Hardware Reference
In-Depth Information
measurement is not within this range, the system never sees the Power_Good signal and therefore
does not start or run properly. In most cases, the power supply is bad and must be replaced.
Continue by measuring the voltage ranges of the pins on the motherboard and drive power connectors.
If you are measuring voltages for testing purposes, any reading within 10% of the specified voltage is
considered acceptable, although most manufacturers of high-quality power supplies specify a tighter
5% tolerance. For ATX power supplies, the specification requires that voltages must be within 5% of
the rating, except for the 3.3V current, which must be within 4%.
Table 18.28
shows the voltage
ranges within these tolerances.
Table 18.28. Voltage Ranges
The Power_Good signal has tolerances that are different from the other voltages, although it is
nominally +5V in most systems. The trigger point for Power_Good is about +2.4V, but most systems
require the signal voltage to be within the tolerances listed here.
Replace the power supply if the voltages you measure are out of these ranges. Again, it is worth
noting that any and all power-supply tests and measurements must be made with the power supply
properly loaded, which usually means it must be installed in a system and the system must be running.
Specialized Test Equipment
You can use several types of specialized test gear to test power supplies more effectively. Because
the power supply is one of the most failure-prone items in PCs today, you should have these
specialized items if you service many PC systems.
Digital Infrared Thermometer
One of the greatest additions to my toolbox is a digital infrared thermometer (see
Figure 20.9
in
Chapter 20
, “
PC Diagnostics, Testing, and Maintenance
,”
p.
975
)
. This is also are called a
noncontact thermometer
because it measures by sensing infrared energy without having to touch the
item it is reading. This enables me to make instant spot checks of the temperature of a chip, a board,
or the system chassis. They are available from companies such as Raytek (
www.raytek.com
) for less
than $100. To use these handheld items, you point at an object and then pull the trigger. Within
seconds, the display shows a temperature readout accurate to +/-3°F (2°C). These devices are
invaluable in checking to ensure the components in your system are adequately cooled.
Variable Voltage Transformer
When you're testing power supplies, it is sometimes desirable to simulate different AC voltage
conditions at the wall socket to observe how the supply reacts. A variable voltage transformer is a
useful test device for checking power supplies because it enables you to exercise control over the AC
line voltage used as input for the power supply (see
Figure 18.41
)
. This device consists of a large
transformer mounted in a housing with a dial indicator that controls the output voltage. You plug the
