Harold Hallikainen wrote:

>> 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'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 

Yes and no:

+9V_Wall -----------+------------ +5V_Out
                7905_Gnd           
0V_Wall -+- 7905_In 7905_Out ---- Gnd_Out (virtual ground)
         |                         
         +----------------------- -4V_Out


>> I've been thinking about a simple zener to clamp the voltage and
>> probably a fuse too before the regulator.

A zener alone is not a good thing: it needs a resistance to be able to drop
the voltage (or a fuse that it can blow). 

> 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. 

It's probably slower though, so it depends on the requirements whether this
is fast enough.

Gerhard

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