PSU Current on negative line

Question

_{(source: tomshardware.com)}

While i am quite familiar with most of the specifications on a power supply unit like this one, I fail to understand what is going on with the negative voltages. I have a few theories, though.

1. The negative voltages are just the grounds which correspond to the matching positive voltages.
2. These are actually at negative potential relative to ground

However, neither of these explanations really explains why you would have these outputs sourcing half an amp of current.

If they were the grounds, then they would have to sink just as much current as the positive side was putting out. In addition, there isn't a negative output to match with every positive output.

However, if they really were at a negative relative potential to ground, I would not expect them to be sourcing current, because that current would have to flow against a voltage differential.

So, what is going on with these negative voltages?

Answer 1

It would probably make more sense if the label were adjusted sligthly:

Those -5V and -12V outputs really are negative voltages (not the ground returns for +5V and +12V) and they do sink current.

Answer 2

I don't know anything about this specific power supply. However, my guess is that:

• There is one ground for everything
• -5V literally means a line that has a voltage of -5V relative to ground
• (-5V, 0.8A) really means that the -5V can sink 0.8A of current.

For another example, take a look at the LM79xx datasheet. These are linear voltage regulators that output a negative voltage. However, on the front page of the datasheet, it says:

The LM79XX series of 3-terminal regulators is available with fixed output voltages of -5V, -8V, -12V, and -15V [...] and is capable of supplying 1.5A of output current.

However, these would typically sink up to 1.5A of current. I imagine that supplying current either means sinking or sourcing, depending on the voltage level - you could say that it's the magnitude of the current.

Answer 3

Older computers needed negative voltage for certain functions. The biggest one that comes to mind is RS-232 signaling, which used +12/-12 volts. Almost nothing (if anything at all) in a modern computer uses negative voltage. Because of this, the voltage regulators for the negative voltages are much smaller than the regulators for the more commonly used positive voltage rails.