Changes in ATX Power Supply Connectors

I have often said that the power supply is the least respected component in a modern PC . By that I mean that most people pay less attention to the power supply in their system than they do to virtually any of the other components. Recently the industry standard ATX power supply used in PCs has been evolving, with new connectors appearing, which are designed for newer systems. There are adapters to convert the new connectors to the old ones (and vice versa) however in virtually all cases these adapters aren't necessary, since by virtue of their design the old and new connectors are actually compatible. For those who are either building new systems from scratch, or upgrading and repairing existing systems, knowledge of the new connectors, and how you can almost always build, upgrade or repair a system without purchasing unnecessary adapters will be invaluable. In this article I'll examine the evolutionary changes to the ATX power supply connectors, as well as highlight the new connectors designed for the latest systems.

The power supply connectors are part of the ATX and related power supply form factor specifications, which are maintained at (www.formfactors.org). For those that aren't familiar with the term, the form factor of a power supply defines not only the physical shape of the box (and other particulars like screw hole locations), but also the types and pinouts of the connectors. Having the right form factor means the power supply will not only fit in the chassis, but will also plug into the motherboard power sockets and work properly. Without standards for power supplies, we would have proprietary designs that would be incompatible with other brand supplies or motherboards. Form factor standardization means that you can easily build, upgrade or repair a system having the freedom to choose from a number of compatible power supplies for a given motherboard and chassis.

Some Background
While there have been a number of different industry standard power supply form factors over the years, for the last 10 years the primary standards have been ATX and ATX12V. The ATX form factor was originally introduced in August 1995 by Intel, and was widely adopted in 1996 by most motherboard, power supply, and chassis manufacturers. Most people who build PCs or work on them regularly are very familiar with the industry standard ATX motherboard and power supply designs, as they are ubiquitous throughout the industry.

Other ATX derivatives have also surfaced such as SFX (small) and TFX (thin), however aside from having smaller physical dimensions, these form factors use the same basic connectors and connector pinouts as ATX. In February 2000 Intel added a new +12V power connector to the ATX design and renamed the specification ATX12V, and by consequence all of the other power supply standards based on ATX added this connector as well. For many years now these industry standard power supply designs have remained relatively unchanged, allowing system builders as well as those upgrading and repairing PCs to have little concern that a new or existing supply would plug into a particular motherboard. More recently things have changed with respect to power supply form factors, with the biggest changes involving the connectors used to plug the power supply into the motherboard. These changes have resulted in a number of different and seemingly incompatible designs.

Standard ATX and ATX12V Motherboard Power Connectors

In addition to connectors providing power to the motherboard, all ATX and ATX12V power supplies also include multiple Peripheral power connectors which are primarily used for disk drives, and one or two Floppy power connectors for floppy drives as well. Newer ATX12V 2.0 and later designs also call for SATA (Serial ATA) power connectors for SATA drives as well. Since the Peripheral, Floppy, and SATA power connectors are relatively fixed in design and not changing, this article will focus on the motherboard power connectors instead.

Standard power supplies conforming to the original ATX and ATX12V 1.x form factor standards use the following three motherboard power connectors:

The Main power connector is always required, but the other two are optional depending on the application. Consequently it is possible for a given ATX or ATX12V power supply to have up to four different combinations of these connectors:

The most common variety would be those including the Main only, or the Main and +12V connectors.

20-pin Main Power Connector

The 20-pin Main power connector is standard for all power supplies conforming to the ATX and ATX12V 1.x form factors, and consists of a Molex Mini-Fit, Jr. connector housing with female terminals. For reference, the connector is Molex part number 39-01-2200 and the terminals are part number 5556. The pinout is shown in the following figure.

ATX and ATX12V 1.x 20-pin Main Power Connector:

Pin 11 might have a second orange or brown wire, used for +3.3V sense feedback - used by the power supply to monitor 3.3V regulation.

Pin 18 might be N/C (no connection) on some later model supplies or motherboards since -5V was removed from the ATX12V 1.3 and later specifications. Supplies with no connection at pin 18 should not be used with older motherboards that incorporate ISA Bus slots.


Each terminal in the Main power connector is rated to handle up to 6 amps of current. By counting the number of terminals for each voltage level, the power handling capability of the connector can be calculated as shown in the following table.


20-pin Main Connector Power Rating
Volts Terminals Amps Watts
+3.3V 3 18 59.4
+5V 4 24 120
+12V 1 6 72
Total Watts: 251.4



This means that the total power handling capacity of this connector is only 251 watts, which is lower than many systems need today. Unfortunately drawing more power than this maximum rating through the connector will cause it to overheat. I'm sure you can appreciate how inadequate this has become today, for example it certainly doesn't make sense to manufacture a 400 or 500 watt power supply, when the Main power connector can only handle 251 watts without melting! That would be like building a car that could go 200 MPH, and equipping it with tires that were rated for only 100 MPH. Everything would be fine until you exceeded the tire's rated speed, after which the results would not be pretty.

6-pin Auxiliary Power Connector
The 6-pin Auxiliary power connector was added as a safety or stopgap measure in the ATX 2.02/2.03 and ATX12V 1.x specifications for systems where the +3.3V and +5V power draw could exceed the 18A and 24A respective maximums allowed using only the Main connector. These conditions would normally be met in systems requiring 300W or higher output power supplies. The Auxiliary power connector is a 6-pin Molex 90331-0010 connector, which is similar to the motherboard power connectors used on older AT/LPX power supplies for Baby-AT motherboards. The pinout is shown in the following figure.

ATX 2.02/2.03 and ATX12V 1.x Auxiliary Power Connector:



Each terminal in the Auxiliary power connector is rated to handle up to 5 amps of current, slightly less than the Main power connector. By counting the number of terminals for each voltage level, the power handling capability of the connector can be calculated as shown in the following table.

6-pin Auxiliary Connector Power Rating
Volts Terminals Amps Watts
+3.3V 2 10 33
+5V 1 5 25
+12V 0 0 0
Total Watts: 58



This means that the total power handling capacity of this connector is only 58 watts. Drawing more power than this maximum rating through the connector will cause it to overheat.

Combining the 20-pin Main plus the Auxiliary power connector would result in a maximum power delivery capability to the motherboard of 309 watts.

4-pin +12V Power Connector
The 4-pin +12V power connector is specified for all power supplies conforming to the ATX12V form factor, and consists of a Molex Mini-Fit, Jr. connector housing with female terminals. For reference, the connector is Molex part number 39-01-2040 and the terminals are part number 5556. This is the same style of connector as the ATX Main power connector, except with fewer pins.

The pinout is shown in the following figure.

ATX12V +12V Power Connector:




Using standard terminals each pin in the +12V connector is rated to handle up to 8 amps of current. Even though it uses the same design and same terminals as the Main power connector, the current rating per terminal is higher on this 4-pin connector than on the 20-pin Main because there are fewer terminals overall. By counting the number of terminals for each voltage level, the power handling capability of the connector can be calculated as shown in the following table.

4-pin +12V Connector Power Rating:
Volts Terminals Amps Watts
+3.3V 0 0 0
+5V 0 0 0
+12V 2 16 192
Total Watts: 192



This means that the total power handling capacity of this connector is 192 watts, which is available to and used only by the processor. Drawing more power than this maximum rating through the connector will cause it to overheat.

Combining the 20-pin Main plus the 4-pin +12V power connector would result in a maximum power delivery capability to the motherboard of 443 watts. Adding the Auxiliary connector as well would raise this to 501 watts total. The important thing to note is that adding the +12V connector provides the ability to support power supplies of up to 500 watts or more without overloading and melting the connectors.

Why 2 or 3 Motherboard Power Connectors?
Only the 20-pin Main power connector was included in the original ATX specification, and it was sufficient to power PC motherboards and processors in the mid-'90s requiring 251 watts or less total power. However, by the late '90s, motherboard and processor power requirements had increased, and in some systems the Main power connector could no longer shoulder the load. Motherboards and processors that drew more than 251 total watts would potentially overheat the terminals and melt the connector housing, something I witnessed several times myself.

Rather than change the design of the Main connector and cause incompatibilities with motherboards that didn't need the additional power, Intel added the Auxiliary power connector as a fix to the ATX 2.02 specification in 1998. The Auxiliary connector was designed to deliver up to 58 watts of additional +3.3V and +5V power to power-hungry motherboards, which often needed the additional power for CPU , memory and AGP slot voltage regulators. Even though incorporating the additional Auxiliary connector was basically a good idea, most motherboards I saw continued to use only the single Main power connector, even if it might be overloaded.

While the Auxiliary connector could provide extra +3.3V and +5V power, it did not provide any additional +12V power. The debut of the Pentium 4 processor in 2000 brought even greater power demands from the processor. CPUs run on very low voltages, which are normally provided via voltage regulators on the motherboard. These regulators take in the voltage provided by the power supply and convert it to that required by the processor. Since power equals volts times amps, for the same power level, the more volts you provide the regulators, the fewer amps they require. Therefore, in order to reduce the overall current (amps) being delivered to the motherboard, a change was made to the CPU voltage regulators such that they would run on +12V power, instead of the +3.3V or +5V they had been previously using.

This unfortunately created another power problem, as even when combining both the Main and Auxiliary power connectors, there was only a single +12V terminal supplying up to 6 amps of current to the motherboard. Therefore, in order to provide additional +12V power and also preserve compatibility with the Main and Auxiliary connectors that had already been defined, in early 2000 Intel added the +12V power connector to the ATX 2.1 specification. This new connector was designed specifically to deliver up to 192 watts of power for the high output voltage regulators required by the Pentium 4 and newer processors.

Power supplies with the +12V power connector were called ATX12V supplies, and a special ATX12V form factor specification was created just for the power supply. Because the +12V power connector was initially used on Pentium 4 motherboards, it became unofficially known as the P4 connector even though AMD processor based motherboards began using it as well. By the end of 2001 most motherboards began requiring this connector, and most power supplies being sold for PCs were the ATX12V type.

Since the +12V power connector was specifically for the CPU, motherboards incorporating this connector would not run if power was not supplied to it. If you were installing a motherboard that used this connector and your power supply did not feature it as well, then you could purchase an adapter that would convert one of the Peripheral power connectors (normally used for disk drives) to a +12V power connector. If the power supply had the +12V connector but the motherboard did not, then you could leave the +12V connector unplugged.

Because the ATX12V standard included the same Main and Auxiliary connectors as had previously been defined, an ATX12V power supply was fully backwards compatible with ATX and could be used to replace a regular ATX type. If a motherboard did not require the extra +12V or Auxiliary connectors, they could simply remain unplugged inside the case. As a consequence of changing the CPU voltage regulators on the motherboard to use +12V power, the load on the +3.3V and +5V rails was reduced such that the Auxiliary connector would no longer be necessary, and many ATX12V supplies came without it. The Auxiliary connector was officially removed from the ATX12V 2.0 specification in 2000. Some ATX12V power supplies still continue to include the Auxiliary power connector, and of course you should be sure it is present if required by your motherboard. Because of the way the Auxiliary and +12V connectors were added to the design, newer power supplies would almost always be interchangeable with those in older systems. In fact one of the greatest things about the ATX and ATX12V standards is that aside from a few anomalies (like certain proprietary Dell systems made in the late '90s with non-standard power connector pinouts), most ATX12V power supplies will work in most PCs built from 1996 up to the present.

New Form Factors
Unfortunately recent changes in motherboard and power supply form factors may mean that compatible power supply connections can't be taken for granted any longer. The recent changes in power supply form factors mostly relate to newer motherboards, processors and even video cards requiring ever increasing amounts of power. Let's examine the new form factors and how they differ from the traditional ATX and ATX12V 1.x standards.

ATX12V 2.x

Starting in June 2004 the new PCI Express bus first appeared on motherboards. PCI Express is a type of serial bus, with standard slots having a single channel or lane of communications. These single lane slots are called x1 slots, and are designed for normal peripheral cards such as network cards, sound cards and the like. PCI Express also includes a special higher bandwidth slot with 16 lanes (called an x16 slot), which is especially designed for use by video cards. 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 problem was that the 20-pin Main connector had only a single +12V pin, and the new video cards would require more +12V power than a single pin could safely deliver. The +12V connector that had already been added was specifically for the CPU, and not available to other devices. Rather than add another supplemental or auxiliary connector as they had done before, Intel decided that it was finally time to upgrade the Main power connector to supply more power.

The result was officially called ATX12V 2.0, released in February 2003. ATX12V 2.0 included two major changes from the previous ATX12V 1.x specifications. This 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 4 more pins supplying additional +3.3V, +5V, and +12V power plus a ground. The inclusion of these extra pins not only satisfied the power requirements for PCI Express video cards drawing up to 75 watts, but also made the older 6-pin Auxiliary connector unnecessary. The pinout of the new 24-pin Main power connector is shown in the following figure.

ATX12V 2.x 24-pin Main Power Connector:



Pin 13 might have a second orange or brown wire, used for +3.3V sense feedback - used by the power supply to monitor 3.3V regulation.

Pin 20 might be N/C (no connection) since -5V was removed from the ATX12V 2.01 and later specifications. Supplies with no connection at pin 20 should not be used with older motherboards that incorporate ISA Bus slots.


It is interesting to note that the 24-pin connector is not really that new, it actually first appeared in the SSI (Server System Infrastructure) EPS (Entry Power Supply) specification released in 1998. SSI (www.ssiforum.org) is an initiative designed to create standard interfaces for server components, including power supplies. The 24-pin Main power connector was first created for servers since at the time only servers needed the higher levels of power. Since today's PCs draw the same power levels as servers did years ago, rather than reinvent an incompatible connector the ATX12V 2.0 standard merely incorporated the 24-pin connector already specified in the SSI EPS standard.

As compared to the previous 20-pin design, the 24-pin Main power connector includes additional +3.3V, +5V and +12V terminals allowing a substantially greater amount of power to be delivered to the motherboard. Each terminal in the Main power connector is rated to handle up to 6 amps of current. By counting the number of terminals for each voltage level, the power handling capability of the connector can be calculated as shown in the following table.

24-pin Main Connector Power Rating:
Volts Terminals Amps Watts
+3.3V 4 24 79.2
+5V 5 30 150
+12V 2 12 144
Total Watts: 373.2



This means that the total power handling capacity of this connector is 373 watts, substantially higher than the 251 watts available in the previous 20-pin connector. Combining the 24-pin Main plus the 4-pin +12V power connector results in up to 565 watts total power available to the motherboard and processor! This is more than enough to support the highest output power supplies on the market today.

PCI Express x16 Graphics Power Specification

Although the ATX12V 2.x specifications includes a new 24-pin Main power connector with more power for devices like video cards, the design was intended to power a video card drawing up to 75 watts maximum. There are already some video cards on the market drawing more power than that, for example the NVIDIA GeForce 6800 Ultra draws 110 watts and future cards may draw even more power. Since greater than 75 watts cannot be provided via the motherboard directly, the PCI-SIG (Special Interest Group) developed a standard for supplying power directly to the video card from the power supply via an additional graphics power connector. This will likely be included in future ATX12V power supply standards as well.

The PCI Express x16 Graphics Power spec. consists of a 6-pin Molex Mini-Fit, Jr. connector housing with female terminals, used to provide power directly to the video card. For reference, the connector is Molex part number 39-01-2060 and the terminals are part number 5556. This is the same style of connector as the Main and +12V power connectors. The pinout of the connector is shown in the following figure.

PCI Express x16 Graphics Power Connector:

Using standard terminals each pin in the PCI Express graphics power connector is rated to handle up to 8 amps of current. By counting the number of terminals for each voltage level, the power handling capability of the connector can be calculated as shown in the following table.

6-pin Graphics Connector Power Rating:
Volts Terminals Amps Watts
+3.3V 0 0 0
+5V 0 0 0
+12V 3 24 288
Total Watts: 288


This means that the total power handling capacity of this connector is 288 watts, which should be more than enough for even the most power hungry video cards in the future.

A new power supply incorporating a 24-pin Main, 4-pin +12V, and 6-pin PCI Express graphics power connector would have a maximum rated power delivery capability to the motherboard of 853 watts! It is important to note that just because a power supply has those connectors doesn't mean that it could truly deliver that much power, only that if it could, the connectors could handle that much power without overheating.

Backwards and Forwards Compatibility
If you have reached this point, I'm sure you have some questions. For example, what happens if you purchase a new power supply that has a 24-pin Main power connector but your motherboard has only a 20-pin Main power socket? Likewise, what if you purchase a new motherboard that has a 24-pin Main power socket, but your power supply has only a 20-pin Main power connector? The answers to these questions are surprising to say the least.

First let me say that there are adapters that can convert a 24-pin connector to a 20-pin type, and the other way around, but surprisingly these adapters are NOT actually necessary, or even desirable. The plain truth is that compatibility has been engineered into the connectors, power supplies and motherboards from the start.

If you look at the 24-pin Main power connector diagram and compare it to the previous 20-pin design (both are shown earlier in this article), you'll see that the extra 4-pins are all placed on one end of the connector, and all of the other pins are defined the same as they were previously. The design of these connectors is such that it allows an interesting bit of backwards compatibility. The result is that you can plug a 24-pin Main connector directly into a motherboard that has a 20-pin socket (and vice versa), WITHOUT using an adapter! The trick is to position the connector such that the 4 extra pins are empty. Depending on the latch design, the latch on the side may not engage, but the connector will otherwise plug in and operate properly.

The following figure shows how you would connect a new power supply with a 24-pin connector to a motherboard that has only a 20-pin socket. The terminals on the 24-pin connector that are highlighted in gray would plug directly into the 20-pin socket, while the white highlighted terminals would remain free and unconnected.

Connecting a 24-pin Main Power Connector to a 20-pin Motherboard Socket:

 



Logically this works because the first 20-pins of the 24-pin connector that match the 20-pin motherboard socket contain the correct signals in the correct positions. The only problem that might arise is if there is some component on the motherboard directly adjacent to the end of the 20-pin power socket which would physically interfere with the 4 extra unused terminals on the 24-pin connector.

What about the opposite condition, where you have a new motherboard with a 24-pin socket, but your power supply has only a 20-pin connector? Well, this plugs in perfectly as well, as shown by the following figure. The gray highlighted terminals in the 20-pin connector plug into the gray highlighted terminals in the 24-pin socket, with the 4 white highlighted terminals in the socket remaining open and unconnected.

Connecting a 20-pin Main Power Connector to a 24-pin Motherboard Socket:



This also works because the 20-pin gray highlighted portion is the same on both the connector and the socket. But this example raises another question; will the motherboard operate properly without the extra power pins? Since the extra signals are merely additional voltage pins which are already present in the remaining part of the connector, the answer should be yes, but if the motherboard draws a lot of power, it may overload the remaining pins. After all, preventing overloads is the reason why the extra pins were added in the first place.

Fortunately even that problem has been solved. All of the motherboards I've seen which use a 24-pin Main power connector also have an additional Peripheral (i.e. disk drive) power connector on-board, which is designed to provide the extra power that would be missing if you connect a 20-pin Main power connector from your power supply. The documentation for the motherboard refers to this as an Alternate power connector. The following figure shows an Intel D925XBC motherboard, which features a 24-pin Main, 4-pin +12V, and 4-pin Alternate power connector.

Intel D925XBC Motherboard Power Connectors:



The +12V power connector is always required since that provides power to the CPU. If you also plug a 24-pin Main power connector into the 24-pin socket on the motherboard, than the Alternate power connection is not necessary. However if you plug a 20-pin Main power connector into the 24-pin Main power socket on the motherboard, then simply select a spare peripheral (disk drive) power connector from the power supply and plug it into the Alternate power connector. Most power supplies have several extra peripheral power connectors for supporting additional drives. Using a 20-pin Main and the Alternate power connector satisfies the power requirements for the motherboard and any PCI Express x16 video cards drawing up to 75 watts.

As a side note, you should be careful when plugging in the mismatched connectors so that they are offset properly. The Main, +12V, and PCI Express graphics connectors are Molex Mini-Fit, Jr. type connectors that are keyed by virtue of a series of different shaped plastic protrusions used around the terminals, which fit similar shaped holes in the mating connectors. This keying is designed to prevent backwards or improper off-center insertion, however I have found two problems with the keying that should be noted. One is that some alternate low-quality connector brands are built to looser tolerances than the original high-quality Molex versions, and the sloppier fit of the low-quality versions may allow improper insertion. The other problem is that with sufficient force, the keying on even the high-quality versions can be overcome. Since plugging a 20-pin connector into a 24-pin socket, or likewise a 24-pin connector into a 20-pin socket is designed to work even though they don't fully match up, you need to make sure you have the offsets correct or you risk damaging the board when you power it up.

Conclusion
New motherboards with PCI Express slots incorporate a 24-pin Main power connector, and new power supplies conforming to the ATX12V 2.x form factor standards also include this connector as well. The 24-pin Main power connector found on these newer supplies will plug directly into the 20-pin Main power socket on older motherboards with no adapter required. Likewise, you can plug an older 20-pin Main power connector into the 24-pin socket on a newer motherboard, as long as you also plug a Peripheral (disk drive) power connector into the Alternate power connector on the motherboard as well.

Using a power supply with a 24-pin Main connector, or combining a 20-pin Main plus the Alternate connector, allows PCI Express motherboards to provide power for up to 75 watt PCI Express x16 video cards. If the video card needs more than 75 watts of power, then the card and power supply will incorporate a 6-pin PCI Express Graphics power connector to deliver the additional power required.

Note that adapters are not required for the Main power connector when converting from 24-pin to 20-pin and vice versa. Adapters can be useful to convert Peripheral (disk drive) power connectors into either +12V power or PCI Express Graphics power connectors if your existing power supply lacks those connectors.

With backwards compatibility insuring that the new 24-pin power connector will plug into older 20-pin motherboard sockets, when purchasing a new power supply I now recommend only those with 24-pin Main power connectors, which are usually sold as ATX12V 2.x or PCI Express models. For the most flexible and future-proof supply, also make sure the power supply includes a 6-pin PCI Express Graphics connector as well as integrated SATA (Serial ATA) drive power connectors. An example of a highly rated power supply incorporating all of these features is the Turbo-Cool 510 Express from PC Power and Cooling (www.pcpowercooling.com). Choosing a power supply with these features provides flexibility that allows it to work not only in newer systems, but in virtually all older ATX systems as well, and with no adapters required.