26.3 Power Connectors

All power supplies provide two types of power connectors. The first powers the motherboard and differs according to form factor. The second powers drives and other internal peripherals, and comes in two varieties, which are described below. Tower/AT and Tower/BAT power supplies include a third type of power connector—a high-voltage cable that connects the power supply to an external main power switch.

26.3.1 AT Main Power Connector

Table 26-6 lists the standard AT Main Power Connector pinouts used by all AT power supply variants, including Desktop/AT, Tower/AT, Desktop/BAT, Tower/BAT, and LPX. PC/XT power supplies use the same pinouts, except that pin P8-2 is unused. In addition to supplying various standard voltages, this connector includes a special-purpose pin. Pin P8-1, PG, carries the Power Good signal, which the power supply asserts once it has started and stabilized. The motherboard will not attempt to boot until the power supply asserts PG.

Table 26-6. Standard motherboard connector pinouts for an AT power supply

Pin	Color	Signal		Pin	Color	Signal
P8-1	orange (white)	PG		P9-1	black	ground
P8-2	red	+5V		P9-2	black	ground
P8-3	yellow	+12V		P9-3	white (blue)	-5V
P8-4	blue (brown)	-12V		P9-4	red	+5V
P8-5	black	ground		P9-5	red	+5V
P8-6	black	ground		P9-6	red	+5V

Note that P8 and P9 are separate connectors from the power supply, but mate to a combined connector on the motherboard.

The P8 and P9 connectors are individually keyed, but it is possible to connect P8 from the power supply to the P9 connector on the motherboard and vice versa, with potentially catastrophic results.

P8 and P9 were IBM's original designation for these connectors. Not all power supplies or motherboards label these connectors, so you may have to determine which is P8 and which is P9 by examining wire colors. The colors shown are those most commonly used, but some AT power supplies use the alternative (ATX) color coding shown in parentheses. We have also seen AT power supplies that use completely nonstandard wire colors, such as green for ground wires.

When installing an AT power supply, the key factor is to ensure that, regardless of wire colors or connector labels, the connectors are aligned so as to place all four ground pins contiguously. It's safer still to examine the documents for the motherboard and power supply. We have seen a few proprietary motherboards and power supplies that used standard connectors but with completely nonstandard pinouts. This was clearly done to force customers to purchase replacement components from the original vendor, and is happily not something you're likely to encounter on any recent system.

26.3.2 ATX Main Power Supply Connector

Table 26-7 lists the pinouts for the ATX Main Power Supply Connector of an ATX power supply. This is a 2 x 10 connector with pin 1 keyed. The motherboard connector is a Molex 39-29-9202 or equivalent. The power supply connector is a Molex 39-01-2200 or equivalent. All wires are 18 AWG, except pin 11, which is specified as 22 AWG. For power supplies 300W or larger, the specification recommends using 16 AWG wires for +3.3VDC, +5VDC, and COM (ground). Note that wire colors are those recommended and commonly used, but may vary.

Table 26-7. ATX Main Power Supply Connector

Pin	Color	Signal		Pin	Color	Signal
1	orange	+3.3VDC		11	orange	3.3VDC
2	orange	+3.3VDC		12	blue	-12VDC
3	black	COM		13	black	COM
4	red	+5VDC		14	green	PS_ON
5	black	COM		15	black	COM
6	red	+5VDC		16	black	COM
7	black	COM		17	black	COM
8	gray	PW_OK		18	white	-5VDC
9	purple	5VSB		19	red	5VDC
10	yellow	+12VDC		20	red	5VDC

In addition to supplying various standard voltages, this connector includes four special-purpose pins:

Pin 8 (PW_OK)

PW_OK is the ATX equivalent of the AT Power Good signal, which the power supply asserts once it has stabilized. The motherboard will not attempt to boot until the power supply asserts PW_OK.

Pin 9 (5V_SB)

5V_SB is the +5V standby circuit, which supplies +5V at low amperage to the motherboard even when the power supply is off. Any ATX power supply must provide at least 10 mA 5V_SB, but motherboards with the Wake-on-LAN (WOL) feature require 720 mA, which all ATX 2.03-compliant power supplies and most good ATX power supplies of earlier vintage also provide.

Pin 11 (Remote Sensing)

On the critical +3.3V rail, small loads can cause large percentage shifts in voltage. This pin provides a means for the power supply to detect the actual voltage present on the +3.3V rail at the main power connector and modify its output to compensate for up to 100 mV of drop due to cable, connectors, and PCB traces, thereby maintaining +3.3V within tolerance.

Pin 14 (PS_ON)

PS_ON is used by the motherboard to turn the power supply on and off.

We recently learned that Dell has been using nonstandard motherboards and power supplies, apparently since September 1998. Although recent Dell power supplies and motherboards use what looks like a standard ATX power connector, the pinouts are different. Replacing a Dell power supply with a standard ATX power supply may destroy the motherboard and/or power supply as soon as you apply power to the system. Similarly, upgrading the motherboard in a Dell system with a standard ATX motherboard while continuing to use the standard Dell power supply destroys the motherboard and/or power supply as soon as you apply power to the system. This situation is particularly insidious because Dell uses what appear to be standard components. For example, they buy Intel motherboards by the million, and those "Dell-version" motherboards resemble standard Intel motherboards in all respects except that the power supply connector is wired differently. For more information, see http://www.hardwareguys.com/dellwarn.html.

26.3.3 ATX Auxiliary Power Supply Connector

The ATX Auxiliary Power Supply Connector, shown in Table 26-8, is recommended by the ATX Specification Version 2.03 for motherboards that require 250W or 300W power supplies. This is an in-line 6-pin connector with pin 6 keyed. The motherboard connector is a Molex in-line 6-pin 15-48-0412 header or equivalent. The power supply connector is a Molex 90331-0010 or equivalent. Few inexpensive power supplies provide this connector, but it is present on many better power supplies. Few motherboards include this connector. Wire colors are standard.

Table 26-8. ATX Auxiliary Power Supply Connector

Pin	Color	Signal		Pin	Color	Signal
1	black	COM		4	orange	3.3V
2	black	COM		5	orange	3.3V
3	black	COM		6	red	5V

26.3.4 ATX Optional Power Supply Connector

In addition to the main and auxiliary power connectors, ATX 2.03 defines the ATX Optional Power Supply Connector, shown in Table 26-9. This is a 2 x 3 connector with pin 1 keyed. The motherboard connector is a Molex 39-30-1060 or equivalent. The power supply connector is a Molex 39-01-2060 or equivalent. All wires are 22 AWG. Wire colors are standard; the first color indicates the base color, and the second the stripe color.

Table 26-9. ATX Optional Power Supply Connector

Pin	Color	Signal		Pin	Color	Signal
1	white	FanM		4	white/black	1394R
2	white/blue	FanC		5	white/red	1394V
3	white/brown	Sense		6	NC	Reserved

ATX Optional Power Supply Connector signals perform the following functions:

FanM

FanM is a two-pulse/revolution signal generated by the power supply fan that notifies the system of current fan speed. If this signal drops, the motherboard realizes immediately that the power supply fan has failed, and can shut down the system in an orderly manner.

FanC

FanC is an optional signal generated by some motherboards to control fan speed for power supplies that are designed to allow this. The signal can range from 0VDC to +12VDC. A signal of +1V or less is recognized by the fan as an order to shut down, and a signal of +10.5V or more is recognized as an order to run at full speed. Intermediate voltage levels, which are supported by some motherboards and some fans, allow the system to instruct the fan to run at some intermediate speed. If this signal is left open (0VDC), properly designed fans run at full speed.

Sense

This is a supplementary +3.3VDC remote sense line, which allows the power supply to monitor the actual voltage at the motherboard connector of the nominal +3.3VDC rail and adjust it to stay within specifications.

1394R

1394R provides an isolated ground return path for the 1394V voltage rail, described below.

1394V

1394V is a segregated voltage supply rail for powering IEEE-1394 devices. The voltage on this rail depends on the IEEE-1394 implementation, may range between +8VDC and +40VDC, and is typically unregulated. If implemented, this rail should deliver voltage only while PS_ON on the main connector is asserted low.

26.3.5 ATX12V Power Supply Connector

With the introduction of the Pentium 4 processor, Intel extended the ATX 2.03 specification to define a new type of power supply called ATX12V, which is a superset of ATX 2.03. ATX12V power supplies include an additional 2 x 2 4-pin +12V power connector, shown in Table 26-10. The purpose of this new connector is to deliver more +12V current to the motherboard than is available from the standard ATX main power connector. The presence of the 4-pin +12V connector indicates that a power supply is ATX12V. The absence of that connector indicates that it is a standard ATX power supply. Wire colors are standard.

Table 26-10. ATX12V Power Supply Connector

Pin	Color	Signal		Pin	Color	Signal
1	black	COM		3	yellow	+12VDC
2	black	COM		4	yellow	+12VDC

According to Intel's policy, all Pentium 4 motherboards and all power supplies used with those motherboards must include the 4-pin +12V connector. Because installing such motherboards in existing systems may also require upgrading the power supply, some third-party motherboard makers have produced motherboards that lack the 4-pin +12V connector. This allows those motherboards to be installed in existing systems without a power supply upgrade, but we regard this as poor practice and suggest you avoid using such motherboards. In addition to the new 4-pin +12V connector, ATX12V power supplies must also provide higher +5VDC standby voltage, required for systems that meet Intel's Instantly Available PC initiative.

If you already have a good high-capacity standard ATX 2.03 power supply, you needn't replace it when you install a motherboard that requires ATX12V. Instead, buy an ATX12V adapter cable. These cables have a standard female drive power connector on one end and the 4-pin +12VDC connector on the other. We use the $8 PC Power & Cooling Pentium 4 12VATX Adapter, shown in Figure 26-1, for this purpose. Note, however, that many older and/or inexpensive ATX power supplies have very loose regulation on the +12V rail, and so may not be suitable for powering a Pentium 4 motherboard.

Figure 26-1. Pentium 4 12VATX Adapter

The requirement for the new connector is due both to increased current draw by Pentium 4 processors and a shift in Intel's plans for future motherboards. Current ATX 2.03 motherboards power components with DC/DC conversion from +5VDC and +3.3VDC sources. However, relative to +12VDC DC/DC conversion, +5VDC and +3.3VDC DC/DC conversion yields lower power transmission and conversion efficiencies. The change in emphasis to +12VDC DC/DC conversion provides increased efficiency and flexibility for forthcoming motherboards.

Most but not all ATX12V-compliant power supplies are backward compatible with ATX 2.03 motherboards because they supply the same voltages and currents within the same specifications on the same rails as an ATX 2.03-compliant power supply. However, some ATX12V power supplies do not provide the 3.3V rails required in the ATX 2.03 specification. Such power supplies are intended for special purposes, and cannot be used with standard ATX motherboards. You can therefore use most ATX12V power supplies with any existing or future ATX-family motherboard, but you cannot use an old-style ATX 2.03 power supply with motherboards that have a 4-pin +12V connector, nor can you use an ATX12V power supply that lacks 3.3V rails with an ATX motherboard, whether or not that motherboard has the 4-pin +12VDC connector. For that reason, any new ATX power supply you buy should be an ATX12V-compliant unit that provides 3.3V rails, which will allow that power supply to be used with any modern motherboard.

26.3.6 NLX Power Connectors

The NLX Main Power Supply Connector uses the same pinouts, wire colors, and physical connectors as the ATX Main Power Supply Connector shown in Table 26-7. The NLX specification also defines the NLX Optional Power Supply Connector, shown in Table 26-11. This connector uses the same physical connectors and 22 AWG wire as the ATX Optional Power Supply Connector described in the preceding section, but uses different wire colors and a slightly different pinout. Wire colors are standard.

Table 26-11. NLX Optional Power Supply Connector

Pin	Color	Signal		Pin	Color	Signal
1	white	FanM		4	NC	Reserved
2	blue	FanC		5	grey	1394
3	brown	Sense		6	NC	Reserved

Pin 5 (1394) on this connector is analogous to pin 5 (1394V) on the ATX connector, and also supplies voltage to unpowered 1394 devices. However, rather than using pin 4 as a ground return path for 1394 voltage, this connector leaves pin 4 unconnected. It appears unwise to share the main system ground return path with the ground return path for unregulated 1394 voltage from pin 5, particularly with pin 4 sitting there so obviously unused. If you understand the reason for this difference, please let us know.

26.3.7 SFX Power Connectors

The SFX Baseboard Connector uses the same pinouts, wire colors, and physical connectors as the ATX Main Power Supply Connector shown in Table 26-7, with one or two exceptions:

• Pin 9 (+5V_SB) is an optional signal for SFX power supplies. If present, it allows the motherboard to control the power supply, just as with ATX and NLX. If pin 9 is not connected, the power supply must be controlled by a standard AC on/off switch.

• Pin 18 (-5VDC) is not connected on SFX power supplies, because -5VDC is required only by ISA expansion slots, which are not supported by SFX systems.

The SFX specification also defines the required SFX Control Connector, shown in Table 26-12. This connector uses the same physical connectors and 22 AWG wire as the ATX and NLX Optional Power Supply Connectors described in the preceding sections, but has only one connection, FanON/OFF, which corresponds to FanC. Wire color is standard.

Table 26-12. SFX Control Connector

Pin	Color	Signal		Pin	Color	Signal
1	NC	Reserved		4	NC	Reserved
2	blue	FanON/OFF		5	NC	Reserved
3	NC	Reserved		6	NC	Reserved

26.3.8 Power-Supply-to-Device Connectors

Power supplies provide two types of connectors to power disk drives and other internal peripherals:

Peripheral Connector

The Peripheral Connector, shown in Figure 26-2, is often called a Molex Connector by technicians. The cable uses a Molex PS-8981-4P connector or equivalent, and 18 AWG wires. Pin 1, at left as you view the connector face, carries +12V and uses a yellow wire. Pins 2 and 3 are COM (ground) and use black wires. Pin 4 is +5V and uses a red wire.

Figure 26-2. Peripheral Connector

Floppy Drive Connector

The Floppy Drive Connector, shown in Figure 26-3, is often called a Berg Connector, and is also used by some other types of drives (5.25" floppy drives use the larger Molex connector also used by hard disks and other drives). The cable uses an AMP 171822-4 connector or equivalent. The wires are 20 AWG. Pin 1, at left as you view the connector face, carries +5V and uses a red wire. Pins 2 and 3 are COM (ground) and use black wires. Pin 4 is +12V and uses a yellow wire.

Figure 26-3. Floppy Drive Connector

Note that wire colors map to the same voltages on the Peripheral Connector and the Floppy Drive Connector, but the pinouts are exactly reversed. While building a home-made cable, we once toasted a drive by assuming the pinouts were identical, thereby putting +12V on a +5V device.

The number and type of device connectors provided are loosely linked to the form factor and power rating of the power supply. Generally, power supplies with low power ratings and those designed for smaller cases provide fewer device connectors, sometimes as few as three. Power supplies with higher power ratings and those intended to fit large tower cases provide more device connectors, sometimes as many as a dozen. So long as you do not exceed the power supply capacity, you can freely clone device connectors by adding Y-splitters, which are available for a couple dollars at any computer store.

26.3.9 Main Power Switch Power Connectors

Desktop/AT and Desktop/BAT power supplies have a built-in paddle switch to turn power on and off. ATX and ATX-variant power supplies seldom have a physical power switch, because they are turned on and off by the motherboard. Some ATX power supplies have a rocker switch on the back of the power supply that disconnects the power supply entirely from mains power. This can be useful because the alternative is disconnecting the power cable when you need to kill all power to the system, including +5V_SB, which is ordinarily always present.

Tower/AT and Tower/BAT power supplies have no built-in main power switch. Instead, they have four power leads that connect to a push-button or toggle switch on the case, as shown in Figure 26-4. These leads, which are usually white, black, blue, and brown, carry AC mains voltage to the power supply.

Figure 26-4. Typical connections for a Tower/AT or Tower/BAT power switch

Although these four wire colors are relatively standard, different switches require connecting them differently. These wires carry full mains voltage, which can kill you, so never work on them without first disconnecting the main power cable from the power supply. Connecting them improperly can also damage the power supply and the computer, so never use trial and error or guess about which wire goes where. Contact the case and power supply manufacturers to verify it. Because these wires carry high voltage, we recommend using electrical tape to insulate the connections.

26.3.10 Real-World Power Supplies Compared

Table 26-13 lists the output by voltage of three nominally 300-watt power supplies. The PC Power & Cooling Turbo Cool 300 ATX is a premium unit, with a street price of about $90; the Antec PP303X is a name-brand unit that sells for about $40; the Astec SA302-3515-288 is not readily available to individuals, but is shown for comparison because many first-tier OEMs (such as Compaq, Dell, Gateway, and IBM) use Astec power supplies for at least some of their systems. We would like also to have compared a no-name Pacific Rim power supply, but finding technical specifications for such units is impossible, probably because they vary so much from one lot to another that no one bothers to test them.

The three key voltage rails are +3.3V (used to power processors and memory), +5V (used to power other electronic components and some drive motors), and +12V (used to power most disk drive motors). Note that, although all the power supplies list individual maximum outputs for +3.3V and +5V, they also note maximum combined output of +3.3V and +5V-150W for the PC Power & Cooling unit, 160W for the Antec, and 170W for the Astec unit.

Table 26-13. Output comparison of three nominal 300-watt power supplies

	PC Power & Cooling	Antec	Astec
	Turbo-Cool 300 ATX	PP303X	SA302-3515-288
Rail	+/-	Amax	Watts	+/-	Amax	Watts	+/-	Amax	Watts
+3.3VDC	1%	14	46.2	4%	14	46.2	5%	23	75.9
+5VDC	5%	30	150	5%	30	150	+5/-4%	32	160
Maximum combined wattage	150			160			170
-5VDC	5%	0.3	1.5	5%	0.5	2.5	10%	0.5	2.5
+5VSB	5%	0.85	4.25	5%	2	10	10%	0.85	4.25
+12VDC	5%	12	144	5%	10	120	5%	12	144
-12VDC	5%	1	12	10%	0.5	6	10%	1	12
Total nominal wattage	357.95			334.7			398.65
Total deliverable wattage	311.75			298.5			332.75

The main differences between these power supplies are as follows:

Total wattage

The most obvious difference is in total wattage deliverable. The PC Power & Cooling and Astec units can both deliver well above the nominal 300 watts, whereas the Antec unit falls a bit short.

Individual wattages

The wattages deliverable on various rails differ significantly. In particular, the Astec unit delivers up to 75.9 watts of +3.3VDC, much more than the 46.2 watts deliverable by the other two units. In practice, however, this difference is less significant than it might seem, both because 46.2 watts of +3.3VDC is adequate for normal systems, and because the combined wattage deliverable at +3.3VDC and +5VDC is only 10 to 20 watts more than the other units.

Another significant difference is at +12VDC, where PC Power & Cooling and Astec units deliver 144 watts, 20% more than the Antec unit. +12V is used primarily for drive motors and some fans, so this differential may or may not matter on any particular system. On a heavily loaded system (we have one with three high-performance SCSI hard drives, a CD-ROM drive, a CD-RW drive, a DVD-RAM drive, and a tape drive) the PC Power & Cooling and Astec units are likely to run without problems, while the Antec unit may be gasping to provide the +12V wattage demanded by all these drives. As the voltage drops, the current increases, which can cause overheating and damage to both the power supply and the drives.

Regulation

The PC Power & Cooling and Antec units are both within ATX specifications for regulation on all voltage rails. The Antec unit exactly meets the ATX regulation requirements on all rails; the PC Power & Cooling unit meets minimum ATX requirements on all rails, and greatly exceeds them on +3.3VDC (which runs processors and memory) and -12VDC. The Astec unit is not ATX compliant, because its 5% regulation on +3.3VDC is less stringent than the ATX-required 4%.

A more subtle aspect of nominal wattage is component loading. A premium power supply may deliver its rated wattage while driving its components at only 50% of their rated capacity; a midrange name-brand power supply may deliver near its rated wattage while driving its components at 70% of their rated capacity. A no-name power supply will likely deliver nowhere near its rated wattage, and will drive its components at 100% (or even more) of their rated capacity to do so.

Component loading has two important aspects. First, components driven at a fraction of their rated capacity are likely to exceed their design life significantly, while those driven at (or above) their designed limits are likely to be short-lived. Second, a component that is "loafing" is likely to perform much better than one that is being driven at or above its design capacity. For example, components designed to supply +3.3V and driven at 50% of capacity are likely to supply 3.3V bang-on; those driven at 100% of capacity may deliver 3.3V nominal, but the actual voltage may vary significantly. Unfortunately, short of disassembling the power supply (never a good idea) and checking the number, size, and quality of the components, about the only thing you can do to assure that the power supply you choose uses good components is to buy a good name-brand power supply.

Comparing the full spec sheets for these three power supplies turns up other differences not shown in the table, of varying significance. For example, the PC Power & Cooling unit provides both the standard 20-pin ATX Main Power Supply Connector and the recommended 6-pin ATX Auxiliary Power Supply Connector, while the Antec and Astec units provide only the standard 20-pin connector. The hold time of the PC Power & Cooling unit is 20 ms, which is the minimum required for ATX compliance. The hold time of the Antec is specified as "< 20 ms" and that of the Astec as 17 ms at 115VAC input, which means that technically neither of these units is ATX compliant.

In short, there are very real differences between power supplies, even if they have the same nominal rating. The PC Power & Cooling unit is a superb power supply, which will supply clean, closely regulated power even when driven at its rated wattage, and is unlikely to fail even if used 24 x 7 at its rated output power. The Antec unit is a very good power supply, likely to work reliably in less demanding applications. The Astec unit is robust, but relatively poorly regulated, particularly relative to the PC Power & Cooling unit. About the best you can say for no-name units is that power is likely to come out of them, although how much, how well-regulated, and for how long is in doubt.