Hardware Reference
In-Depth Information
ThePower_Goodsignalhastolerancesthataredifferentfromtheothervoltages,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.
993
)
. 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 temperat-
ure 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
