Do PSU's ever really shut down?

[Dogdaysdude]

I guess I never thought about this before, but a friend asked...when a computer is shut off, does the power supply also totally shut down? Or does it stay on, ready to power up if rocker switch is left on? (If it has one).
Thanks.

[saphalline]

If there is a rocker switch, then the PSU is always drawing a trickle of energy from the outlet. The only way to stop it completely is to turn it off via the rocker switch or unplug it.

If there is no rocker switch, then the PSU is called an "auto-switching" PSU and will act the same as a PSU with the rocker switch always on. These PSU's can only be stopped by unplugging them.

I prefer high quality PSU's with a rocker switch. Those "auto-switching" ones are a PITA for a high-end system, but they work fine for a low-end or older secondary system.

The reason PSU's always draw a bit of energy is for the +5V stand-by rail. This voltage rail provides power to latent functions within the system, such as keeping the CMOS refreshed (so the mobo battery isn't sucked dry) and for modems/ethernet adapters and keyboards/mice for "wake on ring", "wake on LAN", and "wake on kb/ms" (for advanced ACPI stand-by/sleep mode).