You don’t need a power tester or guides to repair the power supply. If you do diagnose a problem with your PSU your only option is to have it replaced and hopefully it’s still under warranty.

Never open the housing of the power supply. It contains capacitors capable of maintaining a strong electric charge, even when the computer is turned off and disconnected for an extended period of time.

Power Supply Unit Description

Photo of the Corsair Enthusiast TX650 V2 ATX12V EPS12V power supply

The power supply is mounted directly on the back of the housing. If you follow the computer’s power cord, you’ll find that it is plugged into the back of the power supply. The back is usually the only part of the power supply most people will ever see.

There is also a fan opening on the back of the power supply that sends air out on the back of the computer case.

The side of the power supply, facing the outside of the chassis, has a male, three-pin port into which you attach the power cord that is plugged into a power source. Often there is also a power switch and a supply voltage switch.

Large bundles of colored wires exit from the opposite side of the power supply to the computer. The connectors on opposite ends of the cables connect to various components inside the computer to supply them with power. Some are specially designed to connect to the motherboard, while others have connectors to fit fans, floppy drives, hard drives, optical drives, and even some high-powered graphics cards.

Power supplies are rated by wattage to show how much power they can provide to a computer. Since every part of your computer requires a certain amount of energy to function properly, it’s important to have a power supply that can provide the right amount of power. The very useful Cooler Master Supply Calculator tool can help you determine how many you need.

ATX vs ATX12V Power Supplies

ATX and ATX12V are configuration specifications that are important to distinguish between power supplies. For most people, the noticeable difference is only in the physical connection plug on the motherboard. Which one you choose depends on the type of motherboard you are using.

The latest standard, ATX12V v2.4, has been in use since 2013. Motherboards using ATX12V 2.x use a 24-pin connector. ATX motherboards use a 20-pin connector.

One of the situations where pin count matters is deciding if a particular power supply works with your system. Power supplies that comply with the ATX12V standard, while having 24 pins, can actually be used on an ATX motherboard that has a 20 pin connector. The remaining four pins will simply move away from the connector. If your PC case is the case, this is a perfectly doable setup.

However, it doesn’t work the other way around. If you have an ATX power supply that has a 20-pin connector, it won’t work with a newer motherboard that requires all 24 pins to be plugged in. An additional four pins have been added to this specification to provide extra power through the 12V rails, so a 20 pin power supply cannot provide enough power to support this kind of motherboard.

ATX is also the term used to describe the size of the motherboard.

Something else that makes ATX12V and ATX power supplies different is the power connectors they provide. The ATX12V standard (from version 2.0) requires a 15-pin SATA power connector. If you need to use a SATA device, but the power supply doesn’t have a SATA power connector, you’ll need a Molex 4-pin to SATA 15-pin adapter (like this one).

Another difference between ATX and ATX12V is the energy efficiency rating, which determines how much energy is drawn from the wall compared to the computer’s output power. Some older ATX power supplies have efficiency below 70 percent, while the ATX12V standard requires a minimum rating of 80 percent.

An additional benefit of these efficient power supplies is that they generate significantly less heat than other power supplies, and tend to operate at lower volumes as well. Manufacturers are proud to advertise 80 Plus Certified PSUs, but if you’re having trouble finding the information, check out the full list of 80 Plus PSUs.

Form factor – Will your power supply fit?


Another point is simple – you need to choose the shape that will definitely suit your case. Fortunately, there are standards for power supplies, such as for cases and motherboards.

This topic can be quite complicated, but it’s important to remember that you’ll want to match the power supply with your case and motherboard. Below is a basic overview of the most important power factors today.

While AT form factor power supplies are still available, AT form factor power supplies are undoubtedly obsolete products that are coming out soon. Even later ATX power supplies (ATX 2.03 and earlier versions) fall out of favor. The main differences between ATX and AT power supply cases are:

  1. ATX power supplies provide an additional + 3.3V voltage bus.
  2. ATX power supplies use a single 20-pin connector as the main power connector.
  3. ATX power supplies support a soft shutdown feature, allowing software to turn the power off.


Currently, the popular choice is the ATX12V format. There are several different versions of the ATX12V format and they can be very different from each other. The ATX12V v1.0 specification added a 4-pin + 12V connector from the original ATX format to supply power to the CPU only, and a 6-pin auxiliary power connector providing +3.3V and +5V. Later ATX12V specification v1.3 added a 15-pin SATA power connector on top.

A significant change has been made to the ATX12V v2.0 specification which changed the main power connector from 20 pin to 24 pin by removing the 6 pin auxiliary power connector. In addition, the ATX12V v2.0 specification also isolated the current limitation on the 4-pin CPU power connector for the 12V2 bus (the + 12V current is split between the 12V1 and 12V2 buses). Later, the ATX12V v2.1 and v2.2 specifications also increased the performance requirements and introduced a number of other improvements.

All ATX12V power supplies keep the same physical shape and size as the ATX format.

EPS12V, SFX12V, and Others

overview of cryorig taku computer cases (3)

The EPS12V power supply uses the CPU’s 8-pin power connector in addition to the 4-pin connector in the ATX12V housing (not the only difference between the two cases, but most desktop users realize that should be enough). The EPS12V form factor was originally designed for entry-level servers, but more and more high-end desktop motherboards are now equipped with an 8-pin EPS12V CPU power connector, allowing users to choose an EPS12V power supply.

Small Form Factor (SFF) is used to describe a number of smaller power supplies such as SFX12V (SFX stands for Small Form Factor), CFX12V (CFX stands for Compact Form Factor), LFX12V (LFX stands for Low Profile Form Factor), and TFX12V (TFX stands for Thin Form Factor). They are all smaller than standard ATX12V power supplies in terms of physical size, and the small size power supplies must be installed in suitable small computer enclosures.


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The power supply is useless if it does not connect and supplies power to all components of the computer. This means that it must have all the required types of connectors.

The first connector to consider is the main connector that powers the motherboard. This connector comes in two types, 20 pin and 24 pin. The latter is becoming more and more popular, and your PSU will likely provide both options. Just check to be sure.

Next up is the CPU power connector, which comes in 4-pin and 8-pin versions. As with the main power connector, many modern motherboards have switched to a larger format. Make sure your power supply is compatible again.

The most commonly used power connector is the 4-pin Molex connector. It is used for a variety of components including legacy hard drives, optical drives, fans, and some other devices. Newer SATA components have their own SATA power connector, and if you run out of one, you can also use Molex to SATA adapters. You can even use splitters to increase the number of components you can plug in, but be mindful of the power limits.

The mains socket for the power supply is in the upper left corner of the picture, and the power runs clockwise around the picture all the way to the power supply output (large group of colored wires, lower left corner).

The great rail debate

In addition to identifying the output power, manufacturers provide the number of +12 V rails that their power supplies contain. The single-rail power supply has a single, high-power + 12V rail to power hungry system components. A multi-rail unit shares its output power between two or more + 12V lines.

In a single busbar design, all power from the power supply will be available to any component connected to the device, regardless of the connector or cable used. However, in the event of a monorail failure, a monorail power source can shoot significantly more current into your components.

An affordable non-modular PSU

Thermaltake Smart 500W 80+ White Certified PSU

Thermaltake Smart 500W 80+ white certified power supply

Meanwhile, the main disadvantage of a multi-busbar power supply is that it cannot share power between the different buses. For example, if you connect 25 amp components to a +12 volt bus with a maximum rating of 20 amps, the mismatch will start the Overcurrent Protection Mechanism (OCP) and shut down, even though other rails may be available with plenty of power to save. Therefore, when using a multi-rail power supply, you need to pay attention to which components you have plugged into which rail, which is a minor inconvenience that you don’t have to worry about with a monorail power supply.

On the other hand, this disadvantage becomes your major advantage if you ever encounter a catastrophic failure. The OCP mechanisms in the multi-bus power supply monitor each bus and shut down the entire unit if it detects an overload on any bus. The OCP on monorail units is only activated at much higher intensities, which can lead to severe melting in the event of severe overload.

So which type of power supply is better – monorail or multi-rail? Neither usually. From a performance standpoint, both work equally well; and in general both are very safe to use. However, if you’re building a particularly powerful system, the multi-rail OCP provides an extra layer of safety in the event of a short circuit, reducing the risk of frying expensive components in a computer disaster.

Cabling: Piecemeal or whole hog?

The Corsair Corsair HX850 is a partially modular power supply…

Another issue is the wiring. The power supplies are available with wired, semi-modular or fully modular cabling. In modular power supplies, you can add or remove cabling from the power supply as needed to avoid clutter in the case.

From a technical point of view, a permanently wired power supply is optimal as it does not require any additional connections between the device’s internal PCB and the connector that will eventually be connected to one of your components. One end of the cable is soldered to the power supply board, and the other is terminated with a standard connector, with no line breaks. Every time you introduce an additional connection between the power supply and components – as is the case with modular power supplies – you add more resistance and another potential point of failure in the line; and any increase in drag translates into a loss of efficiency.

Corsair… while the AX860i is fully modular.

That said, the additional resistance is usually minimal and is not a cause for concern for most users. Meanwhile, modular cabling makes it much easier to keep the inside of the case tidy and clean – just don’t plug in unnecessary cables to keep it messy. Most people prefer modular power supplies, although they cost slightly more than non-modular models.

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