Power supply of computer: What is Power Supply?

How PC Power Supplies Work


This budget gaming PC includes a Cooler Master MasterBox Lite 3.1 case, a Gigabyte B360M D3H motherboard, an Intel Pentium Gold G5600 CPU and an EVGA BT450 80+ Bronze power supply. Taken on July 30, 2018. Phil Barker/Future Publishing via Getty Images

If there is any one component that is absolutely vital to the operation of a computer, it is the power supply. Without it, a computer is just an inert box full of plastic and metal. The power supply unit, also known as a PSU, converts the alternating current (AC) line from your home to the direct current (DC) needed by the personal computer. In this article, we’ll learn how PC power supplies work and what the wattage ratings mean.

In a personal computer (PC), the power supply is the metal box usually found in a corner of the case. The power supply is visible from the back of many systems because it contains the power-cord receptacle and the cooling fan. A typical PSU will have integrated connectors to send power to the motherboard, microprocessors, and SATA storage. Laptops and mini-PCs usually have their power supplies separate from the computer assembly, instead of integrated into their charging cables.


Power supplies, often referred to as “switching power supplies”, use switcher technology to convert the AC input to lower DC voltages. The typical voltages supplied are:

  • 3.3 volts
  • 5 volts
  • 12 volts

The 3.3 and 5 volts are typically used by digital circuits, while the 12 volt is used to run motors in disk drives and fans. The main specification of a power supply is in watts. A watt is the product of the voltage in volts and the current in amperes or amps. If you have been around PCs for many years, you probably remember that the original PCs had large red toggle switches that had a good bit of heft to them. When you turned the PC on or off, you knew you were doing it. These switches actually controlled the flow of 120-volt power to the power supply.

Today you turn on the power with a little push button, and you turn off the machine with a menu option. The operating system can send a signal to the power supply to tell it to turn off. The push button sends a 5-volt signal to the power supply to tell it when to turn on. The power supply also has a circuit that supplies 5 volts, called VSB for “standby voltage” even when it is officially “off”, so that the button will work.



  1. Switcher Technology
  2. Power Supply Standardization
  3. Power Supply Wattage
  4. Power Supply Problems
  5. Power Supply Improvements


Switcher Technology

Prior to 1980 or so, power supplies tended to be heavy and bulky. They used large, heavy transformers and huge capacitors (some as large as soda cans) to convert line voltage at 120 volts and 60 hertz into 5 volts and 12 volts DC.

The switching power supplies used today are much smaller and lighter. They convert the 60-Hertz (Hz, or cycles per second) current to a much higher frequency, meaning more cycles per second. This conversion enables a small, lightweight transformer in the power supply to do the actual voltage step-down from 110 volts (or 220 in certain countries) to the voltage needed by the particular computer component. The higher-frequency AC current provided by a switcher supply is also easier to rectify and filter compared to the original 60-Hz AC line voltage, reducing the variances in voltage for the sensitive electronic components in the computer.


A switcher power supply draws only the power it needs from the AC line. The typical voltages and current provided by a power supply are shown on the label on a power supply.

Switcher technology is also used to make AC from DC, as found in many of the automobile power inverters used to run AC appliances in an automobile and in uninterruptible power supplies. Switcher technology in automotive power inverters changes the direct current from the auto battery into alternating current. The transformer uses alternating current to make the transformer in the inverter step the voltage up to that of household appliances (120 VAC).


Power Supply Standardization

Over time, there have been at least six different standard power supplies for personal computers. In the late 1990s, the industry settled on using ATX-based power supplies, with the latest version being ATX12V 2.0. ATX is an industry specification that means the power supply has the physical characteristics to fit a standard ATX case and the electrical characteristics to work with an ATX motherboard.

PC power-supply cables use standardized, keyed connectors that make it difficult to connect the wrong ones. Also, fan manufacturers often use the same connectors as the power cables for disk drives, allowing a fan to easily obtain the 12 volts it needs. Color-coded wires and industry standard connectors make it possible for the consumer to have many choices for a replacement power supply. If you don’t want to deal with so much cable management, you can also buy a non-modular PSU that comes with all its wires already attached.


Windows, Mac, and Linux systems use a string of code called the Advanced Configuration and Power Interface (ACPI) to control and monitor power consumption of components inside the computer. ACPI also decides where to send full, partial, or zero power while the machine is in sleep mode.

Power Supply Wattage


A larger power supply may be needed if you use every available slot on the motherboard. making_ultimate/Getty Images

A 400-watt switching power supply will not necessarily use more power than a 250-watt supply. A larger supply may be needed if you use every available slot on the motherboard or every available drive bay in the personal computer case. It is not a good idea to have a 250-watt supply if you have 250 watts total in devices, since the supply should not be loaded to 100 percent of its capacity. Repeatedly maxing out and exceeding a power supply’s given capacity will easily lead to overheating and multiple component failures. Don’t do it.

At the heavy-duty end, power supplies can be bought providing 2,000 watts of energy and beyond, but these are only useful for large servers and supercomputers. An average desktop computer consumes about 200 to 300 watts in use, while laptops and mini PCs are made to take 50 watts or less. Upgrading your machine with things like multi-core CPUs, GPUs, SSDs, larger RAM chips, and bigger fans will naturally require more energy consumption.


With that in mind, a higher-capacity power supply is a common supporting mod when installing new components. PC marketplace NewEgg has a handy calculator on their site that you can use to input various parts of your desktop build and get an estimate of its maximum power requirements.

Power supplies of the same form factor (“form factor” refers to the actual shape of the motherboard) are typically differentiated by the wattage they supply and the length of the warranty.


Power Supply Problems

The PC power supply is probably the most failure-prone item in a personal computer. It heats and cools each time it is used and receives the first in-rush of AC current when the PC is switched on. Typically, a stalled cooling fan is a predictor of a power supply failure due to subsequent overheated components. All devices in a PC receive their DC power via the power supply.

A typical failure of a PC power supply is often noticed as a burning smell just before the computer shuts down. Another problem could be the failure of the vital cooling fan, which allows components in the power supply to overheat. Failure symptoms include random rebooting or failure in Windows for no apparent reason.


For any problems you suspect to be the fault of the power supply, use the documentation that came with your computer. If you have ever removed the case from your personal computer to add an adapter card or memory, you can change a power supply. Make sure you remove the power cord first, since voltages are present even though your computer is off.

Power Supply Improvements

Recent motherboard and chipset improvements permit the user to monitor the revolutions per minute (RPM) of the power supply fan via BIOS and a Windows application supplied by the motherboard manufacturer. New designs offer fan control so that the fan only runs the speed needed, depending on cooling needs.

Web servers include power supplies that offer a spare supply that can be exchanged while the other power supply is in use. Some new computers, particularly those designed for use as servers, provide redundant power supplies. This means that there are two or more power supplies in the system, with one providing power and the other acting as a backup. The backup supply immediately takes over in the event of a failure by the primary supply. Then, the primary supply can be exchanged while the other power supply is in use.


Power Supply FAQ

What is the power supply of a computer?

The basic function of a computer’s power supply is to convert the main AC to low voltage regulated DC power that is required to power a computer’s components. Most laptops have a battery pack that holds power when unplugged from a wall outlet, while desktop computers draw power from an electrical outlet.

What is a 12V power supply?

A 12V power supply takes any input between 100V and 220V AC, which is what comes from a wall socket, and gives an output of 12V DC.

What does “power supply” mean?

A power supply is an electrical device that is able to supply electric power to an electrical powered device. A modern power supply must output at least 18A on the +12V rail for a modern computer.

What power supply do I need for my gaming PC?

Depending on the GPU, most mid-range gaming PC builds can run on 450-600W PSUs. However, choosing a PSU with a 600 or 650-watt output makes it easier to upgrade later.

How do I check my PC wattage?

The most efficient and easiest way of checking PC wattage is with the help of a watt meter. Alternatively, there are a few different online tools that claim to be able to tell you.


Lots More Information

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More Great Links

  • Intel – How to Choose Power Supply for PC: What to Know
  • PC Guide – Fully Modular vs. Semi Modular vs. Non modular Power Supplies
  • PC Guide – Our 5 Best Power Supplies in 2022
  • Build Your Own PC

Cite This!

Please copy/paste the following text to properly cite this HowStuffWorks.com article:

Gary Brown & Talon Homer
“How PC Power Supplies Work”
5 March 2001.
HowStuffWorks.com. <https://computer.howstuffworks.com/power-supply.htm>
11 July 2023


PSUs: Understanding Power Supply Units

One of the key components of any computer system is the power supply unit, also known as the PSU. This component provides power to the rest of the computer, including any installed peripherals such as graphics cards and additional hard drives. In this article we will discuss the role of power supply units as well as the importance of finding the correct unit for your computer.  

Topics Include: 

  • What is a Power Supply Unit?
    • Connecting the Dots: PSU Cables and Power Distribution
    • Power Ratings and Voltage Rails
  • Importance of Using the Correct PSU
    • Types of PSUs
    • PSU Lifespan

What is a Power Supply Unit?

A power supply unit (PSU) is a hardware device that converts AC electricity into DC electricity and then distributes it to the rest of the computer. On a standard desktop computer, the PSU is where the power cord plugs into and usually has an I/O power switch on it. 

Connecting the Dots: PSU Cables and Power Distribution

If you open a standard computer case, you would see that the PSU is connected to the rest of the computer by various power cables. These cables supply the motherboard, hard drives, and case electronics with the electricity they need to function. Most PSUs also have extra cables meant for the installation of peripherals with large power demands such as graphics cards. In recent years, modular PSUs have become more common-place, allowing users to install as many power cables as necessary

In addition to the power provided directly by the PSU, the motherboard assists in distributing power to the CPU and RAM slots as well as the connectors for the CPU and case fan systems. Since the motherboard can help distribute power, the PSU doesn’t need to be directly plugged into every system component. Not only would it be a clutter of wires to deal with, many system components such as integrated graphics chips and CPUs are too small or delicate for a direct PSU connection. By combining a solid PSU with a compatible motherboard, you can rest assured that your computer will have all of the power it needs. 

Power Ratings and Voltage Rails 

One of the main features to pay attention to regarding PSUs is their power rating. The power rating describes the total system power that can be drawn from the unit before it overloads, usually expressed as Watts (W). Modern PSUs commonly range from 300W to over 1000W. PSUs with larger power ratings are commonly found in computers that have multiple graphics cards installed such as those used for gaming or graphics processing. Laptops generally have power supplies ranging from 50W to over 200W. These units usually have an associated power unit or “brick” that converts AC to DC in the same way as a desktop PSU. 

Another key feature of PSUs is their voltage, usually described in terms of voltage “rails”. A voltage rail is a supply of voltage in varying amounts, used by different system components depending on their voltage requirements. For example, a PCI network card will likely draw power from the +5 V rail, whereas the motors for the CPU fans will draw power from the +12 V rail. Put simply, the voltage rails are the levels of voltage available for use by any system component. While power rating determines the total power capacity of a PSU, voltage rails determine how that power is used.  

Importance of Using the Correct PSU

While most PSUs are based around standardized designs to allow for easy installation, there are a few variants that exist for different applications. 

Types of PSUs

ATX Standard – The most common PSU available, usually used in desktop PCs. Designed to work with the ATX motherboard form factor. Provides three positive voltage rails, +3.3 V, +5 V, and +12 V as well as a standby voltage rail +5 V SB to provide computers with power in standby mode. 

Entry-Level Power Supply Specification (EPS) – Derived from the ATX standard, this type of PSU was designed for use in servers rather than personal computers. Provides a more stable environment for critical applications than ATX standard, making them ideal for use in mission-critical servers. 

Small Form Factor – PSU variant designed for use in smaller form-factor computers that use MicroATX motherboards. These are the power supply units found in smaller set-top boxes such as DVD players or cable boxes.

Thin Form Factor – PSU variant designed for use with Mini ITX motherboards and smaller form-factor computers. 

If you are planning to build a home PC, finding an ATX Standard PSU is your best bet as it will fit any standard computer case and motherboard combination. If you are looking to build a server, then maybe the EPS solution is the perfect fit. For smaller computer builds, a small or thin form factor PSU may be more appropriate. 

PSU Lifespan

Since PSUs handle large amounts of electricity on a regular basis, they are more subject to wear and tear than other system components. Because of this, the lifespan of a PSU is an important measurement for determining the reliability of a given power supply. This lifespan is usually described in terms of mean time between failures (MTBF). A higher MTBF value means a given PSU is more reliable and will have a longer lifespan than PSUs with lower MTBF values. 

PSUs manufactured with higher quality materials and better cooling tend to have greater lifespans due to a lower amount of heat-related stress. Generally speaking, the average PSU will function for around 100,000 hours with standard temperature variations. Due to the relatively limited lifespan of PSUs, modern servers are sometimes equipped with hot-swappable PSUs that can be quickly replaced in the event of failure. To prevent unexpected downtime, it can help to ensure that your server uses a reliable, high-quality power supply unit. 

Now that you have a better understanding of computer power supply units you can more easily decide which type of PSU is right for you. By considering the wattage, voltage, form-factor, and lifespan of your PSU, you can ensure that your computer never runs out of the power it needs to function properly.

What is a power supply for a computer. How to choose the best PSU

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  • Definition of
  • Why is the lack of power supply in the PC dangerous?
  • How does a high-quality and reliable power supply differ from a cheap one?
  • Modular cables and connectors

Computer power supply (PSU) is an electronic device that generates the voltage required by a specific PC component from the mains voltage. On the territory of Russia, the power supply unit converts alternating current from the mains 220V and a frequency of 50Hz into several low values ​​​​of direct current: 3.3V; 5V; 12V, etc.

Power supply for a computer

The main parameter of the power supply is power, which is calculated in watts (W). The more powerful the computer, the more powerful the power supply required. Usually it is 300-500 W in budget and office computers and 600 W or more in powerful stations and gaming PCs. Top-class video cards, which need more than a kilowatt of power, are becoming more and more demanding on power.

The power supply is a kind of energy center of any computer. It is he who supplies electricity to all components of the computer, and allows the PC to work. From the mains, the cable goes to the power supply, and already it will distribute the required voltage throughout the rest of the computer.

Cables come out of the PSU to the motherboard, video card, hard drive, drive, coolers and fans, and other devices. High-quality and expensive blocks are resistant to voltage drops in the electrical network. This prevents the failure of both the power supply itself and all computer components.

What is necessary for a stable, trouble-free operation of a computer?

Powerful processor, modern video card, good motherboard. But almost everyone forgets to add a reliable power supply to this list, which, as the power supply center for all other computer components. He must be 100% committed to his assignments. Otherwise, the stable and trouble-free operation of the computer is out of the question.

What is the danger of lack of power in a PC?

If the power of the installed power supply is not enough for all elements of the computer, then this will result in both minor problems and the complete inability to turn on the PC.

Here are the main dangers of a weak PSU:
  • There is a possibility of failure or partial damage to the hard drive. This is due to the fact that in a hard disk, due to a lack of power, the read heads will not be able to function normally and slide over the surface of the disk and begin to scratch it. In this case, characteristic sounds can be heard.
  • Possible problems with the video card (up to the disappearance of the image on the monitor). This is especially evident in modern computer games.
  • Removable hard drives and flash drives connected to USB ports, as well as other devices without additional power, may not be detected by the operating system or may be turned off during operation.
  • The computer may shut down or restart during peak times.

How to get rid of it? Very simple – install a more powerful and reliable power supply.

Attention!!! The above problems may manifest themselves not only due to a poor-quality PSU, but be the result of a malfunction of other PC components. To determine the exact cause, it is better to contact our computer repair at home in Moscow.

Characteristics of the power supply

How does a high-quality and reliable power supply differ from a cheap one?

1. Good, high quality and expensive provides protection against unexpected power surges in the mains. In the event that he fails himself, he must “at the cost of his own life” protect the rest of the computer devices.

2. The power supply must provide the PC user with a modern and convenient cable system in all respects. It is convenient when it is possible to disconnect each power cable from the unit, thereby freeing up a large amount of space inside the case for ventilation and cooling of the system unit.

3. The PSU must have a good cooling system, must be protected from overheating, and must not make a lot of noise from its fan.

Modular cables and connectors

One of the trends in the development of modern PSUs is an increase in the convenience of using cables. The main goal is to remove from the computer case that knot of wires and cables that can now be seen in almost any personal computer.

In the cheapest power supplies, all cables are permanently connected. This leads to the fact that all cables not used to power devices are in any case inside the system unit. This, in turn, impairs air circulation and makes the process of repairing and upgrading a PC extremely inconvenient.

Modular cables and connectors

Much more convenient when all unnecessary and unused cables can be disconnected. When needed, they can be quickly connected via connectors. This not only greatly improves the cooling inside the case, but also makes the contents of the system unit look nice if the case has a window.

The power supply is recommended to be selected and purchased after the power consumption of the entire PC has been accurately calculated. This can be done by adding up the power consumption of all components. After that, you need to add about 30% more as a margin of safety. If you plan to install any more components in the future, then you need to increase the stock.

Not sure how much power supply you need? Don’t know which manufacturer to trust? Call the Compolife. ru computer help center and our specialists will definitely help you make the right choice. In addition, you can order the installation of a new power supply in the system unit from us.

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Leave a comment 002 The power supply is an integral part of any computer, and is no less important for work than, for example, a processor or motherboard. Its main task is to generate the necessary currents for the operation of all PC components.

It often happens that the computer does not turn on, the operating system does not load, and a malfunctioning PSU may be to blame. How to check the PC power supply for performance, what are the main clinical manifestations of some of its malfunctions – this is the main topic of our publication.

PSU basic parameters

The PC power supply provides several voltages necessary for the operation of all components of the computer.

The picture shows the largest 20-pin connector that connects to the motherboard. Readings are given for each contact.

Pinout and color scheme of the 24-pin connector and other PSU connectors.

Using a multimeter to test the PSU

Many users ask how to test the computer’s power supply with a multimeter? It is very simple, knowing what voltage and where it should come.

Before opening the PC case, make sure that it is not connected to 220 V.

  1. Open the PC case.
  2. Disconnect the connectors from each device in turn, having previously photographed or sketched the wiring diagram.
  3. Take the connector that connects to the motherboard (usually the largest one) and make a wire jumper between pins 14 and 15 on a 20-pin connector and pins 16 and 17 on a 24-pin connector. Usually, green and black wires go to them. Green is the control signal. The black wire is ground.
  4. After that, connect the PC to the 220 V network.

If the PSU is turned on, then you can start measuring the voltage at its contacts, according to the diagram presented above. If the computer’s power supply does not turn on, it means that it is out of order and needs to be repaired or completely replaced.

When checking with a multimeter between the black and red wire on the connector connected to the motherboard, there should be – 5 V; between black and yellow – 12 V; between contacts black and pink – 3.3 V; between black and purple – standby voltage of 5 V.

If you do not have sufficient knowledge in electronics, then it is better to entrust the repair of the device to specialists.

Paperclip Method

There is a simple method among users to test the power supply with a paperclip. Our resource will not stand aside, and will tell you what this method is, especially since almost the same thing was discussed in the section on using a multimeter. This is the simplest, one might say, home method, which cannot show the quality of the voltage source, but will quite reliably make it clear whether it turns on or not.

  1. Disconnect the PC from the network.
  2. Open the case and disconnect the connector from the motherboard.
  3. Make a U-shaped jumper from a paper clip to short the green wire of the connector to the nearby black wire.
  4. Connect the PSU to the 220 V network.

If the fan starts working, then the PSU is theoretically in working condition, if not, it is definitely in repair.

Main symptoms and malfunctions

Faulty PSU, most often it just doesn’t work at all. But sometimes, the user is faced with problems that, by all indications, are manifestations of violations in the RAM or motherboard. In fact, the microcircuits receive power from the PSU, so failures in their operation may indicate a malfunction. How to check the power supply in this case, and whether it makes sense to repair it, only a specialist can say. Further, problems will be described in which the cause may be PD.

  • Hangs when turning on the PC.