Build The Ultimate Custom PC (2005)

Chapter 11 — Checking and Testing Components 179

The most common type of damage to affect memory modules is invisible damage sustained from electrostatic discharge (ESD). Take extra precautions when handling memory modules against ESD. For more information, see Chapter 1, “Safety First.”

Checking the PSU

The power supply unit is the component that takes the mains power and changes the voltage to suit the components in a PC. PSUs are normally tested before shipping from the factory, but because damage can occur in transit and because your PSU is a critical component, it’s important to carefully check it to make sure that it is undamaged.

There are two types of check that you can carry out on a PSU:

Visual inspections

Powered tests using a multimeter

We’re well aware that not everyone is comfortable testing components when plugged into the electrical supply. We would recommend that if anyone is unsure or doesn’t feel they are ready to attempt this kind of testing to bypass these tests and assume that the PSU is okay, and only undertake these tests if the system isn’t working (or return the PSU to where you bought it).

Visual Checks

Check the exterior of the PSU for any damage such as dents. Reject any PSUs that show signs of external damage as they might be malfunctioning and destroy other components attached to them.

Also, check all of the power cabling on the power rails (the cables that feed power to the motherboard and drives) coming from the PSU for signs of wear, sharp kinks, or cuts to the outer sheathing of the wires. Check all the connectors carefully and look for any that might have suffered crush damage in transit.

Finally, give the PSU a little shake and listen for anything that sounds loose inside or any rattling. There’s a lot of space inside a PSU, and screws and other small items can work their way inside and cause a short when switched on. Reject the PSU if it sounds like it has something loose inside.

Power Checks

Carrying out power tests on a PSU will give you a definitive answer as to whether the PSU has suffered any damage while in transit and whether the output voltages provided by the power rails conform to the specification required.

These tests involve the PSU being connected to the mains and switched on. This means that mains power is flowing into the PSU and that low-voltage outputs are present in the drive rails.

180 Part I — Choosing Components for Your PC

Technically, the PSU in this state is no more dangerous than when plugged into a PC and running, but there is a slightly increased risk of electric shock if you touch anything inside the PSU with a screwdriver or tweezers (or if something like a screw or wire finds its way into the PSU). Also, because the power rails are free and unconnected to anything, you also need to keep them away from liquids or metal surfaces in case of a short circuit.

If you follow the instructions here carefully, then you can carry out these tests safely but never take short cuts or guesses. If you are unsure about anything, backtrack, reread the instructions, and check again.

If you are in any way uncertain about carrying out these tests or don’t feel that your skills with a multimeter are good enough, then do not perform these tests and assume that the PSU is okay.

To carry out these tests you will need a:

Multimeter (see Figure 11-13)

Surface to place the PSU on while running (a wooden surface, heat resistant kitchen work area, or a ceramic tile is best as the PSU might get a little warm)

Power cord for the PSU

Nearby power outlet

Small length of insulated wire

Hard drive or two 20-ohm (or higher) resistors (all will be revealed shortly!)

FIGURE 11-13: Digital multimeter.

Chapter 11 — Checking and Testing Components 181

The purpose of these tests is to ascertain that the power output from the power rails on the PSU falls within a range acceptable for a PSU.

Table 11-1 lists the three specific voltages you are testing.

Table 11-1 ATX (Advanced Technology Extended)
	Power Output Specification
Power Rail Voltage	Allowable Variance (%)	Minimum Voltage	Maximum Voltage
(Volts)
3.3	5	3.14	3.47
5	5	4.75	5.25
12	5	11.4	12.6

If your PSU has any of the output that falls outside of these values, you will need to get it replaced before using it. As a rule, power supply units are quite well behaved, but some (especially cheaper ones) might not be constructed to a high enough standard.

Never, ever, connect a PSU that is delivering too much voltage to a PC. The additional voltage can harm or destroy the CPU, motherboard, RAM, and hard drives.

Testing the PSU

Testing the PSU is probably one of the most complicated things that we have covered so far. It’s not a case of just plugging the PSU into the mains and probing it with the multimeter probes. There are a few precautionary steps that you have to take to properly load the PSU before testing along with a small temporary modification that you have to do to get the PSU to start when out of the PC.

Loading the PSU

First, it’s not a good idea to run a PC without any load on it (that is, without connecting the PSU to anything to draw power from it). While the ATX specification relating to power supplies indicates that a PSU should be able to run indefinitely without any load, some people encounter problems resulting from this, possibly because the PSU they have purchased is not totally ATX-compliant. These problems range from a high-pitched noise being emitted by the PSU to overheating and cutting off.

Because of this, it’s a good idea to put a load on your PSU. The easiest way to do this is to attach a hard drive to the PSU power rails and let that take the power (see Figure 11-14).

182 Part I — Choosing Components for Your PC

If you are loading a PSU with a hard drive, then make sure that the drive is on a nonconductive surface. Better still, fit the drive into the PC, even if only temporarily.

Because you are testing the PSU for power output, if your PSU is damaged the output might spike and damage the hard drive. If you have an old hard drive, it’s a good idea to keep it for testing. If you don’t want to risk damage to your drive try the method below instead.

FIGURE 11-14: Loading the PSU using a hard drive.

Another way to test the PSU is using two resistors of 20 ohm (or greater). You can use these to load the 5-volt and 12-volt hard drive power rails to simulate a hard drive.

Connecting the resistors is easy (see Figure 11-15). Take the hard drive connector and use one resistor to bridge one of the outside connectors fed by either the yellow wire (12 volts) or red wire (5 volts) to one of the two ground connectors in the middle (black wires). Then simply repeat for the other connector. This will provide load for the PSU and simulate a hard drive.

Chapter 11 — Checking and Testing Components 183

FIGURE 11-15: Loading the PSU with resistors.

Don’t touch! These resistors will get hot when the PSU is switched on. Also take appropriate precautions so that they do not touch other PC components or damage surfaces.

Jumpering the On Switch

A PSU is designed to be switched on and off by the switch on the front of a PC. Because of this, if you plug in the PSU without connecting it to a motherboard, nothing will happen. In order to activate it, you have to bridge one of the motherboard connectors to a ground wire. Fortunately, this is easier than it sounds.

184 Part I — Choosing Components for Your PC

Take a look at the big 20-pin motherboard connector block on the PSU. Hold it so that the keyed connector faces you and the wires are at the bottom and the connectors facing upward. Now look at the seventh pin along from the left. This one is fed by a green wire and either side of this is a pin fed by a black wire (ground) as shown in Figure 11-16. What you need to do is take that piece of wire that we mentioned earlier and strip off a bit of the insulation at each end and bridge the pin fed by the green wire to either of the pins alongside. This will simulate the PSU being connected inside a PC and the PC being switched on.

FIGURE 11-16: Jumpering pin 14 to ground.

Performing the Tests

There are three voltages to test inside a PC. These are:

12-volt supply

5-volt supply

3.3-volt supply

The best place to test the 12-volt and 5-volt outputs is from the hard drive power connector. You do this by bridging the pins with your multimeter. To test the 12-volt supply, you will bridge the pins supplied by the yellow and black wires. To test the 5-volt supply, bridge the pins for the red and black wires (either black wire will do).

Chapter 11 — Checking and Testing Components 185

To easiest way to test the 3.3-volt output is to use the auxiliary connector. With the multimeter, bridge one of the pins fed by the orange wire (pins 4 or 5 on the connector if you look at it with the key facing you and the wires at the bottom) to either of the pins fed by the black wires (pin 1, 2, or 3).

But before you can carry out any testing, you need to set up your multimeter, which you will see how to do next.

Multimeter Setup

The first thing you need to do is set up the multimeter to measure DC voltage in the 20-volt range. All multimeters are different, so consult your documentation if unsure how to do this.

What follows are generic instructions for setting up a multimeter:

1.Plug the black probe into the socket labeled COM.

2.Plug the red probe into the socket labeled V (see Figure 11-17).

FIGURE 11-17: Probes connected to multimeter.

186 Part I — Choosing Components for Your PC

3.Set the multimeter to measure a range around 20V DC. Your number may vary but make sure that it is set to measure DC volts. See Figure 11-18.

FIGURE 11-18: Multimeter set to correct range.

You’re now ready to switch on the multimeter and test the three different output voltages!

12-Volt Supply Tests

Here are the steps necessary for testing the 12-volt output from the PSU:

1.Place the PSU on its side on the heat-resistant surface, taking care not to obstruct any fans or air input or exhaust vents.

2.Prepare the PSU as described in the preceding section — connect a load to one of the hard drive rails and jumper the on switch on the motherboard.

3.Connect the red probe to pin 1 of the hard drive power connector (fed by the yellow wire).

4.Connect the black probe to either pin 2 or 3 (fed by black wires). See Figure 11-19.

Chapter 11 — Checking and Testing Components 187

FIGURE 11-19: Probes inserted correctly.

5.Switch on the PSU and take a reading. You are looking for an output that falls into the range of 11.5–12.6 volts.

6.Switch the PSU off again soon afterward.

5-Volt Supply Tests

Here are the steps necessary for testing the 5-volt output from the PSU:

1.Place the PSU on its side on the heat resistant surface, taking care not to obstruct any fans or air input or exhaust vents.

2.Prepare the PSU as described, connecting a load to one of the hard drive rails and jumpering the on switch on the motherboard.

3.Connect the red probe to pin 4 of the hard drive power connector (fed by the red wire).

4.Connect the black probe to either pin 2 or 3 (fed by black wires). See Figure 11-20.

188 Part I — Choosing Components for Your PC

FIGURE 11-20: Probes inserted correctly.

5.Switch on the PSU and take a reading. You are looking for an output that falls into the range of 4.75–5.25 volts.

6.Switch the PSU off again soon afterward.

3.3-Volt Supply Tests

Here are the steps necessary for testing the 3.3-volt output from the PSU:

1.Place the PSU on its side on the heat-resistant surface, taking care not to obstruct any fans or air input or exhaust vents.

2.Prepare the PSU as described, connecting a load to one of the hard drive rails and jumpering the on switch on the motherboard.

3.Connect the red probe to pin 4 or 5 of the auxiliary power connector (fed by the orange wires).

4.Connect the black probe to either pin 1, 2, or 3 (fed by black wires).

5.Switch on the PSU and take a reading. You are looking for an output that falls into the range of 3.14–3.47 volts.

6.Switch the PSU off again as soon as possible. Don’t leave it on unnecessarily.