PC Hardware in a Nutshell, Second Edition

Volts, Amps, Watts, and Regulation

Power supplies are rated in watts, but that cumulative figure doesn’t tell the real story. Watts (W) is calculated by multiplying Volts (V) and Amperes (A). A power supply must provide specific amperages (or currents) at numerous voltages—how many and which depends on the type of power supply. All PC power supplies provide at least +5VDC, +12VDC, and -12VDC. Some power supplies also provide +3.3VDC, -5VDC, and/or 5V_SB.

Another important aspect of voltage—one that varies greatly between power supplies—is regulation , which specifies how tightly voltages are controlled. For example, a memory module that expects +3.3V may work at +3.2V or +3.4V, but will probably not work at +3.1V or +3.5V. Regulation may be specified as a maximum percentage variation or as a maximum variation in absolute voltage.

No standards body produced a formal specification for all aspects of the AT power supply or its BAT and LPX variants. However, the ATX power supply—along with its variants, the NLX and SFX power supplies—is completely defined in a group of documents, many of which are referenced in the following descriptions, and can be downloaded from http://www.teleport.com/~ffsupprt.

ATX Power Supply Specifications

ATX Specification Version 2.03 and associated documents define the ATX voltage rails and tolerances shown in Table 26-2. An ATX 2.03-compliant power supply must provide these voltages at these tolerances or better. High-quality power supplies provide tighter ...