As you know, modern PC power supplies work mainly with three different voltages : + 12V, + 5V and + 3.3V. In this article we are going to tell you why this is so, and what hardware component of the PC uses each of them.
When you take a look at the power table of a modern power supply you will see that, as a general rule, they will have these three voltages (some still maintain the -12V and + 5Vsb rails , but they are already deprecated in more modern systems) . In addition, each of the rails, in addition to expressing their voltage in volts, are accompanied by the current intensity they are capable of providing, measured in amperes.
Why do PC power supplies operate at different voltages?
The first PC, created by IBM, only supplied two different types of voltage: + 12V and + 5V (it also supplied -12V and -5V but with very limited amounts of power). Most of the microchips of that time operated at 5V, but for some parts with motors such as hard drives and fans, a higher voltage was necessary, and therefore the + 12V rail was included. In addition, as the demand for peripherals grew, the + 12V rail of the power supplies became more important because it was the one they used.
For its part, the -12V rail was mainly for the RS-232 serial port, while the -5V rail was for peripherals on the ISA bus, such as sound cards, but it was never really used by anything other than that. and that’s why it disappeared.
Later, when Intel developed the ATX standard for power supplies in 1995, the microchips began to use lower voltage and it was necessary to implement the + 3.3V rail. Thus, since 1995 and the famous 80486DX4 power supplies began to have the three main rails that modern power supplies have: 12, 5 and 3.3 volts.
The ATX connector on the power supply provides all the necessary voltages directly to the motherboard on its multiple cables and power connections. Another addition to the ATX standard was the addition of the + 5Vsb (standby) rail to provide a small amount of “standby” power even with the PC turned off, but as we discussed at the beginning given the power states of the PC this is already unnecessary. to this day and many sources no longer even have it.
What voltage does each component of the PC use?
After what was explained before, you will already know that power supplies have several different voltages due to the electrical requirements of each of the hardware components of a PC, so now is the time to see which component uses each of the voltages, and especially why everything is not unified so that everything works with a single voltage value.
- + 12V : processor, graphics card, fans and some PCIe expansion cards. It is also the main voltage of the motherboard, although it must go through its own VRMs to regulate it. In general, it is the rail that serves the hardware components with the highest consumption.
- + 5V: mechanical hard drives, optical drives, some PCIe and USB expansion cards. All USB ports on a PC run at 5V, and that includes the peripherals that connect to them.
- + 3.3V: RAM memory and SSDs in M.2 format. Furthermore, all PCIe sockets are also capable of providing + 3.3V.
The reason the sources have different voltage values and therefore different rails is because of the electrical requirements of the components. As the transistors got smaller and smaller on the chips, it became preferable for them to operate with smaller voltage values, and this became more and more imperative as the transistor density in the processors increased.
To supply large amounts of low-voltage power to the processor, since the Pentium era, motherboards began to incorporate a voltage regulator to be able to control themselves what voltage and current is supplied to each component. Most modern processors can require up to 100A at 2V or less, so it is not practical to take these values out of the + 12V rail and be able to do it from another that works at a lower voltage, since it means less work for the regulator.
