Hardware Reference
In-Depth Information
andmakecontact withthemetal terminal inside.Thetechnique iscalled
back probing
be-
cause you are probing the connector from the back. You must use this back-probing tech-
nique to perform virtually all the following measurements.
Figure 18.39
Back probing the power supply connectors.
To test a power supply for proper output, check the voltage at the Power_Good pin (P8-1
on AT, Baby-AT, and LPX supplies; pin 8 on the ATX-type connector) for +3V to +6V of
power.Ifthemeasurementisnotwithinthisrange,thesystemneverseesthePower_Good
signal and therefore does not start or run properly. In most cases, the power supply is bad
and must be replaced.
Continuebymeasuringthevoltagerangesofthepinsonthemotherboardanddrivepower
connectors.Ifyouaremeasuringvoltagesfortestingpurposes,anyreadingwithin10%of
the specified voltage is considered acceptable, although most manufacturers of high-qual-
ity power supplies specify a tighter 5% tolerance. For ATX power supplies, the specific-
ation requires that voltages must be within 5% of the rating, except for the 3.3V current,
which must be within 4%.
Table 18.30
shows the voltage ranges within these tolerances.
Table 18.30 Voltage Ranges
