Determining Power Supply DC Voltage Levels

You can use a multimeter to find out if a power supply is properly converting AC power to DC power. Here's how: Measure the DC power going from the power supply to the motherboard. A power supply that does not meet the measurement standards listed in Table 5.2 should be replaced .

You can take the voltage measurements directly from the power supply connection to the motherboard. Both 12-pin and 20-pin power connectors are designed to be back-probed as shown in Figure 5.14; you can run the red probe through the top of the power connector to take a reading (the black probe uses the power supply enclosure or metal case frame for ground).

Figure 5.14. Testing the +12V line on an ATX power supply. The voltage level indicated (+11.92V) is well within limits.

The multimeter also can be used to check the Power Good or Power OK line by pushing the red lead through the open top of the power connector. See Table 5.2 for the acceptable voltage levels for each item.

Table 5.2. Acceptable Voltage Levels

Rated DC Volts	Minimum Acceptable	Maximum Acceptable
+5.0	+4.8	+5.2
“5.0	“4.8	“5.2
“12.0	“11.4	“12.6
+12.0	+11.4	+12.6
+3.3	+3.14	+3.5
Power Good (pin 1 on P8) (pin 8 Power OK on ATX)	+3.0	+6.0

If a power supply fails any of these measurements, replace it and retest the new unit.