21 July 2012

Why I Have 23 Pins In My ATX PSU Only?

Ever checked your PSU motherboard connectors? Have ever you wondered why you only have 23 pins instead of  20+4 pins that are displayed on the sticker in its device?

I newly bought PSU 600 watts, and I noticed something: It shows only 20+4 pins (so it totals 24), but when I checked its motherboard connectors, I only saw 23 pins. Hence, I asked myself, is this a factory defect? Or I just simply ignored the fact that it is a new design for PSU.

What is ATX?

ATX (Advanced Technology eXtended)  is a motherboard form factor specification developed by Intel in 1995 to improve on previous de facto standards. (source: http://en.wikipedia.org/wiki/ATX)

The ATX Power Supply has three main outputs: 
  • +3.3 V
  • +5 V 
  • +12 V
The Pin 20 a  −5 V output was initially required because it was supplied on the ISA bus(older ATX version 1.3 below). The Modern PC has been removed from ISA bus in a later version of ATX Standard.

With my last post about troubleshooting about the computer won't turn on. , I mentioned there about the PSU on how to test with it. To solve your problem with your PC won't start up. Just check it out.

Why it has only 23 pins only?

It is Really 24 pins but has a reserved one pin
PSU Motherboard Connector (usually called P1)  is a connector that has 24 pins that provide power to the motherboard. 

Information from  PSU 

Taken from Wikipedia

Image Taken from  https://en.wikipedia.org/wiki/ATX 
  • Pins 8, and 16 (shaded) are control signals, not power:
  • Power on is pulled up to +5 V by the PSU and must be driven low to turn on the PSU.
  • Power product is low when other outputs have not yet reached, or are about to leave, correct voltages.
  • Pin 13 supplies +3.3 V power and also has a second thinner wire for remote sensing
  • Pin 20 (formerly −5 V, white wire) is absent in current power supplies; it was optional in ATX and ATX12V ver. 1.2, and deleted as of ver. 1.3.
  • The right-hand pins are numbered 11–20 in the 20-pin version.
Pin 20 is reserved. In older version of ATX, it has a pin in 20 slots pin colored white (formerly -5V). Therefore, if you see 23 pins only and one is absent, then your PSU is a current power supply unit.

Additional data from Wikipedia:

Four wires have special functions:

PS_ON# or Power on is a signal from the motherboard to the power supply. When the line is connected to the ground (by the motherboard), the power supply turns on. It is internally pulled up to +5 V inside the power supply

PWR_OK or Power good is output from the power supply that indicates that its output has stabilized and is ready for use. It remains low for a brief time (100–500 ms) after the PS_ON# signal is pulled low.

+5 VSB or +5 V standby supplies power even when the rest of the supply lines are off. This can be used to power the circuitry that controls the Power On signal.

+3.3 V sense should be connected to the +3.3 V on the motherboard or its power connector. This connection allows for remote sensing of the voltage drop in the power supply wiring.

Generally, supply voltages must be within ±5% of their nominal values at all times. The little-used negative supply voltages, however, have a ±10% tolerance. There is a specification for ripple in a 10 Hz–20 MHz bandwidth:


So if you are wondering why you see only 23 pins instead of 24 pins. The answer is: Pin 20 (which is the absent Pin and formerly -5V colored white wire )has no longer used in Modern PC, and it is now Reserved status. ^_^


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