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What is Power Supply Unit
The power supply unit (or PSU), converts alternating-current (AC) power coming from a wall outlet into direct-current (DC) power, which is a lower voltage. DC power is required for all the components inside the computer.
The power supply unit is critical because it supplies electrical power to every other component in the system. The power supply unit is also one of the most failure-prone components in any computer system.
Because of its importance to proper and reliable system operation, you should understand both the function and the limitations of a power supply unit.
PC power supply units are designed to convert either 120V 60Hz AC (alternating current) or 240V 50Hz AC power into +3.3V, +5V, and +12V DC (direct current) power.
The PSU normally supplies +3.3V, +5V, and +12V to the system.
Electricity is simply a flow of negatively charged particles, called electrons, through matter. All matter enables the flow of electrons to some extent. This flow of electrons is very similar to the flow of water through pipes.
Electric companies gather or generate electricity and then push it to your house under pressure through wires. The pressure of the electrons in the wire is called voltage and is measured in units called volts (V). The amount of electrons moving past a certain point on a wire is called the current or amperage, which is measured in units called amperes (amps or A) . The amount of amps and volts needed by a particular device to function is expressed as how much wattage (watts or W) that device needs. The correlation between the three is very simple math: V × A = W.
Wires of all sorts—whether copper, tin, gold, or platinum—have a slight resistance to the flow of electrons. Resistance to the flow of electrons is measured in ohms (Ω).
- Pressure = Voltage (V)
- Volume flowing = Amperes (A)
- Work = Wattage (W)
- Resistance = Ohms (Ω)
Electricity comes in two flavors: direct current (DC), in which the electrons flow in one direction around a continuous circuit, and alternating current (AC) , in which the flow of electrons alternates direction back and forth in a circuit. Most electronic devices use DC power, but all power companies supply AC power because AC travels long distances much more efficiently than DC.
What is Voltage Rails
Generally, all of the PC’s power comes from a single transformer that takes the AC current from a wall socket and converts it into DC current that is split into three primary DC voltage rails : 12.0 volts, 5.0 volts, and 3.3 volts. These voltages are often called rails, referring to the fact that although multiple wires are carrying a specific voltage, they are normally tied to a single rail (or tap) in the PSU. Multiple wires are used because, if all the current were carried over a single wire, the wire and the terminals, connectors, and even the traces on the circuit boards would all have to be extremely large and thick to handle the load. Instead, it is cheaper and more efficient to spread the load out among multiple smaller and thinner wires.
That means the 12-volt connector on a P4 draws from the same rail as the main 12-volt connector feeding power to the motherboard. This works fine as long as the collective needs of the connectors sharing a rail don’t exceed its capacity to feed them power. To avoid this, EPS12V (Extended Power Supply); divided the 12-volt supply into two or three separate 12-volt rails, each one providing a separate source of power.
The digital electronic components and circuits in the system (motherboard, adapter cards, and disk drive logic boards) typically use the +3.3V or +5V power, and the motors (disk drive motors and any fans) use the +12V power.
What is Negative DC Voltages
Negative voltage in a circuit is voltage that is more negative in polarity than the ground of the circuit. If you look at a specification sheet for a typical PC power supply, you can see that the supply generates not only +3.3V, +5V, and +12V, but also –12V and possibly –5V. Although –12V and (possibly) –5V are supplied to the motherboard via the power supply connectors, the motherboard normally uses only the +3.3V, +5V, and +12V. In fact, –5V was removed from the ATX12V 1.3 and later specifications. The only reason it remained in most power supply designs for many years is that –5V was required on the ISA bus for full backward compatibility. Because modern PCs no longer include ISA slots, the –5V signal was deemed as no longer necessary. However, if you are installing a new power supply in a system with an older motherboard that incorporates ISA bus slots, you want a supply that does include the –5V signal. Although older serial port circuits used +/–12V outputs, today most run only on +3.3V or +5V.
Power Supply Form Factors
The shape and general physical layout of a component is called the form factor. In the PC market, IBM originally defined the form factor standards, and everybody else copied them; this included power supplies. Intel defined a new power supply form factor in 1995 with the introduction of the ATX form factor. More than 10 different power supply form factors have existed that can be called industry standards. Since then, ATX and the many variants based on ATX have become by far the dominant form factors for power supplies.
Any power supply that does not conform to one of these standards is considered proprietary. In general, avoid systems that use proprietary power supply designs because replacements are difficult to obtain and upgrades are generally not available.
The power supply form factors detailed in the following sections are the standards used in current systems. ATX is far and away the most common of these but you are likely to encounter the other types listed here.
The first widespread update to the ATX standard, ATX12V 1.3, came out in 2003. This introduced a 4-pin motherboard power connector, unofficially but commonly called the P4, that provided more 12-volt power to assist the 20-pin P1 motherboard power connector. Any power supply that provides a P4 connector is called an ATX12V power supply. ATX12V power supplies are also used with newer motherboard form factors such as BTX and mini-ITX, ensuring that ATX and its derivatives will remain the most popular power supply form factors for several years to come. The ATX12V specification defines the physical or mechanical form as well as the electrical connectors for the power supply. The ATX12V 2.0 specification (February 2003) dropped the 6-pin auxiliary connector, changed the main power connector to 24 pins, and made Serial ATA power connectors a requirement. The current version is ATX12V 2.2, which was released in March 2005 and contains only minor changes from the previous releases, such as the use of Molex High Current System (HCS) terminals in the connectors.
The ATX form factor includes intelligently designed and keyed power plugs to prevent users from incorrectly plugging in their power supplies. The ATX connectors also supply +3.3V, reducing the need for voltage regulators on the motherboard to power +3.3V-based circuits.
When plugged in, ATX systems have 5 volts running to the motherboard. They’re always “on” even when powered down. The power switch you press to power up the PC isn’t a true power switch like the light switch on the wall in your bedroom. The power switch on an ATX system simply tells the computer whether it has been pressed. The BIOS or operating system takes over from there and handles the chore of turning the PC on or off. This is called soft power.
Because Standby power is always active, the only way to safely prepare a system for internal servicing is to unplug the power supply or set the power supply’s own switch to off.
Intel released the smaller microATX motherboard form factor in December 1997. At the same time, it released the small form factor (SFX) power supply design to go with it. Then in March 1999, Intel released the FlexATX addendum to the microATX specification, which was a small board designed for low-end PCs or PC-based appliances. Since then, the SFX supply has found use in many new compact system designs.
The SFX standard actually defines five different physical shapes, some of which are not directly interchangeable. Therefore, when replacing an SFX/SFX12V-type supply, you need to ensure you are purchasing the correct type—which is to say the type that will physically install in your chassis—as well as have the correct connectors for your motherboard.
SFX12V version 3.0 changed the main motherboard power connector from 20 pins to 24 pins and made Serial ATA power connectors a requirement. The current SFX12V version 3.1 was released in March 2005 and contains a few additional minor revisions, including a change to High Current System (HCS) terminals in the connectors. SFX12V includes several physical designs, including one called the PS3 form factor. The PS3 form factor is actually a shortened version of ATX12V and is generally used in systems with microATX chassis and motherboards that require higher power output than the smaller SFX variants can supply.
Most SFX supplies are designed to provide 80–300 watts of continuous power in four voltages.
Server motherboards are thirsty for power and sometimes ATX12V 1.3 just didn’t cut it. An industry group called the Server System Infrastructure (SSI) developed a non-ATX standard motherboard and power supply called EPS12V.
In 1998, the SSI created the entry-level power supply (EPS) specification, which defines an industrystandard power supply form factor for entry-level pedestal (standalone tower chassis) servers. The initial EPS standard was based on ATX, but with several enhancements. The ATX and the original EPS standards call for a supply that is 86mm tall by 150mm wide by 140mm deep, the same dimensions as the LPX or PS/2 form factors. EPS later added optional extended depths of 180mm and 230mm total.
The TFX12V (thin form factor) power supply unit form factor optimized for low-profile ATX systems.
The CFX12V (compact form factor) power supply unit is an L-shaped power supply optimized for microBTX systems.
The LFX12V (low profile form factor) power supply unit is designed for ultra-small BTX systems, primarily using picoBTX or nanoBTX motherboards.
The Flex ATX power supply unit form factor in the form of proprietary designs for SFF desktop and thin (1U) server systems.
Motherboard Power Connectors
Every PC power supply has connectors that attach to the motherboard, providing power to the motherboard, processor, memory, chipset, integrated components (such as video, LAN, universal serial bus [USB], and FireWire), and any cards plugged into bus slots.
A number of connectors lead out of the power supply. Every power supply provides special connectors to power the motherboard and a number of other general-use connectors that provide power to any device that needs electricity.
Power supplies conforming to the original ATX and ATX12V 1.x form factor standards or variations thereof use the following three motherboard power connectors:
- 20-pin main power connector
- 6-pin auxiliary power connector
- 4-pin +12V power connector
Most motherboards that use the +12V connector do not use the auxiliary connector, and vice versa. The ATX main power connector is either a 20-pin or a 24-pin connector.
During development it was realized that PCI Express x16 video cards could draw more power than what was allowed by the existing 20-pin main and 6-pin auxiliary power supply connectors, especially when it came to +12V power. The ATX12V 2.0 included a new 24-pin main power connector and the elimination of the 6-pin auxiliary power connector. The new 24-pin main power connector included four more pins supplying additional +3.3V, +5V, and +12V power plus a ground.
4 Pin Molex connectors are the most common type of power connection for devices that need 5 or 12 volts of power.
4 Pin Berg connectors used to connect 3.5-inch floppy disk drive units to the power supply unit, usually referred to as simply a "floppy power connector", but often also referred to as LP4.
Be extra careful when plugging in a Berg connector! Whereas Molex connectors are difficult to plug in backward, you can insert a Berg connector incorrectly with very little effort. Doing so will almost certainly destroy the floppy drive.
SATA Power Connectors are use for Serial ATA (SATA) hard drives and SSD drives.
Motherboard power connectors can come with either a 20 pin main power connector or a 24 pin main power connector. Many power supplies come with a 20+4 cable which is compatible with both 20 and 24 pin motherboards. A 20+4 power cable has two pieces: a 20 pin piece, and a 4 pin piece. If you leave the two pieces separate then you can plug the 20 pin piece into a 20 pin motherboard and leave the 4 pin piece unplugged. Be sure to leave the 4 pin piece unplugged even if it fits into another connector. The 4 pin piece is not compatible with any other connectors. If you plug the two pieces of a 20+4 power cable together then you have a 24 pin power cable which can be plugged into a 24 pin motherboard.
Auxiliary Power Connector was added to provide extra wattage to motherboards for 3.3 and 5 volts. This connector is rarely used anymore. It's most commonly found on older dual CPU AMD motherboards
4 Pin ATX +12 Volt Power Connector older computers put most of their load on 3.3 and 5 volts. As time passed, computers drew more and more of their load from 12 volts. This cable added two more 12 volt lines so more of the load could be shifted to 12 volts. The power coming from this connector is usually used to power the CPU but some motherboards use it for other things as well. The presense of this connector on a motherboard means it's an ATX12V motherboard. This power connector plugs into the 4 pin connector and has two black wires and two yellow wires. This connector is sometimes called an "ATX12V" cable or "P4" connector.
8 Pin EPS +12 Volt Power Connector was originally created for workstations to provide 12 volts to power multiple CPUs. But as time has passed many CPUs require more 12 volt power and the 8 pin 12 volt connector is often used instead of a 4 pin 12 volt cable. Depending on the power supply, the connector may contain one 12 volt rail in all 8 pins or two 12 volt rails taking up 4 pins apiece. It is often refered to as an "EPS12V" connector.
4+4 Pin +12 Volt Power Connector, motherboards can come with either a 4 pin 12 volt connector or an 8 pin 12 volt connector. Many power supplies come with a 4+4 pin 12 volt cable which is compatible with both 4 and 8 pin motherboards. A 4+4 power cable has two separate 4 pin pieces. If you plug the two pieces of a 4+4 power cable together then you have a 8 pin power cable which can be plugged into an 8 pin 12 volt connector. If you leave the two pieces separate then you can plug one of the 4 pin pieces into a 4 pin 12 volt connector and leave the other 4 pin piece unplugged.
6 Pin PCI Express Power Connector used to provide extra 12 volt power to PCI Express expansion cards. PCI Express motherboard slots can provide a maximum of 75 watts. Many video cards draw significantly more than 75 watts so the 6 pin PCI Express power connector was created. These high-power cards draw most of their power from the 12 volt rail so this cable provides only 12 volts. These are sometimes called "PCI Express cables". They are also occasionally called "PEG cables" where "PEG" stands for PCI Express Graphics. If your power supply doesn't have a 6 pin PCI Express cable then you can use the adapter shown below on the right to convert two 4 pin peripheral cables into a PCI Express cable. If you use an adapter then be sure to plug the 4 pin peripheral connectors into separate cables coming from the power supply. If you plug them both into the same power supply cable then you are drawing all the power of the PCI Express connector through a single 18 gauge wire. You can usually get away with that but there's no reason to do it. The PCI Express 6 pin connector is polarized so it can only be plugged in pointing in the correct direction. But as with connectors of this type, you can sometimes force them into the wrong kind of socket if you try hard enough. If it doesn't slide in easily then you're probably plugging it into the wrong place.
8 Pin PCI Express Power Connector, the PCI Express 2.0 specification released in January 2007 added an 8 pin PCI Express power connector. It's just an 8 pin version of the 6 Pin PCI Express power connector. Both are primarily used to provide supplemental power to video cards. The older 6 pin version officially provides a maximum of 75 watts (although unofficially it can usually provide much more) whereas the new 8 pin version provides a maximum of 150 watts.
6+2 Pin PCI Express Power Connector, some video cards have 6 Pin PCI Express power connectors and others have 8 Pin PCI Express power connectors. Many power supplies come with a 6+2 PCI Express power cable which is compatible with both kinds of video cards. The 6+2 PCI Express power cable is made up of two pieces: a 6 pin piece, and a 2 pin piece. If you put the two pieces together then you have a full 8 pin PCI Express power cable. But if you split the connector into two parts then you can plug the 6 pin part into the older 6 pin PCI Express connector and leave the 2 pin part unplugged. That way, your power supply only needs to have one 6+2 cable to be compatible with both 6 pin and 8 pin PCI Express connectors.
Splitters and Adapters You may occasionally find yourself without enough connectors to power all of the devices inside your PC. In this case, you can purchase splitters to create more connections. You might also run into the phenomenon of needing a SATA connector but having only a spare Molex. Because the voltages on the wires are the same, a simple adapter will take care of the problem nicely.
What is Active PFC
Simply due to the process of changing this AC current into DC current, creates a electrical phenomena⎯sort of a back pressure⎯that’s called harmonics in the power industry. These harmonics create the humming sound that you hear from electrical components. Over time, harmonics damage electrical equipment, causing serious problems with the power supply and other electrical devices on the circuit. Once you put a few thousand PCs with power supplies in the same local area, harmonics can even damage the electrical power supplier’s equipment!
Good PC power supplies come with active power factor correction (active PFC), extra circuitry that smoothes out the way the power supply takes power from the power company and eliminates harmonics.
Power Supply Ratings
Every device in a PC requires a certain amount of wattage in order to function.
The input specifications are listed as voltages, and the output specifications are listed as amps at several voltage levels. You can convert amperage to wattage by using the following simple formula:
watts = volts × amps
For example, if a component is listed as drawing 8 amps of +12V current, that equals 96 watts of power according to the formula.
By multiplying the voltage by the amperage available at each main output and then adding the results, you can calculate the total capable output wattage of the supply. Note that only positive voltage outputs are normally used in calculating outputs.
The total wattage of all devices combined is the minimum you need the power supply to provide. If you want an easier way to calculate your estimated power requirements, use internet based fairly good power supply wattage calculator, you can use online power supply wattage calculator at the following URL:
After you fill in all the fields with the components in the intended system, the calculator gives you an estimate of the minimum power supply rating you should choose to power the system.
After you’ve added up everything I recommend, multiply the total power consumed by all your components by 1.5 to estimate the size of power supply required. This allows some headroom for future expansion and accounts for the fact that at certain times some devices can draw much more than their nominal power.
No power supply can turn 100 percent of the AC power coming from the power company into DC current. So all power supplies provide less power to the system than the wattage advertised on the box. ATX12V 2.0 standards require a power supply to be at least 70 percent efficient, but you can find power supplies with better than 80 percent efficiency. More efficiency can tell you how many watts the system puts out to the PC in actual use. Plus, the added efficiency means the power supply uses less power, saving you money.
A power supply provides only the amount of power your system needs. If you put a 1000-watt power supply (yes, they really exist) into a system that needs only 250 watts, that big power supply will put out only 250 watts to the system. As a general recommendation for a new system, use at least a 500-watt power supply.
Other Power Supply Specifications
Decent power supplies have an extremely low current leakage to ground of less than 500 microamps. This safety feature is important if your outlet has a missing or an improperly wired ground line.
Here are some of the most common parameters found on power supply specification sheets, along with their meanings:
- Mean Time Between Failures (MTBF) or Mean Time To Failure (MTTF) - Mean time between failures (MTBF) is the predicted elapsed time between inherent failures of a mechanical system, during normal system operation. MTBF can be calculated as the arithmetic mean (average) time between failures of a system.
- Input Range (or Operating Range) - The range of voltages that the power supply is prepared to accept from the AC power source. For 120V AC power, an input range of 90V–135V is common; for 240V power, a 180V–270V range is typical.
- Peak Inrush Current - The greatest amount of current drawn by the power supply at a given moment immediately after it is turned on, expressed in terms of amps at a particular voltage. The lower the current, the less thermal shock the system experiences.
- Hold-Up Time - The amount of time (in milliseconds) that a power supply can maintain output within the specified voltage ranges after a loss of input power. This enables your PC to continue running without resetting or rebooting if a brief interruption in AC power occurs. Values of 15–30 milliseconds are common for today’s power supplies, and the higher (longer), the better.
- Transient Response - The amount of time (in microseconds) a power supply takes to bring its output back to the specified voltage ranges after a steep change in the output current. In other words, the amount of time it takes for the output power levels to stabilize after a device in the system starts or stops drawing power.
- Overvoltage Protection - Is a power supply feature which shuts down the supply, or clamps the output, when the voltage exceeds a preset level. Most power supplies use an over-voltage protection circuit to prevent damage to the electronic components.
- Maximum Load Current - The largest amount of current (in amps) that safely can be delivered through a particular output. Values are expressed as individual amperages for each output voltage. With these figures, you can calculate not only the total amount of power the power supply can supply, but also how many devices using those various voltages it can support.
- Minimum Load Current - The smallest amount of current (in amps) that must be drawn from a particular output for that output to function. If the current drawn from an output falls below the minimum, the power supply could be damaged or automatically shut down
- Efficiency - The ratio of power input to power output, expressed in terms of a percentage. Values of 65%–85% are common for power supplies today. The remaining 15%–35% of the power input is converted to heat during the AC/DC conversion process. Although greater efficiency means less heat inside the computer (always a good thing) and lower electric bills, it should not be emphasized at the expense of precision, stability, and durability, as evidenced in the supply’s load regulation and other parameters.
SLI-Ready and CrossFire/CrossFireX Certifications
Both NVIDIA and AMD have certification programs that test and certify power supplies to be able to power systems with multiple graphics cards in either a Scalable Link Interface (SLI) or a CrossFire (also known as CrossFireX) configuration.
You might also want to check out below tutorial.
How To Test a Power Supply Unit With a Multimeter
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