Form Factor & PSUs

Form Factors

The form factor typically defines the dimensions, screw holes and component and connector positions (including expansion bus slots) of computer parts.  The term is usually used when talking about motherboards and cases.

AT

The original PC-AT motherboard was quite large; it was the half-sized Baby AT design became the predominant AT motherboard.  These are easily identified by the 5-pin DIN keyboard socket and two-part power supply connector (see notes on PSUs).

The AT design was followed for 15 years but it had a number of major flaws:

• The CPU was too far away from the main power connector
• RAM was often obscured by disk drives
• There was no standard for onboard connector placement
• Most cases had a live mains lead running through the case to the on/off switch.

ATX

In 1995 Intel introduced the ATX form factor.  This had a soft power switch, used the PS/2 keyboard & mouse connector, and relocated components for better air flow and easier access.  A full ATX board is 12" × 9.6" (305mm × 244mm).

ATX boards usually have mouse, keyboard, USB, serial, parallel, video and audio connectors mounted in a standard position at the rear of the case.

The new ATX power supply used a keyed 20-pin connector.  Pentium 4 and Athlon 64 systems usually require an extra 4-pin 12V cable and 8-pin tertiary connector.

Motherboards featuring PCI Express require an extended 24-pin PSU connector in addition to the 12V cable.

Micro ATX

A Micro ATX board measures 9.6" × 9.6" (244mm × 244mm) and has fewer expansion card slots.  (A variation of this design is called Flex ATX.)

NLX

The NLX form factor is a low-profile design that has been largely superseded by Micro ATX.

Mini-ITX

Mini-ITX was developed for use in embedded systems and industrial control.  The motherboard measures 170mm × 170mm (6.8" × 6.8").  They are also designed for low power consumption.

Mini-ITX boards are often used to build custom and media centre PCs.

WTX

The WTX form factor was introduced in 1998 for servers using Xeon and AthlonMP / Opteron processors.  It is bigger than ATX and uses a 24-pin PSU connector.

BTX

BTX was intended to replace the ATX form factor.  It offers better airflow but has been largely shunned by manufacturers.

Power Supply Units (PSUs)

The PSU converts mains AC electricity to the DC voltages required by chips, motors and circuits in the PC.

PSUs contain potentially lethal mains voltages.
Unless specially trained, you must not dismantle a PSU while it is powered.

In addition, with power removed some internal components store enough electrical charge to give a very painful shock.

The switch-mode PSUs used in PCs work by switching larger voltages on and off rapidly to give a lower average voltage.  These circuits rely upon feedback loops and are therefore difficult to fault-find without expert knowledge.

Replacing a PSU

It is usually far cheaper to replace a faulty PSU than repair it.  Therefore they are treated as non-serviceable items.

When replacing a PSU it is vital to ensure that the replacement unit is rated at an equal or greater wattage.

Voltages

There are five main DC voltages produced by PSUs: +3.3V, +5V, –5V, +12V and –12V.  The PSU also provide the main system ground that current returns along.  3.3V is used by the processor, 5V by system board ICs, expansion cards and disk drives.  The 12V levels typically power expansion cards and disk drive motors.

Some PSUs allow mains power to be passed through to the display screen.

AT

The older supply used by AT systems has two 6-pin connectors (labelled P8 & P9) that connect to the system board.  It is possible to connect P8 & P9 round the wrong way — if this happens then the system board will fry!

ATX

ATX style PSUs use a single 20-pin adaptor, but may also sport auxiliary connectors for system boards requiring higher currents.

Some companies like Dell use their own custom designs for PSU connection.

Voltages

The main connector carries 12V, 5V and 3.3V supplies.

The first auxiliary connector looks like the old P8/P9 ones.

The second is a four-pin adaptor that supplies additional 12V lines.

Soft power

The main difference between AT and ATX systems is that ATX is designed to be switched on and off by software control.

To enable this, some parts of the system board are powered continuously, so that components can sense a wake-up signal (such as the power button at the front of the case).

Therefore, switch mains power off before removing expansion cards.