PC Hardware in a Nutshell, 3rd Edition

ATX/ATX12V Power Supply Specifications

ATX Specification Version 2.1 and associated documents define the ATX voltage rails and tolerances shown in Table 26-2. An ATX 2.1-compliant power supply must provide these voltages at these tolerances or better. High-quality power supplies provide tighter tolerances, sometimes much tighter, such as 1% across all positive voltages. Cheap power supplies often do not meet the required tolerances for one or more voltages, and are therefore technically not ATX power supplies. However, they look like ATX power supplies, quack like ATX power supplies, and are sold as ATX power supplies. Avoid any power supply that does not meet the following standard. V_min and V_max are calculated values, provided for the convenience of those testing power supplies with a DMM.

Table 26-2. ATX Specification Version 2.1 voltage rails and tolerances

Voltage Rail	Tolerance	Vmin	Vnom	Vmax
+3.3VDC	±4%	+3.168V	+3.300V	+3.432V
+5VDC	±5%	+4.750V	+5.000V	+5.250V
-5VDC	±10%	-4.500V	-5.000V	-5.500V
+5VSB	±5%	+4.750V	+5.000V	+5.250V
+12VDC	±5%	+11.400V	+12.000V	+12.600V
+12VDC (peak load)	±10%	+10.800V	+12.000V	+13.200V
-12VDC	±10%	-10.800V	-12.000V	-13.200V

The ATX/ATX12V Power Supply Design Guide Version 1.2 defines two distinct types of power supply, the ATX power supply and the ATX12V power supply. An ATX12V power supply is a superset of an ATX power supply, and is backward-compatible with an ATX unit. ATX power supplies support ...