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.

• We have 2 types of power supply1.Linear mode power supply2.Switched Mode Power Supply.

• SMPS used in a PC is much lighter and more efficient than Linear mode used in mainframes.

• 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.

Output Supply voltage Lines1.+5 Volt:• 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.

2. +12 Volt• It is used by motors and cooling fans.• Also serial ports and internal modems use +12V.3. -12Volt• It is used by serial ports that use +12 Volt.

4. -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.5. +3.3Volt• 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 .

Power Supply Control Signals

1. Power Good Signal2.Power-On3.+5VSB4.+3.3Sense

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.

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.

+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.

+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.