What are -12v and -5v supplies?

[haaammit]

Hey all. I'm in the process of finding a permanent 5v power supply from my psu and with various research have found that pin 9 on the 24 pin atx connector is the one I want.

Looking at the various pinouts I've seen that pin 14 is -12v (designated blue wire) and pin 20 is/was -5v (designated white cable, often not connected). Neither of these supplies are used much in systems these days, but how does a -12v or -5v work? I thought maybe the polarity would be different, but that doesn't work with common grounds, does it?

I mean it's obviously not going to be sucking 12 or 5 volts back up the wires, but what's going on?

Sorry if this is a dumb question, but I can't seem to find an answer! Many thanks

 

[james.miller]

As i recall, a looong time ago -5v was required for memory and up until not quite soo long ago but still long ago -12v was required for rs232 ports. These days such voltages are produced by the hardware instead.

I thought maybe the polarity would be different, but that doesn't work with common grounds, does it?

That's exactly how it works If you were to measure the potential difference between -12v and +12v, you'd measure 24v.

Careful with that 5v standby rail, they arent typically spec'd to supply much current

 

[Rroff]

Can also be useful for audio - though generally isn't the approach used with modern PC systems - AFAIK it is literally a centre tapped transformer:

+12V O---------------
CMN O---------------
-12V O---------------

So you get 24V measured rail to rail.

EDIT: You can do all kinds of funky things to get voltages in between, etc. but be a bit careful as the PSU might not expect that kind of behaviour as above things may be designed only to work for a specific function and general purpose use might result in some strange behaviour or overloading stuff.

 

[EsaT]

Besides low max output 5V standby isn't designed for any long term source for significant power.
DC-DC PSUs main 5V is far better for 5V power supply.

 

[haaammit]

Electricity confuses me sometimes with stuff like this lol, but thanks for the replies. I'm still not 100% sure on how the - supply works, but it was just out of curiosity!

The item I want to power is a remote reciever that plugs into an always powered usb header, so is designed to only need a very low current as supplied by the 5VSB supply here All my motherboard header cables are custom made and I cba to make another so was going to pinch off the 5vsb supply directly out of the psu and directly wire the adaptor up.

 

[Baldbloke]

Electricity confuses me sometimes with stuff like this lol, but thanks for the replies. I'm still not 100% sure on how the - supply works, but it was just out of curiosity!

I used to confuse me when I was a whippersnapper just learning about electronics. "How can a voltage be minus?" Blooming 'eck, I'd wonder.

Voltages are measured with respect to each pole.

E.g.
Terminal A +12v,
Terminal B 0v,
Terminal C -12v
It's just how you measure WITH RESPECT TO other terminals. Polarity is determined by your point of reference. (Current capacity is dependant on the internal circuitry of the psu source. But that's a whole other can 'o worms.)

Terminal A with respect to terminal B has a magnitude of 12v.
Terminal B to C has also a magnitude of 12v
Terminal A to C has a magnitude of 24v.

I'm sure someone else will explain it better.

There. you probably didn't want to know that but it bumps my post count up by one