Question What is the -12v rail for, and what's the consequence of it being a little too high?

[King_V]

So, I've picked up 4 Dell 460W PSUs for dirt cheap. Apparently unused (guy bought the Dell computers, and immediately put in extra-beefy power supplies), and even if they were used, they're still not terribly old, but they are terribly inexpensive.

I have an electronic PSU tester, and I know that doesn't put any actual load on it. Still, all voltages came up fine . . EXCEPT for the negative 12 volt rail.

It's high on all 4 power supplies, but on two of them, enough so that the tester complained, coming in at 13.8V or 13.9V.

ASSUMING my tester isn't flaky, how bad is that reading? What are the consequences of high voltage on that rail?

 

[punkncat]

I wouldn't take this as "the" answer, but to say that I have a long electrical background in low voltage as it relates to burg/security/fire and most DC equipment can work properly on a range of voltage output typically between just below 10VDC to around 14VDC. I cannot for sure say that this is the case with a motherboard. I cannot recall having a power supply with the output that high on the 12V rail.

A quick search seems to indicate those are more commonly between 11.4-12.6 and this would be more in line with my own personal experience.

Do you happen to have a system you wouldn't mind taking the hit for the cause before putting to use?

 

[Grobe]

The -12V is probably not properly regulated - thereby dirt cheap. The -12VDC is slightly out of spec - according to atx standard the -12V output should lie between -10.8V and -13.2VDC.

Btw - here is a search AI web search that shed some lights on what this negative supply is used for:
https://www.perplexity.ai/search/what-is-the-ydP3IbymQtqsi.zbd4hWLg

 

[punkncat]

The -12V is probably not properly regulated - thereby dirt cheap. The -12VDC is slightly out of spec - according to atx standard the -12V output should lie between -10.8V and -13.2VDC.

Btw - here is a search AI web search that shed some lights on what this negative supply is used for:
https://www.perplexity.ai/search/what-is-the-ydP3IbymQtqsi.zbd4hWLg

Hey, if you don't mind would you link me to the documentation about the ATX standard on this?

 

[Grobe]

Hey, if you don't mind would you link me to the documentation about the ATX standard on this?

The formal documentation? For just the voltage limits, just have a look at wikipedia, about a third down the page you'll find the table for this:
https://en.wikipedia.org/wiki/ATX

 

[punkncat]

The formal documentation? For just the voltage limits, just have a look at wikipedia, about a third down the page you'll find the table for this:
https://en.wikipedia.org/wiki/ATX

Thank you, I was more curious than anything.

 

[King_V]

I wouldn't take this as "the" answer, but to say that I have a long electrical background in low voltage as it relates to burg/security/fire and most DC equipment can work properly on a range of voltage output typically between just below 10VDC to around 14VDC. I cannot for sure say that this is the case with a motherboard. I cannot recall having a power supply with the output that high on the 12V rail.

A quick search seems to indicate those are more commonly between 11.4-12.6 and this would be more in line with my own personal experience.

Do you happen to have a system you wouldn't mind taking the hit for the cause before putting to use?

Oh, yeah, it's not the 12V rails, but just the -12V rail.

Though, just in case, there's a system I've got that can take a hit.

... and I'm about to hit @Grobe's links as well

 

[bill001g]

If I remember correctly the -12 volts was used on the old style serial ports. Not sure what uses it on modern stuff

 

[beyondlogic]

its used to be used for old style serial ports

now its more commonly used for computers that want to use PCI-to-Serial adapter cards

source remember seeing this a few years back.

https://gamersnexus.net/guides/866-psu-dictionary-understand-psu-specs