Power Supply

Power supplies are computer components that provide electricity to the system by converting AC (alternating current) from a wall outlet to DC (direct current) for the computer. They are located at the rear of the computer case and usually contain one or more cooling fans. The back plate features a power cord receptacle and off/on switch. Most also have a rear voltage switch that can be changed for operating in different countries. Some come with LED lights, which are popular with modders

Various components in the computer have different voltage requirements. Power supplies typically provide 3.3v and 5v rails for digital circuitry, and a 12v rail for running drives and fans. Since more components today, including processors, are feeding off the 12v rail, many now provide multiple 12v rails.

There are two major types of computer power supplies: AT and ATX.

AT Power Supply

An AT power supply is what was used for most older computers. This type of power supply powered the first personal computers made by IBM, and the standard was adopted for other manufacturers as well. Specifically, it powered all AT and AT-compatible motherboards

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The AT-compatible motherboard obtained its power from a special two-part power connector from the AT power supply. This power connector contained four +5 volt DC wires, four ground (0 volt) wires, one - 5 volt wire, one +12 volt wire and one -12 volt wire. The remaining wire was a signal wire that allowed the power supply to tell the motherboard that "Power is good." With an AT power supply, you were required to manually turn off your computer by pressing the power switch (which generally was a dedicated On/Off switch).

ATX Power Supply

With advances in software and operating systems, computers could do more, such as enter "Power-Save" or "Sleep" mode. Software could now be used to turn off the computer, rather than having to turn off a computer with a power switch. This has all been made possible by use of ATX power supplies and ATX-compatible motherboards. The ATX power supply, therefore, is more complex.

An ATX power supply has more outputs that connect to an ATX-compatible motherboard. Whereas the AT power supply only had 8 outputs, the ATX power supply uses either 20 or 24 outputs. Most ATX power supplies take into account that an ATX motherboard can contain either 20 or 24 outputs, so the extra 4 outputs are often split out as a separate plug that will only fit one way into a 24 output motherboard connector.

The ATX power supply also supports more voltage settings, and is capable of accepting signals from the ATX-compatible motherboard other than simply "Power Good." The 24-pin ATX power supply has the following number of outputs, all voltages DC: three +3.3 volt, eight ground (0 volt), five +5 volt, one -5 volt, two +12 volt, one -12 volt and four "signal" wires ("Power Good," "+5 volt standby," "+3.3 volt sense" and "Power on").

ATX Supply 20 PIns (Pentuim I to III)

ATX Supply 24  PIns (Pentium IV)

ATX Supply 28  PIns (Pentium IV Advanced)

All about the various PC power supply cables and connectors

Original PC main power cables

The original PC debuted in 1981 and used two cables to connect the PSU (power supply) to the motherboard. The two cables plug side by side into the motherboard connectors. Sometimes they are keyed so they only plug in one way and sometimes they aren't. Even if they're keyed you can insert them the wrong way if you put a little effort into it. You always have to remember to plug them in so the black wires are next to each other. It's either "black to black" or smoke and a shower of sparks.

Pinout
Pin number	Wire color	Description
1	orange	power good
2	red	+5 volts or connector key
3	yellow	+12 volts
4	blue	-12 volts
5	black	ground
6	black	ground
1	black	ground
2	black	ground
3	white	-5 volts
4	red	+5 volts
5	red	+5 volts
6	red	+5 volts

4 pin peripheral power cable

The four pin peripheral power cable dates back to the original PC. It was used for floppy drives and hard disks. It's still around and is now also used for all kinds of things including add-on fans, extra video card power, supplemental motherboard power, and case lighting. It's as old as the hills but is still very widely used. The connector is shaped so that it only fits in one way. You don't have to worry about inserting it the wrong way. People often use the term "4 pin Molex power cable" or "4 pin Molex" to refer to a four pin peripheral power cable

Pinout
Pin number	Wire color	Description
1	yellow	+12 volts
2	black	ground
3	black	ground
4	red	+5 volts

Floppy drive power cable

The four pin floppy drive cable showed up when PCs started including 3.5 inch floppy drives. This kind of cable is also sometimes used as an auxiliary power cable for AGP video cards which use more power than can be drawn from the motherboard slot. The connector is shaped so that it only fits in one way so you don't have to worry about inserting it the wrong way. Floppy cables are built with small connectors and 20 awg wire so they are limited to relatively low current uses.

Pinout
Pin number	Wire color	Description
1	red	+5 volts
2	black	ground
3	black	ground
4	yellow	+12 volts

6 pin auxiliary power cable

The aux power cable 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.

Pinout
Pin number	Wire color	Description
1	black	ground
2	black	ground
3	black	ground
4	orange	+3.3 volts
5	orange	+3.3 volts
6	red	+5 volts

SATA power cable

SATA was introduced to upgrade the ATA interface (also called IDE) to a more advanced design. SATA includes both a data cable and a power cable. The power cable replaces the old 4 pin peripheral cable and adds support for 3.3 volts (if fully implemented). The connector is shaped so it can only be plugged in the correct way.

Pinout
Pin number	Wire number	Wire color	Description
1	5	orange	+3.3 volts
2	5	orange	+3.3 volts
3	5	orange	+3.3 volts
4	4	black	ground
5	4	black	ground
6	4	black	ground
7	3	red	+5 volts
8	3	red	+5 volts
9	3	red	+5 volts
10	2	black	ground
11	2	black	ground
12	2	black	ground
13	1	yellow	+12 volts
14	1	yellow	+12 volts
15	1	yellow	+12 volts

ATX 20 pin main power cable

In 1996 PC makers started switching to the ATX standard which defined a new 20 pin motherboard power connector. It includes a 3.3 volt rail which is used to power newer chips which require a lower voltage than 5 volts. It also has a standby 5 volt rail which is always on even when the power supply is turned off to provide standby power to the motherboard when the machine is sleeping. The new connector also allows the motherboard to turn the power supply on and off rather than depend on the user to flip a power switch. This connector is polarized so it can only be plugged in pointing in the correct direction.

Pinout
Pins 1 through 10			Pins 11 through 20
Description	Wire color	Pin number	Pin number	Wire color	Description
+3.3 volts	orange	1	11	orange	+3.3 volts
+3.3 volts	orange	2	12	blue	-12 volts
ground	black	3	13	black	ground
+5 volts	red	4	14	green	PS_ON#
ground	black	5	15	black	ground
+5 volts	red	6	16	black	ground
ground	black	7	17	black	ground
PWR_OK	gray	8	18	white	-5 volts (optional)
VSB +5 volts	purple	9	19	red	+5 volts
+12 volts	yellow	10	20	red	+5 volts

ATX 24 pin main power cable

The 24 pin main power connector was added in ATX12V 2.0 to provide extra power needed by PCI Express slots. The older 20 pin main power cable only has one 12 volt line. The new 24 pin connector added one line apiece for ground, 3.3, 5, and 12 volts. The extra pins made the auxiliary power cable unnecessary so most ATX12V 2.x power supplies don't have them. The 24 pin connector is polarized so it can only be plugged in pointing in the correct direction.

Pinout
Pins 1 through 12			Pins 13 through 24
Description	Wire color	Pin number	Pin number	Wire color	Description
+3.3 volts	orange	1	13	orange	+3.3 volts
+3.3 volts	orange	2	14	blue	-12 volts
ground	black	3	15	black	ground
+5 volts	red	4	16	green	PS_ON#
ground	black	5	17	black	ground
+5 volts	red	6	18	black	ground
ground	black	7	19	black	ground
PWR_OK	gray	8	20	white	-5 volts (optional)
VSB +5 volts	purple	9	21	red	+5 volts
+12 volts	yellow	10	22	red	+5 volts
+12 volts	yellow	11	23	red	+5 volts
+3.3 volts	orange	12	24	black	ground

Some of the voltage lines on the connector may have smaller sense wires which allow the power supply to sense what voltage is actually seen by the motherboard. These are pretty common on the 3.3 volt line in pin 13 but are sometimes used for other voltages too. The -5 volt line on pin 20 was made optional in ATX12V 1.3 (introduced in 2003) because -5 had been rarely used for years. Newer motherboards virtually never require -5 volts but many older motherboards do. Most new power supplies don't provide -5 volts in which case the white wire is missing.

If you have an ATX power supply with a 24 pin main cable, it's okay to plug it into a motherboard with a 20 pin connector. It was designed to work that way. You can see an example in the picture above. The extra 4 pins on the cable just hang over the end of the motherboard connector. The 24 pin cable only fits into a 20 pin socket at one end so you can't plug it in incorrectly. The extra 4 pins were added to the 24 pin version of the cable to provide one extra wire for ground, 3.3, 5, and 12 volts. But it's okay to leave those 4 pins disconnected because a motherboard with a 20 pin connector doesn't need them. The only problem you can bump into (literally) is if there is something blocking the spot where the 24 pin cable hangs over the end. Or sometimes the end of the 20 pin motherboard connector is too thick to fit between the pins of the 24 pin cable. You can solve that problem by carefully shaving down one end of the 20 pin motherboard connector. It's just plastic. You won't miss it. If you can't get them to fit together then you can get an adapter cable which will make it work. The 24 pin cable plugs into one end of the adapter and then the adapter plugs into the 20 pin motherboard. But you should avoid using that kind of adapter if you can because the extra wire and connector are just more things which can go wrong. Adapters also slightly increase the voltage drop which is something worth avoiding. It's better to first see if you can get a 24 pin cable to fit into a 20 pin motherboard before resorting to an adapter.

ATX 20+4 pin main power cable

Motherboards 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.

4 pin ATX +12 volt power cable

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 Before this power cable was introduced there was just one 12 volt line provided to the motherboard. 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 presence of this connector on a motherboard means it's an ATX12V motherboard. For dual 12 volt rail power supplies, this connector provides the voltage referred to as 12V2. The power cable which plugs into the 4 pin connector has two black wires and two yellow wires. This cable is sometimes called an "ATX12V" cable or "P4" cable although neither of those are technically accurate descriptions.

If you have one of these connectors on a motherboard then you must plug a power cable into it or your CPU won't get any power. The one exception is that when this connector was new, some motherboards shipped with a socket into which you could plug a 4 pin peripheral power cable as an alternative. That helped people who had older power supplies which didn't have the 4 pin 12 volt cable.

The 4 pin 12 volt cable is polarized so it can only be plugged into the 4 pin motherboard connector correctly. If you look carefully at the picture above you can see that two of the pins are square and the other two have rounded corners. The motherboard connectors also have the same square and rounded arrangement so the power cable only fits in one way. At least that's true unless you try really hard to force it into the connector. With enough force you can sometimes get a cable with a small number of pins into a connector which doesn't match. If you look carefully you can also see that the square and rounded pattern matches various positions on other motherboard connectors like the 20 pin main power connector and 24 pin main power connector. Do yourself a favor and only plug the 4 pin 12 volt cable into the motherboard connector where it belongs (unless you enjoy smoke and fried components).

If your power supply doesn't have a 4 pin 12 V cable then you can provide one with the adapter shown above. It converts a 4 pin peripheral cable into a 4 pin 12 V cable.

Pinout
Pins 1, 2			Pins 3, 4
Description	Wire color	Pin number	Pin number	Wire color	Description
ground	black	1	3	yellow	+12 volts
ground	black	2	4	yellow	+12 volts

8 pin EPS +12 volt power cable

This cable 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 cable 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" cable.

The 8 pin 12 volt cable is polarized so it can only be plugged into the 8 pin motherboard connector correctly. If you look carefully at the picture above you can see that four of the pins are square and the other four have rounded corners. The motherboard connectors also have the same square and rounded arrangement so the power cable only fits in one way. At least that's true unless you try really hard to force it into the connector. With enough force you can sometimes get a cable with a small number of pins into a connector which doesn't match. The 8 pin cable has enough pins that it's pretty hard to insert it in the wrong direction but determined people might be able to do it. If you look carefully you can also see that the square and rounded pattern matches various positions on other motherboard connectors like the 20 pin main power connector and 24 pin main power connector. You should only plug the 8 pin 12 volt cable into the motherboard connector where it belongs unless you enjoy the smell of fried electronics.

You can also plug an 8 pin 12 volt cable into a 4 pin 12 volt motherboard connector. Four of the pins on the 8 pin cable fit into the motherboard connector and the other four pins hang off the end. The 8 pin cable only fits into one end of the 4 pin motherboard connector unless you try hard to force it into the wrong position. The 8 pin cable is electrically compatible but it may not fit into a 4 pin motherboard. There is often a component which blocks the area where the 4 pins would hang off the end. And sometimes the plastic end of the 4 pin connector is too thick to fit between the pins of the 8 pin cable.

Make sure that you don't try to plug an 8 pin 12 volt cable into the 8 Pin PCI Express power connector on a video card. The two cables look very similar so it's easy to get the two confused. 8 Pin PCI Express power cables are usually labeled to distinguish them from 8 pin 12 volt cables. The PCI Express cable usually has "PCI-E" printed on the connector. If there are no labels then you can usually use wire colors to tell the two kinds of cables apart. An 8 pin 12 volt cable has yellow wires on the same side as the connector clip. An 8 Pin PCI Express cable has black wires on the clip side. The two power cables are also keyed differently so you can't plug one kind of power cable into the other kind of connector. But as with this kind of connector, you can sometimes force the wrong kind of cable into a connector if you push hard enough. Make sure you have the right kind of cable before plugging it in. The two are definitely not compatible with each other.

Pinout
Pins 1 through 4			Pins 5 through 8
Description	Wire color	Pin number	Pin number	Wire color	Description
ground	black	1	5	yellow	+12 volts (12V1)
ground	black	2	6	yellow	+12 volts (12V1)
ground	black	3	7	yellow	+12 volts (12V1 or 12V2)
ground	black	4	8	yellow	+12 volts (12V1 or 12V2)

If you don't have an 8 pin 12 volt cable then you can use the adapter shown above. It converts a couple of 4 pin peripheral power cables into an 8 pin 12 volt cable. If you use one of these adapters 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 8 pin 12 volt connector through a single 18 gauge wire.

4+4 pin +12 volt power cable

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.

If you look carefully at the image above then you can see the polarization of the pins which prevents you from plugging the cable in improperly. Some of the pins are square and some of them have rounded off corners. The motherboard connectors have matching square and rounded off corners to prevent the cable from being plugged in the wrong way. But if you look really carefully at the right half of this particular cable and then look at the 8 pin 12 volt cable pictured above you'll notice that they don't match. A regular 8 pin cable has four square pins and four rounded ones but the 4+4 cable shown above has two square pins and 6 rounded ones. The left half of the 4+4 matches the left half of an 8 pin cable but the right half is different. Hmmmm... And this isn't some bizarre cable either. I've seen plenty of 4+4s which look like this one. And then there are other 4+4 cables which look just like an 8 pin cable split in two (which makes sense). Since rounded pins fit into square holes in motherboard connectors, this particular cable will fit just fine into an 8 pin 12 volt motherboard connector. But both halves of this 4+4 will fit into a 4 pin 12 volt motherboard connector. You're supposed to use the left half of the cable shown above when plugging it into a 4 pin motherboard connector but the right half will also fit. As it happens, either half will work fine in a 4 pin motherboard because both halves of the 4+4 just provide 12 volts. The pinouts are the same for both halves so either one will work. I'm not sure why they make cables like this one because you'd figure a 4+4 cable would just be an 8 pin cable which splits in two. And you only need one half of a 4+4 cable to plug into a 4 pin motherboard. The other half is unused. But the kind of 4+4 cable shown above is pretty common so don't let it throw you.

6 pin PCI Express power cable

This cable is 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 cable 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 above 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.

Some video cards come with the 8 pin PCI Express power connector to support higher wattage than the 6 pin PCI Express connectors. It's okay to plug a 6 pin PCI Express power cable into an 8 pin PCI Express connector. It's designed to work that way but will be limited to the lower wattage provided by the 6 pin version of the cable. The 6 pin cable only fits into one end of the 8 pin connector so you can't insert it incorrectly but you can sometimes force the 6 pin cable in the wrong way if you try hard enough. Video cards can sense whether you have plugged a 6 pin or 8 pin cables into an 8 pin connector so the video card can impose some kind of restriction when running with only a 6 pin power cable. Some cards will refuse to run with only a 6 pin cable in an 8 pin socket. Others will work with a 6 pin cable at normal speeds but will not allow over clocking. Check the video card documentation to get the rules. But if you don't have any other information then just assume that if your video card has an 8 pin connector then you must plug in an 8 pin cable.

Pinout
Pins 1 through 3			Pins 4 through 6
Description	Wire color	Pin number	Pin number	Wire color	Description
+12 volts	yellow	1	4	black	ground
+12 volts or not connected	yellow or not connected	2	5	black	ground
+12 volts	yellow	3	6	black	ground

The PCI Express specification is, unfortunately, not a free, public specification. So most people have never seen it. Including me. ATX specification: freely available to all. PCI Express specification: expensive so hardly anyone has seen it. ATX: good. PCI Express: bad. It's a shame when a widely used standard isn't freely available to the public. Nonetheless, information leaks out from the specification and the 6 pin PCI Express power cable is actually rated at an extremely conservative 75 watts. I have no idea why the wattage is rated so low because the specifications from Molex clearly allow substantially more power. Part of the reason may be that pin 2 (listed above as a 12 volt line) may be listed as not connected in the specification. I've never seen a 6 pin PCI Express power cable with pin 2 not connected. They've all had a 12 volt line connected to pin 2. I've also seen claims that there may be unimplemented sense lines in the specification. Welcome to the uncertainty which happens when you don't have freely available specifications. Even with only two 12 volt lines the standard implementation of PCI Express power cables use large enough gauge wire and a good enough connector to provide much more than the three amps per wire required to provide 75 watts. Nonetheless, the 6 pin PCI Express power cable officially provides only 75 watts. In all likelihood, however, real implementations of this power cable can provide far more than 75 watts.

8 pin PCI Express power cable

The PCI Express 2.0 specification released in January 2007 added an 8 pin PCI Express power cable. It's just an 8 pin version of the 6 Pin PCI Express power cable. 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. It is very easy to confuse the 8 pin version with the very similar-looking EPS 8 pin 12 volt cable.

The 8 pin PCI Express and the EPS 8 pin 12 volt connectors are polarized differently so you won't be able to plug one kind of cable into the other kind of connector. That is, you won't be able to plug the wrong kind of cable in unless you try really hard. Unfortunately, the Molex Mini-fit Jr. connectors used by both kinds of power cables can sometimes be forced into a differently-polarized connector if they only have a few pins and you push hard enough. If the cable won't slide in easily then you're probably trying to insert the wrong kind of cable. The 8 pin PCI Express connector does have a small plastic bridge which prevents it from being plugged into an EPS 8 pin 12 volt motherboard connector. You can see the bridge in the image above between the rightmost two pins in the top row of the connector. But there's no such protection to prevent EPS 8 pin 12 volt cables from being plugged into an 8 pin PCI Express connector on a video card. That combination may fit if you shove hard enough. And if you plug in the wrong kind of cable then expect fireworks. Some of the grounds and 12 volts wires for an EPS 8 pin 12 volt are reversed compared to an 8 pin PCI Express. Fortunately, most 8 pin PCI Express connectors are labeled "PCI-E" so people won't confuse them with EPS 8 pin 12 volt cables. If the connectors aren't labeled then you can tell an 8 pin PCI Express power cable from an EPS 8 pin 12 volt cable by checking the color of the wires which plug into the clip side of the connector. On the EPS 8 pin cable, the yellow wires (the 12 volt wires) go into the clip side of the connector. On the 8 pin PCI Express cable, the wires on the clip side are all black (grounds). That's the same as it is with the 6 Pin PCI Express power cable. Of course, none of this helps you if your cable uses the trendy all-the-same-color-wires design which is popular with high-fashion power supplies. In that case you'll just have to be very careful or hope the connectors are labeled.

Pinout
Pins 1 through 3			Pins 4 through 6
Description	Wire color	Pin number	Pin number	Wire color	Description
+12 volts	yellow	1	5	black	ground
+12 volts	yellow	2	6	black	ground
+12 volts	yellow	3	7	black	ground
ground	black	4	8	black	ground

6+2 pin PCI Express power cable