So I have been working on an old power supply that outputs negative voltage. The positive output is a 0 volts. The negative is at -15v.
Can sombody explain this concept to me and why one would use such a setup?
Thanks,
-Dave
Power supplies don't really care which side you take as the reference.
If you have a supply with,say, two output terminals which are not connected to the case of the supply,("Floating" in other words),you can make either the positive or negative terminal the reference in a circuit you are operating off the supply, by linking that terminal to the common (usually "earth" or "ground")of the circuit.
It was fairly common in Communications sites to run the main DC supply with the positive terminal "common" (in fact,earthed),so as to avoid the problems of Electrolysis,where ,with negative earthing,there was a greater risk of corrosion of the earth connection.
Equipment designed to run off this DC supply would be designed to operate with a "positive earth".
The power supply you have may have been used to test this equipment.
Another possibility is that it may have been designed for early solid state equipment.
Germanium technology was found to be easier to design in PNP type than in NPN.
With PNP,the Collector is connected to the negative supply,so it made sense to make positive "common"(or chassis,or ground,depending on the terminology).
Note: the terms "earth"& "ground" are used fairly carelessly in Electronics as a synonym for "common".
In a particular circumstance "common" may be connected to the actual "dirt",as in the Comms sites I referred to.
In many cases it will not.