Arcade games use one of two types of power supplies, either a switching power supply (much like modern computers use), or a linear power supply (used mostly in older games).

An arcade switching power supply looks similar to a small car audio amplifier, these are often referred to as "switchers". It will have 2 screws for hookup to AC power, and then a row of screws for the DC power it puts out. The vast majority of these power supplies are capable of putting out +5, +12, and -5 volts (most games do not use the -5 volts at all). These units are self contained, and require little maintenance (they do have a little knob on the side that you can adjust them with as needed). Defective switching power supplies can be cheaply repaired by a competent technician (although the industry standard is just to throw them away). You can replace an arcade switching power supply with a standard PC power supply in a pinch (but a few games want more out of the -5 volts setting than the PC power supply can put out).

Now a linear power supply is a whole different kind of beast (most older games like Ms. Pac-Man and Turbo use a linear power supply). Linear power supplies vary wildly in capabilities and in construction, but they all have a few things in common. Inside they contain a transformer, rectifier, and smoothing capacitor. The exact amount and types of DC current they put out is quite different from game to game (they don't have a set standard like the switching supplies do). Linear power supplies have a much higher failure rate (it seems even higher due to the fact that most of them are almost 20 years old now), than switching power supplies do. But fortunately they are not very difficult to repair (due to their simplicity).

The main difference between the two types (as far as normal people are concerned), is that a linear supply can operate without any load at all (while the switching units will fry themselves if left on without anything attached). The switching supplies require almost no adjustment ever, while the linear ones need adjustment fairly frequestly (but they do put out cleaner power when properly tuned).

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!


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.


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.


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


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


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 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.
 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 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.
 Depth: 230 mm
 Height: 86 mm
 Width: 224 mm
 Wattage: 460W, 610W, 800W

Motherboard Connectors


  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


      +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


   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


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