Articles :: Power Supplies 101: A comprehensive guide ::

Jon Gerow · 06-26-2006 · Category: Guides

Defining the connectors of an ATX power supply

As stated earlier, there are several connectors on an ATX power supply. Several connectors with several different wires providing several different voltages. In this section, we'll take a look at each of the connectors on a typical ATX power supply.

The above connector is a 20-pin ATX power supply connector. As you can see from the color-code to the left of the photo, it supplies many different voltages (+3.3V, +5V, +12V, -12V, -5V) as well as a number of ground wires and a couple signal wires ("Pwr On" which is how the motherboard tells the power supply to turn on and "Pwr Good" which reports back that the voltage of 3.3V and 5V are within spec) There's also a +5V stand by (+5VSB) that constantly supplies power to the board even when the PC is off. This voltage is used for several different CMOS functions like Wake On LAN, Wake On Ring, etc.

The connector above is still an ATX main power connector, but is of the 24-pin variety. One each additional +3.3V, +5V, +12V and Ground have been added to the connector.

Some people will tell you that 24-pin connectors are for PCI-e boards. Funny. PCI-e slots don't use +5V and I can think of a few AGP based boards that have a 24-pin main power connector.

Fact is, the extra 4-pins simply provide more power to the motherboard over more wires. This reduces resistance, heat and can provide better voltages.

It's not unusual to see motherboards with 24-pin connectors on them. Likewise, it's not unusual to still see power supplies with only 20-pin power connectors. If you end up with such a mismatch, don't sweat. Use the 20-pin power connector in the 24-pin socket, as shown in the above photo, and chances are your PC will work just fine. The above is actually a production model eMachine, built by FIC, that came from the factory with a 20-pin power supply, despite the 24-pin connector on the board.

Some 24-pin main power connectors allow the user to "break away" the extra four pins so the connector can easily be used in a 20-pin motherboard. The four pins that are "broken off" need to be tucked away somewhere inside the PC. These wires can not be connected anywhere else in the PC.

The above left connector is called a "+12V" and is the defining characteristic that differentiates an ATX power supply from an ATX12V power supply. It's also known as the "P4" connector because they were initially introduced only on Pentium 4 boards. The connector comprises of two +12V wires and two grounds and provides voltage to the CPU.

The connector on the right is called the "EPS connector." It's typically used on SSI or SMP boards, mostly of the server variety. Essentially, it is two of the +12V connectors side by side and comprises of four +12V leads and four grounds. A quad rail SSI power supply tends to have each pair of +12V leads on this connector on it's own rail, essentially providing each CPU in a dual CPU arrangement it's own dedicated rail. Recently, high performance boards (like the Asus P5N32-SLI Deluxe) have added an EPS connector to provide better amperage, through more leads.

Like the 20 and 24-pin ATX main power connectors, EPS and +12V connectors are interchangeable. A 4-pin +12V connector will fit in an EPS receptacle and an EPS connector can fit into a +12V receptacle if you leave four pins hanging off the edge.

In fact, some power supplies actually provide an 8-pin EPS that can be split into two 4-pin connectors (see below left,) one of which can be used for the +12V connector.

The connector in the upper right photo is the PCI Express power connector. Similar to the +12V and EPS connectors, it consists of only +12V leads and ground leads. Three of each, to be precise. This connector is used to provide additional power for PCI-e video cards. Only so much power can be pushed through the tiny little slot soldered to your motherboard's PCB. By using this connector, better voltage can be delivered to your video card's GPU because there is less resistance.

The above left connector is the most common, and most certainly the most unchanged connector, of any PC power supply. It's technically called a "peripheral power connector," although most of us call it a "Molex." Molex is quite a misnomer, actually. Like Kleenex and Tabasco, Molex is a brand and the company actually has part numbers for every connector you'd ever find on a power supply or motherboard and not just the peripheral power connector. And although there are a number of companies other than Molex making these connectors, we still call them "Molex."

This connector is used for optical drives, hard drives, fans... pretty much any device in a PC that doesn't have a connector specifically designed for it. This connector supplies both +12V (yellow wire) and +5V (red wire) and has two grounds. Typically, the +12V is used to power motors while the +5V is used to power logic boards.

On the right, we have a floppy power connector. It's wired the same way as a peripheral power connector with one +12V, one +5V and two grounds, but is considerably smaller in size for use with devices like floppies, LS120's and Zip drives.

Finally, we have the SATA power connector. This connector provides power to Serial ATA devices. It provides +12V and +5V just like a peripheral power connector, but also provides +3.3V. +3.3V has yet to be utilized by SATA drives, which is why SATA drives currently function properly with standard peripheral power connectors, but may be implemented in the near future.


  1. Introduction
  2. The PC power supply:
  3. The PC power supply label:
  4. Defining the connectors of an ATX power supply
  5. ATX power supplies DO NOT turn on at the flip of a switch
  6. Testing your power supply's voltage: Software vs. Multimeter
  7. Power Supply Efficiency
  8. The Derating Curve
  9. Power Factor Correction
  10. The resistance of modular connectors, adapters and splitters
  11. Conclusion

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