What is Standby Power (+5VSB)?
Standby Power (+5VSB) is a dedicated 5-volt standby power rail supplied by an ATX power supply that remains active whenever the PSU is connected to AC power and switched on, even when the computer is turned off. It powers low-energy motherboard circuits that enable features such as soft power-on, Wake-on-LAN, USB charging, and system wake events.
Key Takeaways
+5VSB stands for +5 Volt Standby.
It remains powered while the main PSU outputs are off.
It enables remote startup and sleep/wake functions.
Modern motherboards depend on +5VSB for power management.
Typical output ranges from 2A to 4A, depending on the PSU model.
Why Does Standby Power (+5VSB) Exist?
Computers need certain circuits to remain powered even after shutdown. Without a standby power rail, users would lose convenient features like pressing the power button to start the PC, waking it over a network, or charging USB devices while the system is off.
The +5VSB rail provides continuous low-power electricity without energizing the CPU, GPU, or storage devices.
How Does Standby Power (+5VSB) Work?
When an ATX power supply is connected to mains electricity and its rear switch is on, the PSU activates a small standby converter.
This converter continuously supplies +5VSB to the motherboard. The motherboard uses this power to operate its power controller and monitoring circuits.
When the user presses the power button:
The motherboard receives the button signal.
It sends the PS_ON# signal to the PSU.
The PSU enables the main voltage rails (+12V, +5V, and +3.3V).
After the voltages stabilize, the PSU sends the PWR_OK signal, allowing the system to boot.
Key Characteristics
Constant 5V output while AC power is available
Independent from the main PSU outputs
Low current compared to primary power rails
Supports modern power management standards
Operates during shutdown, sleep, and soft-off (S5) states
Important Specifications
| Specification | Typical Value |
|---|---|
| Output Voltage | +5V DC |
| Rail Name | +5VSB |
| Typical Current | 2A–4A |
| Available State | PSU connected to AC and switched on |
| Primary Purpose | Power standby motherboard circuits |
What Devices and Features Use +5VSB?
Standby power supports numerous motherboard and peripheral functions, including:
Power button controller
Wake-on-LAN (WoL)
Wake-on-Keyboard and Wake-on-Mouse
USB charging while the PC is off
RGB controller memory
BIOS and embedded controller functions
Sleep and hibernation wake events
Advantages
Enables convenient soft power control
Supports remote PC startup
Allows USB charging when powered off
Improves power management capabilities
Essential for modern ATX systems
Limitations
Provides only limited power output
Cannot run high-power components like CPUs or GPUs
Standby circuits consume a small amount of electricity even when the PC is off
Excessive standby loads may exceed the rail's current rating
Standby Power (+5VSB) vs Main Power Rails
| Feature | +5VSB | Main Power Rails |
|---|---|---|
| Active when PC is off | Yes | No |
| Purpose | Standby electronics | System operation |
| Current Capacity | Low | High |
| Powers CPU/GPU | No | Yes |
| Always Available | Only when PSU has AC input | Only after system startup |
Common Misconceptions
"The PC is completely off."
Not entirely. Some motherboard circuits continue receiving standby power."+5VSB powers the entire computer."
No. It powers only low-power standby electronics."All USB ports remain powered."
Not always. USB standby power depends on motherboard and BIOS settings.
Real-World Examples
Pressing the PC's power button after shutdown.
Waking a desktop remotely using Wake-on-LAN.
Charging a smartphone from a USB port while the computer is off.
Turning on a PC using a keyboard or mouse when supported by the motherboard.
Related Technology Terms
ATX Power Supply — The PSU standard that defines the +5VSB rail.
PS_ON# Signal — Motherboard signal that tells the PSU to start its main outputs.
Power Good (PWR_OK) Signal — Indicates stable PSU output voltages before system boot.
Wake-on-LAN (WoL) — Technology that powers on a computer through a network packet.
Soft Power — Electronic power control that replaces a traditional mechanical power switch.