A power supply unit (PSU) for a PC doesn't operate at a single voltage; instead, it **provides multiple distinct voltage outputs** simultaneously to power different components within the computer.
Based on standard configurations, **all PC power supplies provide at least:**
* **+5VDC**
* **+12VDC**
* **-12VDC**
Some power supplies are designed to offer additional voltage rails to support various system components. These may include:
* **+3.3VDC**
* **-5VDC** (less common in modern systems)
* **5VSB** (5 Volt Standby)
## Understanding PSU Voltage Outputs
Each voltage rail serves a specific purpose in powering different parts of your computer:
* **+12V:** This is typically the most heavily used rail, powering high-demand components like the CPU, graphics card (GPU), and motors in hard drives and fans. Modern systems rely heavily on this voltage.
* **+5V:** This voltage traditionally powered logic circuits on the motherboard, drives (like SSDs, HDDs, CD/DVD drives), and USB ports. While some functions have shifted to +3.3V, it's still essential.
* **+3.3V:** Used by components like RAM (memory modules), chipsets, and some parts of the motherboard circuitry.
* **-12V:** Historically used for serial port interfaces (RS-232) and some older expansion cards. It is not heavily utilized in modern systems but is still provided by ATX specifications.
* **-5V:** Used by older ISA bus cards and is generally absent in modern PSUs due to its lack of use in current hardware.
* **5VSB (Standby Voltage):** This voltage is *always on* when the PSU is connected to a power source, even when the computer appears off. It powers circuits that allow the computer to wake up from sleep mode, power on via LAN, or respond to power button presses.
## Voltage Regulation
Beyond the specific voltage levels, **voltage regulation** is a critical aspect of a PSU's quality. Regulation specifies how tightly the PSU controls its output voltages under varying loads. Poor regulation can lead to unstable voltages, potentially causing system errors, instability, or even damage to components. High-quality PSUs maintain voltages very close to their target values.
In summary, a PC PSU is not "a specific number of volts" but rather a device that converts AC wall power into multiple stable DC voltage levels required by computer components.
| Voltage Rail | Common Uses |
| :----------- | :------------------------------------------------ |
| **+12V** | CPU, GPU, Motors (Fans, HDDs) |
| **+5V** | USB, SSDs, HDDs, Motherboard Logic (historical) |
| **+3.3V** | RAM, Chipsets, Motherboard Circuits |
| **-12V** | Serial Ports (historical), some Expansion Cards |
| **5VSB** | Standby Power (Wake-on-LAN, Soft Power Off) |
