why does my power supply ATX connector have a 20+4 breakout?

[jtspade]

I just got a new power supply and the 24 pin ATX connector is actually a 20+4 connector. The first 20 pins are on one connector connect just fine into the MOBO. However, the last 4 are on another connector that (due to the shape of the connector) I am unable to plug in both the 20 and 4 pin connectors into the MOBO. I just have the 20 pin connector seated, and the 4 pin connector dangling. Is this ok? What's the deal with 20+4 configuration?

 

[maxwellmelon]

the 20+4 is to support some abnormal lower power motherboards. if you have a 24 pin motherboard you need to use both connectors. usually you have to link the two connectors together then plug them into the board at once

 

[InvalidError]

What's the deal with 20+4 configuration?

before ATX12V and PCIe auxiliary power connectors, CPUs and add-in boards were stretching the limits of the 20 pins connector's specifications, so the ATX spec upgraded the connector with one extra 3.3V, 5V, 12V and GND pin and we got the 24 pins connector. Now that nearly everything is powered from 12V and the major loads (CPUs and GPUs) have their own 12V supply connector, the +4 is largely superfluous.

As Max said, you usually need to "snap" the +4 to the 20 pins connector before you connect it to the motherboard. The two parts are usually keyed by small pins to make it difficult if not impossible to insert them incorrectly.

 

[jtspade]

Thanks for the context. I am looking at the connector and there are 2 painted arrows, one on each piece. Based on the natural flow of the fannout, the 'natural' position is these arrows point toward each other. It's difficult to go against the grain of the cable so the arrows point in the same direction. My assumption is theset arrows should point toward each other..?

 

[InvalidError]

My assumption is theset arrows should point toward each other..?

Probably. As I wrote, the connector is designed to make it physically difficult if not impossible to plug the connector in incorrectly. The painted arrows you mention are nothing more than a clue to help you find the correct position.

If you look at the shape of the holes on the motherboard and PSU connector, you will notice a pattern of complete squares and squares with corners filled (motherboard) or chamfered (connector). These make it impossible to insert the connector in anything but the correct orientation without great force.

Edit: there are two ways you can insert the connector incorrectly without excessive force: by inserting it while being four or eight positions off either way or being off by one row and two or six positions since the connector keying pattern repeats every four positions lengthwise and is the same but offset by two positions on both rows. It is difficult to imagine being that far off by accident and not noticing it.