>Given a source power supply nominally +12v (a car battery), can I use a
7805
>and a 7905 together to get a +5/0/-5 regulated power supply? or am I
>misunderstanding what a negative regulator actually does?

You could, BUT be very careful that your 0V point does not come into contact
with the car body. If doing something like this then make sure that it is
not possible to short the internal "ground" to anything outside your box.

However on thinking about it a bit more, each regulator will have only 1V
drop across it, which is not enough to maintain a stable output voltage.
7800/7900 series regulators need something near 3V across the regulator to
maintain output voltage stability.

I would suggest that you use a 7805 to get +5V, and then a switching
regulator to get -5V from the +12V. This also means that your circuit ground
is at the same potential as the car body, so accidental connection of the
two is not a problem.

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