I'm not seeing how you're getting the positive supply. It seems like you'd
get -5V and -9V. You could do a voltage divider to "center tap" the 9V
supply (I think TI made or does make "artificial ground" chips consisting
of a voltage divider driving an op amp voltage follower that would give
you a low impedance center tap). As far as protecting it from overvoltage,
some regulators handle overvoltage, reverse battery, etc. Otherwise, I'd
use a PTC thermistor in series and the zener to ground. I prefer this over
a fuse since it's self resetting.

Harold

> Hi,
>
> I used to be part of the list a while ago, but for one reason or another
> I left :-(
>
> Anyway, I have a strange power supply circuit that I need to protect and
> its confusing me.
> I have a 9V linear wall cube supply which is powering a 7905 -5V
> regulator. The +9V from the wall cube is connected to the 7905 GND pin,
> the GND from the wall cube is connected to the 7905 Input pin. This then
> ends up giving me a +5V, 0V and -4V power supply.
> This works fine, but I am conscious that if someone plugs in a power
> supply greater than 9V I will get a larger negative supply rail voltage.
> EG if someone plugs in a 24V supply I will get -19V!!!
>
> I've been thinking about a simple zener to clamp the voltage and
> probably a fuse too before the regulator.
>
> Any suggestions? It's been confusing the hell out of me this afternoon.
>
> Thanks for any help,
>
> G
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