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Motherboard Power Connectors: Six-Pin Auxiliary And 24-Pin Main

Power Supply 101: A Reference Of Specifications
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Six-Pin Auxiliary Power Connector

As motherboards and processors have evolved, the need for power has become greater. The terminals in the main power connector are rated for six amps (A) using standard terminals, which allows for a maximum supply of approximately 250 watts to the motherboard. Because motherboards with high-speed processors and multiple cards installed could draw more power than that and power supply manufacturers were building supplies with 300-watt and higher ratings, melted connectors were becoming more and more common. The terminals in the main connector overheated under such a load.

To allow for additional power from the supply to the motherboard, Intel modified the ATX specification to add a second auxiliary power connector for high power-drawing ATX motherboards and 250-watt or higher rated supplies. The criteria is such that, if the motherboard could draw more than 18 A of +3.3 V power or more than 24 A of +5 V power, the auxiliary connector is required to carry the additional load. These higher levels of power are needed in systems using 250- or 300-watt or greater supplies.

The six-pin auxiliary power connector was added as a safety or stopgap measure in the ATX motherboard 2.02/2.03 and ATX12V 1.x power supply specifications for systems in which the +3.3 V and +5 V power draw could exceed the respective 18 A and 24 A maximums allowed using only the main connector with standard terminals. These conditions would normally be met in systems requiring 300 W or higher output power supplies. The auxiliary power connector is a six-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 pinouts of the auxiliary connector are shown below.

ATX Auxiliary Power Connector Pinout
Pin
Signal
Color
Pin
Signal
Color
1
GndBlack4
+3.3 VOrange
2
GndBlack5
+3.3 VOrange
3
GndBlack6
+5 VRed


Each terminal in the auxiliary power connector is rated to handle up to five amps of current, slightly less than the main power connector. By counting the number of terminals for each voltage level, you can calculate the power-handling capability of the connector, as shown below.

Six-Pin Auxiliary Power Connector Maximum Power-Handling Capabilities
Volts
No. pins
Watts
+3.3 V
2
33
+5 V
1
25
Total watts:
58
Terminals are rated five amps.
Ratings assume 18-gauge wire under standard temperature conditions.


This means 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.

Few motherboards actually used this connector, and few power supplies included it. Generally, if a motherboard includes this connector, you need a power supply that has it as well, but if the power supply includes the auxiliary connector but the motherboard does not, it can be left unconnected.

Starting in 2000, both motherboards and power supplies began including a different additional connector that was a better solution than the auxiliary connector. The most recent power supply form factor specifications have removed the auxiliary connector, rendering it an obsolete standard in modern systems.

ATX12V 2.x 24-Pin Main Power Connector

Starting in June 2004, the 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 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 six-pin auxiliary power supply connectors, especially when it came to +12 V power.

The problem was that the 20-pin main connector had only a single +12 V pin, but the new video cards required more +12 V power than a single pin could safely deliver. The +12 V connector that had already been added, as discussed in the next section, was specifically for the CPU and was unavailable to other devices. Rather than add another supplemental or auxiliary connector as it had done before, Intel eventually decided that it was finally time to upgrade the main power connector to supply more power.

The result was officially called ATX12V 2.0 and was released in February 2003. ATX12V 2.0 included two major changes from the previous ATX12V 1.x specifications: a new 24-pin main power connector and the elimination of the six-pin auxiliary power connector. The new 24-pin main power connector included four more pins supplying additional +3.3 V, +5 V, and +12 V power plus a ground. The inclusion of these extra pins delivered extra power to satisfy the power requirements for PCI Express video cards drawing up to 75 watts, but it also made the older six-pin auxiliary connector unnecessary. The pinout of the new 24-pin main power connector started to be implemented in motherboards in mid-2004. The motherboard connector pinout is shown below, as is the PSU connector.

Note: Even though one of the design goals for increasing the main power connector to 24 pins was to provide extra power for PCI Express video cards, many if not most high-end video cards need more than the 75 watts available directly through the PCIe x16 slot. Video cards requiring more will have one or more additional power connectors on the card, which are used to draw power directly from the PSU.

ATX12V 2.x 24-pin main power connector.ATX12V 2.x 24-pin main power connector.

ATX12V 2.x 24-Pin Main Power Supply Connector Pinout (Motherboard Connector)
Color
Signal
Pin
Pin
Signal
Color
Orange+3.3 V1311
+3.3 VOrange
Blue–12 V14
2
+3.3 VOrange
BlackGND
15
3
GND
Black
Green
PS_On
16
4
+5 V
Red
Black
GND
17
5
GND
Black
BlackGND
18
6
+5 V
Red
BlackGND
19
7
GND
Black
-
N/C
202
8
Power_GoodGray
Red
+5 V21
9
+5 VSB (Standby)Purple
Red+5 V22
10
+12 VYellow
Red+5 V23
11
+12 VYellow
Black
GND
24
12
+3.3 VOrange
1. Pin 13 might have a second orange or brown wire, used for +3.3 V sense feedback. The power supply uses this wire to monitor 3.3 V regulation.
2.  Pin 20 will be N/C (no connection) because –5 V was removed from the ATX12V 1.3 and later specifications.


It is interesting to note that the 24-pin connector is not really that new; it first appeared in the SSI EPS specification released in 1998. SSI (http://ssiforum.org/) is an initiative designed to create standard interfaces for server components, including power supplies. The 24-pin main power connector was created for servers because, at the time, only servers needed the additional power. Today’s PCs draw the same power levels as servers did years ago, so rather than reinvent an incompatible connector, the ATX12V 2.0 standard merely incorporated the 24-pin connector already specified in the SSI EPS standard.

Compared to the previous 20-pin design, the 24-pin main power connector includes additional +3.3 V, +5 V, and +12 V 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 six amps of current. By counting the number of terminals for each voltage level, you can calculate the power-handling capability of the connector, as shown in the following table.

Maximum Power-Handling Capabilities of the 24-Pin Main Power Connector
VoltsNo.
Pins
Using Std.
Terminals (W)
Using HCS
Terminals (W)
Using Plus HCS
Terminals (W)
+3.3 V4
79.2
118.8
145.2
+5 V5
150
225
275
+12 V2
144
216
264
Total watts:373.2
559.8
684.2
Standard terminals are rated six amps.
HCS terminals are rated nine amps.
Plus HCS terminals are rated 11 amps.
All ratings assume Mini-Fit Jr. connectors with 12–24 circuits using 18-gauge wire under standard temperature conditions.


This means the total power-handling capacity of this connector is 373 watts using standard terminals or 560 watts using HCS terminals, which is substantially higher than the 251 watts available in the previous 20-pin connector. Combining the 24-pin main and the four-pin +12 V power connector results in up to 565 watts (standard terminals) or 824 watts (using HCS terminals) total power available to the motherboard and processor! This is more than enough to support the most power-hungry motherboards and processors on the market today.

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