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What is the negative 12 volt supply used for in an ATX motherboard?

scottiebabe

Technology...  Never-ending compatibility problems....

Here is the 24-pin ATX connector of my new power supply

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Two of the pins are missing.

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Pin 20 was formally -5V and has been omitted from ATX power supplies for a very long time.

No negative 12 volt supply for me! Somehow I missed that looking at the product spec, and no one seem to mention it either.

Here is the ATX V3.0 power specification

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https://www.cybenetics.com/attachs/52.pdf 

The negative 12V supply is optional... great!

What is the negative 12V supply used for?

I am just going off the b350 chipset motherboard schematics available here: https://github.com/YeaTheMen/am4-motherboard/tree/main/Other%20Board%20Schematics 

In many cases nothing.

In one case an RS232 line driver/receiver 

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In another case, an amplifier/buffer for a second audio line out for the front panel

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Something to watch out for. In the two examples above, having the -12V rail tied to ground wouldn't be catastrophic, though the op-amp and rs232 would not function as intended... 

  • in reply to anniel747

    I just referenced them, because they are easily accessible. My PC is nothing fancy a Ryzen 5 3600 with a b450 chipset...

  • This is a known issue for a very small minority of users - generally speaking, you don't cross ATX major version numbers if you want to be sure. That means old 20-pin boards generally require an ATX 1.x supply, 24-pin boards with 12V P4 connector require an ATX 2.x supply and only the latest boards with ATX 3.x where 12VHPWR is required.

    The differences are slightly more than just the absence of rails made optional later (to be honest, most RS-232 ports use charge pumps like the venerable MAX232 and clones so as not to need a negative supply, and the other use of negative rails was for ISA cards which have long been deprecated). The original incarnations of ATX supplies had a bias towards 5V current as that's what many boards of the era needed (i.e. Pentium 1-3 era, roughly speaking). This later transitioned to 12V being the primary rail, so many PSUs from 2.x onward prioritise 12V output, which can lead to cross-regulation issues when used with boards expecting a 1.x power supply. Moving to ATX 3.x it seems multi-12V rails are now standard which is important because older versions of ATX had strict limits on rail maximum current which were a bit too restrictive for power-hungry graphic cards and CPUs.

    - Gough

  • Once upon a time, EEPROMs like the 6834 would need +5 and -12; EPROMs would use +5, -5 and +12.