Multiple voltage power-supply question

[dsharp02]

ATX supplies have multiple voltage rails.  If you need 7 volts, can you just hook something up between the 12v rail and the 5v rail?  I'm assuming the 5v rail has a diode or something to prevent power from flowing back into the power supply, so where would the current go? 

Thanks,
Dave

[katzohki]

You would hook something up between 12V and ground, such as a voltage divider. Even better would be a voltage regulator.

[David Hess]

What you suggest may be done if the current load on the lower voltage rail exceeds the current load between the rails.  The various power supply outputs will normally only source current and not sink current so a load between the +12 and +5 volt rails will pull the +5 volt rail positive unless an equal or greater amount of current is drawn from the +5 volt rail to ground.