DC Outputs and Output Voltages
(Last edited 5/30/2026)
PC power supplies have a number of output voltages. Fortunately, there are fewer now than there used to be. And if ATX12VO ever gets adopted, we’ll be down to just one! In this section, I’d like to review all of the DC output voltages, past and present, specified by Intel for the ATX specification.
Negative 5V was removed from the ATX12V v1.3 in 2003. This voltage was primarily used in ISA slots. Most commonly, it was used for analog audio card op-amp biasing. When this voltage was a requirement, the ATX specification allowed for a ±10% deviation.
Negative 12V was moved from required to optional in the ATX 3.0 specification in 2022. This voltage was used for RS-232 ports, PCI slots and some analog audio chips’ op-amp biasing. If this voltage is implemented, the ATX specification allows for a ±10% deviation.
On the motherboard, the most important thing the +3.3V coming from the PSU supports is the M.2 NVMe drives we use. This is the only voltage these devices rely on. Other than this, this voltage is currently only used for low-power ICs. Some of these ICs may include the super I/O chip, PHY logic or an audio code chip. The PCIe slots have a few +3.3V pins for the same reason.
Off the motherboard, there is only one cable that has a +3.3V lead, and this is a legacy voltage. In very early SATA drives, the logic was powered by both +3.3V and +5V. Now it is just powered by +5V. And now with the PWDIS (power disable) feature on newer drives, people with +3.3V leads on their SATA drives are left thinking their drives are dead because they repurposed the function of this pin. The ATX specification allows for a ±5% deviation of the +3.3V rail.
These two diagrams represent two different motherboard layouts. The one on the left is ATX12VO with just two +12V rails and a +12V standby, while the one on the right is a typical ATX12V motherboard with +12V, +3.3V and +5V outputs and a +5V standby.
The +5V rail is quite busy on the motherboard. This is the rail that powers your RAM! Sure, your RAM actually runs at 1.1V (DDR5) or 1.2V (DDR4), etc. but this voltage is bucked from the +5V. The motherboard’s PCH or chipset also uses voltages bucked from the +5V rail. And, of course, the USB ports on your board’s I/O panel, certain audio codecs and ARGB headers. Throughout the rest of your PC, the +5V is used for the logic for most drives. Everything from SSD’s to optical drives use +5V. The ATX specification allows for a ±5% deviation of this voltage.
The +5V Stand By rail operates even when your PC is off. And by off, I mean “in standby”. ATX PCs are meant to be powered down through a “soft off” routine of shutting down through the operating system as opposed to literally throwing a switch to kill power to the PC. They are meant to go into a standby mode. It’s sort of like when you have a flat panel TV and you hit the power on the remote and the screen goes black. You wouldn’t typically then walk up to the TV and pull the plug out of the wall. While your PC is in this standby mode, the +5VSB keeps a small set of circuits alive so the PC can wake up or respond to external events. These “wake” functions include Wake-on-LAN (WoL) via the Ethernet controller, Wake from USB (keyboard, mouse, or USB device), Wake-on-PCIe (some add-in cards), and RTC (Real Time Clock) alarm wake (scheduled power-on). On many boards, USB ports can deliver power while in standby and this power comes from the +5VSB rail.
In the S3 sleep state, known as “suspend to RAM”, the computer is mostly off, but system memory (RAM) stays powered so the machine can wake up quickly and continue exactly where it left off. The RAM’s VRMs that were once regulated from the PSU’s +5V are now being regulated from the +5VSB.
The +12V rail is the biggest one in the PC. If our PSU is worth its weight, we should find that at least 90% of our power supply’s total output is on the +12V rail. All of our fans, AIO pump motors, and most RGB uses +12V. The CPU and GPU’s VRMs regulate their power from the +12V. Of course, we can also mention the motors of our “spinning rust” hard drives and optical drives using +12V, but not many of us use those any more. The ATX specification (pre-ATX 3.1) allows for a ±5% deviation of this voltage. In the ATX 3.1 specification, the +12V rail was allowed to drop as much as 7% (to 11.16V). This is due to the power excursions from high end graphics cards pushing the +12V rail well beyond the PSU's rated capability and the PSU's capability to quickly recover that rail back to its nominal voltage.
The +12V is so important because of the lower current for the same power (P = V × I) which results in less heat and loss in wiring and enables efficient point‑of‑load voltage regulation. This is why we keep seeing the big push for ATX12VO. ATX12VO (ATX 12‑Volt‑Only) is considered superior to legacy ATX from an engineering and efficiency standpoint, especially for modern PCs. Since modern PCs are already 12V‑centric with the CPU, GPU, fans, pumps, PCIe devices all fundamentally using +12V, 80–95% of power consumption is on +12V.
Current ATX power supplies have to regulate +12V, +5V and +3V regardless of load; whether they are in use or not. The byproduct of this is poor light-load efficiency and higher idle power draw. With ATX12VO, the PSU only generates 12V and the motherboard generates +5V and +3.3V locally and only as needed. This gives us better efficiency at idle and low-load, easier compliance with modern energy regulations (EU ErP, DOE), and lower electricity usage over system lifetime. Furthermore, regulating lower voltages on the motherboard where these voltages are needed provides the benefit of delivering more stable voltages with faster transient response.