What is +5VSB?

What Is +5VSB?

+5VSB, short for +5-Volt Standby, is a dedicated power rail provided by ATX-compliant power supply units (PSUs). Unlike the main power rails (+3.3V, +5V, +12V) that only become active when the system is fully powered on, the +5VSB rail delivers a continuous 5 volts as long as the PSU is plugged into a live AC outlet and its master switch is on. This standby voltage is what keeps certain portions of the motherboard alive even when the computer appears to be completely shut down.

Understanding +5VSB is essential for anyone studying computer hardware, troubleshooting boot problems, or preparing for certifications such as CompTIA A+, because it underpins many of the power-management and wake-up features found in modern PCs.

Why It Matters

The +5VSB rail exists to support the concept of soft power control. In older AT-style systems, the power switch physically cut mains power. ATX systems replaced this with a momentary push button connected to the motherboard. For the motherboard to detect a button press while the system is off, it must have some power available at all times — that power comes from +5VSB.

• Soft power on/off — The power button signals the motherboard rather than switching mains directly.
• Wake-on-LAN (WoL) — The network interface card stays partially powered to listen for a magic packet.
• Wake-on-USB / Wake-on-keyboard — USB and PS/2 devices can wake the machine.
• Real-time clock and CMOS backup assistance — Helps preserve settings and reduce battery drain.
• RTC wake timers — Scheduled power-on events.

How It Works

Inside the PSU, a small dedicated standby converter (often a flyback circuit) generates the +5VSB output independently of the main switching circuitry. This is why the rail is available before the PSU receives the PS_ON# signal from the motherboard.

1. When AC power is applied and the PSU switch is on, the standby converter energizes the +5VSB rail.
2. The motherboard's power-management chipset uses +5VSB to monitor the power button and other wake sources.
3. When a power event occurs, the motherboard pulls the PS_ON# line (green wire on the 24-pin connector) low.
4. The PSU then activates all main rails and asserts Power_Good (PWR_OK) once voltages stabilize.

Note: On the standard 24-pin ATX connector, +5VSB is carried on pin 9, typically identified by a purple wire. The PS_ON# signal is the green wire on pin 16.

Current Ratings

The ATX specification requires the +5VSB rail to supply a minimum current, historically around 2 amps (older specs) up to 3 amps or more in modern high-feature systems. Because features like WoL and fast startup draw from this rail, undersized standby capacity can cause wake failures or instability.

Common Use Cases and Applications

• Enterprise remote management — Administrators use Wake-on-LAN to power on machines overnight for patching, backups, and imaging, all dependent on +5VSB keeping the NIC alive.
• Fast Startup / Hybrid Sleep — Windows fast startup and S3/S4 sleep states rely on standby power to preserve state and resume quickly.
• Server always-on baseboard management — Standby power supports lights-out management controllers.
• USB charging while off — Some boards route +5VSB to USB ports so devices can charge with the PC powered down.

Troubleshooting Considerations

The +5VSB rail is one of the first things a technician checks when a system shows no signs of life:

• Dead motherboard LED — Many boards have a standby LED that lights when +5VSB is present. If it is dark, suspect the PSU standby circuit or AC input.
• System won't power on with button press — If +5VSB is missing, the motherboard cannot detect the power button.
• Failing standby rail — A degraded standby converter may produce voltage that is out of spec (e.g., below 4.75V), causing intermittent no-boot conditions even though main rails test fine.
• Measurement — Use a multimeter on the purple wire (pin 9) referenced to ground; it should read approximately 5V (±5%) with the PSU plugged in but the system off.

Best Practices

• When servicing internal components, remember that +5VSB may still be live even when the PC is shut down — always unplug the AC cord or flip the PSU master switch before working inside the case.
• Choose a quality PSU with a robust standby rail if you rely heavily on WoL or standby features.
• If Wake-on-LAN or fast startup behaves unreliably, verify standby rail voltage and current capacity as part of diagnostics.
• Disable AC power (unplug) to fully clear standby power when clearing CMOS or reseating components.

Real-World Example

Consider a corporate IT department that schedules Windows updates at 2 a.m. Machines are shut down each evening but remain plugged in. A management server broadcasts magic packets to each computer's MAC address. Because the +5VSB rail keeps each NIC's wake logic powered, the systems boot, apply patches, and shut down before employees arrive — all made possible by this quiet, always-on standby rail.