ATX Power Supply Power-On Switch
Last updated: 5/10/02

Q. Why won't the front panel power switch on my computer turn-off the computer?  The computer has an ATX power supply and motherboard.

A.  It probably will if wired correctly... That switch is a Power-on switch.  It connects to the motherboard and is used to remotely control the power supply in conjunction with functions such as soft On/Off (Windows, etc.), Wake on LAN (WOL), Wake on MODEM (WOM), USB, etc.  The motherboard in turn sends a signal, PS_ON#, a TTL (Transistor Transistor Logic) low signal (which is debounced by the power supply), to the power supply telling it to turn-on full power.  If PS_ON# is pulled to a TTL high state, the power supply will turn-off all of the voltages except the 5-volt standby voltage (+5VSB).  +5VSB is active whenever AC power is present.  Once the power supply voltages are stable, the power supply will signal that fact to the motherboard with the PWR_OK signal.

Most motherboards will put an ATX power supply in the standby mode if the front panel Power-on switch pushed and held in for 4 seconds.  It is a long four seconds for most motherboards power supply combinations. Count one thousand, two thousand,... five thousand... The CMOS Setup for many motherboards has a setting that can change the 4 second "Power-off" (standby) to "Instant off."  Many recent ATX power supplies have a separate power switch on the power supply that will turn-off the supply (including +5VSB) as surely as a light switch will turn-off the lights in a room. That switch is handy when installing expansion boards in a computer, etc., but should not be used for a normal power-down.  To avoid damage, pull the power cord to kill all power (+5VSB) when installing expansion boards, drives, etc. if there is no such switch.

Reference: Intel ATX/ATX12V Power Supply Design Guide