A short history of PC power supply voltage rails

Let's clear up any confusion about the term "rail". A voltage "rail" refers to a single voltage provided by the PSU (short for power supply - actually it stands for power supply unit). An ATX PSU has one 3.3 volt rail. It also has one 5 volt rail. The 3.3 volt rail has its own circuitry in the PSU which generates the voltage. It also has a lot of wires and connectors to distribute the 3.3 volts to any hardware which needs it. The 5 volt rail has its own separate set of circuitry, wires, and connectors to deliver 5 volts. Modern ATX12V PSUs may have as many as four separate 12 volt rails. Each 12 volt rail has its own set of wires and connectors just like the 3.3 and 5 volt rails. The 12 volt rails just happen to be generating the same voltage as each other. If you'd like to have a look at the official ATX specifications they can be found at formfactors.org.

The original IBM PCs drew most of their power from two voltage rails: 5 volts and 12 volts. Their power supplies also provided -5 and -12 volts but those only delivered small amounts of power. They had a 5 volt rail because that was the voltage needed to power most of the standard silicon chips of the time. The 12 volt rail was used primarly to operate fans and floppy disk drive motors. The original PC PSU could deliver a maximum 63.5 watts most of which was on the 5 volt rail. As time passed, PCs included bigger, faster chips which increased the load on 5 volts. People also added new-fangled devices like hard disks and eventually CD-ROM drives so the 12 volt rail had to deliver more power too. But the 5 volt rail still continued to deliver most of the power because most of the power is consumed by the chips.

Chip technology improves by cramming larger numbers of smaller transistors onto the chips. As transistors shrink they need to operate on lower voltages. When the newer ATX standard was created, a 3.3 volt rail was added to power the newer chips. So a PC of the time had a mix of 3.3 volt and 5 volt chips directly connected to their respective voltage rails. The table below shows the rail sizes from an old 300 watt ATX PSU. Most of the wattage is delivered on the 3.3 and 5 volt rails. It also has a fairly powerful 12 volt rail for computers with multiple disk drives.

300 Watt ATX PSU
Voltage Maximum current Maximum wattage
+3.3 volts 20.0 amps 66 watts
+5 volts 30.0 amps 150 watts (180 watts maximum combined +5 and +3.3)
+12 volts 10.0 amps 120 watts
5 volts standby 1.0 amps 5 watts
-5 volts 0.5 amps 2.5 watts
-12 volts 0.8 amps 0.96 watts

As technology improved, the transistors in the chips continued to shrink and they needed to run off of voltages lower than 3.3. It just wasn't practical to continue to run all the chips directly off of a voltage provided by the PSU because they would have to add more and more lower voltage rails as time passed. On top of that, you had to deal with CPUs which needed different voltages depending on which CPU was plugged into the motherboard. They temporarily avoided the problem by providing motherboard voltage regulators which dropped 5 or 3.3 volts down to a lower voltage by discarding the extra voltage as heat. As power requirements increased, that solution quickly became impractical.

That's when PC's power distribution fundamentally changed. The older PCs powered their chips by connecting them directly to voltage rails provided by the PSU. But the newer PCs started putting DC/DC converters onto the motherboard which took a voltage provided by the PSU and efficiently converted it into the lower voltage needed by the chips. Many of the early DC/DC converters converted 5 volts into the lower voltage. Presumably this was because the power supplies of the time delivered most of their power on 5 volts. But converting 12 volts instead of 5 volts makes the wiring much simpler because a higher voltage delivers the same amount of power by using a smaller current. Smaller current lets you use less wires and connectors to deliver the same power. Power distribution is much easier at higher voltages. The highest voltage provided by a PC PSU is 12 volts so that became the most common input voltage used by the biggest DC/DC converters. A modern CPU has its own converter on the motherboard which converts 12 volts to whatever voltage the CPU requires. Modern video cards also have their own converters on the card which convert 12 volts into the desired voltages. The CPU and video card tend to be the biggest consumers of power, when fully loaded, so a modern PSU has to provide most of its power at 12 volts. So in the old days you had a bunch of chips directly connected to 3.3 or 5 volts and that's where a PSU provided most of its wattage. But in a new computer the PSU provides most of its power at 12 volts and then various DC/DC converters throughout the computer convert it to whatever voltage is needed by that particular set of chips. The table below is a more modern 480 watt PSU. The maximum power available on 3.3 and 5 volts has increased a little but the bulk of the expanded wattage is provided on the 12 volt rail.

480 Watt ATX12V 1.3 PSU
Voltage Maximum current Maximum wattage
+3.3 volts 34.0 amps 112.2 watts
+5 volts 35.0 amps 175 watts (200 watts maximum combined +5 and +3.3)
+12 volts 28.0 amps 336 watts
5 volts standby 2.0 amps 10 watts
-12 volts 1 amps 12 watts
A short history of ATX versions
ATX version Introduction date Main changes from previous version Max watts at 3.3V + 5V for 300 watt PSU Max watts at 12V for 300 watt PSU
ATX 1995 Created 20 pin main power power cable
Included 4 pin peripheral power cable
Included floppy power cable		 ? watts ? watts
ATX12V 1.0 2000 Added 4 pin 12 volt power cable
Added 6 pin auxiliary power cable
Increased 12 volt output power		 180 watts 180 watts
ATX12V 1.3 2003 Added SATA power cable
Made -5 volt rail optional
Increased 12 volt output power 		190 watts 215 watts
ATX12V 2.0 2003 Changed to 24 pin main power cable
Removed 6 pin auxiliary power cable
Added 20 amp current limit to rails
Increased 12 volt output power
Reduced 3.3/5 volt output power 		120 watts 265 watts

The change in wattage distribution among the rails is why you have to be careful when sticking an old PSU into a new computer. Older PSUs deliver most of their wattage on 3.3 and 5 volts and newer ones deliver most of it on 12 volts. You can definitely have problems using an old 300 watt PSU in a new computer which needs a 300 watt PSU even if the power connectors are compatible. It's very easy for a new computer to overload the 12 volt rail of an old PSU. You can also have overloading problems sticking a new PSU into an older computer. Most ATX12V 1.3 and earlier PSUs provide enough 3.3 or 5 volt power to run an older motherboard but some newer ATX12V 2.0 and newer supplies have reduced the available power on 3.3 and 5. If you're using a new supply with an older motherboard then it's best to check that it has enough wattage on 3.3 and 5. You may also run into a different kind of trouble related to the -5 volt rail. -5 volt support has been optional in PSUs since ATX12V 1.3 because it's rarely used anymore. It's rarely included in new PSUs. But some old motherboards or ISA expansion cards require -5. So even if motherboards and PSUs from different eras have compatible connectors, you may have problems using them together. New and old ATX PSUs may look the same but what's going on inside is very different.