Hooking them together would provide 24VDC. On a 350W PSU, I see -12V rated to supply 0.3A, which would allow for 24V fans drawing up to 7.2W of power, if desirable.
I don't see any reason to prefer 24V fans, however. The current losses over such short ranges are minimal, and I would think the larger economies of scale of 12V devices will keep 12V fans less expensive.
Do not use 24V fans for anything, if you have any control over it.
24V fans in general are poorly stocked. 24V 4-wire PWM fans are, these days, more or less special order items. There are thousands of 12V 4-wire PWM fans in stock at Mouser and Digi-Key. There are less than a dozen 24V 4W fans in stock, and most of those are sizes you probably don't want.
I'm a consulting EE these days. One of the jobs I get to do is clean up designs that aren't working. We always flag these fans when we see them... then nothing gets done because they're "available enough"... then when build time comes we can't source the fans because the "stock item" is completely out of stock and all possible substitutes are out of stock. But 12V versions are available.
So, yeah, stick to 12V fans whenever possible (or 5V for the little guys). Also genuinely never use AC fans, but that's a different story.
This led me to wonder why we still need the -12V supply - I know the -5V supply was removed a while ago because it was only needed for ISA slots. According to Wikipedia, -12V is for RS-232 ports and PCI slots, both of which are hardly seen on any current motherboards. It should be safe to use it for fans, but 7.2 watts is not very much power. Looking on Mouser, I see 24VDC fans that consume 1 watt or 18 watts depending on speed, so one could easily overload the -12V bus if they are trying to get a lot of CFM. I don’t see a compelling reason to use 24V fans in a PC.
From what I've read, ATX12VO really is 12V only; peripheral power connectors (sata/molex) will be connected to the motherboard, where the 5v and 3.3v (if needed) will be generated. 3.3V on sata power plugs was never reliably available and was rarely if ever used, and one of the pins was redfined to be a disable pin, so it's usually best for everyone if 3.3V isn't provided at all.
-12V for RS-232 is easily generated from +5V or similar sources; max232 chips have been available since the 80s
I'd suspect they'd go up to like 48V. It's the max voltage for USB-PD 3.1, which corresponds with being the effective highest voltage you can have before you're no longer "low voltage". This is also why PoE is approximately the same voltage.
People can easily overload 12V motherboard fan headers, too.
> if they are trying to get a lot of CFM.
If so, they probably have a PSU larger than the entry-level 350W unit I specified. Higher wattage PSUs will provide more power on the -12V rail as well.
> I don’t see a compelling reason to use 24V
Neither does anyone else. Still, correcting a bit of misinformation in the article.
huh, i didn't know they could be connected like that! i only knew of the -12V as a source for RS-232. thank you for the tip; i'll investigate this (i assume you're correct, just want to understand it for myself) and update the page. thank you very much!
> huh, i didn't know they could be connected like that!
'Ground' is not necessarily 0V, it is just convenient to use it like that. If 'ground' is 0V then 12V - 0V = 12V, but if you make 'ground' -12V then that is 12V - (-12V) = 24V.
This is false. ATX provides both +12V and -12V: * https://pinoutguide.com/Power/atxpower_pinout.shtml
Hooking them together would provide 24VDC. On a 350W PSU, I see -12V rated to supply 0.3A, which would allow for 24V fans drawing up to 7.2W of power, if desirable.
I don't see any reason to prefer 24V fans, however. The current losses over such short ranges are minimal, and I would think the larger economies of scale of 12V devices will keep 12V fans less expensive.