The ATX power supply generates three main voltage outputs that are: +3.3 V ; +5 V ; and +12 V. Low-power −12 V and +5 VSB (standby) supplies are also generated by this power supply. The −5 V output was originally needed since it was supplied on the ISA bus, however it became obsolete with the removal of the ISA bus in modern day PCs and has been removed in later versions of the standard ATX power supply.
Originally the motherboard was powered by one 20-pin connector. An ATX power supply has several peripheral power connectors. In modern desktop computer system, there are two connectors for the motherboard: a 4-pin auxiliary connector providing additional power to the CPU, and a main 24-pin power supply connector, an extension of the original 20-pin version.
Here the ATX Power Supply Pinout:
There are 4 wires which have special functions:
- PS_ON# or “Power On” is a signal from the motherboard to the power supply. When the line is connected to GND (by the motherboard), the power supply will turned on. It is internally pulled up to +5 V inside the power supply. To check your stand alone ATX power supply, simply connect the PS_ON# wire (green wire) to the grounding wire (black).
- PWR_OK or “Power Good“ is an output from the power supply that indicates that its output has stabilized and is ready for use. It remains low for a brief time (100–500 ms) after the PS_ON# signal is pulled low.
- +3.3 V sense should be connected to the +3.3 V on the motherboard or its power connector. This connection allows for remote sensing of the voltage drop in the power supply wiring.
- +5 VSB or “+5 V standby” supplies power even when the rest of the supply lines are off. This can be used to power the circuitry that controls the Power On signal.