how the power switch works

[mrsteve0924]

wasn't sure if i should post this here or in motherboards.

i am trying to better understand how the PC switch turns on the system. I know it's a momentary switch and it shorts the two pins on the header on the MB.

i just cant get my head around how the system stays on though after the momentary switch switch goes back to open.

ive searched around the internet but couldnt seem to find a good answer (or one i understood anyway)

i guess i want to understand how the electricity is moving through the PSU to the power header to the momentary switch. thanks

 

[freakdiablo]

I'm guessing it's something like this - the motherboard does get some power from the PSU even when the system is off (if you look at a lot of PSUs, they'll have a section that says something like 3A @ 5VSB, meaning they'll supply up to 3 amps at 5v in standby). I'm guessing at least some of this powers acontroller on the motherboard.

That controller looks for that momentary switch from the power button, then shorts out a wire from the PSU to ground. This tells the supply to turn on and supply power to the rest of the system. The controller probably keeps the pin shorted until it receives another signal, then breaks the connection.

 

[bob4933]

Relays are your answer

 

[Supertrucker]

Shorting the green atx wire to one of the black (ground) wires on the 24 pin motherboard connector forces the power supply into an on state. A useful feature for powering a water cooling loop to bleed air and leak test without powering up motherboard components.

Id imagine the motherboard does the same thing after shorting the power on pins with a switch.

 

[mrsteve0924]

thanks guys. clears things up a bit for me.

 

[larrymoencurly]

Flip-flop, AKA latch, AKA bistable multivibrator, AKA static memory: http://en.wikipedia.org/wiki/Flip-flop_(electronics)

The front panel power button blips the flip-flop, which remembers the state and holds the green wire of the power supply at low voltage until the power button is blipped again. There are several types of flip-flops, and the kind that would do that would be a T flip-flop, or a JK flip-flop with both inputs turned on together. The circuitry on the motherboard that handles the on/off button is always powered by the +5Vstandby supply from the PSU (purple wire, which also powers the LED found on some motherboards).