1. Power Supply in PCs
It is primarily designed to take AC power from the wall outlet and convert it into much lower DC voltages needed by the system.
This conversion must be reliable and efficient.
It should shut down the system if there is any short circuit, over heating or overloading.
It must include some way of providing cooling airflow, because the conversion process has always some heat as byproduct.

2. We have 2 types of power supply
1.Linear mode power supply
2.Switched Mode Power Supply.
SMPS used in a PC is much lighter and more efficient than Linear mode used in mainframes.

4. Cheaper power supplies not only have poor voltage and load regulation due to cheaper parts but also often use inferior means of EMI protection.
They work fine under normal loads and clean input power, but in less than perfect circumstances they may become unreliable causing spontaneous rebooting or even physical damage.

5. Output Supply voltage Lines
This is the basic supply voltage for nearly all electronic components.
The reference for the “high” level in binary signals is derived from this voltage, so accuracy is very important.
It is used in 2.5” and 3.5” drives.
It always be red in any standard system.
In AT and mini AT, circuits on the motherboard converts +5V to +3.3V.

6. Volt
It is used by motors and cooling fans.
Also serial ports and internal modems use +12V.
Volt
It is used by serial ports that use +12 Volt.

7. -5Volt
It is not used by any device and is retained only for purpose of backward compatibility.
Earlier it is used by some of the DRAM chips.
Volt
CPU and memory ,PCI bus use this voltage.
It does not appear on the AT systems.
Removing the voltage regulation circuit on motherboard helps reduce the amount of heat .

8. Power Supply Control Signals 1. Power Good Signal 2. Power-On 3
Power Supply Control Signals 1. Power Good Signal 2.Power-On 3.+5VSB Sense

9. Power Good Signal
Also known as PWR_OK or PWR_Good
It is a logic level signal.
It is a signal from PSU to motherboard when it has stable dc voltages.
Otherwise the CPU will be reset.
It normally takes 0.1 to 0.5 seconds after we switch on the system.

10. Power-On
Also known as PS_ON
It is a low voltage logic level signal from the motherboard to power supply, telling it when to turn on and when to turn off.
With this design the power supply is actually always powered up in a standby mode.
The motherboard uses the PS_ON signal to tell the power supply to power up fully.
This also allows an external wake-on-LAN to wake up the system on special commands from the network.

11. +5VSB
+5V, Standby Mode
It is a +5V supply line that is active any time the AC power input is active, regardless of whether the system is turned on.
The purpose of this supply line is to power standby circuitry, such as PS_ON circuit, the wake-on-LAN e.tc.

12. +3.3 Sense
It is used to sense the 3.3V after it has reached the motherboard.
This allows the power supply to "fine tune" the +3.3 V output in the event of excessive voltage drop between the supply and the components that use +3.3 V.
It is an important signal because CPU uses 3.3V.