PC Repair and Maintenance: In-depth Look at Power Supply

Like this article? We recommend

Upgrading and Repairing PCs, 22nd Edition

Learn More Buy

Like this article? We recommend

Like this article? We recommend 

Upgrading and Repairing PCs, 22nd Edition

Learn More Buy

Motherboard Power Connectors

Every PC power supply has special connectors that attach to the motherboard, giving power to the system processor, memory, and all slotted add-on boards (ISA, PCI, AGP). Attaching these connectors improperly can have a devastating effect on your PC, including burning up both your power supply and motherboard. The following sections detail the motherboard power connectors used by various power supplies.

AT Power Supply Connectors

Industry standard PC, XT, AT, Baby-AT, and LPX motherboards all use the same type of main power supply connectors. These supplies feature two main power connectors (P8 and P9), each with 6 pins that attach the power supply to the motherboard. All standard PC power supplies that use the P8 and P9 connectors have them installed end to end so that the two black wires (ground connections) on both power cables are next to each other. Some power supplies have them labeled as P1/P2 instead. Because these connectors usually have a clasp that prevents them from being inserted backward on the pins on the motherboards, the major concern is getting the two connectors in the correct orientation side by side and also not missing a pin offset on either side. Following the black-to-black rule keeps you safe. You must take care, however, to make sure that no remaining unconnected motherboard pins exist between or on either side of the two connectors after you install them. A properly installed connector connects to and covers every motherboard power pin. If any power pins are showing on either side of the connectors, the entire connector assembly is installed incorrectly, which can result in catastrophic failure for the motherboard and everything plugged into it at the time of power-up. Figure 3.6 shows the P8 and P9 connectors (sometimes also called P1/P2) in their proper orientations when connecting.

Figure 3.6 The P8/P9 power connectors (sometimes also called P1/P2) that connect an AT/LPX power supply to the motherboard.

ATX Main Power Connector

The industry standard ATX power-supply–to–motherboard main connector is the Molex 39-29-9202 (or equivalent) 20-pin ATX style connector (see Figure 3.7). First used in the ATX form factor power supply, it also is used in the SFX form factor or any other ATX-based variations. The colors for the wires listed in Table 3.3 are those recommended by the ATX standard; however, they are not required for compliance to the specification, so they could vary from manufacturer to manufacturer. Note that I like to show these connector pinouts in a wire side view, which shows how the pins are arranged looking at the back of the connector (from the wire and not terminal side). This is because it shows how they would be oriented if you were back-probing the connector with the connector plugged in.

Figure 3.7 ATX style 20-pin motherboard main power connector, perspective view.

Figure 3.8 shows a view of the connector as if you were looking at it facing the terminal side.

Figure 3.8 ATX/NLX 20-pin main power connector, terminal side view.

Table 3.3 ATX Main Power Supply Connector Pinout (Wire Side View)

*Might have a second orange or brown wire, used for +3.3v sense feedback—used by the power supply to monitor 3.3v regulation.

ATX Auxiliary Power Connector

As motherboards and processors evolved, the need for power became greater. In particular, chipsets and DIMMs were designed to run on 3.3v, increasing the current demand at that voltage. In addition, most boards included CPU voltage regulators designed to convert +5v power into the unique voltage levels required by the processors the board supported. Eventually, the high current demands on the +3.3v and +5v outputs were proving too much for the number and gauge of the wires used. Melted connectors were becoming more and more common as these wires overheated under these loads.

Finally, Intel modified the ATX specification to add a second power connector for ATX motherboards and supplies. The criteria was that if the motherboard needed more than 18A of +3.3v power, or more than 24A of +5v power, an auxiliary connector would be defined to carry the additional load. These higher levels of power are normally necessary in systems using 250-watt to 300-watt or greater supplies.

This is a 6-pin Molex-type connector (see Figure 3.9). It is keyed to prevent misconnection.

Figure 3.9 ATX auxiliary power connector.

The pinouts of the auxiliary connector are shown in Table 3.4.

Table 3.4 ATX Auxiliary Power Connector Pinout

If your motherboard does not feature a mating auxiliary connector, it probably wasn't designed to consume a large amount of power, and the auxiliary connector from the power supply can be left unconnected. If your power supply is rated at 250 watts or larger, you should ensure that it has this connector and that your motherboard is capable of accepting it. This eases the load on the main power connector.

ATX12V Connector

Power for the processor comes from a device called the voltage regulator module (VRM), which is built into most modern motherboards. This device senses the CPU voltage requirements (usually via sense pins on the processor) and calibrates itself to provide the proper voltage to run the CPU. The design of a VRM enables it to run on either 5v or 12v for input power. Most have used 5v over the years, but many are now converting to 12v because of the lower current requirements at that voltage. In addition, the 5v already might be loaded by other devices, whereas, typically, only drive motors use the 12v. Whether the VRM on your board uses 5v or 12v depends on the particular motherboard or regulator design. Many modern voltage regulator ICs are designed to run on anything from a 4v to a 36v input, so it is up to the motherboard designer as to how they will be configured.

Although most motherboard VRM designs up through the Pentium III and Athlon/Duron use 5v-based regulators, a transition is underway to use 12v-powered regulators. This is because the higher voltage will significantly reduce the current draw. As an example, using the same 65W AMD Athlon 1GHz CPU, you end up with the levels of draw at the various voltages shown in Table 3.5.

Table 3.5 Levels of Draw at Various Voltages

As you can see, using 12v to power the chip results in only 5.4A of draw, or 7.2A assuming 75% efficiency on the part of the regulator.

So, modifying the motherboard VRM circuit to use the +12v power feed would seem simple. Unfortunately, the standard ATX 2.03 power supply design has only a single +12v lead in the main power connector. The auxiliary connector has no +12v leads at all, so that is no help. Pulling up to 8A more through a single 18ga. wire supplying +12v power to the motherboard is a recipe for a melted connector.

To augment the supply of +12v power to the motherboard, Intel created a new ATX12V power supply specification. This adds a third power connector, called the ATX12V connector, specifically to supply additional +12v power to the board. This connector is shown in Figure 3.10.

Figure 3.10 An ATX12V power connector.

The pinout of the +12v power connector is shown in Table 3.6.

Table 3.6 ATX +12v Power Connector Pinout (Wire Side View)

If you are replacing your motherboard with a new one that requires the ATX12V connection for the CPU voltage regulator, and yet your existing power supply doesn't have that connector, an easy solution is available. Merely convert one of the peripheral power connectors to an ATX12V type. PC Power and Cooling has released just such an adapter that can instantly make any standard ATX power supply into one with an ATX12V connector. The issue is not whether the power supply can generate the necessary 12v—that has always been available via the peripheral connectors. The ATX12V adapter shown in Figure 3.11 solves the connector problem quite nicely.

Figure 3.11 ATX12V adapter from PC Power and Cooling.

ATX Optional Connector

The ATX specification also defines an optional six-pin connector. This connector has two rows of three pins each to provide the signals and voltages. The computer can use these signals to monitor and control the cooling fan, monitor the +3.3v power to the motherboard, and provide power and grounding to IEEE 1394 (FireWire) devices.

This connector has gone through several revisions in pinout since first being published, and I have yet to see any motherboards or power supplies on the market that actually support it. In fact, the latest ATX/ATX12V Power Supply Design Guide published by Intel states, "Details of the 2x3 'Optional Power Connector' mentioned in the ATX 2.03 Specification are omitted from this design guide until such time as the signals on that connector are more rigidly defined."

Table 3.7 lists the pinout of the optional connector as defined in the ATX 2.03 Specification.

Table 3.7 ATX Optional Power Supply Connections

The FanM signal enables the operating system to monitor the status of the power supply's cooling fan so that it can take appropriate actions, such as shutting down the system if the fan fails.

The motherboard (under operating system control) can use the FanC signal with variable voltages to control the operation of the power supply's fan, shifting it into a low power state or shutting it off completely when the system is in sleep or standby mode. The ATX standard specifies that a voltage of +1v or less indicates that the fan is to shut down, whereas +10.5v or more instructs the fan to operate at full speed. The system designer can define intermediate voltages to operate variable-speed fans at various levels. If the power supply does not include a variable-speed fan circuit, any voltage level higher than +1v on the FanC signal is interpreted as a command to run the fan at its full (and only) speed.

The 1394 connectors are for powering an optional IEEE-1394 (FireWire or iLink) bus on a motherboard. The 1394V pin provides voltages from 8v to 40v to run FireWire peripherals off the bus, and the 1394R pin is a return or ground line for this power circuit. This separate power rail keeps the 1394 bus power separate from the system main power to prevent interference.

Dell Proprietary (Nonstandard) ATX Design

If you currently own a desktop system made between 1996 and 2000 from Dell, you will definitely want to pay attention to this section. A potential booby trap is waiting to nail the unsuspecting Dell system owner who decides to upgrade either the motherboard or power supply in his system. This hidden trap can cause the destruction of the motherboard, power supply, or both! Okay, now that I have your attention, read on....

As those of you who have attended my seminars or read previous editions of this book will know, I have long been a promoter of industry-standard PCs and components and wouldn't think of purchasing a desktop PC that didn't have what I consider an industry-standard form factor motherboard, power supply, and chassis (ATX, for example). I've been down the proprietary road before with systems from Packard Bell, Compaq, IBM, and other companies that used custom, unique, or proprietary components. For example, during a momentary lapse of reason in the early '90s, I purchased a Packard Bell system. I quickly outgrew the capabilities of the system, so I thought I'd upgrade it with a new motherboard and a faster processor. It was then that I discovered, to my horror, that LPX systems were not an interchangeable standard. Because of riser card differences, virtually no interchangeability of motherboards, riser cards, chassis, and power supplies existed. I had what I now refer to as a "disposable PC"—the kind you can't upgrade and have to throw away instead. Suddenly, the money I thought I had saved when initially purchasing the system paled in comparison to what I'd now have to spend to completely replace it. Lesson learned.

After several bad upgrade and repair experiences, I decided never again would I be trapped by systems using proprietary or nonstandard components. By purchasing only systems built with industry- standard parts, I could easily and inexpensively upgrade, maintain, or repair the systems for many years into the future. I have been preaching the gospel of industry-standard components in my seminars and in this book ever since.

Of course, building your own system from scratch is one way to avoid proprietary components, but often that route is more costly in both time and money than purchasing a prebuilt system. And what systems should I recommend for people who want an inexpensive prebuilt system but one that uses industry-standard parts so it can be inexpensively upgraded and repaired later? Although many system vendors and assemblers exist, I've settled on companies such as Gateway, MicronPC, and Dell. In fact, those are really the three largest system vendors that deal direct, and they mostly sell systems that use industry-standard ATX form factor components in all their main desktop system product lines. Or so I thought.

It seems that when Dell converted to the ATX motherboard form factor in mid-1996, it unfortunately defected from the newly released standard and began using specially modified Intel-supplied ATX motherboards with custom-wired power connectors. Inevitably, it also had custom power supplies made that duplicated the nonstandard pinout of the motherboard power connectors.

An even bigger crime than simply using nonstandard power connectors is that only the pinout is nonstandard; the connectors look like and are keyed the same as is dictated by true ATX. Therefore, nothing prevents you from plugging the Dell nonstandard power supply into a new industry-standard ATX motherboard you installed in your Dell case as an upgrade, or even plugging a new upgraded industry-standard ATX power supply into your existing Dell motherboard. But mixing either a new ATX board with the Dell supply or a new ATX supply with the existing Dell board is a recipe for silicon toast. How do you like your fried chips: medium or well-done?

Frankly, I'm amazed I haven't heard more about this because Dell has climbed to the lead in worldwide PC sales. In any case, I figure by getting this information out I can save thousands of innocent motherboards and power supplies from instant death upon installation.

If you've already fallen victim to this nasty circumstance, believe me, I feel your pain. I discovered this the hard way as well—by frying parts. At first, I thought the upgraded power supply I installed in one of my Dell systems was bad, especially considering the dramatic way it smoked when I turned on the system: I actually saw fire through the vents! Good thing I decided to check the color codes on the connectors and verify the pinout on another Dell system by using a voltmeter before I installed and fried a second supply. I was lucky in that the smoked supply didn't take the motherboard with it; I can only surmise that the supply fried so quickly it sacrificed itself and saved the motherboard. You might not be so lucky, and in most cases I'd expect you'd fry the board and supply together.

Call me a fool, but I didn't think I'd have to check the color-coding or get out my voltmeter to verify the Dell "pseudo-ATX" power connector pinouts before I installed a new ATX supply or motherboard. You'll also find that motherboard and power supply manufacturers don't like to replace these items under warranty when they are fried in this manner due to nonstandard connector wiring.

Dell's official explanation for its lack of conformance to the ATX standard was, "In the mid-90s the industry moved to a higher use of 3.3v motherboard components. Dell engineers designed a connector that supported the increased use of 3.3v current which differed from the industry proposed designs that we deemed less than robust." Unfortunately, this explanation doesn't hold much water because the standard ATX connector incorporated three 3.3v pins, allowing for up to 18A of current, and the addition of the Auxiliary Connector added two more pins with 10A of additional current. Dell's pseudo-ATX design had only three 3.3v pins in the Auxiliary Connector, which could supply only up to 15A to the board. You can see that even the main ATX Connector alone had more 3.3v current than Dell's design using two connectors!

Because its technical explanation fails to address the issue, the only other reason I can imagine it did this is to lock people into purchasing replacement motherboards or power supplies from Dell. What makes this worse is that Dell uses virtually all Intel-manufactured boards in its systems. One system I have uses an Intel D815EEA motherboard, which is the same board used by many of the other major system builders, including Gateway and Micron. It's the same, except for the power connectors, that is. The difference is that Dell has Intel custom-make the boards for Dell with the nonstandard connectors. Everybody else gets virtually the same Intel boards, but with industry-standard connectors.

Tables 3.8 and 3.9 show the nonstandard Dell main and auxiliary power supply connections. This nonstandard wiring is used on Dell's pseudo-ATX systems.

Table 3.8 Dell Proprietary (Nonstandard) ATX Main Power Connector Pinout (Wire Side View)

Table 3.9 Dell Proprietary (Nonstandard) ATX Auxiliary Power Connector Pinout

At first I thought that if all Dell did was switch some of the terminals around, I could use a terminal pick to remove the terminals from the connectors (with the wires attached) and merely reinsert them into the proper connector positions, enabling me to use the Dell power supply with an upgraded ATX motherboard in the future. Unfortunately, if you study the Dell main and auxiliary connector pinouts I've listed here and compare them to the industry-standard ATX pinouts listed earlier, you'll see that not only are the voltage and signal positions changed, but the number of terminals carrying specific voltages and grounds has changed as well. You could modify a Dell supply to work with a standard ATX board or modify a standard ATX supply to work with a Dell board, but you'd have to do some cutting and splicing in addition to swapping some terminals around. Usually, it isn't worth the time and effort.

If you do decide to upgrade the motherboard in any Dell system purchased between 1996 and 2000, a simple solution is available—just be sure you replace both the motherboard AND power supply with industry-standard ATX components at the same time. That way nothing gets fried, and you'll be back to having a true industry-standard ATX system. If you want to replace just the Dell motherboard, you're out of luck unless you get your replacement board from Dell. On the other hand, if you want to replace just the power supply, you do have one alternative. PC Power and Cooling now makes a version of its high-performance 300W ATX power supply with the modified Dell wiring for about $100. The internals are identical to its industry-standard, high-performance 300W ATX supply (which it sells for about 30% less)—only the number and arrangement of wires has changed.

Fortunately, starting in 2000, Dell switched to using industry-standard ATX power connections in its Dimension 4300, 4400, 8200, and newer systems. That means barring any other unforeseen glitches, these systems should be more easily upgradable by just replacing either the power supply or the motherboard alone. I, for one, am glad to see Dell moving back toward industry standardization because its systems are now more appealing to purchase as a starting point for a system that will be user upgradable and repairable in the future.

Power Switch Connectors

Three main types of power switches are used on PCs. They can be describes as follows:

• Integral Power Supply AC switch

• Front Panel Power Supply AC switch

• Front Panel Motherboard Controlled switch

The earliest systems had power switches integrated or built directly into the power supply, which turned the main AC power to the system on and off. This was a simple design, but because the power supply was mounted to the rear or side of the system, it required reaching around to the back to actuate the switch. Also, switching the AC power directly meant the system couldn't be remotely started without special hardware.

Starting in the late '80s systems began using remote front panel switches. These were essentially the same power supply design as the first type. The only difference is that the AC switch was now mounted remotely (usually on the front panel of the chassis), rather than integrated in the power supply unit, and connected to the power supply via a four-wire cable. The ends of the cable are fitted with spade connector lugs, which plug onto the spade connectors on the power switch. The cable from the power supply to the switch in the case contains four color-coded wires. In addition, a fifth wire supplying a ground connection to the case might be included. The switch was usually included with the power supply and heavily shrink-wrapped or insulated where the connector lugs attached to prevent electric shock.

This solved the ergonomic problem of reaching the switch, but it still didn't enable remote or automated system power-up without special hardware. Plus, you now had a 120v AC switch mounted in the chassis, with wires carrying dangerous voltage through the system. Some of these wires are hot anytime the system is plugged in (all are hot with the system turned on), creating a dangerous environment for the average person when messing around inside her system.

The four or five wires are color-coded as follows:

• Brown and blue. These wires are the live and neutral feed wires from the 110v power cord to the power supply. These are always hot when the power supply is plugged in.

• Black and white. These wires carry the AC feed from the switch back to the power supply. These leads should be hot only when the power supply is plugged in and the switch is turned on.

• Green or green with a yellow stripe. This is the ground lead. It should be connected to the PC case and should help ground the power supply to the case.

On the switch, the tabs for the leads are usually color-coded; if not, you'll find that most switches have two parallel tabs and two angled tabs. If no color-coding is on the switch, plug the blue and brown wires onto the tabs that are parallel to each other and the black and white wires to the tabs that are angled away from each other. If none of the tabs are angled, simply make sure the blue and brown wires are plugged into the most closely spaced tabs on one side of the switch and the black and white wires on the most closely spaced tabs on the other side.

See Figure 3.12 as a guide.

Figure 3.12 Power supply remote push button switch connections.

As long as the blue and brown wires are on the one set of tabs and the black and white leads are on the other, the switch and supply will work properly. If you incorrectly mix the leads, you will likely blow the circuit breaker for the wall socket because mixing them can create a direct short circuit.

All ATX and subsequent power supplies that employ the 20-pin motherboard connector use the PS_ON signal to power up the system. As a result, the remote switch does not physically control the power supply's access to the 110v AC power, as in the older-style power supplies. Instead, the power supply's on or off status is toggled by a PS_ON signal received on pin 14 of the ATX connector.

The PS_ON signal can be generated physically by the computer's power switch or electronically by the operating system. PS_ON is an active low signal, meaning that the power supply voltage outputs are disabled (the system is off) when the PS_ON is high (greater than or equal to 2.0v). This excludes the +5VSB (standby) on pin 9, which is active whenever the power supply is connected to an AC power source. The PS_ON signal is maintained by the power supply at either 3.3v or 5v. This signal is then routed through the motherboard to the remote switch on the front of the case. When the switch is pressed, the PS_ON signal is grounded. When the power supply sees the PS_ON signal drop to 0.8v or less, the power supply (and system) is turned on. Thus, the remote switch in an ATX-style system (which includes NLX and SFX systems as well) carries up to only +5v of DC power, rather than the full 115v–230v AC current like that of the older AT/LPX form factors.