Home > Articles > CompTIA > A+

  • Print
  • + Share This
This chapter is from the book

Power Supplies

Okay, now that we've tested our AC outlet and put some protective power devices into play, let's go ahead and talk power supplies. The power supply is in charge of converting the alternating current (AC) drawn from the wall outlet into direct current (DC) to be used internally by the computer. It feeds the motherboard, hard drives, optical drives, and any other devices inside of the computer. Talk about a single point of failure! That is why many higher-end workstations and servers have redundant power supplies.

Planning Which Power Supply to Use

It is important to use a reliable brand of power supply that is UL listed (certified). There are a few other things to take into account when planning which power supply to use in your computer:

  • Type of power supply and compatibility
  • Wattage and capacity requirements
  • Amount and type of connectors

Now, in our scenario we said that we need a power supply that can support many devices in our workstation; one that will output a lot of power. In this scenario the computer has two IDE hard drives, a CD-Burner, a DVD-ROM, one SATA drive, and a PCIe video card. And let's just say that we use an ATX 12V 2.0 motherboard. So we need to look for a high–capacity, compatible ATX power supply with a decent amount of connectors for our devices. Let's discuss planning now.

Types of Power Supplies and Compatibility

The most common form factor today is Advanced Technology Extended (ATX). Depending on the type of ATX, the main power connector to the motherboard will have 20 pins or 24 pins. Table 5.1 shows a few different form factors and their characteristics. The key is compatibility. In our scenario we have a previously built computer, which means that the case and motherboard are already compatible. If this computer was proprietary, we could go to the computer manufacturer's website to find out the exact form factor, and possibly a replacement power supply for that model computer. Some third-party power supply manufacturers also offer replacement power supplies for proprietary systems. However, if this computer was custom built, we would need to find out the form factor used by the motherboard and/or case, and should open the computer and take a look at all the necessary power connections. Then we need to find a compatible power supply according to those specifications from a third-party power supply manufacturer. Table 5.1 displays the form factors you need to know for the exam.

Table 5.1. Common Power Supply Form Factors

Form Factor

Main Power Connector

Other Characteristic


P1 20-pin connector

An older standard but you will still support it!

ATX 12V 1.0 - 1.3

P1 20-pin connector & P4 4 pin 12V connector

Supplemental 6-pin AUX connector provides additional 3.3V and 5V supplies to the motherboard.

ATX 12V 2.0

P1 24-pin connector (backward compatible)

  • 6-pin AUX was removed.
  • SATA power cable is required.

Figure 5.4 gives examples of a P1 20-pin (the white connector) and P1 24-pin connector (the black connector). Toward the left of the black connector you notice it has an additional four pins that can be separated from the main group of 20 pins. Both have locking tabs to keep the P1 connector fastened to the motherboard. (In the figure this is shown only on the 20-pin connector.)

Figure 5.4

Figure 5.4 24-pin and 20-pin power connectors

There are many other types of form factors such as microATX, BTX, and NLX (covered in Chapter 2, "Motherboards") and older form factors such as AT; however, the form factors listed in Table 5.1 are the important ones to know regarding power supplies for the A+ exam. For any other form factors, just remember that the power supply, case, and motherboard all need to be compatible.

Another important piece to consider is the type of case that is used. Larger cases require longer power cables to reach the devices. You can find the measurements for the cables on the power supply manufacturer's website. There are several different types of cases that you need to be familiar with:

  • Desktop: Lies horizontally, usually has one 5¼-inch drive bay.
  • Mini-tower: Stands vertically, usually has two or three drive bays.
  • Mid-tower: Usually has three or four bays.
  • Full tower: Usually has six bays.
  • Slim line: Compaq and the Playstation III and other third-party case manufacturers use this case design.

Many power supply manufacturers also make computer cases and often sell them as a package or to be purchased separately.

Wattage and Capacity Requirements

Power supplies are usually rated in watts. They are rated at a maximum amount that they can draw from the wall outlet and pass on to the computer's devices. Remember that the computer will not always use all that power the way in which a light bulb does. And the amount depends on how many devices work and how much number crunching your processor does! In addition, when computers sleep or suspend, they use less electricity. What you need to be concerned with is the maximum amount of power all the devices need collectively. Most power-supply manufacturers today offer models that range from 300 watts all the way up to 1,000 watts. Although 300 watts is a decent amount of power for many computers, it might not suffice in our scenario. Devices use a certain amount of power defined in amps and/or watts. By adding all of the devices power consumption together, we can get a clearer picture of how powerful a power supply we need. Consult the manufacturer's web page of the device for exact requirements. We said that in our scenario the computer has two IDE hard drives, a CD-Burner, a DVD-ROM, a floppy drive, and one SATA drive and a PCIe video card. It also has a quad core processor and 2GB of RAM (in two sticks).

After doing the math, it appears that the computer in our scenario needs about 400 watts or so to run smoothly. The power supply we purchase should be rated slightly higher just in case, so in this scenario we would obtain a 450-watt or 500-watt power supply. Most power supplies are rated for 15 amps, so it is important to connect the computer to a 15-amp circuit or higher.

Amount and Type of Power Connectors

It is important to know how many of each type of power connector you need when planning which power supply to use. In our scenario we need four IDE power connectors (for the two hard drives, CD-Burner, and DVD-ROM), one floppy power connector, and one SATA power connector. You need to be familiar with each of these types of power connectors for the A+ exams. Be prepared to identify them by name and by sight. Table 5.2 defines the usage and voltages for the most common power connectors: Molex, mini, SATA, and PCIe, which are displayed in Figures 5.5 through 5.8.

Table 5.2. Power Connectors

Power Connector


Pins and Voltages


IDE hard drives, optical drives, and other devices

Red (5V), black (G), black (G), yellow (12V)


Floppy drives

Red (5V), black (G), black (G), yellow (12V)


Serial ATA hard drives

15-pin, 3.3V, 5V, and 12V


PCI Express cards


Figure 5.5

Figure 5.5 Molex power connector

Figure 5.6

Figure 5.6 Mini power connector

Figure 5.7

Figure 5.7 SATA 15-pin power connector

Figure 5.8

Figure 5.8 PCIe 6-pin power connector

Installing the Power Supply

When the power supply arrives, we can install it. But first, let's take a look at the back of the power supply to identify the components we see, as shown in Figure 5.9.

Figure 5.9

Figure 5.9 Rear view of power supply

On the top-left portion of Figure 5.9, we see a hard on/off switch sometimes referred to as a kill switch. This is a nice feature when troubleshooting PCs. Instead of disconnecting the power cable, we can shut off this switch. It works nicely in emergencies as well. Below that we see a red voltage selector switch. This should be set to 115V in the United States. It also has a 230V option to be used in other countries. (An additional adapter might be necessary for the different wall outlets you might encounter.) Never change the voltage selector switch while the computer is running. Be sure to check this setting before using the power supply. Some newer power supplies are now equipped with a universal input enabling you to connect the power supply to any AC outlet between 100V to 240V, without having to set a voltage switch. Below that we see the power cable inlet; this is known as a C14 inlet and is where we attach our power cord to the power supply. These inlets and cables that connect to them are defined by the IEC 60320 specification (previously the IEC 320 spec), and because of this many techs refer to the power cord as an IEC cable (which by the way stands for International Electrotechnical Commission).

This cord actually has a standard three-prong connector suitable for an AC outlet on one end and a C13 line socket on the other to connect to the power supply. To the right we see the power supply fan that is of great importance when troubleshooting power supplies.

If there is a power supply connected to the computer, turn off the computer and unplug the power supply. ATX motherboards are always receiving 5 volts even, when they are off, if the computer is plugged in. Be sure that you are employing antistatic methods. Remove the old power supply and prepare to install the new one.

You might want to test the power supply before installing it. This can be done by connecting a power supply tester (described in the next section), plugging in the power supply to the AC outlet, and turning on the hard on/off switch. Or you can test the power supply after it is installed by simply turning the computer on.

The power supply is placed inside the case and mounted with four standard screws that are screwed in from the back of the case. In some instances, a plastic housing inside the case might need to be removed. In addition, the power supply might not fit without the removal of other devices, such as the processor, and such, but in most cases (pun intended) you should install the power supply without too much trouble. Next, connect the P1 connector to the motherboard and attach the Molex, mini, SATA, and PCIe as necessary to their corresponding devices. Note that the P1 connector (20-pin or 24-pin main connector) can be plugged in only in one way and that there is a locking tab. Also, most other connectors are molded in such a way as to make it difficult to connect them backward. If you need a lot of strength to plug in the connector, check and make sure that it is oriented correctly. Don't force the connection. Afterward, remove any antistatic protection, and finally, plug the power supply into the AC outlet, turn on the hard on/off switch (if the power supply has one), and turn on the computer. Check to see if the fan in the power supply is working and if the computer boots correctly.

Troubleshooting Power Supply Issues

Installation of the power supply was easy, and there aren't usually many issues when doing so, but power supplies don't last forever. Moreover, many issues that occur with power supplies are intermittent making the troubleshooting process a little tougher. Your best friends when troubleshooting power supplies are going to be a multimeter, power supply tester, and your eyes and ears. Of course, always make sure that the power supply connects to the AC outlet properly before troubleshooting further. Here are a couple of the issues you may encounter with power supplies:

  • Fan failure
  • Fuse failure
  • Quick death
  • Slow death

Fan failure can be due to the fact that the power supply is old, extremely clogged with dirt, or that the fan was of cheaper design (without ball bearings). However, for the A+ exam it doesn't make a difference. As far as A+ is concerned, if the fan fails, the power supply needs to be replaced, and it makes sense. Chances are, if the fan has failed, other components of the power supply are on their way out also. It is more cost-effective to a company to simply replace the power supply than to have a technician spend the time opening it and trying to repair it. More important, although it is possible to remove and replace the fan by opening the power supply, this can be a dangerous venture because the power supply holds an electric charge, so the A+ rule is to never open the power supply.

Fuse failure can occur due to an overload or due to the power supply malfunctioning. Either way, the proper course of action is to replace the power supply. Do not attempt to replace the fuse. Chances are that the power supply is faulty if the fuse is blown. If it so happens that you need to test an individual fuse that was lying around, then use your multimeter. Make sure that your red lead is connected to the ohms (W) input and set the meter to Ohm (W). Touch the probes to both ends of the fuse. A good fuse should show zero ohm or display continuity. A bad or "blown" fuse will not show any reading. This is an example of testing impedance.

If the power supply dies a quick death, it might be because of several reasons from an electrical spike to hardware malfunction. First make sure that the IEC cable is connected properly to the power supply and to the AC outlet. Sometimes, it can be difficult to tell whether the power supply has failed or if it's something else inside or outside the computer system. You should check the AC outlet with your trusty receptacle tester and make sure that a circuit hasn't tripped, and verify that any surge protectors and/or UPS devices work properly. Depending on what you sense about the problem, you might decide to just swap out the power supply with a known good one. Otherwise, move on to the following numbered steps.

If the power supply is dying a slow death and is causing intermittent errors, it could be tough to troubleshoot. If you suspect intermittent issues, first make sure that the power cord is connected securely and then try swapping out the power supply with a known good one. Boot the computer and watch it for awhile to see if the same errors occur.

Whether the power supply has apparently failed completely or is possibly causing intermittent errors, and you can't figure out the cause to this point, continue through the following steps:

  1. Remove the computer case.
  2. Connect a power supply tester, as shown in Figure 5.10, to the P1 connector and look at the results. (Make sure you have the correct power supply tester; this depends on whether you have a 20-pin or 24-pin power connector.) These power supply testers normally test for +12V, –12V, +5V, –5V, and 3.3V, but they might not test every individual pin. If there are error lights, no lights, or missing lights for specific voltages on the tester, replace the power supply. If all the lights are green, move on to the next step.
    Figure 5.10

    Figure 5.10 Testing a 24-pin P1 connector with a power supply tester

  3. Use a multimeter to test the power supply. Use the same methodology for testing with a multimeter as in the beginning of this chapter.
    1. Turn off the hard on/off switch. (If there is one; if not, unplug the IEC cord.)
    2. With the main motherboard connector (P1) inserted into the motherboard, connect the black lead to a ground wire (or other source of ground) and insert the red lead to a colored voltage wire in the main power connector, as shown in Figure 5.11. You need to dig a little bit to get the lead in there but don't press too hard. When the leads are stationary, move on to the next step.
      Figure 5.11

      Figure 5.11 Testing the 3.3 volt wire with a multimeter set to volts DC

    3. Turn on the hard on/off switch (or plug the IEC cord back in) and turn on the computer.
    4. Turn on the multimeter to volts DC and view the results. In the figure you notice that we test an orange wire (which is rated for +3.3 volts). Generally, supply voltages should be within +/- 5 percent of the nominal value. Our result was +3.43 volts, which is within tolerance.
    5. Shut off the multimeter and computer every time before moving to another wire. Check each of the wires for proper voltages. A chart of all the voltages for 20-pin and 24-pin connectors is available in Chapter 2.
    6. If one of the wires fails or gives intermittent results, first verify you have a decent connection with the multimeter leads; then see if the wire just needs to be inserted into the main motherboard connector better, and if it continues, replace the power supply. If all the wires are fine (which is doubtful), move to the next step.
  4. Swap the power supply with a known good power supply. Boot the computer and watch it for several minutes or longer to see if there are any strange and intermittent occurrences.

Remember that sometimes connections can be jarred loose inside and outside the computer. Check the IEC cord on both ends and all power connections inside the computer. This includes the main motherboard connector, Molex, mini, SATA, and PCIe connectors. Any one loose connector can have interesting results on your computer!

Heating and Cooling

Another thing to watch for is system overheating. This can happen for several reasons:

  • Power supply fan failure
  • Auxiliary case fan failure
  • Inadequate amount of fans
  • Missing or open slot covers
  • Case isn't tightly closed and screwed in

Air flow is important on today's computers because processors can typically operate at 3 gflops. That creates a lot of heat. Add to that the fact that the video card and other cards have their own on-board processors, it can get hot inside the computer case. Circulation is the key word here. Air should flow in the case from the front and be exhausted out the back. Any openings in the case or missing slot covers can cause circulation to diminish. If you have a computer that has a lot of devices, or does a lot of processing, or runs hot for any other reason, your best bet is to install a case fan in the front of the case, which pulls air into the case, and a second case fan in the back of the case, which with the power supply fan helps to exhaust hot air out the back. Also, try to keep the computer in a relatively cool area and leave space for the computer to expel its hot air! Of course there are other special considerations and options, such as liquid cooling, and special processor cooling methods, such as the Intel Chassis Air Guide, but they are not covered in the A+ exam.

Cram Quiz

Answer these questions. The answers follow the last question. If you cannot answer these questions correctly, consider reading this section again until you can.

  1. Which device tests multiple wires of a power supply at the same time?


    A. Multimeter


    B. Power supply tester


    C. Line conditioner


    D. Surge protector

  2. Which power connector would be used to power an IDE hard drive?


    A. Molex


    B. mini


    C. P1


    D. P8/P9

  3. Which of the following uses a 24-pin main motherboard power connector?


    A. ATX


    B. ATX 12V 1.3


    C. ATX 12V 2.0


    D. ATX 5V 2.0

  4. The red wire in a Molex connection is rated for what voltage?


    A. 12 volts


    B. 5 volts


    C. 3.3 volts


    D. 24 volts

Cram Quiz Answers

  1. B. The power supply tester tests 3.3V, 5V, –5V, 12V, and –12V simultaneously. A multimeter tests only one wire at a time. Line conditioners and surge protectors are preventative devices, not testing devices.

  2. A. Molex connectors power IDE devices. Mini connectors are for floppy drives, P1 is a name used for the main motherboard connector, and P8/P9 are legacy main power connectors for AT systems.

  3. C. ATX 12V 2.0 combined the 20-pin and 4-pin connectors used in ATX 12V 1.3 into one 24-pin connector.

  4. B. The red wire is rated for 5 volts. The yellow wire is rated for 12 volts and 3.3 volts is associated with the main motherboard connector (to feed the processor); 24 volts is not involved in the devices we discussed in this chapter.

  • + Share This
  • 🔖 Save To Your Account