Of all the components in a PC, most technicians understand the power supply the least. That’s unfortunate because power supplies are not all that complicated, and they are often the cause of puzzling, tough-to-troubleshoot problems. In this Daily Drill Down, I’ll explain some of the basics of power supplies, including how they work, what types are available, and how to test one for proper operation.

How a power supply works
The power supply takes wall current (120-volt, 60-Hz AC) and converts it to an appropriate level of DC voltage for the various components in a PC. Depending on the component, this can be +3.3V, +5V, or +12V. Generally speaking, the motherboard and any circuit cards use +3.3V or +5V, (newer motherboards and processors tend toward +3.3V, while older ones are usually +5V) and fans and disk drives use +12V.

Many power supplies also generate -5V and -12V, but those negative voltages are rarely used in modern systems and some of the newer power supplies do not even provide -5V support. Support for -5V is part of the ISA standard, but new systems being produced today are typically PCI-only, so they do not require this support.

What the wires do
Have you ever wondered why the plug from the power supply to the motherboard has so many different colored pins and wires? It’s to provide different voltages of power signals to the motherboard, which then parses them out to connected devices. The motherboard itself uses only +5V. Other voltages are routed to the ISA bus: -5V on pin B5, -12V on B7, and +12V on B9. Integrated serial ports on older systems use +12V; while on newer systems, they use +3.3V or +5V. All the other plugs coming out of the power supply—called Molex connectors—are for drives, and they provide +12V (yellow) and +5V (red) power, as well as two ground wires (black).

Types of power supplies
Power supplies are sold using two main specifications: the form factor and the wattage. Wattage is volts multiplied by amps. For example, you might see an ATX-style 250-watt power supply or an LPX-style 200-watt power supply.

LPX style is a descendant of the Baby-AT, AT/Tower, and AT/Desk type of power supply and is used primarily with Baby-AT style motherboards. The ATX style is used with ATX, Micro-ATX, and NLX-style motherboards. When selecting a power supply, you must make sure that it not only matches up with the motherboard type (so the connectors will fit) but also that it fits inside the case you are using. LPX-style power supplies have two six-pin connectors to the motherboard, while ATX-style power supplies have a single 20 pin. See Table A and Table B for a breakdown on what each pin does.

Table A
P8-1 Power_Good (+5V)
P8-2 +5V
P8-3 +12V
P8-4 -12V
P8-5 Ground
P8-6 Ground
P9-1 Ground
P9-2 Ground
P9-3 -5V
P9-4 +5V
P9-5 +5V
P9-6 +5V
For an LPX-style power supply (AT computers), there are two connectors: P8 and P9. Each has six pins, and you plug them into the motherboard so the black wires are together.

For an ATX-style power supply, there is a single 20-pin connector, two rows of ten wires. The colors listed here are part of the ATX standard but are not required, so some off-brand systems might be different.

Table B
Pin 1 (Orange) +3.3V
Pin 2 (Orange) +3.3V
Pin 3 (Black) Ground
Pin 4 (Red) +5V
Pin 5 (Black) Ground
Pin 6 (Red) +5V
Pin 7 (Black) Ground
Pin 8 (Gray) Power_Good
Pin 9 (Purple) +5VSB (Standby)
Pin 10 (Yellow) +12V
Pin 11 (Orange or Brown) +3.3V
Pin 12 (Blue) -12V
Pin 13 (Black) Ground
Pin 14 (Green) PS_On
Pin 15 (Black) Ground
Pin 16 (Black) Ground
Pin 17 (Black) Ground
Pin 18 (White) -5V
Pin 19 (Red) +5V
Pin 20 (Red) +5V
Notice that on an ATX-style connector, all the wires with the same colors have the same voltages or functions. For example, all the red wires are +5V and all the black wires are ground.

Power supply manufacturers will provide you with specs on request for their power supplies, but a typical 250-watt LPX power supply might break down like this:

  • +5V—Maximum of 25 amps (125 watts)
  • +12V—Maximum of 10 amps (120 watts)
  • -5V—Maximum of .5 amps (2.5 watts)
  • -12V—Maximum of .5 amps (2.5 watts)

For a 235-watt ATX, you might see something like this:

  • +5V—Maximum of 22 amps (110 watts)
  • +3.3V—Maximum of 14 amps (46.2 watts)
  • +5V and +3.3V combined—Maximum of 125 watts
  • +12V—Maximum of 8.0 amps (96 watts)
  • +-5V—Maximum of .5 amps (2.5 watts)
  • -12V—Maximum of 1 amp (12 watts)


Notice that for the above specs, the combination of +5V and +3.3V cannot exceed 125 watts. That allows for maximum power flexibility while still maintaining the 235-watt limit.


It isn’t always easy to get power consumption data for the various components in your system, but you can use the following rough numbers for conservative calculation. These numbers represent the maximum for each component; the actual amount of draw will likely be less.

  • Motherboard—5 amps of +5V or +3.3V and .7 amps of +12V
  • ISA circuit boards—2 amps of +5V and .175V of +12V.
  • PCI circuit boards—5 amps of +5V, 0.5 amps of +12V, and 7.6 amps of +3.3V
  • CD-ROM drives—1 amp of +5V and 1 amp of +12V
  • 3 œ” floppy drives—0.5 amps of +5V and 1 amp of +12V
  • 5 Œ” floppy drives—1 amp of +5V and 2 amps of +12V


When a drive is spinning up, it requires about twice the normal amount of +12V power, so when calculating the needed +12V amps, double the measurement.


The wattage of the power supply refers to the maximum wattage of which it is capable. An extremely high-wattage power supply in a lightly loaded system is a waste, because the system draws only what it needs in terms of amps. That is not to say, however, that a high-quality power supply is a waste. High-quality power supplies can provide cleaner and more reliable power to a system and can help reduce sags and spikes from the wall current.

There are many other measures of a power supply’s performance, but these are not typically shopping specs. If you become a real hard-core hardware enthusiast, you might also want to compare the ratings of various power supplies for features such as MTBF, input range, peak inrush current, holdup time, transient response, over-voltage protection, maximum and minimum load current, and so on.

What happens when you start the computer?
When you turn on a PC, the power supply starts up and waits until any startup spike or sag has passed and the power output has stabilized. Then, it sends +5V through pin 8 (on an ATX connector) or pin 1 on the P8 connector (on an AT-style power supply). This is called the Power_Good signal. The motherboard looks for this signal, and if it finds that between +3.0V and +6.0V is passing through the Power_Good pin, it knows it’s okay to turn on and start making use of the rest of the power coming through the other pins on the power-supply-to-motherboard connector.

If the motherboard is receiving power from the other pins but the right voltage is not coming through the Power_Good pin, it waits, continually resetting itself until it receives the correct voltage on Power_Good. This system helps prevent electrical damage to sensitive components from a malfunctioning power supply. The original designers of the PC thought this was a very conservative system that would ensure freedom from power supply problems, but I will explain later in this article, problems can occur anyway.

The power supplies in PCs are of the switching type (as opposed to linear). Because of this, they do not run without a load—that is, without some device drawing power from them. If you turn on a power supply that is not connected to anything, it either won’t work at all (best case) if it has protection circuitry built in, or it will fry itself within a few seconds (worst case) if it does not. Therefore, when testing power supplies, you should always have something connected to them, even if it’s an old broken-down motherboard and an obsolete drive. How much do you need to connect? It depends on the age of the power supply. On modern systems, most motherboards draw the needed amount of current by themselves; but on older systems, or with larger power supplies, at least one disk may need to be connected as well.

Symptoms of a faulty power supply
A failing power supply can cause all sorts of problems that do not appear to be directly related, leading the less experienced technician on a wild goose chase through memory, processor, motherboard, and hard disk errors. Often, the problem seems to jump around, such as a memory problem that reports a different memory address as faulty each time or spontaneous rebooting after a random amount of time. There are three reasons a power supply can cause a problem:

  1. Physical failure—In a failing power supply, the power supply is not generating its rated amount of power or it is providing the wrong voltages on some wires. The PC generally will not start at all if such a condition exists. (See the next section for help determining whether a power supply is working correctly.) Replacing a faulty power supply is the best solution as repairing power supplies can be dangerous for inexperienced technicians and is seldom cost effective.
  2. Overloading—On an overloaded power supply, there is not enough wattage to support all the devices plugged into it. On a system with an overloaded power supply, problems will often occur at startup, when all the drives are spinning up, or when accessing a drive. (See the preceding section to calculate how much wattage you need in the system. Then replace the power supply with a higher-wattage model if needed.)
  3. Overheating—This happens when the power supply fan (or processor cooling fan) is not doing its job adequately or when the system case’s airflow is obstructed. Most computer cases are designed to pull fresh air through the case across the major heat-producing components. The air flowing through the restricted space is very important. If you remove the case cover or leave empty-slot covers off, the air does not flow as designed and overheating can result. If the system starts up okay but then starts having problems after several minutes of operation, inadequate cooling is almost always the problem. Make sure the airflow path is unobstructed, that the processor heat sink or cooling fan is in place and operational, and that the power supply fan is working quietly and correctly.

Testing a power supply
To test a power supply, you will need a digital multimeter. The analog type, with the needle-style readout, can damage computer circuits. The multimeter has two probes: red and black. Touch the black probe to the computer chassis for grounding and then use the red probe for testing.

When testing the power supply, you must check it in place; readings obtained while disconnected from a load will not be accurate. You can’t disconnect the connectors while the computer is running, of course, so you must use a technique called back probing to take your measurements. With back probing, you stick the red probe into the back of the connector and touch the wire down inside the plastic plug.

From the charts earlier in this article, you know what the various power supply wires should test out at voltage-wise. The first wire to test is the Power_Good; if it is between +3V and +6V, the power supply is probably doing its job.

Replacing a power supply
Replacing a power supply is fairly easy. Just undo the four screws that hold it into the case and slide it out; then fasten the new one into place. On an LPX (AT-style) power supply, the power switch is attached to the power supply, so you must unfasten it from the front of the case to remove the old power supply and then fasten the new power supply’s switch in its place. On an ATX-style power supply, there is no attached power supply; instead, a wire runs from the case’s on/off switch to pins on the motherboard, and when you press the power button, those pins are shorted, telling the motherboard to start up the PC. On an ATX power supply, power to the motherboard is always on as long as the computer is plugged in.

In this Daily Drill Down, I’ve attempted to take some of the mystery out of power supplies by explaining wire by wire what happens and by providing some starter points for troubleshooting power difficulties. The next time you have a baffling hardware problem to troubleshoot, remember to check the power supply!

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