[sdiy] Newbie warning! What is negative DC power?

[Andreas Gaunitz]

I'm planning to buy a DC power supply, with limited current and digital 
readout etc, for my upcoming projects.

Now, I am choosing between two fairly cheap ones: One that has +-15 
volts, and the other that has 0-30 v. Now, I always thought that a 
negative DC power was a breeze to produce: just set the ground to a 
positive voltage value. Did I dream this?

I checked out the 7805/7809 component range and it seems the ones 
producing neg current have to be supplied with neg current - duh. So 
how to get the neg current in the first place?

I'm getting more and more confused the more I read...

regards
Andreas