PC Hardware in a Nutshell, 2nd Edition

26.2 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.

26.2.1 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 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 standards in Table 26-2. 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 2.03 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

Intel ATX Power Supply Design Guide Version 0.9 recommends (but does not require) the power distribution levels listed in Table 26-3. A_min specifies the highest minimum amperage load the power supply should require to function. A_max specifies the lowest maximum amperages the power supply should provide continuously. A_peak describes startup surge current required to spin up disk drives. The 250W and 300W units include the ATX Auxiliary Power Supply Connector, and may or may not include the ATX Optional Power Supply Connector, both of which are described later in this section.

Table 26-3. Intel ATX Power Supply Design Guide Version 0.9 recommended power distribution

Voltage rail	160W			200W			250W			300W
	Amin	Amax	Apeak	Amin	Amax	Apeak	Amin	Amax	Apeak	Amin	Amax	Apeak
+3.3VDC	0.3	14.0		0.3	14.0		0.3	16.0		0.3	28.0
+5VDC	1.0	18.0		1.0	21.0		1.0	25.0		1.0	30.0
+3.3V & +5V	110W total maximum			125W total maximum			145W total maximum			220W total maximum
-5VDC	0.0	0.3		0.0	0.3		0.0	0.3		0.0	0.3
+5VSB	0.0	0.72		0.0	0.72		0.0	0.72		0.0	0.72
+12VDC	0.0	6.0	8.0	0.0	6.0	8.0	0.0	10.0	12.0	0.0	10.0	12.0
-12VDC	0.0	0.8		0.0	0.8		0.0	0.8		0.0	0.8

26.2.2 NLX Power Supply Specifications

NLX Power Supply Recommendations Version 1.1 defines the NLX voltage rails and tolerances shown in Table 26-4. An NLX 1.1-compliant power supply must provide these voltages at these tolerances or better. Note that NLX has tighter requirements than ATX on some rails. This document also recommends (but does not require) the power distribution levels listed for a typical 145W sustained (160W peak) power supply. A_min, A_max, and A_peak are as described above. NLX power supplies may or may not include the NLX Optional Power Supply Connector described later in this section.

Table 26-4. Intel NLX Power Supply Recommendations Version 1.1 voltage rails, tolerances, and recommended power distribution

Voltage rail	Tolerance	Vmin	Vnom	Vmax	Amin	Amax	Apeak
+3.3VDC	±4%	+3.168V	+3.300V	+3.432V	0.3	9.2	16.0
+5VDC	±5%	+4.750V	+5.000V	+5.250V	1.0	16.0	18.0
-5VDC	±5%	-4.750V	-5.000V	-5.250V	0.0	0.1
+5VSB	±5%	+4.750V	+5.000V	+5.250V	0.0	0.72
+12VDC	±5%	+11.400V	+12.000V	+12.600V	0.0	1.4	4.0
-12VDC	±5%	-10.800V	-12.000V	-13.200V	0.0	0.2

26.2.3 SFX Power Supply Specifications

SFX systems are designed to be low-cost, and the SFX specification reflects this with less stringent requirements than the ATX and NLX specifications. SFX power supplies are designed to provide 90W maximum continuous power, with peak power of 135W for 15 seconds duration on a 5 minute duty cycle. The Intel SFX Power Supply Design Guide Version 1.1 Release defines the SFX voltage rails, tolerances and power distribution shown in Table 26-5. An SFX 1.1-compliant power supply must provide these voltages at these amperages with these tolerances or better.

Table 26-5. Intel SFX Power Supply Design Guide Version 1.1 Release voltage rails, tolerances, and required (not just recommended) power distribution

Voltage rail	Tolerance	Vmin	Vnom	Vmax	Amin	Amax	Apeak
+3.3VDC	±5%	+3.135V	+3.300V	+3.465V	0.0	6.0
+5VDC	±5%	+4.750V	+5.000V	+5.250V	1.0	10.0	12.0
+5VSB	±5%	+4.750V	+5.000V	5.250V	0.0	0.72
+12VDC	±5%	+11.400V	+12.000V	+12.600V	0.02	1.5	4.8
-12VDC	±10%	-10.800V	-12.000V	-13.200V	0.0	0.2

Although derived from the ATX and NLX specifications, SFX has several differences, all of which are oriented to the low-end focus of SFX, and allow lower-cost power supplies that still meet the needs of these systems. +3.3VDC is regulated to ±5% rather than ±4%; a peak current is specified for +5VDC; -5VDC, included in ATX for backward compatibility with ISA cards, is absent because SFX systems do not use ISA cards; +5V_SB, although specified, is optional; and the A_min for +12VDC, 0.0A for ATX and NLX, is now 0.02A.