|Mr. Fixit's PC Upgrade and Repair
The PC's power supply is the most important part of any computer. Its job is to provide a stable supply of power to the system. Some
power supplies have a master switch and an Input Voltage Selector switch (115/230). The IVS switch sets the power supply to accept
either 115VAC (USA/Canada) or 230VAC (other countries), some power supplies have an automatic voltage selector. Most desktop pc's
are based on the ATX form factor. These power supplies provide +3.3V, +5V, and +12V of direct current known as rails.
The +3.3V rail powers Dual In-line Memory Modules (DIMM), PCI/AGP bus, Chipsets (North and South Bridge), etc. The +5V rail powers
disk drive logics, old Single In-line Memory Modules (SIMM), PCI/AGP bus, Chipsets, etc. The +12V rail powers disk drive motors, fans,
and voltage regulators.
Most power supplies also provide a -12V which is used for LAN cards (Ethernet) and serial ports. Older power supplies had a -5V line
that was used on the old 16-bit ISA bus, but has been removed since ATX version 2.0 because the PCI bus replaced the ISA. There is
also a +5VSB line known as the +5V Stand-By. It's function is to provide the motherboard power during the 'OFF' state. Functions like
Wake-On-LAN and Wake-On-Modem use the +5VSB to turn on the PC whenever activity on the network was detected. It has been known
that some motherboards have the VSB routed to the Keyboard. Press a key combination and the PC comes to life. The VSB will supply
power as long as the Power Supply unit in plugged into the AC. If you plan to work inside the PC, be sure to disconnect the AC cord or flip
the main power switch on the back of the PC.
Motherboard power connector will either be a 20-pin or a 24-pin (ATX12V v2.0 and later) that provides the
motherboard with 3.3V, 5V, 12V, -12V and +5VSB. Some power supplies use connectors referred to as
20+4, because the added 4-pin connector can be detached to fit 20-pin motherboards. The extra 4-pins
provide an additional 3.3V, 5V, 12V and ground to motherboards using the 24-pin connector. If the power
supply has a 20-pin connector, but the motherboard has a 24-pin, you can buy a 20 to 24-pin adapter
without the need to buy a new PSU.
Peripheral power connectors provide 12V (yellow), 5V (red) and ground to IDE / EIDE disk drives, cooling
fans, and some graphic cards. These are known as 4-pin Molex connectors or IDE power connectors. If you
need an extra connector, you can buy a 4-pin "Y" adapter that will convert one 4-pin connector into two 4-pin
molex connectors. Some PSU Manufactures are doing away with the connector because many of the new
PCs do not use these connectors, but you may see some devices that still use these connectors.
Mini 4-pin Molex power connectors provide 12V (yellow), 5V (red) and ground to power Floppy drives.
The connectors are small and are wired in reverse order opposite of the peripheral power connector. These
connectors were often branched off a Peripheral connector but, are slowly being phased out since many
PCs no longer use Floppy drives, however, other devices and adapters may use the connector.
Serial ATA (SATA) power connectors provide 3.3V (orange), 5V (red), and 12V (yellow) to SATA drives.
The 3.3V provision was added as a compatibility measure for drives that want to use it. Most SATA drives
use 3.3V for their logic circuitry using a 5V > 3.3V conversion circuitry. Manufactures wanted to rid the
conversion circuitry to reduce costs but, have become reluctant since the popularity of the 4-pin Molex to
SATA Power adapter do not provide a 3.3V line for the drive.
ATX12V power connector was introduced in ATX v2.03 and became standard since ATX12V v1.0. The
connector provides 12V to the Processor's Voltage Regulators with the socket near the processor.
Motherboards built since the release of intel's Pentium 4 processor use the P4 connector (nicknamed P4
because the Pentium 4 was the first processor to use the connector). High-End motherboards that require
more power use an 8-pin connector called EPS12V, which is the same as combining two P4 connectors.
Some PSUs have two P4 connectors that can be combine to make an EPS12V.
PCIe power connector is a 6 or an 8-pin Molex mini-fit Jr. used to power high-end graphic cards, providing
up to 150 watts. Some graphic cards require 2 PCI-e power connectors. Some PSUs provide a 6+2
connector to power 6-pin or 8-pin Graphic Cards. If the PSU provides a 6-pin but not an 8-pin connector, you
can buy a 6-pin to 8-pin Power Adapter. If the PSU doesn't have PCIe power you can buy a Dual Molex
Power into a PCIe Power Connector adapter.