Mar 24, 2010
Searching on internet a found some things, but nothing helpfull. I know how switches works, but power button only when pressed creates connection, and when let there are no connection, computer still work. Is there any tutorial or something to learn, how power button work? How initialized computer power?


The actual switching of the power (on and off) is done by circuits on the motherboard. The circuits are triggered to change state by a momentary short circuit of two points in the circuit. That is done by the front panel push button. A short-term connection of that button is enough. The motherboard circuit also has a second and third function it can do. The BIOS usually is set so that, if the circuit is already on the ON state and the momentary contact is kept closed for at least 4 seconds, one of two actions will be carried out. Either the system will shut down right away (like turning off a switch), or the motherboard will enter the "sleep" state and wait to be re-awakened. Which of these two options is done is something you set in the BIOS Setup screens.

This is the reason that the motherboard is never entirely "Off" in most operating situations. Even when it is "Off", there is a small portion of it devoted to checking for inputs from a few sources (like the front push button, some USB devices and maybe a modem) so that it can respond by turning power on to the entire system. The way to really shut down all power is either to pull out the power cord, or to use the physical switch mounted on the back of the PSU to switch it off.