The power supply is an essential, but oft-overlooked part of the
personal computer. Most often it comes with the case so compatibility is not an issue. But when a power supply fails and you're out on
the cold, wet streets of the unwired, you'll thank me, that is, if you've had the foresight to print this out before it dies.
Godspeed!
Function
The
Prime Directive of the power supply is to convert the
AC power from your house and turn it into various
DC voltages that the computer can use. It also serves to insulate the computer, to some degree, from the spikes and dips that naturally occur with AC power.
A similar sort of conversion takes place all over your house. That big black thing that makes it impossible to efficiently use your power strip (generally called a wall wart) is an AC to DC adapter. As is the block on the cord for your laptop (soap on a roap). The problem with theze is that they are most often linear power supplies, which are vastly cheaper to build but they are inefficient. In power supplies, inefficiency means power wasted as heat. This is a problem when you put one inside a computer where there are already enough components giving off heat to warm your pizza pocket (don't try, it's sticky). To solve this, computer power supplies are designed to be switching power supplies, or, for the pedantic, "constant-voltage, half-bridge forward-converting switching power supply". Basically it reduces the heat produced by only drawing as much AC power from the wall as it needs.
For the
electrically-minded in our audience, here is a diagram of a simple design for a switching power supply:
AC - Prim. - Prim. - Solid State - Transformer - Secondary - Secondary - DC
Rectifier Filter Switch Rectifier Filter |
/\ |
| |
| |
----------Sensing/Switching Circuit----------
The switching/sensing closed loop allows for stable output and regulation of the AC power drawn.
Output Voltages and their Usage
These have remained mostly unchanged throughout the history of the PC, with the exception of adding some important ones and dropping some less important ones. Here's the basic set:
-12v : Seldom used, intended for some serial ports, less than 1 Amp
-5v : Also obsolete, used for ISA
0v :
Ground
+3.3v : Introduced by Intel to reduce power consumption in newer CPUs and some components
+5v : On newer systems, just runs motherboard, used to run almost all components
+12v : Runs drive motors and fans
Form Factor
A Form Factor is a standard set of dimensions into which a power supply must fit. In order for the PC to survive, non-proprietary as it is, there must be standards to which manufacturers can adhere and be assured that their products will work with parts made by other manufacturers
. All the power supply form factors have corresponding cases and
motherboards with which they will work properly.
PC/XT
The Original power supply form factor. Used in the first
IBM PCs, the establishment of a form factor aided in the manufacturing of clones for these first
Open architecture PCs.
Depth: 142 mm
Height: 120 mm
Width: 222 mm
Wattage: 63.5 W
Standard
Molex internal connectors
Extends from the front of the computer to the rear, where there is a fan an both a power input and output for a monitor. The switch stuck out the side of the computer.
AT
Another IBM instituted standard, the
AT form factor marked the rapid growth in PC manufacturing and use. It first came about in
1984 when IBM release the
PC/AT. AT stands for "Advanced Technology" and can still be found in the computers of
people who still don't know any better.
Depth: 150 mm
Height: 150 mm
Width: 213 mm
Wattage: 192 and up
The AT standard was also the first to be used in "Tower" cases. A revision of the form factor allowed for a "remote" power switch (i.e. connected by
wires, not actually on the power supply), the first ever. This allowed for innovations in
case design which still haven't taken place.
Baby AT
It's like AT, only narrower.
Depth: 150 mm
Height: 150 mm
Width: 165 mm
Wattage: 192 and up
LPX
The main competitor with Baby AT form factor. The LP stands for "
Low Profile". LPX has also been called
slimline and
PS/2 because of the cases it comes in.
Depth: 140 mm
Height: 86 mm
Width: 150 mm
ATX (NLX)
Intel introduced this one in 1995. It was the biggest change in fundamental design since the inception of power supply form factors.
Depth: 140 mm
Height: 86 mm
Width: 150 mm
Wattage: 100w-500w
The dimensions are essentially the same as that which came before, but there were a number of changes.
- +3.3V Power
- +5 Standby and Power On Signals, allowing "soft power".
- Additional Non-power Signals
- +3.3 V Sense (motherboard power regulation)
- FanC (Fan Control)
- FanM (Fan Monitor)
- 1394V and 1394R (firewire)
- No Monitor Pass-through
- Reversed Fan Direction
- New Motherboard Connector style
ATX is now the
industry standard power supply and has seen the most widespread use since its relatively recent inception.
SFX
SFX was released by Intel in 1997 to accompany the new
microATX motherboard form factor. It's connectors are identical to the ATX factor, but it is
smaller to allow for smaller system
footprints.
Dimensions:
Depth: 125 mm
Height: 76.4 mm
Width: 100 mm
Wattage: 90 W
Besides size, the only difference in the SFX form factor is the lack of a -5 voltage because it is only required for the
ISA bus, which most microATX boards do not have.
WTX
WTX is the big, hulking cousin of ATX.
Intel introduced this form factor in 1997 and revised it in 1998. The main differences are that it is
bigger, more powerful, and has a different motherboard connector and more
component connectors.
Dimensions:
Depth: 230 mm
Height: 86 mm
Width: 224 mm
Wattage: 460W, 610W, 800W
Motherboard Connectors
AT
P8 _ _ P9
|1|Power Good |1|Ground
|2|+5v |2|Ground
|3|+12v |3|-5v
|4|-12v |4|+5v
|5|Ground |5|+5V
|6|Ground |6|+5v
ATX
_____
+3.3v|1 11|+3.3v and +3.3v sense
+3.3v|2 12|-12v
Ground|3 13|Ground
+5v|4 14|Power On
Ground|5 15|Ground
+5v|6 16|Ground
Ground|7 17|Ground
Power Good|8 18|-5v
+5v Standby|9 19|+5v
+12v|10 20|+5v
WTX
P1 _____ P2 _____
+3.3v|1 13|+3.3v +5v Sense|1 12| +5v Sense Return
+3.3v|2 14|+3.3v +3.3v Sense|2 13|+3.3v Sense Return
+3.3v|3 15|+3.3v Reserved|3 14|+3.3v
+3.3v|4 16|+3.3v Ground|4 15|Ground
+3.3v|5 17|+3.3v Aux Ground|5 16|+12v IO
Ground|6 18|Ground +12v IO|6 17|+12v IO
Ground|7 19|Ground -12v|7 18|Sleep
Ground|8 20|Ground Reserved|8 19|Reserved
Ground|9 21|Ground FanC|9 20|FanM
Ground|10 22|+5v standby Pwr Good|10 21|Power On
+5v|11 23|+5v Reserved|11 22|Reserved
+5v|12 24|+5v
Molex® Drive Connector
__
|1 \ +12v
|2 | Ground
|3 | Ground
|4 / +5v
Source: http://www.pcguide.com/ref/power/sup/index.htm